Showing papers in "Marine Ecology Progress Series in 2000"

Separation of chlorophylls and carotenoids from marine phytoplankton: a new HPLC method using a reversed phase C8 column and pyridine-containing mobile phases

Abstract: A high-performance liquid chromatographic (HPLC) method based on a reversed-phase C8 column and pyridine-containing mobile phases was developed for the simultaneous separation of chlorophylls and carotenoids. The method is selective enough to resolve monovinyl (MV) and divinyl (DV) pairs of polar chlorophylls and DV chlorophyll a (chl a ) (the marker pigment for the prokaryote Prochlorococcus marinus) from chl a (the MV analogue). Only the pair DV chl alchl b was not resolved. This resolution capability for chlorophylls was only prev~ously achieved using polymeric C),, columns in combination with ammonium acetate or pyridine-containing mobile phases. The proposed method also allows the separation of taxon-specific carotenoids belonging to 8 algal classes, including some critical pigment pairs for previous HPLC methods using C,, columns. The method employs a binary gradient, so it can be used with both low-pressure and high-pressure mixing instruments. Method transferability was tested using 3 HPLC systems. Only a slight adjustment of gradient profile was required to obtain similar results with HPLC equipment having different dwell volumes. The selectivity of the method towards some recently discovered chlorophyll and carotenoid pigments makes it especially suitable for studying not only fleld samples, but also for re-examining the pigment composition of different algal classes.

Predator size - prey size relationships of marine fish predators: interspecific variation and effects of ontogeny and body size on trophic-niche breadth

Abstract: We utilized a long-term data base collected over a broad geographic range to examine predator size - prey size relationships for 18 species of marine fish predators from continental shelf waters off the northeast US coast. Regression analysis was used to illustrate interspecific variation in ontogenetic patterns of prey size use, gape allometries, and ratio-based trophic niche breadths. Size- based feeding strategies were assessed through comparison of frequency distributions of relative prey sizes eaten and were related to general predator feeding tactics and gape morphology. The results demonstrated that the range of prey sizes eaten expanded with increasing predator body size for each of the marine predators examined, leading to asymmetric predator size - prey size distribu- tions. Absolute maximum prey size and slopes of maximum prey size versus predator size varied widely among predator taxa. Distinct size-based feeding strategies were evident, as diets of some predators were dominated by prey that were 10 to 20% of predator size, whereas other predators frequently consumed prey >50% of predator size. Gape sizes and allometric relationships with body size were also diverse among predators and often were closely associated with maximum prey sizes. Ratio-based trophic-niche breadths generally did not expand with predator ontogeny and tended to narrow for the largest predators, which may be common for animal taxa.

Detection of spatial variability in relative density of fishes: comparison of visual census, angling, and baited underwater video

Abstract: The ability to make accurate estimates of fish relative abundance is the basis of both ecological and environmental effects studies, and flawed sampling methods may give misleading results even in otherwise well-designed surveys. This paper compares surveys of snapper Pagrus auratus (Sparidae) and blue cod Parapercis colias (Pinguipedidae) conducted using 3 methods (underwater visual census, experimental angling, and baited underwater video) inside and outside the Cape Rodney-Okakari Point marine reserve in northeastern New Zealand. Angling and baited video consistently detected adult P. auratus at protected and fished sites, providing estimates of 36.7 and 39.2 times greater density of fishable P. auratus within the reserve, respectively. Visual surveys provided the least reliable measure of density of P. auratus, with adults only detected at the reserve centre where fish have been habituated to divers by hand-feeding. Measures of the size structure of P. auratus were consistent between angling and video, but mean size was significantly smaller using visual census methods. Relative density of P. colias was similar for all 3 methods, but angling estimated larger mean size, probably due to hook selectivity against smaller fish. The study indicates that methodological standardisation across all species is not always appropriate for environmental effects studies, and that different survey methods should be considered according to the biology and behaviour of the species of interest.

Primary production and community respiration in the Humboldt Current System off Chile and associated oceanic areas

Abstract: The high biological productivity of the Humboldt Current System (HCS) off Chile supports an annual fish catch of over 7 million t. The area is also important biogeochemically, because the outgassing of recently upwelled water is modulated by contrasting degrees of biological activity. However, very few field measurements of primary production and planktonic respiration have been undertaken within the Eastern Boundary Current (EBC) system off Chile. In this study an estimate of primary production (PP) and surface planktonic community respiration is presented from several research cruises in the HCS and adjacent oceanic areas. The highest production levels were found near the coast correlating closely with known upwelling areas. Both gross primary production (GPP) and community respiration (CR) showed important spatial and temporal fluctuations. The highest water column integrated GPP was measured in the southern and central fishing area (19.9 g C m -2 d -1 ) and off the Antofagasta upwelling ecosystem (9.3 g C m -2 d -1 ). The range of GPP agrees well with values reported for Peru (0.05 to 11.7 g C m -2 d -1 ). The Coquimbo upwelling system, despite being an area of persistent upwelling, showed lower production and community respiration values than the Antofagasta and Concepcion Shelf areas.The lowest surface PP values were measured within the oceanic region adjacent to the northern coastal upwelling zones (0.8 ± 0.5 μg C l -1 h -1 ), though slightly enhanced biological production was found within the Nazca Ridge (1.5 ± 2.1 μg C l -1 h -1 ) that separates the Chilean and Peruvian deep basins.

Importance of shallow-water biotopes of a Caribbean bay for juvenile coral reef fishes: patterns in biotope association, community structure and spatial distribution

Abstract: Fish community structure of a non-estuarine inland bay on the Caribbean island of Curacao was determined in the mangroves, seagrass beds, algal beds, channel, fossil reef boulders, notches in fossil reef rock, and on the adjacent coral reef, using visual censuses in belt transects. Fish communities varied among biotopes, but some overlap was present. Fish density and species richness were highest at the boulders and on the coral reef, and extremely low on the algal beds, whereas the total number of individuals calculated for the entire bay was highest on the seagrass beds. Differ- ences in fish densities between biotopes were related to differences in structural complexity and amount of shelter. Fishes in the bay largely consisted of 17 (mainly commercially important) reef fish species, which used the bay biotopes only as a nursery during the juvenile part of their life cycle. Small juveniles of these species were most often found in the mangroves, whereas at intermediate sizes some were found in the channel. Large individuals and adults were found on the reef, and den- sities of several of these species were higher on the reef near the bay than on reefs located farther down-current. Fishes which spent their entire life cycles in either the bay or on the coral reef were also found, and the latter group showed a strong decrease in abundance with increasing distance into the bay. The density distribution of individual fish species was not homogeneous within the bay. In the mangroves and seagrass beds, spatial distribution of fishes was correlated with distance to the mouth of the bay, water transparency, amount of shelter, and the structural complexity of the biotope. Juveniles of 3 reef species showed an increase in size on the seagrass beds with distance from the mouth into the bay, whereas 1 bay species showed a decrease in size with this distance.

Seabird-fishery interactions : quantifying the sensitivity of seabirds to reductions in sandeel abundance, and identification of key areas for sensitive seabirds in the North Sea

Abstract: Sandeels Ammodytes marinus are important food for many breeding seabirds in the North Sea, and are harvested in large quantities by an industrial fishery. There is very little evidence of the fishery reducing availability of sandeels to breeding seabirds, but there is concern that fishery managers should take account of the needs of breeding seabirds. Here we present a quantitative index of the sensitivity of different seabird species' breeding success to reduced abundance of sandeels. The index is based on seabird size, cost of foraging, potential foraging range, ability to dive, amount of 'spare' time in the daily budget, and ability to switch diet. Testing the index with empiri- cal data from Shetland during periods of reduced sandeel abundance shows a close correlation between seabird breeding performance and predictions from the index. Mapping the distributions around the North Sea of seabirds with breeding success highly sensitive to sandeel abundance shows that the majority of sensitive seabirds breed in Shetland and Orkney. Industrial fishing in those regions should be closely controlled to avoid depleting the local sandeel stocks on which seabirds depend. This analysis considers only impacts on seabird breeding. There is a need for analysis of possible influences on other aspects of seabird demography.

The use of phytoplankton pigments for identifying and quantifying phytoplankton groups in coastal areas: testing the influence of light and nutrients on pigment/chlorophyll a ratios

Abstract: The influences of light and nutnents on ratios of pigments/chlorophyll a (chl a) were investigated for several species of different phytoplankton groups from estuaries and coastal areas lor the purpose of calculating the biornass of individual phytoplankton groups as chl a based on pigmenWch1 a ratios. Pigment ratios were constructed for varying Light intensities and qualities and for nutrient-starved algae cultured in the laboratory. The pigrnent ratios were tested on 4 data Sets obtained in estuanes and coastal areas using the CHEMTAX program, for calculating phytoplankton group abundance as chl a. Field samples were collected over a vanety of time and spatial scales as well as being subjects to variations in light and nutnent conditions. The pigmentkhl a ratios derived frorn the different treatments in culture experiments generally had a minor effect on the CHEMTAX biomass calculations, although the biomass of cyanobactena and prymnesiophytes was significantly influenced by the ratios chosen. In addition. interspecies variations in pigment/chl a ratios within the individual phytoplankton groups were even more pronounced than variations caused by the different growth conditions, indicating that the ratios chosen for CHEMTAX calculations should, if at ali possible. reflect the dominant phytoplankton species present in a given area. For 2 of the data sets, where larger algal cells dominated, the composition and the biomass of the individual phytoplankton groups, using our pigrnent ratios in the CHEMTAX prograrn, corresponded well to rnicroscopic determinations of the biomass of phytoplankton. One of the data sets, where small algal cells dominated, was counted under the microscope by 2 different laboratones. Their biomass estimates were not consistent, and both disagreed with the CHEMTAX results. This probably reflects the subjectivity of microscopic analysis, which is greatest when small phytoplankton cells dominate. In a fourth data set, owing to the high sensitivity and reproducibility of the pigment analysis, differences were detected between phytoplankton groups over a transect of 5 km. which might have been unresolved using standard microscopic techniques.

Delayed effects on growth and marine survival of pink salmon Oncorhynchus gorbuscha after exposure to crude oil during embryonic development

Abstract: We report delayed effects on the growth and marine survival of pink salmon Oncorhynchus gorbuscha, which were exposed to oil as embryos under conditions similar to those observed after the 'Exxon Valdez' oil spill. Pink salmon eggs were incubated in water that became contaminated with polynuclear aromatic hydrocarbons (PAHs) after percolating through gravel coated with weathered oil. Weathering ensured that the PAH composition of the water was dominated by alkyl-substituted naphthalenes and larger compounds. Most survivors of the exposures appeared healthy, and were released to the marine environment with coded-wire tags. Their survival was evaluated when they returned at maturity 2 yr later. Other survivors, also healthy in appearance, were retained in net pens to measure delayed effects on growth during the early juvenile stage. Pink salmon exposed to an initial concentration of total PAH equal to 5.4 ppb experienced a 15% decrease in marine survival compared to unexposed salmon. A delayed effect on growth was measured in juvenile salmon that survived embryonic exposure to doses as low as 18 ppb PAH. Reductions in juvenile growth could account for the reduced marine survival observed in the released fish. The demonstration of delayed effects on growth and survival support claims of delayed effects in pink salmon after the 'Exxon Valdez' oil spill, and indicate the potential for population-level effects resulting from embryonic exposure to oil.

Formation of transparent exopolymer particles, TEP, from dissolved precursor material

Abstract: Transparent exopolymer particles (TEP) form from polysaccharides released by many phytoplankton species, but this process by which dissolved organic matter becomes particulate is poorly understood Here, the abiotic formation of TEP from precursors <02 μm and the minimum molecular weight (MW) of TEP-precursors were studied In most samples TEP formed from material <02 μm (polycarbonate membrane filters, Poretics) when exposed to laminar shear in Couette flocculators This result was unexpected as no TEP formed from material <045 μm (polycap capsules, Whatman) due to surface coagulation onto bubbles (Zhou et al 1998; Limnol Oceanogr 43:1860-1871) Some TEP-precursors were able to pass through dialysis bags with a nominal pore size of 8 kDa (natural cellulose, Spektrum), although their MW is presumably 2 orders of magnitude larger, suggesting that TEP-precursors can be fibrillar It is suggested that freshly released precursors are fibrillar and that these fibrillar precursors form larger colloids and eventually TEP within hours to days after their release

Stable isotopes' as trophic tracers: combining field sampling and manipulative labelling of food resources for macrobenthos

Abstract: We combined 3 different approaches to determine the relative importance of micro- phytobenthos production as food for intertidal macrobenthic animals: (1) the natural abundance of stable-isotope ratios of carbon and nitrogen, (2) an in situ deliberate tracer addition of 13 C-bicarbonate, which was transferred through the benthic food chain after its incorporation by benthic algae, and (3) a dual labelling experiment in a flume, where pelagic and benthic algae were labelled with 15 N and 13 C, respectively. The results of the 3 approaches confirmed the high importance of microphytobenthos as a food source for (surface) deposit feeders. Despite the clearly demonstrated resuspension of benthic algae at high current velocities, suspension feeders appeared to depend almost exclusively on pelagic algae (and possibly detrital carbon) as a food source. Based on the results of the experiments, we determined an approximate degree of dependence on microphytobenthos for different species of intertidal macrobenthos. The macrobenthic biomass at 5 study locations, when weighted by these coefficients, correlated very well with measured productivity of the microphytobenthos.

Reef fish assemblages: a re-evaluation using enclosed rotenone stations

Abstract: The reef fish assemblage at Orpheus Island, Great Barrier Reef, was examined using visual censuses and the ichthyocide rotenone. Small 3.5 m 2 quantitative rotenone samples, using a fine-mesh net to enclose the site, were compared with visual-point censuses (prior to the placement of the net), random-point censuses and strip censuses. Furthermore, the fishes collected inside and outside the net were examined to determine the relative efficiency of enclosed versus open rotenone samples. Rotenone samples comprised 128 species in 28 families. Of these, only 49% of the species overlapped between the enclosed (inside net) and open (outside net) samples. Only 17.7% of the species in the enclosed rotenone sample were seen prior to collection. Rotenone samples reveal that visual censuses underestimated the abundance of small taxa. Overall, rotenone samples increased the estimated number of species by 40.4% and the total fish abundance by 50.1% (75.3% for fishes <50 mm). Biomass increased by less than 1.0%. However, through relatively high energetic de- mands, high turnover rates and their role as potential prey, small fish taxa may play a greater role in reef processes than previously assumed. The limits of visual censusing techniques are highlighted, emphasising the value of small, enclosed, intensive rotenone samples for providing reliable quanti- tative samples of small taxa.

Day-night shifts of fishes between shallow-water biotopes of a Caribbean bay, with emphasis on the nocturnal feeding of Haemulidae and Lutjanidae

Abstract: Day-night changes in fish communities were quantified in 6 associated shallow-water biotopes within a single bay: mangroves, seagrass beds, algal beds, channel, fossil reef boulders, and notches in fossil reef rock. All biotopes, except the algal beds, showed a strong reduction in fish density and species richness at night, caused by absence of diurnally active fishes and migrations of Haemulidae and Lutjanidae to the seagrass beds. The fish fauna of the different biotopes showed a relatively high dissimilarity between day and night. This dissimilarity is largely caused by absence of Acanthuridae, Chaetodontidae, Labridae, Pomacentridae, Scaridae and Sparidae at night. These fishes seek shelter at night in, amongst others, the channel, notches and boulders. The balloonfish Diodon holocanthus utilised almost all biotopes as shelter as well as feeding sites. The wide distribution of its preferred food (molluscs) probably explains its distribution in most biotopes at night. The nocturnally active Haemulidae and Lutjanidae, on the other hand, migrated from their daytime shelter sites to the seagrass beds at night to feed. Some of these fishes also migrated to the algal beds to feed. The preference of Haemulidae and Lutjanidae for the seagrass bed as a feeding biotope, instead of other bay biotopes, appears to be related to the relatively high availability of their preferred food (Tanaidacea and Decapoda) as determined by digestive tract analysis. Other bay biotopes showed much lower densities of such food items compared to the seagrass beds.

Development of swimming abilities in reef fish larvae

Abstract: Recent studies have revealed that reef fish larvae have excellent sustained swimming capabilities and considerable potential for modifying their dispersal patterns by active swimming. However, these studies concentrate solely on the late pelagic phase. We examined the development of swimming abilities from hatching through to settlement in 3 reef fish species (Pomacentrus amboinensis, Sphaeramia nematoptera, Amphiprion melanopus). Larval rearing provided larvae at all stages of development. Experiments were conducted in flow chambers designed for measuring the critical and sustained swimming capability of young larvae. In all 3 species, critical swimming ability increased steadily with age, size, relative propulsive area and developmental stage of the larvae. In contrast, sustained swimming ability showed a marked inflection during development. Differences among species throughout development appear to reflect variations in the developmental patterns of the 3 species. Propulsive area was highly correlated with swimming ability and may prove useful for estimating swimming capabilities among species. The results suggest that some species have the potential to actively modify their dispersal patterns from an early age.

Seasonal variation in δ13C and δ15N of size-fractionated plankton at a coastal station in the northern Baltic proper

Abstract: Seasonal cycles of δ 13 C and δ 15 N in dissolved organic carbon and size-fractionated plankton, ranging from bacteria to the jellyfish Aurelia aurita, were studied during a 1 yr cycle at a coastal station in the Baltic Sea. The observed isotopic changes were found with time lags in all size-fractions of plankton. The δ 13 C showed a bimodal cycle with 2 local maxima, the first coinciding with the spring bloom and the second with the autumn bloom. In δ 15 N, the annual cycle was trimodal with 3 local maxima. The first occurred in connection with the spring bloom, the second in mid-summer and the third was a broad autumn-to-winter maximum. The causes of these patterns are discussed in relation to measured oceanographic variables. In the summer, a depleted nitrogen isotopic signal was propagated through all size-classes of plankton, indicating direct or secondary utilisation of fixed nitrogen from cyanobacteria. The strength of the signal indicated that nitrogen-fixing cyanobacteria are more ecologically important as instantaneous nitrogen sources in the Baltic than previously assumed. Enrichment of δ 15 N in size-classes of plankton was found to be a linear function of logarithmic organism size from 20 to 500 μm, reflecting size-related consumption patterns of marine plankton food-webs. The explanatory power of the linear regression and the enrichment per unit size were stronger in spring and autumn than in the summer, reflecting time lags and diversity in the zooplankton community. The size-specific approach was found to be a simpler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per trophic level.

Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments : an annual study

Abstract: The annual cycle of oxygen and nitrogen flux, denitrification and microphytobenthic variables (primary production, biomass, composition and calculated N demand) were studied for 2 shallow-water microtidal sediment sites in NE Kattegat, 1 sandy and 1 silty, by incubating undisturbed sediment in the laboratory in light and darkness. Both sites (2 stations within each) were characterised by low concentrations of inorganic nitrogen (IN) in the overlying water during summer (NO 3 generally < 1 μM), with winter concentrations of 5 and 30 pM for the sandy and silty site, respectively. Through the activity of microphytobenthos, the sediment systems appeared to be net autotrophic during most of the year. Net oxygen production varied between -400 and 6600 pmol m -2 h -1 , being highest during the warmest season. Although the composition of the microphytobenthos depended on the sediment type, it did not have a crucial effect on the magnitude of the microphytobenthic biomass or function. The temporal pattern of the function of the microalgal community, on the other hand, was significantly influenced by the sediment type; sandy sediment exhibited a smooth seasonality, controlled mainly by temperature and light, while the silty microtidal sediment was also controlled by stochastic events, such as sediment resuspension. Microphytobenthos had a significant effect on the IN flux, the clearest effect being found for NH 4 . Total denitrification (isotope-pairing technique) generally varied between <1 and 40 μmol m -2 h -1 , being dominated by nitrification-coupled denitrification (D n ), and being 1 order of magnitude higher at the silty site. Microphytobenthic activity generally inhibited denitrification in the low-N areas in this study. The results suggest that the microphytobenthos functions as a major control throughout the annual cycle, by forming communities that are net photoautotrophic throughout the year, and by significantly influencing both the IN flux and denitrification rates. Sandy sediment appeared to function as an IN sink during winter and early spring, while no clear seasonal pattern was found for silty sediment. Calculated N demand of the microphytobenthos far exceeded the measured sediment net uptake of N, supporting the idea that sandy systems in low-nitrogen areas can be highly productive through a closed recycling of N. The ratio between calculated microphytobenthic N demand and measured denitrification rates suggests that denitrification has a minor role as a N sink, particularly in sandy, cold-climate microtidal sediments.

Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). III. Dynamics of phytoplankton growth and microzooplankton grazing

Abstract: Phytoplankton growth and microzooplankton grazing were investigated during the IronEx II mesoscale enrichment experiment using the seawater dilution technique combined with group-specific pigment markers. Growth rate estimates for the phytoplankton community increased greater than or equal to 2-fold, from 0.6 d(-1) in the ambient environment to 1.2-1.6 d(-1) in the iron-enhanced bloom. Grazing lagged growth, allowing phytoplankton biomass to accumulate at a high rate (similar to 0.8 d(-1)) initially. However, grazing mortality ultimately increased 3- to 4-fold to 1.2-1.4 d(-1), largely balancing growth by Day 6 of the experiment. Increased rates were broadly distributed among phytoplankton taxa, but they differed in timing. Whereas picophytoplankton showed more of a steady balance between growth and grazing, increasing grazing pressure on diatoms followed a 3-5-fold increase in larger (>20 mu m) heterotrophic dinoflagellates and ciliates, which grew in response to enhanced diatom biomass. In the ambient environment, phytoplankton production was 15 to 20 mu g C l(-1) d(-1), with diatoms accounting for 17 % of growth and 7 % of grazing losses. Total phytoplankton production increased to 150-200 mu g C l(-1) d(-1) at the peak of the patch bloom, where 79 % of growth and 55 % of microzooplankton grazing involved diatoms. Phytoplankton grazing mortality was significantly correlated with grazer biovolume, and high carbon-specific grazing estimates at the bloom peak indicated growth rates similar to 1.0 d(-1) for the heterotrophic community and up to 1.4 d(-1) for >20 mu m heterotrophs. During several days when high phytoplankton biomass was relatively constant in the patch bloom, the pennate diatom dominated the community and remained physiologically healthy and growing at a fast rate, even though nutrient conditions were suboptimal. Efficient cropping of diatoms by large protistan grazers and rapid remineralization of iron and biogenic silica were required to maintain this dynamic balance. Contrary to expectations, the carbon export ratio did not increase with the iron-induced diatom bloom. Thus, mesoscale iron-enrichment of high-nutrient, low chlorophyll waters in the eastern equatorial Pacific clearly demonstrated that phytoplankton growth rates and standing stocks were iron-limited. However, the food web also demonstrated a remarkable resiliency to environmental perturbation by establishing a new balance in which the essential features of a microzooplankton-dominated, low export system were maintained.

Dietary guild structure of the fish community in the Northeast United States continental shelf ecosystem

Abstract: Trophic guild analysis identifies groups of species that use similar resources within a community. We evaluated the trophic guild structure in an assemblage of 40 fish species in the North- east United States shelf ecosystem using a 25 yr database of food habits. We explicitly accounted for ontogenetic diet shifts by separating predator species into size classes. There were 14 significant trophic guilds. These distinguished predators based upon prey size and location in the water column (i.e., benthic to pelagic feeding). Ontogenetic diet shifts were important in guild structure, particu- larly within dominant piscivores. The mean dietary overlap both between and within guilds was notably lower than in other fish communities due to the broad spatial and temporal scale of the study, the diversity of prey types consumed, and the generalist nature of predators in this system. The guild concept is a useful framework to simplify highly connected, complex ecosystems like the Northeast US continental shelf and identify ecologically similar functional units.

Evidence for Allee effects in an over-harvested marine gastropod: density-dependent mating and egg production

Abstract: Conservation programs often focus on studying extinction risks encountered by small populations and de- termining minimum population sizes below which they can- not recover. In certain cases, per capita rates of population growth become negative at low population density. This 'Allee effect' (or 'depensation') is rarely considered in marine systems. We conducted surveys of adult density, reproductive behavior, and spawning in natural populations of Caribbean queen conch Strombus gigas at 2 locations in the Exuma Cays, Bahamas, to test for Allee effects. Mating never occurred when density was < 56 conch ha -1 , and spawning never occurred at <48 conch ha -1 , clearly demonstrating the operation of depensatory mechanisms. Reproductive behav- ior then increased rapidly to asymptotes at densities near 200 conch ha -1 . Heavily exploited populations of queen conch in the Caribbean have been slow to recover despite fishery closures. Failure to recover could result from spawning stock densities that are reduced to the point at which Allee effects begin to operate on reproductive behavior.

On the trophic coupling between protists and copepods in arctic marine ecosystems

Abstract: Grazing experiments were conducted at different seasons with the large Calanus fin- marchicus, C. glacialis and C. hyperboreus, and the small Acartia longiremis in Disko Bay, West Greenland and Young Sound, NE Greenland. Female copepods incubated in 200 µm screened nat- ural water preferred large protists. Thus, particularly during the post-bloom period, the relatively large heterotrophic protists (ciliates and heterotrophic dinoflagellates) contributed substantially to the trophic coupling between protists and copepods. However, low grazing by C. glacialis and C. hyperboreus in mid-June suggests that large parts of the populations of these species had terminated feeding at this time, prior to overwintering. Clearance increased with ciliate and dinoflagellate size above 10 µm equivalent spherical diameter (ESD), equal to the size of the smallest heterotrophic pro- tists. At a size of 30 to 40 µm ESD maximum clearance was observed. Grazing on Phaeocystis single cells of 5 µm by C. finmarchicus showed a lower size-limit for capture of this species < 5 µm which contrasts with C. glacialis and C. hyperboreus, which had a lower size-limit near 10 µm. In addition to size and relative concentrations of phytoplankton and heterotrophic protists, prey and/or predator behavior is suggested to play an important role for copepod feeding.

Ambient sound as a cue for navigation by the pelagic larvae of reef fishes

Abstract: Sound is a potentially important navigational cue for organisms in aquatic environments. Most reef fishes produce pelagic larvae that must locate suitable settlement habitat for the completion of their life-cycle. We used light traps and underwater loudspeakers to determine whether reef fish larvae are attracted to sounds produced on a reef. 'Sound traps' caught more triplefin (a benthic reef fish) larvae than did 'silent traps', demonstrating that the larvae of some reef fishes may use sound as a navigational cue in the field. Catches of pilchard larvae, a pelagic fish, did not vary between treatments. These results are the first demonstration, of which we are aware, of sound as a potential navigational cue in the aquatic environment.

Limited wind-driven dispersal of intertidal mussel larvae: in situ evidence from the plankton and the spread of the invasive species Mytilus galloprovincialis in South Africa

Abstract: Dispersal of intertidal mussel propagules was examined by sampling larvae (0.2 to 0.8 mm) of Perna perna in shallow (10 to 20 m) inshore waters on the south coast of South Africa. Mussels were randomly distributed through the water column with no signs of diel vertical migration. Horizontal distribution was sampled on fine scale grids of plankton stations on 5 occasions. Lines of stations ran offshore and were 300 m apart. Within each line, stations were 10 m apart. On each occa- sion grids were sampled 3 to 4 times in rapid succession so that each station was sampled at intervals of about 1 h. Distribution was patchy and denser clouds of larvae moved in the same direction and at the same speeds as surface currents. Displacement of passive particles moving with wind-driven currents was estimated from wind data. Over periods of 1 mo estimated total displacement was 118 to 220 km depending on month and year. However, because of frequent changes in wind direction, net displacement for any month was 54 to 164 km to the northeast. These estimates were compared with the rate of spread over 4 yr of an invasive intertidal mussel (Mytilus galloprovincialis) dispers- ing initially from a single-point source. Spread was estimated as the furthest occurrence of M. gallo- provincialis from the parent population. After 4 yr, 90% of individuals sampled were < 5 km from the original population. Yearly increase in range between 1988 (introduction to the area) and 1992 was strongly directional: 55 to 97 km to the northeast and 12 to 29 km to the southwest. The good match with estimates from wind data implies that mussel larvae in this region are dispersed like passive par- ticles and that dispersal direction and ranges can be predicted from hydrographic data. The results strongly suggest that maximum effective dispersal of mussel larvae in this area is relatively limited (<100 km), with the great majority of successful recruits appearing within <5 km of the parent population.

Succession in marine benthic habitats and fauna in response to oxygen deficiency: analysed by sediment profile-imaging and by grab samples

Abstract: This study shows, for the first time, in situ images of changes in marine benthic habitats in response to oxygen deficiency. Tight coupling is demonstrated between gradual degradation of benthic habitat and faunal behaviour, species richness, abundance and biomass. The critical oxygen level that forced changes in the benthic faunal successional stages was ∼10% that of air saturation (∼0.7 ml 02 l -1 ). Before this critical saturation level was reached, tube-building polychaetes their tubes extended higher into the water column, the width of the sub-oxic sediment layer decreased, and vertical animal burrows (formerly oxidised and brown in colour) became sulphidic and black. Over a 10 mo hypoxic period (June 1997 to April 1998) in the Gullmarsfjord (Swedish west coast), benthic community successional stages declined from equilibrium to virtually azoic conditions. As normoxic conditions returned, pioneering stages gradually recolonised the area.

Elemental fingerprints of otoliths of fish may distinguish estuarine 'nursery' habitats

Abstract: The elemental fingerprints or composition of otoliths of fish may provide a natural tag of the 'nursery' habitat of juvenile fish The natural tag could then be used to determine the 'nursery' habitat of adult fish found on coastal reefs We collected juvenile Pelates sexlineatus from 2 to 5 sites within each of 7 estuaries to determine whether elemental composition of otoliths differed among estuaries and among sites within estuaries In addition, fish were collected in 2 successive years to determine whether temporal differences may be found in elemental fingerprints that could then con- found subsequent assignment of adults to 'nursery' estuaries Significant differences in elemental fingerprints (Sr, Ba and Mn) were found within and among estuaries, but there was often an inter- action with time suggesting that there were differences among estuaries/sites but the difference varied between times Quadratic discriminant functions (QDFs) generated to discriminate among estuaries suggested that the QDFs calculated from a single year class were more successful at assign- ing fish to the correct estuary than either (1) QDFs based on both years combined or (2) using the QDFs generated from the first year class to classify the second year class Small-scale variability among sites within an estuary suggested that for an estuarine fishery we may be able to classify fish to their site of origin; this warrants further investigation Adult fish from a coastal fishery can now be assigned to their 'nursery' estuary by analysing the juvenile region of their otoliths, provided that discriminant functions are generated from the appropriate year class of recruits

Ecological significance of benthic foraminifera: 13C labelling experiments

Abstract: We examined the ability of foraminifera in taking up freshly deposited algal carbon in intertidal estuarine sediments. Lyophylized C-13-labelled axenic Chlorella was added to intact sediment cores containing natural benthic biota. The response of the system as a whole was rapid; similar to 5% of the added carbon was respired to CO2 within 6 h. Bacteria assimilated similar to 2 to 4 % of the added carbon within 12 h. Among the foraminifera, the dominant foraminifer Ammonia exhibited rapid uptake and it is estimated that similar to 1 to 7% of the added carbon was ingested within 3 to 53 h. This rapid, substantial uptake by Ammonia and the significant contribution of foraminifera to meiobenthic abundance and biomass (37 and 47% respectively) suggest that foraminifera play an important role in the carbon cycle in these sediments. [KEYWORDS: meiobenthos; benthic foraminifera; bacterial biomarkers; algal carbon; carbon cycle; CO2; C-13 labelling Deep-sea; organic-carbon; sedimentation event; metazoan meiofauna; adriatic sea; fatty-acid; phytodetritus; community; microalgae; deposition]

Relative importance of benthic microalgae, phytoplankton and the detritus of smooth cordgrass Spartina alterniflora and the common reed Phragmites australis to brackish-marsh food webs

Abstract: We conducted a study to determine the trophic pathways leading to juvenile fish in 2 mesohaline tidal marshes bordering Delaware Bay. The relative roles of the major primary producers in supplying energy, ultimately, to the mummichog Fundulus heteroclitus were assessed by measur- ing the stable isotopic compositions of juveniles (21 to 56 mm total length, TL; most of which were young-of-the-year) and those of macrophyte vegetation, phytoplankton, and benthlc microalgae at each site. We collected samples of primary producers and F. heteroclitus, the dominant fish species in this and other marshes along the east coast of the USA, in June and August 1997, at 2 study sites (upstream and downstream) within Mad Horse Creek (a Spartina alterniflora-dominated site) and Alloway Creek (a Phragmites australis-dominated site), for a total of 4 study sites. Our results indicate that F. heteroclitus production is based on a mixture of primary producers, but the mixture depends on the relative abundance of macrophytes. In S. alterniflora-dominated marshes, C and S isotope ratios indicate that F. heteroclitus production is supported by S. alterniflora production (ca 39%, pre- sumably via detritus), whde in P. australis-dominated marshes, secondary production is based upon P. australis (73%). To our knowledge, this finding provides the first evidence that P. australis may contribute to aquatic food webs in tidal marshes. Benthic microalgae also contribute to the food chain that leads to E heteroclitus in both marsh types, while phytoplankton may be of lesser importance. Benthic microalgal biomass was lower in the P. australis-dominated system, consistent with a greater effect of shading in P. australis- versus S. alterniflora-based creek systems. Based on the difference in nitrogen isotope values between F. heteroclitus and the primary producers, the trophic level of E heteroclitus appears to be similar in the 2 marsh types, despite the differing vegetation types. In summary, the relative roles of the primary producers in supplying energy to F. heteroclitus varies locally and, in particular, with respect to the type of marsh macrophyte vegetation.

Short-circuit in the mangrove food chain

Abstract: Crabs of the subfamily Sesarminae are important components of mangrove ecosystems in the Indo-west Pacific, Africa, the Caribbean and South America. By retaining a large proportion of mangrove leaf-litter within mangrove forests, they profoundly influence the functioning of mangrove ecosystems. Despite obvious importance to ecosystem functioning, little is known about predation on sesarmid crabs. Three large, predatory fishes of tropical Indo-Pacific estuaries, the groupers Epinephelus coioides and E. malabaricus and the snapper Lutjanus argentimaculatus are known to feed on brachyuran crabs. However, the contribution of sesarmids to the brachyuran component of the diets of these fishes is unknown. To determine the extent to which these fishes prey on sesarmid crabs, the gut contents and stable isotope values (δ13C and δ15N) of E. coioides, E. malabaricus, and L. argentimaculatus from 3 mangrove estuary systems on the northeast coast of tropical Australia were investigated. All 3 species fed extensively on sesarmid crabs. Sesarmid crabs were the dominant food items for E. malabaricus and L. argentimaculatus, occurring in 50% of the stomachs that contained prey, and being the most common prey in terms of overall numbers. Although still the dominant prey, sesarmids occurred in only 30% of E. coioides stomachs. As well as being numerically dominant, sesarmids were large relative to other prey types. The 3 species also had stable isotope values enriched by about +0.75 to +2 δ13C and +1.5 to +2.5 δ15N, which were also consistent with extensive feeding on these crabs. Most other sympatric species had quite different diets and stable isotope profiles. Extensive feeding on sesarmid crabs by these fishes has a range of implications for the ecology of tropical mangrove ecosystems. Food webs are apparently more complex, and food chains leading from mangroves to top predators may be shorter than previously thought. Furthermore, a substantial part of the mangrove productivity sequestered by sesarmid crabs may be exported from mangrove ecosystems as a result of offshore migration by these fishes. The low incidence of piscivory in these fishes adds support to theories that reduced predation pressure may enhance the nursery ground value of tropical mangrove systems for fishes.

Relationship between primary producers and bacteria in an oligotrophic sea--the Mediterranean and biogeochemical implications

Abstract: The proverbial blue colour of the Mediterranean reflects some of the most extreme oligo-trophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satelhte data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is hrectly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionahty between bacterial and primary production of 0.22, whlch, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low bio-mass of benthic organisms in the region.

Marine microbenthic community structure regulated by nitrogen loading and grazing pressure

Abstract: Generalisations on the combined effects of consumers and resources on autotrophs in aquatic food webs largely rely on freshwater studies. In this study, we tested these general concepts with marine benthic microalgae, which are important components of coastal food webs. We manipu- lated nitrogen availability and herbivore presence in a factorial field experiment in the Western Baltic Sea. Moreover, we investigated how herbivore control varied among 3 sites and 2 seasons and tested for trophic cascades by enhancing demersal fish density at 2 sites. Nitrogen availability and herbivore presence had strong and antagonistic effects on microalgal biomass, species composition and diver- sity. Herbivores significantly reduced algal biomass, whereas nutrient enrichment led to an increase in biomass. Herbivore effects on microalgal biomass increased with increasing nitrogen availability, indicating a functional response of herbivores to nutrient enrichment. The response of microalgae at the species level suggested a trade-off between nutrient use and grazing resistance which appeared to be linked to algal growth form. Compared to other growth forms, large erect species were most responsive to both nitrogen loading and herbivory. Grazing reduced microalgal diversity at low nutri- ent supply, but enhanced it at high nutrient supply. Herbivore effects varied considerably among dif- ferent sites and were stronger in spring than in summer. Manipulations of fish density during sum- mer did not have any effects on microalgal community structure. In conclusion, our results demonstrate that herbivores and nutrients have strong and balancing effects on marine microbenthic community structure.

Somatic growth model of juvenile loggerhead sea turtles Caretta caretta: duration of pelagic stage

Abstract: The pelagic juvenile stage of sea turtles is poorly studied. We present a growth model and estimates for duration of the pelagic juvenile stage for loggerhead sea turtles Caretta caretta in the North Atlantic based on length-frequency analyses and sizes of young-of-the-year stranded in the Azores. The size-specific growth model is a monotonic, nonlinear, declining function. The growth model is consistent with growth rates calculated from recaptures of tagged loggerheads. Logger- heads leave the pelagic habitat and recruit to neritic habitats over a range of sizes from 46 to 64 cm curved carapace length (CCL). From this size range and the growth model, we estimate the duration of the pelagic stage varies from 6.5 to 11.5 yr. Nonparametric smooths of the size frequency distribu- tions of loggerheads in pelagic (n = 1692) and neritic (n = 1803) habitats intersect at 53 cm CCL, which is equivalent to an 8.2 yr duration for the pelagic stage. More growth data from loggerheads <2 yr old would strengthen the database for our growth model and perhaps lengthen our estimates of the dura- tion of the pelagic stage. Incorporating our estimates for duration of the pelagic juvenile stage into the stage-based population model developed for North Atlantic loggerheads would have a major effect on estimates of population growth.

Zoobenthic community establishment and habitat complexity — the importance of seagrass shoot-density, morphology and physical disturbance for faunal recruitment

Abstract: Seagrass meadows are among the most diverse coastal ecosystems in the Baltic Sea. There is, however, an apparent lack of quantitative data on functional aspects of these systems, such as plant-animal interactions. We tested the importance of seagrass density and morphology for ben- thic infaunal recruitment in a 2 mo (June/July 1997) field-experiment with both bare and vegetated (3 densities of artificial Ruppia maritima and Zostera marina) colonization trays with azoic sediment. These artificial seagrass patches were placed at 3 m depth in an unvegetated area of a sandy bottom seagrass site on the Aland Islands, northern Baltic Sea. Faunal succession was followed by SCUBA- diving and core sampling with 10 to 20 d intervals. The data showed strong effects of seagrass com- plexity and wind disturbance on (1) physical processes such as accumulation of drifting algae, parti- cle trapping and sediment binding, (2) development of community parameters (abundance, species richness, diversity) and (3) species-specific colonization patterns. Our data further demonstrated the importance of post-settlement events for distribution of juvenile macrofauna (e.g. resuspension or transport by means of drifting algae), and showed negative and positive effects of wind-mediated dis- turbance in low- and high-complexity habitats, respectively. It is concluded that wind disturbance may act as a mechanism creating and maintaining high animal diversity in seagrass meadows.