Showing papers in "Marine Ecology Progress Series in 1999"

Chemistry and composition of fish otoliths : pathways, mechanisms and applications

Abstract: The fish otolith (earstone) has long been known as a timekeeper, but interest in its use as a metabolically inert environmental recorder has accelerated in recent years. In part due to technological advances, applications such as stock identification, determination of migration pathways, reconstruction of temperature and salinity history, age validation, detection of anadromy, use as a natural tag and chemical mass marking have been developed, some of which are difficult or impossible to implement using alternative techniques. Microsampling and the latest advances in beam-based probes allow many elemental assays to be coupled with daily or annual growth increments, thus providing a detailed chronological record of the environment. However, few workers have critically assessed the assumptions upon which the environmental reconstructions are based, or considered the possibility that elemental incorporation into the otolith may proceed differently than that into other calcified structures. This paper reviews current applications and their assumptions and suggests future directions. Particular attention is given to the premises that the elemental and isotopic composition of the otolith reflects that of the environment, and the effect of physiological filters on the resulting composition. The roles of temperature, elemental uptake into the fish and the process of otolith crystallization are also assessed. Drawing upon recent advances in geochemistry and paleoclimate research as points of contrast, it appears that not all applications of otolith chemistry are firmly based, although others are destined to become powerful and perhaps routine tools for the mainstream fish biologist.

On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles

Abstract: Many benthic marine invertebrates develop by means of free-living, dispersive larval stages. The presumed advantages of such larvae include the avoidance of competition for resources with adults, temporary reduction of benthic mortality while in the plankton, decreased likelihood of inbreeding in the next generation, and increased ability to withstand local extinction. However, the direction of evolutionary change appears generally biased toward the loss of larvae in many clades, implying that larvae are somehow disadvantageous. Possible disadvantages include dispersal away from favorable habitat, mismatches between larval and juvenile physiological tolerances, greater susceptibility to environmental stresses, greater susceptibility to predation, and various costs that may be associated with metamorphosing in response to specific chemical cues and postponing metamorphosis in the absence of those cues. Understanding the forces responsible for the present distribution of larval and non-larval (aplanktonic) development among benthic marine invertebrates, and the potential influence of human activities on the direction of future evolutionary change in reproductive patterns, will require a better understanding of the following issues: the role of macro-evolutionary forces in selecting for or against dispersive larvae; the relative tolerances of encapsulated embryos and free-living larvae to salinity, pollutant, and other environmental stresses; the degree to which egg masses, egg capsules, and brood chambers protect developing embryos from environmental stresses; the relative magnitude of predation by planktonic and benthic predators on both larvae and early juveniles; the way in which larval and juvenile size affect vulnerability to predators; the extent to which encapsulation and brooding protect against predators; the amount of genetic change associated with loss of larvae from invertebrate life cycles and the time required to accomplish that change; and the degree to which continued input of larvae from other populations deters selection against dispersive larvae. The prominence of larval development in modern life cycles may reflect difficulties in losing larvae from life cycles more than selection for their retention.

Community reorganization in the Gulf of Alaska following ocean climate regime shift

Abstract: A shift in ocean climate during the late 1970s triggered a reorganization of community structure in the Gulf of Alaska ecosystem, as evidenced in changing catch composi- tion on long-term (1953-1997) small-mesh trawl surveys. Forage species such as pandalid shrimp and capelin declined because of recruitment failure and predation, and populations have not yet recovered. Total trawl catch biomass declined > 50% and remained low through the 1980s. In contrast, recruitment of high trophic-level groundfish improved during the 1980s, yielding a > 250% increase in catch biomass during the 1990s. This trophic reorganization apparently had negative effects on piscivorus sea birds and marine mammals.

Changes in community structure in temperate marine reserves

Abstract: No-take' marine reserves provide a valuable tool for managing marine resources as well as for providing relatively undisturbed habitat with which to assess modifications to ecosystems. We studied 2 marine reserves in northeastern New Zealand, the Leigh Marine Reserve (established 1975) and Tawharanui Marine Park (established 1982) in order to assess whether changes in protected predator populations had resulted in other indirect changes to grazers and consequently to algal abundance. Estimates of abundance of the most common demersal predatory fish Pagrus auratus indicated that adults of this species (i.e. large enough to prey upon urchins) were at least 5.75 and 8.70 times more abundant inside reserves than in adjacent unprotected areas. Overall, P auratus were also much larger inside reserves with mean total lengths of 316 mm compared with 186 mm in fished areas. The spiny lobster Jasus edwardsii displayed similar trends, and was approximately 1.6 to 3.7 times more abundant inside the reserves than outside. Lobsters within the reserves had a mean carapace length of 109.9 mm, compared with 93.5 mm outside the reserves. In one of the reserves, densities of the sea urchin Evechinus chloroticus had declined from 4.9 to 1.4 m-2 since 1978 in areas formerly dominated by it. Consequently, kelp forests were more extensive in 1998 than they were at the time of reserve creation. Urchin-dominated barrens occupied only 14 % of available reef substratum in reserves as opposed to 40"4 in unprotected areas. These changes in community structure, which have persisted since at least 1994, demonstrate not only higher trophic complexity than anticipated in Australasian ecosystems but also increased primary and secondary productivity in marine reserves as a consequence of protection. Trends inside reserves indicate large-scale reduction of benthic primary production as an indirect result of fishing activity in unprotected areas.

Diet, provisioning and productivity responses of marine predators to differences in availability of Antarctic krill

Abstract: Knowledge of relationships between prey availability and predator performance is the key to using predators as indicators of the state of marine systems and to assessing potential consequences of competition between natural predators and man for common resources. Fluctuations in the abundance of Antarctic krill are believed to have a substantial influence on the reproductive performance of krill-dependent top predator species in the Southern Ocean; few quantifications of such interactions exist. At South Georgia, for 2 years in which acoustic surveys revealed a major difference in krill abundance, we compared diet, provisioning of offspring and breeding success in 4 main predator species (2 penguins, 2 albatrosses, with supporting data from Antarctic fur seal) whose dependence on krill typically ranges from 20 to 90%. The 4-fold difference in krill biomass between 1986 (ca 30 g m-2) and 1994 (ca 7 g m-2) was accompanied by (1) an 88 to 90% reduction in the mass of krill in predator diets (and some increase in the fish component), (2) greater prey diversity for most species, (3) reduced diet overlap between species and (4) a switch from krill to amphipods in macaroni penguin but no major dietary change in other species. Rates of provisioning of offspring decreased by 90% in gentoo penguin and 40 to 50% in the other 3 species; this was due to reduced meal size in penguins (by 90% in gentoo and 50% in macaroni) and to doubling of foraging trip duration in albatrosses. Breeding success was reduced by 50% in grey-headed albatross (the species least dependent on krill), by 90% in black-browed albatross and gentoo penguin (only 3 to 4% of eggs producing fledged chicks) but by only 10% in macaroni penguin, presumably reflecting its ability to switch to small prey unprofitable for the other species. However, all species (except for black-browed albatross), particularly macaroni penguin, produced fledglings significantly lighter than usual, probably affecting their subsequent survival. Some effects on adult survival could also be inferred. Our results show a coherent, though complex, pattern of within and between species similarities and differences. These mainly reflect the degree of dependence on krill, the feasibility of taking alternative prey and constraints on trip duration and/or meal size imposed by foraging adaptations (especially relating to travel speeds and diving abilities, whereby flightless divers and pelagic foragers differ markedly). The generality of these principles are explored through comparison with other studies, particularly of Shetland seabirds.

The taxonomic distinctness measure of biodiversity: weighting of step lengths between hierarchical levels

Abstract: Taxonomic distinctness is a univariate (bio)diversity index which, in its simplest form, calculates the average 'distance' between all pairs of species in a community sample, where this distance is defined as the path length through a standard Linnean or phylogenetic tree connecting these species. It has some appealing properties: it attempts to capture phylogenetic diversity rather than simple richness of species and is more closely linked to functional diversity; it is robust to variation in sampling effort and there exists a statistical framework for assessing its departure from 'expectation'; it appears to decline monotonically in response to environmental degradation whilst being relatively insensitive to major habitat differences; and, in its simplest form, it utilises only simple species lists (presence/ absence data). Many of its practical characteristics remain to be explored, however, and this paper concentrates on the assumptions made about the weighting of step lengths between successive taxonomic levels (species to genera, genera to families etc.), which when accumulated give the overall path lengths. Using data on free-living marine nematodes from 16 localities/habitat types in the UK, it is shown that the relative values of taxonomic distinctness for the 16 sets are robust to variation in the definition of step length. For example, there is a near perfect linear relationship between values calculated using a constant increment at each level and a natural alternative in which the step lengths are proportional to the number of species per genus, genera per family, family per suborder etc. These weightings are then manipulated in more extreme ways, to capture the structure of phylogenetic diversity in more detail, and a contrast is drawn between the biodiversity of island (the Isles of Scilly) and mainland (UK) locations and habitats. This paper concludes with a discussion of some of the strengths and weaknesses of taxonomic distinctness as a practical tool for assessing biodiversity.

Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean

Abstract: We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South America in autumn 1996. Each diatom cell contained the heterocystous, N 2 -fixing cyanobacterial endosymbiont Richelia intracellularis. Surface Richelia heterocyst (and filament) densities increased from 10 6 heterocyst 1 -1 in the bloom. Total abundance ranged from 10 6 heterocyst m -2 outside the bloom to over 10 10 heterocyst m -2 within the bloom. Rates of primary production averaged 1.2 g C m -2 d -1 , higher than typical for oligotrophic open ocean waters. N 2 fixation during the bloom by the Richelia/Hemiaulus association added an average of 45 mg N m -2 d -1 to the water column. The relative importance of NH 4 + uptake over the course of the bloom increased from 0 to 42% of total N uptake by the Hemiaulus/Richelia association. N 2 fixation by Richelia exceeded estimates of 'new' N flux via NO 3 diffusion from deep water and, together with additional N 2 fixation by the cyanobacterium Trichodesmium, could supply about 25 % of the total N demand through the water column during the bloom. Suspended particles and zooplankton collected within the bloom were depleted in 15 N, reflecting the dominant contribution of N 2 fixation to the planktonic N budget. The bloom was spatially extensive, as revealed by satellite imagery, and is calculated to have contributed about 0.5 Tg N to the euphotic zone. Such blooms may represent an important and previously unrecognized source of new N to support primary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates that heterocystous cyanobacteria can also make quantitatively important contributions of N in oceanic water column environments.

Predator and prey perception in copepods due to hydromechanical signals

Abstract: Copepods can perceive moving predators and prey by means of the hydrodynamical disturbances these generate. We formulate a simplified, general model of the fluid disturbance generated by a plankter that is moving or generating a feeding current and we estimate the magnitude and attenuation of the different components of the fluid disturbance. We use this model to argue that prey perception depends on the absolute magnitude of the fluid velocity generated by the moving prey, while predator perception depends on the magnitude of one or several of the components of the fluid velocity gradients (deformation rate, vorticity, acceleration) generated by the predator. On the assumption that hydrodynamic disturbances are perceived through the mechanical bending of sensory setae, we estimate the magnitude of the signal strength due to each of the fluid disturbance components. We then derive equations for reaction distances as a function of threshold signal strength and the size and velocity of the prey or predator. We provide a conceptual framework for quantifying threshold signal strengths and, hence, perception distances. The model is illustrated by several examples, and we demonstrate for example, (1) how larval fish behaviour is adapted to allow their undetected approach up to the strike distance of their copepod prey, (2) that prey velocity is much more significant for prey encounter rates than traditionally assumed, even for cruising predators, (3) that prey perception is strongly biased towards large and rapidly swimming/sinking prey particles, and (4) that the model can accommodate the 3 orders of magnitude variation in clearance rates observed in the copepod Oithona similis feeding on motile protists and sinking particles. We finally discuss the implications of hydromechanical predator and prey perception to trophic interactions and vertical particle fluxes, and suggest important research questions that may be addressed.

Photoinhibition and photoprotection in symbiotic dinoflagellates from reef-building corals

Abstract: Pulse-amplitude-modulation fluorometry and oxygen respirometry were used to investigate diel photosynthetic responses by symbiotic dinoflagellates to light levels in summer and winter on a high latitude coral reef. The symbiotic dinoflagellates from 2 species of reef-building coral (Porites cylindrica and Stylophora pistillata) showed photoinhibitory decreases in the ratio of variable (F-v) to maximal (F-m) fluorescence (F-v/F-m) as early as 09:00 h on both summer and winter days on the reefs associated with One Tree Island (23 degrees 30' S, 152 degrees 06' E; Great Barrier Reef, Australia). This was due to decreases in maximum, F-m, and to a smaller extent minimum, F-0, chlorophyll fluorescence. Complete recovery took 4 to 6 h and began to occur as soon as light levels fell each day. Chlorophyll fluorescence quenching analysis of corals measured during the early afternoon revealed classic regulation of photosystem II (PSII) efficiency through non-photochemical quenching (NPQ). These results appear to be similar to data collected for other algae and higher plants, suggesting involvement of the xanthophyll cycle of symbiotic dinoflagellates in regulating the quantum efficiency of PSII. The ability of symbiotic dinoflagellates to develop significant NPQ, however, depended strongly on when the symbiotic dinoflagellates were studied. Whereas symbiotic dinoflagellates from corals in the early afternoon showed a significant capacity to regulate the efficiency of PSII using NPQ, those sampled before sunrise had a slower and much reduced capacity, suggesting that elements of the xanthophyll cycle are suppressed prior to sunrise. A second major finding of this study is that the quantum efficiency of PSII in symbiotic dinoflagellates is strongly diurnal, and is as much as 50% lower just prior to sunrise than later in the day. When combined with oxygen flux data, these results indicate that a greater portion of the electron transport occurring later in the day is likely to be due to the increases in the rate of carbon fixation by Rubisco or to higher flutes through the Mehler-Ascorbate-Peroxidase (MAP) cycle.

A comparison of the chemical characteristics of oceanic DOM and extracellular DOM produced by marine algae

Abstract: The chemical characteristics of extracellular high molecular weight (HMW) dissolved organic matter (DOM) from 3 species of marine phytoplankton were compared to HMW DOM in seawater. Thalassiosira weissflogii, Emiliania huxleyi and Phaeocystis sp., were grown in nutrient enriched seawater that had been previously ultrafiltered to remove HMW DOM. The extracellular HMW DOM produced in these cultures was isolated by ultrafiltration and characterized using nuclear magnetic resonance (NMR) spectroscopy, and molecular level analyses. All species exude DOM rich in polysaccharides, and the exudates of T. weissflogii and E. huxleyi closely resemble acyl heteropolysaccharides (APS) previously identified as major constituents of naturally occurring marine HMW DOM. Degradation of the T. weissflogii exudate alters the chemical composition of the DOM, which we attribute to differences in the reactivity of specific polysaccharides. The component within the exudate that most resembles seawater DOM has a slower degradation rate relative to the total polysaccharide fraction. Our study indicates that APS isolated from the surface ocean can have a direct algal source and that APS may accumulate in seawater as a result of its metabolic resistance.

Seagrass herbivory: evidence for the continued grazing of marine grasses

Abstract: Unlike the majority of marine plants, seagrasses are believed to experience little damage from the feeding activities of marine herbivores. Based on our previous work, plus a review of the literature, we suggest that this paradigm significantly underestimates the importance of seagrass herbivory in nearshore environments. In this review, we provide evidence from over 100 publications, showing that grazing on seagrasses is widespread in the world's oceans. Overwhelmingly, reports of grazing on seagrasses are based on observations, laboratory measurements, and bioenergetic calculations. To date, few field experiments have been conducted to evaluate the importance of seagrass grazing in the nearshore environment. Of these, even fewer have considered the possibility that herbivores may stimulate rates of primary production of the role of belowground nutrient reserves in determining the impacts of grazers on seagrasses. We contend that the currently accepted view that herbivory plays a minor role in the energetics of seagrass habitats needs to be reexamined by measuring seagrass responses to grazer induced tissue losses in controlled field manipulations. Only then will we be able to determine the in~portance of the seagrass-grazing pathway in marine food webs.

Ecological patterns in multivariate assemblages: information and interpretation of negative values in ANOSIM tests

Abstract: Analysis of similarities (ANOSIM) has been widely used for testing hypotheses about spatial differences and temporal changes in assemblages and particularly for detecting environmental impacts. ANOSIM generates a value of R which is scaled to lie between -1 and +1, a value of zero representing the null hypothesis. Generally, R lies between zero and +1. Values much smaller than zero have been considered unlikely because they would indicate greater dissimilarity among replicate units within samples than occurs between samples. Nevertheless, in some habitats, frequent and large negative values of R are common. In this paper, assemblages that consistently gave negative R values when analysed using ANOSIM were examined to identify patterns of differences among replicates within and between samples to test the hypothesis that particular patterns of differences generated consistent negative R values. The hypothesised patterns were then tested by analysing simple assemblages generated by computer simulation and examining the frequencies of R values. In natural assemblages, negative R values were found when assemblages were very patchy so that replicates were variable, but each sample had similar amounts of variability among replicates. Large negative values of R were particularly common when either or both samples contained an outlier, or when the assemblage being sampled had 2 different states and the replicates had sampled each of these states. Negative values of R may therefore indicate the need for stratification of the sampling design, or problems of positive correlation between the different sets of samples. When negative values occur, they should not simply be ignored as anomalies. In fact, they identify important ecological information and identify issues about the design of sampling.

Natural diet and grazing rate of the temperate sponge Dysidea avara (Demospongiae, Dendroceratida) throughout an annual cycle

Abstract: Sponges are one of the major invertebrate groups inhabiting hard-bottom communities worldwide. In this study, we measured in situ rates of grazing on DOC (dissolved organic carbon), POC (particulate organic carbon), and pico-, nanoand microplankton for the common temperate sponge Dysidea avara throughout a yearly cycle. The natural diet of the species was h~ghly heterogeneous and included procaryotes (heterotrophc bacteria, Prochlorococcus sp., Synechococcus sp.) and eucaryotes (protozoa, phytoplankton, and ciliates) ranging in size from 0.5 * 0.3 (heterotrophic bacteria) to 70 k 0.3 pm (pennate diatoms). Procaryotic cell clearance rates were higher than those for the other groups, suggesting a higher grazing efficiency upon these prey types. Specific clearance rates showed a pattern of decrease with sponge size increase, although they did not vary with prey concentration or with temperature. Overall, procaryotes contributed 74 k 14 % of the total ingested carbon, picoand nanoeucaryotes contributed 11 i 3"&, and phytoplankton contnbuted 11 i IOU:>. Therefore, Dysidea avara obtained 85% of its ingested carbon from the fract~on smaller than 5 pm and 15% from the fractlon larger than 5 pm. Howcver, the partial contributions of the different groups varied seasonally, following the planktonic composition of the water column Dur~ng winter, phytoplankton was an important component of the total uptake [26°<)), whereas durlng the rest of the year it contnbuted less than 7 % of the total uptake. The capacity of this sponge to feed on a broad size range of prey allowed it to maintain rather constant food uptake throughout the year. These results show the importance of particle type (size) for selective uptake in sponges, as well as the relevance of phytoplankton in the sponge diet. This trophic plasticity may represent an advantage for the species because it attenuates the effects of seasonal fluctuations in the planktonic community. This plasticity in trophic ecology may be one of the main factors contributing to the worldwide abundance and distribution of sponges despite large spatial and temporal variations in food sources.

Seasonal variation in nutrients, pelagic primary production and grazing in a high-Arctic coastal marine ecosystem, Young Sound, Northeast Greenland

Abstract: The Young Sound estuary was covered by sea ice for approximately 10 mo during 1996. Investigations of pelagic primary production and estimated grazing were performed during the summer thaw (20 June to 25 August) and in the winter month of February, The phytoplankton community was dominated by diatoms in the surface samples, as well as in the subsurface bloom succeeding the spring bloom. Pelagic primary production was limited by light during sea ice cover. After break-up of the sea ice, silicate initially limited primary production in the surface water due to a well-established pycnocline, and maximum photosynthesis occurred in a subsurface layer at 15 to 20 m depth. In August, production sank to deeper water layers presumably due to nitrogen limitation. The carbon budget describing the fate of the annual pelagic primary production in Young Sound reveals that the pelagic production of ∼10 g C m -2 yr -1 was tightly coupled to the grazer community, since total consumption by the grazer community amounted to 10-12 g C m -2 yr -1 . The classical food web dominated this northeastern Greenlandic fjord, and it was estimated that copepods account for >80% of the grazing pressure upon phytoplankton. Based on this study and other values of annual pelagic primary production and sea ice cover found in the literature, we suggest that annual pelagic primary production in the Arctic can be described as proportional to the length of the open water light period. We propose that annual pelagic primary production, and hence secondary production, in a wide range of Arctic marine areas may increase in the future as a consequence of reduction and thinning of sea ice cover due to global warming.

Effects of trawling on seafloor habitat and associated invertebrate taxa in the Gulf of Alaska

Abstract: Short-term effects of bottom trawling on a 'hard-bottom' (pebble, cobble, and boulder) seafloor were studied on the outer continental shelf in the eastern Gulf of Alaska. Eight sites were trawled in August 1996; then, from a research submersible we videotaped each trawl path and a nearby reference transect to obtain quantitative data. Boulders were displaced, and large epifaunal invertebrates were removed or damaged by a single trawl pass. These structural components of habitat were the dominant features on the seafloor. There was a significant decrease in density, and an increase in damage, to sponges and anthozoans in trawled versus reference transects. Changes in density, or damage to most motile invertebrates were not detected. Delayed mortality, of apparently undamaged invertebrates, may have resulted in greater impact than we detected. Alternatively, over time, some invertebrates may have recovered from any damage previously suffered. A subsequent survey at these sites will address these questions.

Water flow in tide- and wave-dominated beds of the seagrass Thalassia testudinum

Abstract: Biological processes in seagrass meadows are regulated by the exchange of momentum, heat and mass between the surrounding water and the plants and thus may strongly depend on the characteristics of water flow and turbulence. Comparisons of mean flow profiles, turbulence distribution and mixing in meadows of the seagrass Thalassia testudinum colonizing 2 hydrodynamically different sites (wave-dominated and tide-dominated) suggest that the hydrodynamic microclimate and consequently mixing within seagrass beds strongly depend on the hydrodynamic forces (waves and currents) acting on the plants. Unidirectional flows deflect the water over the meadow (skimming flows), which potentially leads to lower mixing between the water above and within the meadow. In contrast, waves cause the blades to move back and forth, increasing the exchange between the water column and that within the meadow. Therefore, the hydrodynamic conditions prevailing in the seagrass habitat (waves, tides) change the pattern of flow attenuation and mixing within the vegetation.

Organic enrichment by macrophyte detritus, and abundance patterns of megafaunal populations in submarine canyons

Abstract: Submarine canyons can provide large quantities of food in aggregated form on the deep-sea floor by acting as conduits for marine macrophyte production produced in the intertidal and shallow subtidal zone. Longshore transport delivers substantial quantities of macrophyte detritus from surfgrass Phyllospadix torreyi, kelps Macrocystis pyrifera and Egregia menziesii, and other macroalgae to the heads of Scripps and La Jolla Submarine Canyons. Strong tidal and gravity currents distribute this material throughout much of the canyon system, where it is utilized as food and habitat by benthic fauna. Video data taken from remotely operated vehicles and submarines were used to evaluate differences in detrital cover and megafaunal abundance in the canyons, and at nearby reference stations. Within the canyons dense mats of detritus were common down to 550 m, and M. pyrifera holdfasts were observed at 700 and 900 m. Virtually no drift material was observed out of the canyons. Comparisons of megafaunal invertebrates in and out of the canyons revealed generally higher densities at non-canyon sites due to large numbers of urchins. Species richness of all megafauna and abundance of non-urchin megafauna were greater in the canyons than out. It is likely that urchin abundance in canyons is reduced through disturbance by currents and detrital flows in the canyons. Species richness and abundance of fishes were greater in the canyons at all depths for which comparative data were available (100 to 500 m). From 150 to 200 m in Scripps Canyon, juvenile Pacific hake Merluccius productus were so abundant at times that their bodies obscured visibility. Turbot Pleuronichthys sp. and zoarcids Lycodes pacifica were also abundant in Scripps Canyon from 100 to 300 m. Dala from this study support the hypotheses that macrophyte detritus covers large areas of the La Jolla and Scripps Canyon axis, and that megafaunal abundance is associated with detritus at both large and small spatial scales.

Hydrodynamic signal perception in the copepod Acartia tonsa

Abstract: Copepods may remotely detect predators from the velocity gradients these generate in the ambient water. Each of the different components and characteristics of a velocity gradient (acceleration, vorticity, longitudinal and shear deformation) can cause a velocity difference between the copepod and the ambient water and may, therefore, be perceived by mechanoreceptory setae. We hypothesised that the threshold value for escape response to a particular component depends solely on the magnitude of the velocity difference (= signal strength) it generates. In experiments we isolated the different components and noted the minimum intensities to which the copepod Acartia tonsa responded. As hypothesised, threshold signal strengths due to longitudinal and shear deformation were similar, ∼0.015 cm s -1 , and were invariant with developmental stage. The latter implies that the threshold deformation rate for response scales inversely with size, i.e. that large stages respond to lower fluid deformation rates than small stages and, hence, may detect predators at longer distances. Signals due to vorticity and acceleration did not elicit escape responses, even though their magnitude exceeded threshold signal strength due to deformation. We suggest that A. tonsa cannot distinguish such signals from those due to their own behaviour (sinking, swimming, passive reorientation due to gravity) because they cause a similar spatial distributions of the signal across the body. Reinterpretation of data from the literature revealed that threshold signal strength due to deformation varies by ca 2 orders of magnitude between copepods and exceeds the neurophysiological response threshold by more than a factor of 10. In contrast, threshold deformation rates vary much less, ∼ 0.5 to 5 s -1 . Model calculations suggest that such threshold deformation rates are just sufficient to allow efficient predator detection while at the same time just below maximum turbulent deformation rates, thus preventing inordinate escapes.

Benthic recovery following cessation of fish farming: a series of successes and catastrophes

Abstract: After the removal of fish cages at an intensive aquaculture site, the sedimentary environment was monitored over 23 no for redox potential, total organic carbon and nitrogen, total phosphorus, algal pigments and macrofauna. Three sampling stations were established: one under the previous location of the cages (R-0), a second one (R-10) at 10 m distance from the edge of the cages and a control site (R-c) at > 1 km distance. At both stations near the farming site the sediment was initially found to be anoxic and overlain by a highly organic black layer. Most geochemical variables at Stn R-10 attained values close to those at Stn R-c within 1 1 mo. Large fluctuations in the values of most variables were observed at Stn R-0 over the 23 no, indicating that the environment had not fully recovered before the end of the observations. Similar results were obtained from the macrofaunal analysis, which revealed that after 23 no a high proportion of benthic fauna at Sin R-0 was still composed of opportunistic species; abundance biomass and species composition showed marked successive changes in the direction of succession. This regression was attributed to a secondary disturbance due to a benthic algal bloom, caused by the seasonal release of nutrients from the farm sediment. It is concluded that the recovery process of heavily enriched benthos in a dynamic coastal environment is subject to the influence of different factors, resulting in progress and regression, and therefore the succession model proposed by Pearson & Rosenberg (1978; Oceanogr Mar Biol Annu Rev 16:229-311) may not be applicable in the early stages of succession.

Occurrence of UVA- and UVB-absorbing compounds in 152 species (206 strains) of marine microalgae

Abstract: Marine microalgae (152 species, 206 strains) from 12 classes were examined for the presence of UVA- and UVB-absorbing compounds. Cultures were grown under white fluorescent light without supplementary UVA or UVB radiation and were extracted after harvest in tetrahydrofuran:methanol (20:80, v/v). Ratios of UV absorbance (280 to 390 nm) to chlorophyll a (chl a) (665 nm) obtained by spectrophotometry ranged from 0.18 to 6.75. Three groups of species were distinguished: those with low UV:chl a ratios (0.18 to 0.9, diatoms, green algae, cyanophytes, euglenophytes, eustigmatophytes, rhodophytes, some dinoflagellates, some prymnesiophytes), those with intermediate ratios (0.9 to 1.4, chrysophytes, some prasinophytes, some prymnesiophytes) and those with very high ratios (1.4 to 6.75, surface bloom-forming dinoflagellates, cryptomonads, prymnesiophytes and raphidophytes). UV-absorbing pigments varied across species of the same algal class and strains of the same species. HPLC analysis of extracts of 5 species (1 diatom, 2 bloom-forming raphidophytes and 2 bloom-forming dinoflagellates) showed suites of mycosporine-like amino acids in 4 of them, which included mycosporine-glycine, asterina-330, shinorine, porphyra-334 and palythine. The dinoflagellate Gymnodinium catenatum also contained major quantities of unknown UV-absorbing compounds.

Determination of food sources for benthic invertebrates in a salt marsh (Aiguillon Bay, France) by carbon and nitrogen stable isotopes: importance of locally produced sources

Abstract: delta(13)C and delta(15)N were measured in benthic invertebrates and food sources collected in the salt marsh of the Aiguillon Bay, France. The results showed that, although Spartina anglica was dominant, this marine phanerogame did not contribute significantly to the carbon and nitrogen requirements of the invertebrates considered in this study. In fact, Macoma balthica, Scrobicularia plana and Mytilus edulis preferentially fed on a mixture of benthic diatoms and marine phytoplankton, but in different proportions, as was estimated through isotope mixing models. However, this isotopic data set revealed complex trophic relationships for the nematode community and for the crab Carcinus maenas which did not allow accurate isotopic determination of food sources. [KEYWORDS: carbon and nitrogen isotope ratios; muddy salt marsh; Spartina anglica; benthic diatoms; benthic invertebrates Oyster crassostrea-virginica; marennes-oleron; spartina-alterniflora; natural variations; marine-sediments; organic nitrogen; fresh-water; estuarine; detritus; webs]

Gradients in coral reef fish density and size across the Barbados Marine Reserve boundary:effects of reserve protection and habitat characteristics

Abstract: Reductions in fishing mortality within no-take coral reef marine reserves can produce gradients in the density and size of fishes across reserve boundaries. Such gradients may be affected by other factors, however, including differences in habitat quality between reserve and non-reserve areas and the movement of fish across reserve boundaries. To examine the effects of protection from fishing mortality and of habitat quality on an assemblage of exploited reef fishes, we measured the spatial patterns of fish density and size on fringing reefs near the boundary of the Barbados Marine Reserve (Barbados, West Indies) and statistically controlled for habitat correlates of fish density and size. Reserve sites supported a higher total density and size of fishes than non-reserve sites. Most species had a non-significantly higher mean density and size at reserve sites. The density and/or size of many species were correlated with the depth, rugosity, and/or substrate composition of sites. After statistically controlling for the effects of habitat correlates, the difference in total density between reserve and non-reserve sites remained significant, and the mean density and size of most species remained non-significantly higher at reserve sites. Neither the mobility of species nor their vulnerability to capture by Antillean fish traps was correlated with their relative difference in density or size between reserve and non-reserve sites. Spearfishing target species had a significantly higher relative difference in size between reserve and non-reserve sites than non-target species. Protection from fishing mortality and higher habitat quality appear to contribute to the increased density and size of fishes on study reefs in the Barbados Marine Reserve, and this difference is not compromised by emigration from the reserve.

Establishment of a primary cell culture from a sponge: primmorphs from Suberites domuncula

Abstract: In spite of the fact that cells from the phylum Porifera (sponges) contain high levels of telomerase activity, no successful approach to cultivate sponge cells has yet been described. Telomerase is the enzyme which catalyzes the addition of new telomeres onto chromosome ends which have been lost after each round of DNA synthesis. One reason may be seen in the observation that after dissociation the cells lose their telomerase activity. In addition, no nutrients and metabolites have been identified that would stimulate sponge cells to divide. We report here the culture conditions required for the formation of multicellular aggregates from Suberites domuncula from dissociated single cells; they are termed primmorphs. These aggregates, formed in seawater supplemented with antibiotics, have a tissue-like appearance; they have been cultured for more than 5 mo. Cross sections through the primmorphs revealed an organized zonation into a distinct unicellular epithelium-like layer of pinacocytes and a central zone composed primarily of spherulous cells. After their association into primmorphs, the cells turn from the telomerase-negative state to the telomerase-positive state. Important is the finding that a major fraction of the cells in the primmorphs undergo DNA synthesis and hence have the capacity to divide. By applying the BrdU (5-bromo-2'-deoxy-uridine)-labeling and detection assay it is demonstrated that up to 33.8 % of the cells in the primmorphs are labeled with BrdU after an incubation period of 12 h. It is proposed that the primmorph system described here is a powerful novel model system to study basic mechanisms of cell proliferation and cell death; it can also be used in aquaculture, for the production of bioactive compounds and as a bioindicator system.

Copepod grazing selection and particle discrimination on the basis of PSP toxin content

Abstract: Omnivorous copepods are capable of discriminatory feeding using mechano- and chemosensory mechanisms. The presence of phycotoxins in phytoplankton often results in reduced consumption of such potential prey by copepods, though it has not been clear if this is the result of discriminatory feeding by either tactile (mechanosensory) or chemosensory recognition of toxic prey, or perhaps a physiological response to ingested neurotoxic compounds. In this study, experiments were performed to determine whether 3 species of marine copepods (Acartia tonsa, Centropages hamatus, and Eurytemora herdmani) that commonly co-occur with toxic Alexandrium spp. dinoflagellates were capable of discriminating between cultured Alexandrium spp. strains on the basis of paralytic shellfish poisoning (PSP) toxin content, i.e. by chemosensory means, using live fluorescently labeled cells. Additional experiments investigated whether toxic cells in mixtures with non-toxic alternate species of dinoflagellates affected either prey selection or total carbon consumption rates of copepods, and whether daily carbon rations could be maintained on both toxic and non-toxic Alexandrium spp. monoculture diets. Results indicated that all 3 copepod species could discriminate between toxic and non-toxic Alexandrium spp. cells by chemosensory means, suggesting that selective behavior, rather than physiological effects, governs the grazing response of copepods exposed to toxic prey. Prey selection in mixtures of several dinoflagellate species depended on whether the Alexandrium spp. cells present were toxic or non-toxic. C. hamatus and E. herdmani (but not A. tonsa) maintained daily carbon rations despite the presence of toxic Alexandrium spp., chiefly through increased consumption of alternate prey. For A. tonsa and C. hamatus, carbon rations were not equivalent between toxic and non-toxic Alexandrium spp. monoculture diets, indicating strong aversions to PSP toxins, and the potential for physiological effects when no other food is available. In all experiments feeding behavior varied among copepod species, suggesting that grazing pressure on toxic Alexandrium spp. is not uniform throughout the zooplankton community. The grazer-deterrent effects observed have implications for the function of PSP toxins.

Photosynthesis, respiration, and carbon turnover in sinking marine snow from surface waters of Southern California Bight: implications for the carbon cycle in the ocean.

Abstract: Photosynthesis and respiration were measured in 1 to 6 mm large aggregates (marine snow) collected in the Southern Californian Bight, USA. The aggregates were freely sinking in a vertical flow system with an upward flow velocity which opposed the sinking velocity of individual aggregates during the measurements. The aggregates were net heterotrophic communities at light intensities 500 μE m 2 S 1 . Bacterial densities on aggregates were >2000-fold higher than in the surrounding water. Cytophaga was highly abundant in the aggregate-associated bacterial community as identified by in situ hybridization techniques. Both the respiration rate per aggregate volume and the bacterial densities decreased with increasing aggregate size. The respiration rates normalized to the number of bacteria in single aggregates were 7.4 to 70 fmol C cell 1 d -1 , The aggregate community respired 433 to 984 ng C d -1 per aggregate in darkness, which yielded a turnover time of 8 to 9 d for the total organic carbon in aggregates. Thus, marine snow is not only a vehicle for vertical flux of organic matter; the aggregates are also hotspots of microbial respiration which cause a fast and efficient respiratory turnover of particulate organic carbon in the sea.

Marine diversity shift linked to interactions among grazers, nutrients and propagule banks

Abstract: Diverse coastal seaweed communities dominated by perennial fucoids become replaced by species-poor turfs of annual algae throughout the Baltic Sea. A large-scale field survey and factorial field experiments indicated that grazers maintain the fucoid community through selective consumption of annual algae. Interactive effects between grazers and dormant propagules of annual algae, stored in a 'marine seed bank', determine the response of this system to anthropogenic nutrient loading. Nutrients override grazer control and accelerate the loss of algal diversity in the presence but not in the absence of a propagule bank. This implies a novel role of propagule banks for community regulation and ecosystem response to marine eutrophication.

Community recovery following catastrophic iceberg impacts in a soft-sediment shallow-water site at Signy Island, Antarctica

Abstract: Ice disturbance is possibly the major structuring element of polar nearshore biological communities. Effects range from encapsulation by ice forming on rock substrata to gouging and trampling by bergs. Some 15 to 20% of the world's oceans are affected by this phenomenon, yet measurements of the extent of biological destruction from iceberg impacts and subsequent community recovery are very rare. Communities can be held at early successional stages, or even completely destroyed by scouring, and these effects occur from the intertidal to depths around 500 m in Antarctica. The wide scales of disturbance intensity are thought to add to the overall high levels of Antarctic benthic biological diversity, which has recently been shown to be similar to tropical areas. Data here indicate >99.5 % removal of all macrofauna and >90 % removal of most meiofauna by iceberg impact on a soft-sediment habitat at Signy Island, Antarctica. Species return was via locomotion, advection or larval recolonisation, and all 3 mechanisms worked on different timescales. Locomotion caused groups to return within 10 d of an impact. Storms with wind speeds around 100 km h(-1) induced water movements intense enough to advect meiofauna to the 9 m depth site. However, it was only during the strongest storm which occurred during the study (maximum wind speed 148 km h(-1)) that water movements were powerful enough to redistribute small macrofauna such as the bivalve Mysella charcoti.

Responses of seagrass to nutrients in the Great Barrier Reef, Australia

Abstract: Declines in seagrass biomass and growth have been widely reported in response to anthropogenic impacts. In contrast, the distribution and biomass of seagrass in the carbonate sediment around Green Island reef, part of Australia's Great Barrier Reef (GBR) has measurably increased during the past 50 yr, possibly due to increases in the availability of nutrients from local and regional anthropogenic sources. Using historical aerial photography, increases in seagrass distribution at Green Island have been mapped. The growth, morphological and physiological responses of 2 seagrass species (Halodule uninervis and Syringodium isoetifolium) to elevated sediment nitrogen (N; 100x control) and/or phosphorus (P; 10x control) were measured to investigate whether increased nutrients could account for the observed increase in distribution. Increases in the growth rate, amino acid composition and tissue nutrient content of both species occurred in response to elevated sediment N, but not P. Concentrations of the N-rich amino acids asparagine and glutamine increased 3- to 100-fold in seagrass leaves from N treatments. The δN values of leaves decreased in response to additions of nitrogen, probably due to increased discrimination against the N isotope, because N availability was surplus to demand. The low δN value of seagrasses in the Green Island back reef suggests that their primary source of N is either from N fixation or fertilisers and that the N from sewage is not a large component of their N budget. This study is the first to demonstrate N, rather than P, as the primary limiting nutrient for growth of seagrass in carbonate sediments and supports the hypothesis that the increase in the seagrass distribution and biomass at Green island was caused by an increase in nutrient availability. We also hypothesise that seagrass distribution and biomass in many regions of the GBR may be limited by nutrients and that the lack of substantial seagrass meadows in the southern GBR could be due to these reefs receiving less nutrients from the mainland.

The predominantly facultative nature of epibiosis: experimental and observational evidence

Abstract: Epibiosis is d spatially close association between 2 or more organisms belonging to the same or different species. Through direct and indirect interactions, this association has major effects on the species involved and on community dynamics. When the effects are predominantly beneficial for epibiont and basibiont, coevolution can be expected to lead to associational specificity. Circumstantial evidence, however, suggests that many epibionts are non-specific substratum-generalists. In this article, we investigate the commonness of specificity in epibiotic associations. In a first approach, we investigated the in situ recruitment preferences of potential epibionts when choosing between artificial and living substrata. After exposure for 3 wk in early summer, an early successional community had established, comprising cyanobacteria, diatoms, sessile colonial ciliates and red algae. All species recruited on almost all substrata available. However, artificial substrata were usually preferred over living surfaces. Consequently, the species studied are classified as facultative epibionts. An analysis of a list of over 2000 epibiotic associations corroborated these results: the majority of described 'epibionts' are not basibiont-specific and generally occur on non-living substrata as well. Also, basibiont species usually bear more than 1 epibiont species. Relative to each other, epibionts and basibionts are characterised by a typical set of life history traits. We conclude that specific and obligate epibionts are rare. Their scarcity is discussed in view of multilevel antifouling defences and presumptive evolutionary transitions from epibiosis towards endoparasitism or endosymbiosis.

A high frequency time series at Weathership M, Norwegian Sea, during the 1997 spring bloom: the reproductive biology of Calanus finmarchicus

Abstract: The reproductive biology of Calanus finmarchjcus was investigated at a permanent station in the Norwegian Sea (Weathership Stn M, 66\" N, 2\" E) during a time series between March and June 1997 The temporal development of female abundance, egg production rate and gonad development stage in relation to the phytoplankton production cycle is described Abundance of females, copepodite stage 5 and males as well as female gonad morphology were examlned from MrP2 net samples taken daily from the upper 100 m. Daily egg production rate and number of spawn.ing females we]-e determined from 50 individual females placed in multiwells or beakers. Once a week a multinet haul from 1000-500-250-100-50-0 m was performed to study the depth d~stribution of females and gonad development stages. Results show that the reproductive period of C. finmarchicusin the Atlantic rcqion of the Norwegian Sea can be subdivided in 3 periods in relation to phytoplankton devrlopmt,nt. ( 1 ) During the prebloom over a period of 49 d mean egg production rate was 8 eggs female-' d ' and an average of 47 of the females were mature. (2) Coincident with the bloom in mid May the egg production rate increased up to a maximum of 44 eggs female-' d-' while more than 80% of the females were mature. (3) After the bloom at the beginning of June, egg production decreased, and mature females were only rarely found. Feedlng expenments indicate that food quantity limited egg production prior to the bloom, while presumably food quality was not sufficient dunng postbloom. However, due to high female abundance the total population egg production prior to the bloom was the same as during the bloom. This implies that the reproduction of C. finmarchicus in the Norwegian Sea is to some extent decoupled from the phytoplankton bloom.