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Showing papers in "Marine Ecology Progress Series in 2001"


Journal ArticleDOI
TL;DR: For example, a recent review of the early phase of the coastal eutrophication problem can be found in this article, where the authors suggest that the early (phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophicication by the 1960s.
Abstract: A primary focus of coastal science during the past 3 decades has been the question: How does anthropogenic nutrient enrichment cause change in the structure or function of nearshore coastal ecosystems? This theme of environmental science is recent, so our conceptual model of the coastal eutrophication problem continues to change rapidly In this review, I suggest that the early (Phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophication by the 1960s The Phase I model emphasized changing nutrient input as a signal, and responses to that signal as increased phytoplankton biomass and primary production, decomposition of phytoplankton- derived organic matter, and enhanced depletion of oxygen from bottom waters Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (leading to large differences among estuarine-coastal systems in their sensitivity to nu- trient enrichment); and (2) a complex suite of direct and indirect responses including linked changes in: water transparency, distribution of vascular plants and biomass of macroalgae, sediment biogeochem- istry and nutrient cycling, nutrient ratios and their regulation of phytoplankton community composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic invertebrates, and subtle changes such as shifts in the seasonality of ecosystem functions Each aspect of the Phase II model is illustrated here with examples from coastal ecosystems around the world In the last section of this review I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes constrain or amplify the responses of coastal ecosystems to nutrient enrichment? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydro- logic manipulations) to change coastal ecosystems? (3) How are responses to multiple stressors linked? (4) How does human-induced change in the coastal zone impact the Earth system as habitat for humanity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?

2,658 citations


Journal ArticleDOI
TL;DR: The combination of Δ+ and Λ+ is seen to provide a statistically robust summary of taxonomic (or phylogenetic) relatedness patterns within an assemblage, which has the potential to be applied to a wide range of historical data in the form of simple species lists.
Abstract: A further biodiversity index is proposed, based on taxonomic (or phylogenetic) relatedness of species, namely the Œvariation in taxonomic distinctness¹ (VarTD, Λ+) between every pair of species recorded in a study. It complements the previously defined Œaverage taxonomic distinctness¹ (AvTD, Δ+), which is the mean path length through the taxonomic tree connecting every pair of species in the list. VarTD is simply the variance of these pairwise path lengths and reflects the unevenness of the taxonomic tree. For example, a species list in which there are several different orders represented only by a single species, but also some genera which are very species-rich, would give a high Λ+ by comparison with a list (of equivalent Δ+) in which all species tended to be from different families but the same order. VarTD is shown to have the same desirable sampling properties as AvTD, primarily a lack of dependence of its mean value on the sample size (except for unrealistically small samples). Such unbiasedness is of crucial importance in making valid biodiversity comparisons between studies at different locations or times, with differing or uncontrolled degrees of sampling effort. This feature is emphatically not shared by indices related to species richness and also not by properties of the phylogeny adapted from proposals in other, conservation contexts, such as Œaverage phylogenetic diversity¹ (AvPD, Φ+). As with AvTD, the VarTD statistic for any local study can be tested for Œdeparture from expectation¹, based on a master taxonomy for that region, by constructing a simulation distribution from random subsets of the master list. The idea can be extended to summarising the joint distribution of AvTD and VarTD, so that values from real data sets are compared with a fitted simulation Œenvelope¹ in a 2 d (Δ+, Λ+) plot. The methodology is applied to 14 species lists of free-living marine nematodes, and related to a master list for UK waters. The combination of AvTD and VarTD picks out, in different ways, some degraded locations (low Δ+, low to normal Λ+) and the pristine island fauna of the Scillies (normal + , high Λ+). The 2 indices are also demonstrated to be measuring effectively independent features of the taxonomic tree, at least for this faunal group (although it is shown theoretically that this will not always be the case). The combination of Δ+ and Λ+ is therefore seen to provide a statistically robust summary of taxonomic (or phylogenetic) relatedness patterns within an assemblage, which has the potential to be applied to a wide range of historical data in the form of simple species lists.

908 citations


Journal ArticleDOI
TL;DR: A Browns Bank benthic habitat map is developed as a conceptual model summarizing the understanding of the bank ecology and highlights the utility of multibeam bathymetric sonar for interpretation of sea floor sediments and for extrapolating benthics habitat characteristics across large areas of seafloor.
Abstract: This study presents results of a new approach for sea floor habitat mapping based on an integrated analysis of multibeam bathymetric data, associated geoscientific information, and benthos data from Browns Bank on the southwestern Scotian Shelf, off the Canadian Atlantic coast. Based on sea floor sediment maps and statistical analysis of megabenthos determined from photographs, 6 habitats and corresponding associations of benthos were derived and mapped. The habitats are dis- tinguished primarily on the basis of sediment type and water depth. Additional factors are sea floor geomorphology, habitat complexity, and relative current strength. A Browns Bank benthic habitat map is developed as a conceptual model summarizing the understanding of the bank ecology. This study highlights the utility of multibeam bathymetric sonar for interpretation of sea floor sediments and for extrapolating benthic habitat characteristics across large areas of sea floor.

398 citations


Journal ArticleDOI
TL;DR: The benthic macrofauna of a group of small seamounts south of Tasmania was surveyed with a dredge and camera to assess the impact of trawling for orange roughy (Hoplostethus atlanti-cus; Trachichthyidae) and the efficacy of a proposed marine reserve as mentioned in this paper.
Abstract: The benthic macrofauna of a group of small seamounts south of Tasmania was surveyed with a dredge and camera to assess the impact of trawling for orange roughy (Hoplostethus atlanti- cus; Trachichthyidae) and the efficacy of a proposed marine reserve. The seamounts were generally 300 to 600 m high and the peaks ranged from 660 to 1700 m depth. The fauna was diverse: 262 spe- cies of invertebrates and 37 species of fishes were enumerated, compared with 598 species of inver- tebrates previously reported from seamounts worldwide. On seamounts that peaked at depths 1400 m. These seamounts were dominated by sea urchins and had lower biomass and fewer species per sample. However, few species were restricted to either the shallowest or deep- est depths sampled. The fauna unique to the region's seamounts appears to be adequately repre- sented within a recently established 'Marine Protected Area' that encloses 12 seamounts that peak at depths >1150 m.

348 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated anthropogenic disturbance in the Mediterranean and found that the disappearance of canopy algae causes an increase in cover of turf-forming species in disturbed habitats, which is consistent at the spatial and temporal scales examined in the study.
Abstract: Anthropogenic disturbances affect natural populations and assemblages by interacting with fundamental ecological processes. Field experiments simulating the effects of human activities at the appropriate spatial and temporal scales are useful to understand these interactions and eventually to predict their ecological consequences. In the Mediterranean, low-shore habitats of rocky coasts are often dominated by canopy algae Cystoseira spp., but these species are frequently replaced by assemblages of turf-forming algae and mussel beds. We propose that anthropogenic disturbance is the proximate cause of loss of Cystoseira in the Mediterranean, and that the disappearance of canopy algae causes an increase in cover of turf-forming species in disturbed habitats. Two hypotheses were investigated to test this proposition: (1) canopy algae will be dominant in relatively pristine habitats while turf-forming algae will be more abundant in urban areas, and (2) removal of canopy algae in unpolluted areas will result in the development of assemblages similar to those found in urban areas. We tested the first hypothesis by comparing patterns in abundance of Cystoseira and turf-forming algae at a number of locations in urban areas and in areas far from distinct sources of anthropogenic disturbance in the northwest Mediterranean. The second hypothesis was tested by conducting a large- scale manipulative experiment, involving the experimental removal of Cystoseira and several spatial and temporal repetitions of the manipulation. Turf-forming algae were always dominant in urban areas where Cystoseira was nearly absent. In contrast, canopy algae often dominated rocky shores in relatively pristine areas of the northwest Mediterranean. The removal of Cystoseira generally caused an increase in the percentage cover of turf-forming algae and a decrease in the abundance of inverte- brates. These changes were already evident 4 mo after manipulation and were consistent at the spatial and temporal scales examined in the study. Assemblages in cleared patches were qualitatively similar to those occurring where Cystoseira was naturally absent, but quantitative differences in the relative abundance of several taxa were still evident by the end of the study. These results support a cause- effect relationship between anthropogenic disturbance and loss of Cystoseira in the northwest Mediterranean and allow for quantitative predictions of the indirect consequences of disturbing canopy algae for the whole understory assemblage. Management options aimed at conserving these plants should simultaneously preserve other components of the assemblage.

345 citations


Journal ArticleDOI
TL;DR: This review deals with the measurement of filtration rates in suspension feeding bivalves, and some reliable filTration rate data obtained under optimal laboratory conditions are presented.
Abstract: This review deals with the measurement of filtration rates in suspension feeding bivalves. Currently used methods are described, and some reliable filtration rate data obtained under optimal laboratory conditions are presented. The different methods have often caused troubles, and a number of problems and shortcomings are brought to light. The conflicting data on filtration rates seem partly to be due to the incorrect use of methods, and partly to be caused by differences in experimental conditions.

336 citations


Journal ArticleDOI
TL;DR: Significant interspe- cific variations of both the sulfur isotope ratios and the fatty acid composition of these consumers suggest that there are a variety of pathways by which seagrass organic matter reaches invertebrates at lower trophic levels of the community food web.
Abstract: Carbon and sulfur stable isotope ratios, as well as fatty acid composition of tissues, of dominant consumer species were determined and compared to those of potential food sources in an isolated community of Zostera marina in a shallow, semi-enclosed inlet of the Sea of Japan. Of the 6 dominant species of invertebrates, 4 species were enriched in 13 C, compared to all sampled carbon sources alternative to Z. marina. Among them, the grazing gastropods Littorina squalida and Homa- lopoma sangarense exhibited the most enriched δ 13 C values. On the dual δ 13 C versus δ 34 S plot, these mollusks occupy an intermediate position between Z. marina and epiphytes, suggesting nearly equal proportions of organic carbon from both nutritional sources. In lipids of H. sangarense there was a high content of the 18:1(n-7) acid characteristic of aerobic bacteria; however, another grazer (L. squalida) showed the lowest content of bacterial fatty acids among all consumers. Other highly 13 C-enriched consumers were the surface-deposit-feeding mollusks, the gastropod Batillaria cumingii and the bivalve Macoma incongrua; however, their δ 34 S values were markedly lower than those of any of the primary producers sampled, including Z. marina. Although the high δ 13 C values of grazers and surface-deposit feeders are suggestive of a great contribution of Z. marina organic car- bon, no substantial concentrations of seagrass marker fatty acids were detected. Significant interspe- cific variations of both the sulfur isotope ratios and the fatty acid composition of these consumers sug- gest that there are a variety of pathways by which seagrass organic matter reaches invertebrates at lower trophic levels of the community food web. Dominant filter feeders, the bivalves Ruditapes philippinarum and Pillucina pisidium, had carbon drastically different in isotopic composition from Z. marina organic matter. Body tissues of P. pisidium and especially its gills, which bear symbiotic bacteria, were dramatically depleted both in 13 C and 34 S compared to all sources of photosyntheti- cally fixed carbon in the bay. Very low δ 13 C and δ 34 S values of P. pisidium body together with a high content of 18:1(n-7) acid suggest a leading role of sulfur-oxidizing symbiotic bacteria in the nutrition of this species. R. philippinarum was only slightly 13 C-enriched, compared to POM, and was the only consumer which had the high concentration of fatty acids characteristic of plankton, particularly 22:6(n-3). At the same time, it was much more 34 S-depleted than would be expected, assuming neg- ligible contribution of Z. marina detritus to its food. This mollusk showed a high content of branched fatty acids, especially the iso17:0 and anteiso17:0 acids characteristic of bacteria from sediment; this suggests that R. philippinarum assimilated notable amounts of bacteria from resuspended sediment. Furthermore, the contribution of 34 S-depleted bacteria, which inhabit reduced sediment, to R. philip- pinarum nutrition was high enough to result in the observed depletion of 34 S in mollusks. Further progress in food web studies of seagrass ecosystems using a complex of multiple stable isotope and fatty acid analyses would appear possible on the basis of analysis of separate components of the sea- grass epiphytic community and micro- and meiobenthic organisms, inhabiting surface sediments.

333 citations


Journal ArticleDOI
TL;DR: Experimental evidence for high survivorship of Acropora dig- itifera coral colonies that are subjected to both high SSTs and high-water flow is shown, while corals that were subjected toboth high S STs and low-waterflow showed low survivorship.
Abstract: In the western Pacific during 1998, coral bleach- ing, or the paling of corals through loss of pigmentation or loss of symbiotic algae (zooxanthellae), coincided with some of the warmest sea-surface temperatures (SSTs) on record. How- ever, there was considerable spatial variation in coral sur- vivorship; for example, corals of the same species at different locations around the Ryukyu Islands (Japan), within kilome- ters of each other, showed vastly different responses. Some locations experienced 100% coral mortality while other loca- tions, nearby, suffered little coral mortality. Here we show experimental evidence for high survivorship of Acropora dig- itifera coral colonies that were subjected to both high SSTs (ranging from 26.22 to 33.65°C) and high-water flow (50 to 70 cm s -1 ), while corals that were subjected to both high SSTs and low-water flow (2 to 3 cm s -1 ) showed low survivorship. All experiments were conducted under high irradiance (~95% photosynthetically active radiation). We also empiri- cally show that no coral mortality occurred when SSTs were below 30°C (ranging from 26.64 to 29.74°C) under similar flow regimes. The spatial differences in coral mortality during the 1998 bleaching event may have been, in part, a result of differences in water-flow rates that induced differential rates of passive diffusion, which varied among habitats.

330 citations


Journal ArticleDOI
TL;DR: The stable isotope data, in combination with measured high biomass and primary production rates of the epiphytic algae, strongly suggest that these algae are the primary source of organic matter for higher trophic levels in seagrass beds of Mississippi Sound.
Abstract: Multiple stable isotope analyses were employed to examine food web dynamics in a northern Gulf of Mexico seagrass system in which epiphytic algae were the single most important primary productivity component, being responsible for 46 and 60 % of total system and benthic primary production, respectively. The seagrass Halodule wrightii Ascherson contributed only 13% to total system primary production on an annual basis. Stable isotope ratios of carbon (δ 13 C), nitrogen (δ 15 N), and sulfur (δ 34 S) were measured for producer and consumer samples collected from May 1989 through November 1992. Epiphytes and leaves of H. wrightii had distinct δ 13 C values (-17.5 vs -12‰, respectively) as well as distinct δ 34 S values‰ (+18 vs +11‰, respectively). δ 13 C values for the sand microflora, occasional macroalgae, and phytoplankton were -16, -17, and -22‰, respectively; δ 15 N values were lowest for epiphytes and H. wrightii (+6‰) and highest for phytoplankton (+10‰). Virtually all consumers had δ 13 C values that fell within a narrow range of - 20 to -15%o, which included all δ 13 C values of epiphytes and the sand microflora but none of those for either H. wrightii or phytoplankton. Values for δ 15 N for consumers fell within a range of + 8 to +16‰ spanning herbivorous species with diets of microalgae to carnivorous species feeding at secondary to tertiary levels in the local food webs. Consumer values for δ 34 S ranged from +4 to +20‰ (mean = 14.2‰), and indicate a stronger influence of seawater-derived sulfate than sediment-associated sulfides. The stable isotope data, in combination with measured high biomass and primary production rates of the epiphytic algae, strongly suggest that these algae are the primary source of organic matter for higher trophic levels in seagrass beds of Mississippi Sound. The contribution af H. wrightii to the food web appears to be minimal. The overall picture that has emerged based on the present and previous stable isotope studies is one of the major trophic importance of benthic microalgae (i.e. epiphytic and sediment-associated) in seagrass beds.

329 citations


Journal ArticleDOI
TL;DR: It is recommended that secondary production rates of Calanus finmarchicus and possibly other lipid-storing copepods not be estimated from egg production mea- surements alone, as has been suggested for other species of copepod, because growth, including structural growth, is not equivalent for all stages.
Abstract: Development rates, nitrogen- and carbon-specific growth rates, size, and condition were determined for the copepod Calanus finmarchicus reared at 3 temperatures (4, 8, and 12°C) at non-limiting food concentrations and 2 limiting food concentrations at 8°C in the laboratory. Devel- opment rates were equiproportional, but not isochronal. Naupliar stage durations were similar, except for non-feeding stages, which were of short duration, and the first feeding stage, which was prolonged, while copepodite stage durations increased with increasing stage of development. Under limiting food concentrations at 8°C, development rates were prolonged but similar relative patterns in stage durations were observed. Body size (length and weight) was inversely related to temperature and positively related to food concentration. Condition measurements were not affected by tempera- ture, but were positively related to food concentration. Growth rates increased with increasing tem- perature and increased asymptotically with increasing food concentration. At high food concentra- tions, growth rates of naupliar stages were high (except for individuals molting from the final naupliar stage to the first copepodite stage, in which growth rates were depressed), while growth of copepodites decreased with increasing stage of development. Neither nitrogen nor carbon growth rates, the former a proxy for structural growth, were exponential over the entire life cycle, but rather sigmoidal. Carbon-specific growth rates were greater than nitrogen-specific growth rates, and this difference increased with increasing stage of development, reflecting an augmentation in lipid depo- sition in the older stages. However, nitrogen and carbon growth rates were more similar under food- limited conditions. Based on this study, we recommend that secondary production rates of Calanus finmarchicus and possibly other lipid-storing copepods not be estimated from egg production mea- surements alone, as has been suggested for other species of copepods, because growth, including structural growth, is not equivalent for all stages.

321 citations


Journal ArticleDOI
TL;DR: Comparison of fish densities from the 11 different inland bays of Curacao indicates that for the nursery species the degree of depen- dence on a combination of mangroves and seagrass beds as nurseries for juvenile fish is high for Ocyurus chrysurus and Scarus iserti, and the dependence on seagRass beds ishigh for Haemulon parrai, H. flavolineatum and L. mahogoni.
Abstract: Mangroves and seagrass beds are considered important nursery habitats for coral reef fish species in the Caribbean, but it is not known to what degree the fish depend on these habitats. The fish fauna of 11 different inland bays of the Caribbean island of Curacao were compared; the bays contain 4 different habitat types: seagrass beds in bays containing mangroves, seagrass beds in bays lacking mangroves, mud flats in bays containing mangroves and seagrass beds, and mud flats in bays completely lacking mangroves and seagrass beds. Principal component analysis showed a high similarity of fish fauna among bays belonging to each of the 4 habitat types, despite some dif- ferences in habitat variables and human influence between bays. Juveniles of nursery species—fish species using mangroves and seagrass beds as juvenile nurseries before taking up residence on reefs—showed highest abundance and species richness on the seagrass beds, and on the mud flats near mangroves and seagrass beds, but were almost absent from bays containing only mud flats. The high abundance and species richness on the mud flats near nursery habitats can be explained by fishes migrating from the adjacent mangroves/seagrass beds to the mud flats. Seagrass beds near to mangroves showed a higher richness of nursery species than did seagrass beds alone, suggesting an interaction with the mangroves resulting in an enhancement of species richness. Comparison of fish densities from the 4 different habitat types indicates that for the nursery species the degree of depen- dence on a combination of mangroves and seagrass beds as nurseries for juvenile fish is high for Ocyurus chrysurus and Scarus iserti, the dependence on seagrass beds is high for Haemulon parrai, H. sciurus, Lutjanus apodus, L. griseus, Sparisoma chrysopterum and Sphyraena barracuda, and the dependence on mud flats near mangroves/seagrass beds is high for L. analis. The dependence on mangroves and/or seagrass beds is low for Chaetodon capistratus, Gerres cinereus, H. flavolineatum and L. mahogoni, which can also use alternative nursery habitats.

Journal ArticleDOI
TL;DR: In this article, X-ray electron microprobe analysis of the otoliths of 69 yellow and silver eels, collected from 10 localities in seawater and freshwater habitats around Japan, and classified their migratory histories.
Abstract: To confirm the occurrence of marine residents of the Japanese eel, Anguilla japonica, which have never entered freshwater ('sea eels'), we measured Sr and Ca concentrations by X-ray electron microprobe analysis of the otoliths of 69 yellow and silver eels, collected from 10 localities in seawater and freshwater habitats around Japan, and classified their migratory histories. Two-dimen- sional images of the Sr concentration in the otoliths showed that all specimens generally had a high Sr core at the center of their otolith, which corresponded to a period of their leptocephalus and early glass eel stages in the ocean, but there were a variety of different patterns of Sr concentration and concentric rings outside the central core. Line analysis of Sr/Ca ratios along the radius of each otolith showed peaks (ca 15 × 10 -3 ) between the core and out to about 150 µm (elver mark). The pattern change of the Sr/Ca ratio outside of 150 µm indicated 3 general categories of migratory history: 'river eels', 'estuarine eels' and 'sea eels'. These 3 categories corresponded to mean values of Sr/Ca ratios of ≥ 6.0 × 10 -3 for sea eels, which spent most of their life in the sea and did not enter freshwater, of 2.5 to 6.0 × 10 -3 for estuarine eels, which inhabited estuaries or switched between different habitats, and of <2.5 × 10 -3 for river eels, which entered and remained in freshwater river habitats after arrival in the estuary. The occurrence of sea eels was 20% of all specimens examined and that of river eels, 23%, while estuarine eels were the most prevalent (57%). The occurrence of sea eels was confirmed at 4 localities in Japanese coastal waters, including offshore islands, a small bay and an estuary. The finding of estuarine eels as an intermediate type, which appear to frequently move between different habitats, and their presence at almost all localities, suggested that A. japonica has a flexible pattern of migration, with an ability to adapt to various habitats and salinities. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, and this form of diadromy should be defined as facultative catadromy, with the sea eel as one of several ecophenotypes. Furthermore, this study indicates that eels which utilize the marine environment to various degrees during their juve- nile growth phase may make a substantial contribution to the spawning stock each year.

Journal ArticleDOI
TL;DR: In this article, three cruises were conducted to simultaneously quantify the fine-scale optical and physical structure of the water column, and data from 120 profiles were used to investigate the temporal occurrence and spatial distribution of thin layers of phytoplankton as they relate to varia- tions in physical processes.
Abstract: In 1996 three cruises were conducted to simultaneously quantify the fine-scale optical and physical structure of the water column. Data from 120 profiles were used to investigate the temporal occurrence and spatial distribution of thin layers of phytoplankton as they relate to varia- tions in physical processes. Thin layers ranged in thickness from a few centimeters to a few meters. They may extend horizontally for kilometers and persist for days. Thin layers are a recurring feature in the marine environment; they were observed and measured in 54% of our profiles. Physical pro- cesses are important in the temporal and spatial distribution of thin layers. Thin layer depth was closely associated with depth and strength of the pycnocline. Over 71% of all thin layers were located at the base of, or within, the pycnocline. The strong statistical relationships between thin layers and physical structure indicate that we cannot understand thin layer dynamics without understanding both local circulation patterns and regional physical forcing.

Journal ArticleDOI
TL;DR: It is suggested that krill and higher predators have a controlling influence on lower trophic levels, both stabilising population sizes and maintaining high rates of energy flow in this food web, through selective grazing and possibly also through nutrient regeneration.
Abstract: The South Georgia region is characterised by high biomass and productivity of phyto- plankton, zooplankton and vertebrate predators. Important commercial fisheries have been based at the island since the late 1700s, initially exploiting seals and whales, and currently taking krill Euphausia superba and finfish. Despite studies dating from the beginning of the last century, the causes of the high productivity remain unclear. The island lies within the Antarctic Zone of the Antarctic Circumpolar Current, to the south of the Polar Front. The offshore waters to its north and east are affected by a north- wards deflection of the Southern Antarctic Circumpolar Current Front, together with waters from the Weddell-Scotia Confluence. Despite a retentive circulation over the shelf, the high productivity of phyto- plankton and copepods is widespread, occurring far downstream and possibly extending to the Polar Front. High phytoplankton concentrations (>20 mg chlorophyll a m -3 ) may be linked to enhanced supply of iron or reduced forms of nitrogen (up to ~3 mmol ammonium m -3 ). Although macronutrients are gen- erally not limiting in the Antarctic Zone, silicate concentrations of 2 mg chl a m -3 occur for 4t o5 mo. Biomass of krill plus net-caught zooplankton in summer is around 15 to 20 g dry mass m -2 , equally dominated by krill and copepods. This greatly exceeds typical values for Antarctica, and is high compared to productive northern shelves. Zooplankton, and in particular krill, appear to have a pivotal role in regulating energy flow in this food web, through selective grazing and possibly also through nutrient regeneration. Abundances of krill and copepods are negatively related across a wide range of scales, suggesting direct interaction through competition or predation. Evidence suggests that when phytoplankton stocks are low, energy flow through krill is maintained by their feeding on the large pop- ulations of small copepods. Metazoans and higher predators at South Georgia can feed across several trophic levels according to prey abundance, and they regenerate substantial quantities of reduced nitrogen. Therefore we suggest that these groups have a controlling influence on lower trophic levels, both stabilising population sizes and maintaining high rates of energy flow. Hydrography, nutrient con- centrations, phytoplankton, copepod, and krill biomasses fluctuate between years. Periodically (once or twice a decade), shortages of krill in summer result in breeding failures among many of their predators. This appears to be a downstream effect from wider scale, Scotia Sea phenomena, although the processes involved are unclear. The elevated biomass and energy flows at South Georgia appear to be caused by locally enhanced growth rates; there is no evidence so far for any physical concentration mechanism. Even for krill, which do not breed there, local growth rates are probably of a similar order to the biomass removed by their main land-based predators in summer. Thus the transfer of energy to higher predators depends on local enhancement of fluxes through the food web as well as the supply of plankton to the island by the Antarctic Circumpolar Current.

Journal ArticleDOI
TL;DR: It is reported here, for the first time, that whale sharks feed on the freshly released spawn of cubera snapper Lutjanus cyanopterus and dog snappers L. jocu (Lutjanidae), and there is consensus locally that this remarkable interaction is in need of immediate protection from overfishing of snappers and unregulated tourism development.
Abstract: This study confirms reports by fishermen of a large and predictable aggregation of whale sharks Rhincodon typus along the Belize Barrier Reef. Although whale sharks are rarely sighted at this location during most of the year, we counted as many as 25 whale sharks in a 50 m diameter area on 1 occasion and tagged 6 sharks during a 22 min period on another. The whale shark aggregation coincides seasonally and temporally with a multispecies reef-fish spawning aggregation at a reef promontory, Gladden Spit, at sunset, during the full and last-quarter moon periods of April and May each year. We report here, for the first time, that whale sharks feed on the freshly released spawn of cubera snappers Lutjanus cyanopterus and dog snappers L. jocu (Lutjanidae), and have documented the phenomenon with still and digital video photography of hundreds of feeding events. There is consensus locally that this remarkable interaction is in need of immediate protection from overfishing of snappers and unregulated tourism development. Our continued investigations are providing management recommendations for a new marine reserve at the site.

Journal ArticleDOI
TL;DR: The trophic structure of intensively trawled benthic invertebrate communities may be a robust feature of this marine ecosystem, thus ensuring the efficient processing of production within those animals that have sufficiently high intrinsic rates of population increase to withstand the levels of mortality imposed by trawling.
Abstract: Bottom trawling causes chronic and widespread disturbance to the seabed in shallow shelf seas and could lead to changes in the trophic structure and function of benthic communities, with important implications for the processing of primary production and the wider functioning of the marine ecosystem. We studied the effects of bottom trawling on the trophic structure of infaunal and epifaunal benthic communities in 2 regions (Silver Pit and Hills) of the central North Sea. Within each region, we quantified long-term (over 5 yr) differences in trawling disturbance at a series of sites (using sightings data from fishery protection flights), and related this to differences in the biomass and trophic structure of the benthic community. There were 27- and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites respectively, and we estimated that the frequency with which the entire area of the sites was trawled ranged from 0.2 to 6.5 times yr -1 in the Silver Pit and 0.2 to 2.3 times yr -1 in the Hills. The impacts of fishing were most pronounced in the Sil- ver Pit region, where the range of trawling disturbance was greater. Infaunal and epifaunal biomass decreased significantly with trawling disturbance. Within the infauna, there were highly significant decreases in the biomass of bivalves and spatangoids (burrowing sea-urchins) but no significant change in polychaetes. Relationships between trophic level (estimated using nitrogen stable isotope composition, δ 15 N) and body mass (as log2 size classes) were rarely significant, implying that the larger individuals in this community did not consistently prey on the smaller ones. For epifauna, the relationships were significant, but the slopes or intercepts of the fitted linear regressions were not significantly related to trawling disturbance. Moreover, mean δ 15 N of the sampled infaunal and epi- faunal communities were remarkably consistent across sites and not significantly related to trawling disturbance. Our results suggest that chronic trawling disturbance led to dramatic reductions in the biomass of infauna and epifauna, but these reductions were not reflected in changes to the mean trophic level of the community, or the relationships between the trophic levels of different sizes of epifauna. The trophic structure of intensively trawled benthic invertebrate communities may be a robust feature of this marine ecosystem, thus ensuring the efficient processing of production within those animals that have sufficiently high intrinsic rates of population increase to withstand the levels of mortality imposed by trawling.

Journal ArticleDOI
TL;DR: It is suggested that oceanography may be largely involved in the geographic variability detected in patterns of community structure, as well as the influence of mesoscale oceanographic features through satellite image analyses, by classifying the study sites as either strongly or weakly influenced by coastal upwelling.
Abstract: We conducted field surveys of rocky intertidal communities at 18 sites distributed between 29°S and 36°S on the coast of central Chile in order to document broad patterns of functional and trophic group abundance and evaluate their association with oceanographic features. Results showed that the main sessile components of the landscape throughout the region were kelps (16%) and crustose algae (48%) in the low intertidal zone, while mussels beds (30%) and corticated algae (32%) dominated the mid-intertidal zone. Geographic trends in abundance across the entire region varied from no clear regional patterns for some groups (e.g. barnacles), to smooth gradients increasing or decreasing toward higher latitudes for others (kelps and ephemeral algae) and to an abrupt switch from dominant to scarce northward of 32° S in 1 group (mussels). Significant among-site negative correlations were detected between the abundances of most algal functional groups and between mussels and barnacles. Herbivore density was negatively cor- related with kelp and corticated algal cover, and positively correlated with crustose algal abundance. These correlations reinforce the notion that local biological interactions can partially account for the among-site variability in functional group abundance. We assessed the influence of mesoscale oceanographic features through satellite image analyses, by classifying the study sites as either strongly or weakly influenced by coastal upwelling. At sites strongly influenced by upwelling, we found significantly higher cover of kelps in the low intertidal zone and of corticated algae in the mid-intertidal zone. Cover of ephemeral algae in the mid-intertidal zone was significantly lower at sites strongly affected by upwelling. Contrary to pre- dictions of bottom-up community regulation models, we found no significant differences in abundance of sessile filter feeders (mussels or barnacles), herbivores or carnivores. A significant proportion of the regional variation in abundance in some of the most abundant functional groups within each tidal level was asso- ciated with the latitudinal gradient in annual mean sea surface temperature (SST). We detected a signif- icant correlation between SST and the abundance of kelps and crustose algae in the low intertidal zone, and mussels and ephemeral algae in the mid-intertidal zone. The abrupt change in the abundance of mus- sels in the mid-intertidal zone at about 32° S may indicate a similarly abrupt change in biological or envi- ronmental conditions. Lack of significant correlation between the abundance of mussels and other func- tional groups suggests that biological interactions may not be responsible for this geographic discontinuity. We suggest that oceanography may be largely involved in the geographic variability detected in patterns of community structure. Recent remote sensing studies documenting oceanographic discontinuities around the 32° S zone support our suggestion. Our results highlight the need for further experimental and oceano- graphic studies in areas where strong biophysical gradients are observed. Such areas may well reflect the existence of steep oceanographic gradients sensitive to large-scale environmental forcing.

Journal ArticleDOI
TL;DR: It is found that the equatorial upwelling causes an increase in phytoplankton biomass and productivity without altering the typical size structure found in less productive regions such as the subtropical gyres.
Abstract: A total of 94 vertical profiles of size-fractionated chlorophyll a concentration and primary production rate were obtained along a meridional transect from the United Kingdom to the Falkland Islands (50° N to 50° S) during 4 cruises carried out in April and October 1996 and in April and October 1997. This data set allowed us to characterize the patterns of phytoplankton size-structure and pro- ductivity in temperate, oligotrophic, upwelling and equatorial regions. On average, picophytoplank- ton (0.2 to 2 µm) accounted for 56 and 71% of the total integrated carbon (C) fixation and autotrophic biomass, respectively. Enhanced biomass and productivity contributions by nano- and microplankton took place in the temperate regions and in the upwelling area off Mauritania. Small ( 2 µm) phytoplankton accounted for a proportion of total biomass that was similar to their shares of productivity. In the oligotrophic and equatorial regions, in contrast, large phytoplankton tended to account for a fraction of the total pro- duction that was significantly higher than their share of the biomass. We found that the equatorial upwelling causes an increase in phytoplankton biomass and productivity without altering the typical size structure found in less productive regions such as the subtropical gyres. In the oligotrophic ocean, significant changes in C fixation rates take place without accompanying variations in the magnitude of the phytoplankton standing stocks or the size structure of the microbial community.

Journal ArticleDOI
TL;DR: Strain A3, a species of Pseudoalteromonas isolated from the CCA Hydrolithon onkodes (Heydrich), was able to induce significant levels of metamorphosis of Acropora willisae Veron & Wallace, 1984 and A. millepora larvae in laboratory assays, providing evidence for a widening range of morphogenic sources and demonstrates the role that micro- organisms may play in fine-scale coral recruitment.
Abstract: External chemical signals provide a mechanism for broadcast-spawning scleractinian corals to recognise suitable substrata for larval settlement and metamorphosis. These morphogens can be extracted from crustose coralline algae (CCA) and the skeletons of some coral species, how- ever the precise origin of the chemical inducers has not yet been conclusively demonstrated. Micro- organisms have been reported to induce metamorphosis in various species of echinoderms, molluscs, polychaetes and cnidarians. We report that Strain A3, a species of Pseudoalteromonas isolated from the CCA Hydrolithon onkodes (Heydrich), was able to induce significant levels (up to 51.5% ± 5.8 SE) of metamorphosis of Acropora willisae Veron & Wallace, 1984 and A. millepora (Ehrenberg, 1834) larvae in laboratory assays. This experiment was repeated daily over 4 d, and the spat developed normally into juvenile polyps in flow-through aquaria. Approximately the same number of larvae underwent partial metamorphosis, forming flattened discs that were not attached to the substrata. Larvae underwent full settlement, attachment and metamorphosis only in the presence of Pseudo- alteromonas A3 plus inert chips of the coral skeleton Porites sp., indicating that the calcareous matrix may play a role in the synthesis of inducers from Pseudoalteromonas Strain A3. This discovery pro- vides evidence for a widening range of morphogenic sources and demonstrates the role that micro- organisms may play in fine-scale coral recruitment. In addition, the synthesis of chemical inducers by Pseudoalteromonas Strain A3 may have biotechnological applications for reef re-seeding.

Journal ArticleDOI
TL;DR: In this article, the carbon and nitrogen content of transparent exopolymer particles (TEP) were determined and related to the concentration of TEP as quantified by a colorimetrical method.
Abstract: The carbon and nitrogen content of transparent exopolymer particles (TEP) was determined and related to the concentration of TEP as quantified by a colorimetrical method. TEP were produced in the laboratory from dissolved precursors by laminar or turbulent shear. Dissolved precursors were obtained by 0.2 µm filtration from diatom cultures, with or without nutrient reduction, and from natural diatom populations. The relationship between carbon and TEP was significant, linear and species-specific. Carbon concentration of TEP derived from this relationship concurred with previous findings. Shortage of silicic acid or nitrate in the culture media had no effect on the carbon content of TEP. Molar C:N ratios of TEP were above the Redfield ratio, with a mean value of 26. It is suggested that the nitrogen fraction of TEP can be explained by adsorption of dissolved organic nitrogen (DON) onto TEP. Based on the newly established relationship, concentrations of TEP-derived carbon (TEP-C) were calculated for the Baltic Sea, the coastal Pacific, the North East Atlantic and the Northern Adriatic Sea.

Journal ArticleDOI
TL;DR: Calcification rate changed with light following the typical hyperbolic tangent function, and a decrease in calcification rate occurred at all light levels, indicating that rising CO 2 will impact corals living at all depths.
Abstract: Temperature, light and aragonite saturation state of seawater are important determinants of the global distribution of coral reefs. Saturation state of surface seawater is decreasing in response to increases in atmospheric pCO 2 , causing concern for a global reduction in the rates ol reef accretion. In this study we address the influences of light and saturation state on skeletal growth of the hermatypic coral Porites compressa. We conducted 2 experiments using coral nubbins; one in a controlled laboratory with Biosphere 2 ocean water and the other in the 711 m 2 Biosphere 2 ocean itself. The laboratory experiment consisted of 3 light treatments (PAR = 698, 150, 81 μmol photons m -2 s -1 ) and 2 chemical treatments (pCO 2 = 199 μatm, Ω = 5.05, CO 3 2- = 355 μmol kg -1 ; and pCO 2 = 448 μatm, Ω = 2.48, CO 3 2- = 183 μmol kg -1 ). In the Biosphere 2 ocean, coral nubbins were deployed at 4 depths (6, 4, 2 and 0.5 m) which corresponded to different light conditions (PAR from 2.7 to 29.5 mol photons m -2 d -1 ) during 3 different chemical states, mimicking the conditions of the Last Glacial Maximum (LGM), the present day and the year 2100 (Y2100). Calcification rate changed with light following the typical hyperbolic tangent function. Calcification rate was positively correlated with saturation state, which ranged from 5 (LGM) to 2.4 (Y2100). From the glacial experimental conditions (Ω = 5.05, pCO 2 = 186) to those of the future (Ω = 2.25, pCO 2 = 641), calcification dropped by 30%; from present day conditions (Ω = 3.64, pCO 2 = 336) to those of the future, calcification dropped by 11%, This decrease in calcification rate occurred at all light levels, indicating that rising CO 2 will impact corals living at all depths.

Journal ArticleDOI
TL;DR: This study evaluated the modifications caused by C. racemosa invasion on the structure of the benthic macroalgal community near Leghorn (northwestern Mediter- ranean) and showed that the invasive alga completely covered the surface of both types of substratum 6 mo after the start of the invasion.
Abstract: Caulerpa racemosa is a tropical green alga introduced into the Mediterranean probably as an immigrant from the Red Sea. This study evaluated the modifications caused by C. racemosa invasion on the structure of the benthic macroalgal community near Leghorn (northwestern Mediter- ranean). To achieve this objective, we studied invasion by C. racemosa of algal assemblages on 2 dif- ferent substrata by monitoring the structure of the phytobenthic community before and after the inva- sion of the alga and comparing our results with assemblages not invaded by C. racemosa. Results showed that the invasive alga Caulerpa racemosa completely covered the surface of both types of substratum 6 mo after the start of the invasion. With rapid horizontal elongation of the stolons (up to 2 cm d -1 ) C. racemosa overgrew native macroalgal species within 4 mo. As a consequence, phyto- benthic community structure deeply changed: species cover, number and diversity greatly decreased. Furthermore, the relative importance among vegetation layers of the macroalgal assem- blage changed: turf and encrusting species were deeply affected while erect species survived after 1 yr of the invasion. During the period from December to May, when both cover and biomass of C. racemosa diminished, the macroalgal community did not return to initial conditions. Instead, the structural changes increased in the following season, indicating the importance of C. racemosa inva- sion on a local scale.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the general model that much of the variation on rocky sea shores occurs across the vertical gradient, unless major physical attributes of the habi- tat change.
Abstract: Analyses of spatial pattern in populations and assemblages along distinct environmen- tal gradients are common in ecology. Few studies have examined whether these patterns are consis- tent at a hierarchy of spatial scales, despite increasing evidence indicating that spatial variability is not scale independent. In this study I have investigated the general model that much of the variation on rocky sea shores occurs across the vertical gradient, unless major physical attributes of the habi- tat change. This model was contrasted with 2 alternative models that incorporate patchiness at dif- ferent scales: either vertical variation is larger than horizontal variability at small but not at large scales of horizontal spatial variability or there is no characteristic scale at which vertical variation is larger than horizontal variability. In order to distinguish among these alternatives, I compared vari- ability in abundance of organisms across heights on the shore on rocky coasts in the north-west Mediterranean, with estimates of horizontal variability obtained at different scales, ranging from the scale of the patch (among quadrats 10s of cm apart) up to the regional scale (among shores 100s of km apart). Results indicated that vertical and horizontal variability were comparable in magnitude at the smallest spatial scale, while horizontal variability was generally larger when measured at scales of 100s to 1000s of m. When multivariate patterns in the structure of assemblages were examined, there was more vertical than along-shore variation at small (10s to 100s of cm) but not at large (>1000s of m) spatial scales. Univariate and multivariate analyses also revealed that much of the hor- izontal variation was among quadrats 10s of cm apart. These results indicated that any description of spatial pattern in abundance for organisms living on rocky sea shores in the NW Mediterranean must contemplate both vertical and horizontal sources of variation, regardless of the scale investigated. It is concluded that failure to recognise the importance of sources of variation other that those expressed by the most obvious gradients detracts from potentially important causal processes hin- dering progress in ecological understanding.

Journal ArticleDOI
TL;DR: Strong positive correlations between δ 15 N and δ 13 C data for plankton, fishes and crustaceans indicated a single primary source material for these com- munities, which is attributed to marine snow.
Abstract: Fishes and invertebrates were collected from the continental slope southwest of the Balearic Islands, Mediterranean, in October 1996 and May 1998 from 3 faunally-distinct depth zones: 200 to 800 m (upper slope), 800 to 1425 m (middle slope) and 1425 to 1800 m (lower slope). Differ- ences in δ 15 N and δ 13 C between October 1996 and May 1998 were observed only in the crustaceans Aristeus antennatus and Nematocarcinus exilis. There were few differences among depths in δ 15 N or δ 13 C of benthic fishes or crustaceans: the shrimps A. antennatus and Acanthephyra eximia showed higher δ 15 N values at greater depth, and δ 13 C values of the fishes Alepocephalus rostratus (Ale- pocephalidae) and Coelorhynchus coelorhynchus (Macrouridae) also tended to increase with in- creasing depth. Differences in plankton and suprabenthos were also observed among zones (at depth, greater δ 15 N values of gelatinous plankton, calanoid copepods and peracarids were recorded, along with smaller δ 13 C values of gelatinous plankton and peracarids). These are attributed to a microbially-mediated increase in δ 15 N and a reduction in δ 13 C of the particulate material remaining after preferential utilization of labile organic matter in the stable and relatively high (13°C) tem- perature of the Mediterranean water column. Strong positive correlations between δ 15 N and δ 13 C data for plankton, fishes and crustaceans indicated a single primary source material for these com- munities, which is attributed to marine snow.

Journal ArticleDOI
TL;DR: A new isopycnic separation technique was developed to minimize the effect of sediment granulometry on extraction efficiency of benthic meiofauna by the addition of a 5 min period of fluidization of the sediment.
Abstract: A new isopycnic separation technique was developed to minimize the effect of sediment granulometry on extraction efficiency of benthic meiofauna. The method uses colloidal silica Ludox ® (DuPont) HS 40 as a high density solution to extract meiofaunal organisms from sediment. Previous protocols using silica sols have had problems with physical interactions between the sediment and the specimens, causing large variations in extraction efficiencies between sediment types. The pro- cedure presented herein reduces this problem by the addition of a 5 min period of fluidization of the sediment. The time used to complete the method is minimized with a final centrifugation step, which accelerates the settling rate of the fine sediment, resulting in a total time required for separation of approximately 20 min. Forty-eight samples were processed, with sediments ranging from sand to silty clay. The average extraction efficiency was 96.8 ± 3.9% for the total meiofaunal abundance. Extraction efficiency was independent of sediment granulometry over the range of sediments tested.

Journal ArticleDOI
TL;DR: The effects of freezing, oven-drying, and forma- lin and ethanol preservation on stable isotope signatures of 3 unrelated marine species were examined to investigate temporal changes in δ 13 C and δ 15 N.
Abstract: The effects of freezing, oven-drying, and forma- lin and ethanol preservation on stable isotope signatures of 3 unrelated marine species (fish, octopus and kelp) were stud- ied over a period of 3 mo to investigate temporal changes in δ 13 C and δ 15 N. The effects on stable isotope ratios during the short-term confinement of live littorinid snails were also examined. Freezing and oven-drying did not result in a sig- nificant change in the initial isotope ratio for both carbon and nitrogen over the period of investigation. Formalin preser- vation significantly decreased, while ethanol preservation significantly increased δ 13

Journal ArticleDOI
TL;DR: In this paper, more sensitive genetic techniques, in the form of microsatellite analysis, were used to reanalyse the population structure through the specific targeting of mature fish on spawning grounds.
Abstract: Previous studies of cod Gadus morhua population structure in European continental waters using molecular genetic markers have indicated high dispersal and limited structuring, in contrast to many tagging studies. More sensitive genetic techniques, in the form of microsatellite analysis, were used to reanalyse the population structure through the specific targeting of mature fish on spawning grounds. Significantly greater levels of differentiation were found than in previous studies, supporting the presence of 4 genetically distinct populations within the North Sea: Bergen Bank, Moray Firth, Flamborough Head and Southern Bight. Gene flow between the southern North Sea and eastern English Channel is largely restricted to populations within the Southern Bight (southern North Sea) and Beachy Head (eastern English Channel). The spawning stock in the central English Channel (Start Point) remains distinct from those in the Celtic Sea, Outer Hebrides, and central and northern North Sea. No evidence of sub-structuring within the Irish and Celtic Seas was found, in contrast to previous genetic studies, although the populations remained divergent from that of the Outer Hebrides. All European populations were significantly divergent from Canadian Scotian Shelf and Barents Sea (Bear Island) populations. The study indicates that the current fishery moni- toring systems may need to be reassessed, particularly with respect to the North Sea, and highlights the value of using microsatellites combined with the targeting of spawning stocks to investigate subtle population structuring.

Journal ArticleDOI
TL;DR: The main objectives of the present study were to document the mortality suffered by the red coral populations in the Provence region and assess ecological correlates of the mortality which could help to identify the putative agent or agents of the event.
Abstract: A mass mortality event of the red coral Corallium rubrum (L.) occurred in the NW Mediterranean region in summer 1999. The main objectives of the present study were to document the mortality suffered by the red coral populations in the Provence region and assess ecological correlates of the mortality which could help to identify the putative agent or agents of the event. The mortality outbreak resulted in partial to complete loss of the coenenchyme. The first observations of red coral mortality in the study area were in early October 1999 and continued until November 1999. To deter- mine the extent of mortality on red coral populations, the incidence (= % colonies suffering mortality) and the virulence (= % coenenchyme loss) were quantified. Twelve surveys were carried out from November 1999 to January 2000. A total of 874 colonies of the 2375 observed showed some mortality. Incidence and virulence differed significantly among surveys. In some cases about 80% of the colonies were affected, while in others only 5% of colonies showed some mortality. Depth had a strong influ- ence on the incidence of mortality. Shallower populations had the greatest incidence, while popula- tions deeper than 30 m depth showed no mortality. However, virulence was similar in the 10 to 30 m depth range. With regard to habitat, the entrances of caves displayed a significantly higher incidence but not virulence than the interiors. Given the life history traits of red coral (mainly slow growth and low recruitment), it seems that recovery from the outbreak will be a long-term process. However, fur- ther studies are urgently required to provide basic information regarding red coral population dynamics as a basis for hypotheses on the actual recovery capability of affected populations. The cause of the mass mortality is unknown. However, during late summer 1999, the NW Mediterranean area affected by the mass mortality event experienced high temperatures and hydrographic stability over a period of several weeks. This temperature anomaly could have caused physiological stress or/and triggered the development of pathogenic agents that otherwise would have remained non-virulent. This hypothesis is reinforced by the absence of signs of mass mortality below 30 m depth in red coral populations. Since the NW Mediterranean seems to be affected by the global warming trend, if the temperature hypothesis is confirmed, the long-term consequences of a repetition of this kind of event could severely endanger the persistence of shallow-water red coral populations.

Journal ArticleDOI
TL;DR: Experimental evidence is provided that pontoons are fundamentally different habitats from natural rocky reefs, independent of the size, shape, age and composition of the substratum, and the development of epibiota on settlement panels in the 2 habitats.
Abstract: The orientation and position of hard substrata used to test ecological hypotheses about sessile marine plants and animals have often been based on logistical convenience. Much of our understanding of the ecology of epibiota is based on artificial habitats (particularly the undersides of floating pontoons), despite epibiotic organisms being an important and conspicuous component of natural hard substrata (e.g. vertical surfaces of rocky reefs). We assessed the model that pontoons act as inherently different habitats from rocky reefs, independent of the size, shape, age and composition of the substratum, by comparing the development of epibiota on settlement panels in the 2 habitats. Panels orientated the same way on pontoons and rocky reefs were found to support different epibiotic assemblages, and panels orientated differently (vertical vs horizontal undersides) also supported different assemblages. Position (reef vs pontoon) affected a broader range of taxa than did orientation, although effects of each were generally inconsistent among sites. Covers of spirorbid polychaetes, encrusting bryozoans, mussels, colonial ascidians and red filamentous algae were affected greatly by position. Tubiculous polychaetes, barnacles and species of brown and green filamentous algae were influenced by orientation. This study provides experimental evidence that pontoons are fundamentally different habitats from natural rocky reefs. The results highlight the need for caution in the use and interpretation of studies using artificial habitats when testing hypotheses about naturally occurring assemblages.

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TL;DR: It is suggested that grazing by herbivorous fishes can exclude macroalgae from mid-depth reefs with high cover of hard corals, but that on low- cover reefs, the amount of space occupied by algae overwhelms the ability of grazing fishes to crop it down.
Abstract: Widespread rises in the abundance of fleshy macroalgae on Caribbean reefs within the last 20 yr have variously been attributed to eutrophication, overfishing, or the 1983-4 mass mortality of the grazing sea urchin Diadema antillarum. However, none of those factors can satisfactorily explain why macroalgae are abundant today even on lightly fished mid-depth reefs in sparsely pop- ulated areas. Here we explore another explanation for rises in macroalgal cover on such reefs, namely that they are a side effect of declines in coral cover. We suggest that grazing by herbivorous fishes can exclude macroalgae from mid-depth reefs with high cover of hard corals, but that on low- cover reefs, the amount of space occupied by algae overwhelms the ability of grazing fishes to crop it down. We simulated the effect of 10% and 25% rises in coral cover by attaching 'pseudo-corals' (PVC tiles covered in a non-toxic anti-fouling coating) to reef substratum in 5 × 5 m plots (3 groups: control, 10%T and 25%T) on a 12 m deep forereef site in front of Ambergris Caye, Belize. Within 3 mo of attaching tiles, macroalgae had declined by approximately 10% in 10%T plots and 25% in 25%T plots compared with control plots, and these new states persisted for the duration of the experiment (macro-algae averaging 38.0% in control plots, 28.7% in 10%T plots, and 13.6% in 25%T plots). Dra- matic reductions in macroalgae may have made the 25%T plots more attractive to herbivorous fishes, as, in the later stages of the experiment, herbivorous fish biomass and acanthurid feeding rate were higher in those plots than in 10%T and control plots.