Showing papers on "Pelagic zone published in 2000"

Small pelagics in upwelling systems: patterns of interaction and structural changes in "wasp-waist" ecosystems

Abstract: In upwelling ecosystems, there is often a crucial intermediate trophic level, occupied by small, plankton-feeding pelagic fish dominated by one or a few schooling species. Their massive populations may vary radically in size under intensive exploitation. We have used decadal-scale time series to explore patterns of interactions between these fish, their prey, and their predators so as to quantify functional roles of small pelagic fish in those ecosystems. Top-down control of zooplankton is detected off South Africa, Ghana, Japan, and in the Black Sea. Conversely, bottom-up control of predators, such as predatory fish and marine birds, is observed in the Benguela, Guinea, and Humboldt currents. Thus small pelagic fish exert a major control on the trophic dynamics of upwelling ecosystems and constitute midtrophic-level “wasp-waist” populations. Ecosystem effects of fishing are addressed by considering potential structural changes at different scales of observation, ranging from individuals, via school dynamics, to food webs. The overall impact is explored using a trophic model (Ecosim) given different types of food web control. Ecosystem dynamics can be entirely different depending on how the food web is controlled (bottom-up, top-down or wasp-waist). The threat of eroding intra-specific diversity is emphasized because it may lead to a long-term decline in the productivity of the pelagic fish resources. School composition is shown to reflect the relative species abundance within the pelagic community and functions as a “school trap” that could maintain a collapsed population in a depleted state for lengthy periods, and affect spatial dynamics such as migrations. As a result, overfishing can alter the abundance, composition, and distribution in pelagic communities, and may induce drastic changes of state. Predation on zooplankton by the jack mackerel (Trachurus symmetricus murphyi) population in the South Pacific provides an example of the alteration of matter fluxes in trophic webs caused by fishery removals. In conclusion, it is doubtful that pelagic fisheries will continue to increase without major disruptions to ecosystems.

The relationship in lake communities between primary productivity and species richness

Abstract: An understanding of the relationship between species richness and productivity is crucial to understanding biodiversity in lakes. We investigated the relationship between the primary productivity of lake ecosystems and the number of species for lacustrine phytoplankton, rotifers, cladocerans, copepods, macrophytes, and fish. Our study includes two parts: (1) a survey of 33 well-studied lakes for which data on six major taxonomic groups were available; and (2) a comparison of the effects of short- and long-term whole-lake nutrient addition on primary productivity and planktonic species richness. In the survey, species richness of all six taxa showed a significant quadratic response to increased annual primary productivity ( 14 C estimate, g C-m -2 -yr -1 ) when lake area is taken into account. However, the richness-productivity relationship for phytoplankton and fish was strongly dependent on lake area. The relationship for phytoplankton, rotifers, cladocerans, copepods, and macrophytes was significantly unimodal. Species richness generally peaked at levels of primary productivity in the range of 30-300 g C-m -2 -yr -1 . For the average lake size, the highest biodiversity tended to occur in lakes with relatively low primary productivity, such as those found in the Northern Temperate Lakes Long-Term Ecological Research (LTER) site in the upper Midwest (United States) and in the Experimental Lakes Area of Ontario (Canada). Based on short-term (3 yr) and long-term (21-24 yr) experiments, we tested whether individual lakes respond to whole-lake enrichment experiments in the manner suggested by analyses of survey data. Experimental addition of nutrients produced varied and unpredictable responses in species richness, probably due to transient dynamics and time lags. Responses to nutrient addition were taxon and lake specific. Phytoplankton showed a variety of relationships between species richness and pelagic primary productivity (PPR), depending on the history of enrichment and recovery. No significant effect of primary productivity on rotifer richness occurred in any of the experimental lakes, whereas richness of crustacean zooplankton was negatively correlated with primary productivity in both the short- and long-term experiments.

Foraging ecology of northern elephant seals

Abstract: Sexual segregation in foraging is predicted from the great size disparity of male and female northern elephant seals, Mirounga angustirostris. Our aim was to test this prediction by measuring diving and foraging behavior, foraging locations, and distribution of the sexes during biannual migrations in the northeastern Pacific Ocean. Daily movements of 27 adult males and 20 adult females, during 56 migrations from Afio Nuevo, California, USA, were determined by Argos satellite telemetry via head-mounted platform transmitter terminals. Diving records were obtained with archival time-depth-speed recorders attached to the backs of seals that were recovered when the seals returned to the rookery. Pronounced sex differences were found in foraging location and foraging pattern, as reflected by hor- izontal transit speed and diving behavior. Males moved directly north or northwest at a mean speed of 90 ? 27 km/d to focal foraging areas along the continental margin ranging from coastal Oregon (534 km away) to the western Aleutian Islands (4775 km away). Males remained in these areas (mean size = 7892 ki2) for 21-84% of their 4-mo stays at sea. The predominance of flat-bottom dives in these areas suggests concentrated feeding on benthic prey. Migration distance and estimated mass gain were positively correlated with male size, and individual males returned to the same area to forage on subsequent migrations. In contrast, females ranged across a wider area of the northeastern Pacific, from 380 to 600 N and from the coast to 172.5? E. Focal foraging areas, indicated by a reduction in swim speed to <0.4 m/s, were distributed over deep water along the migratory path, with females remaining on them a mean of 3.5 d before moving to another one. Jagged-bottom dives that tracked the deep scattering layer prevailed in these areas, suggesting that females were feeding on pelagic prey in the water column. Females took roughly similar initial paths in subsequent migrations, but large deviations from the previous route were observed. We conclude that there is habitat segregation between the sexes. Females range widely over deep water, apparently foraging on patchily distributed, vertically migrating, pelagic prey, whereas males forage along the continental margin at the distal end of their migration in a manner consistent with feeding on benthic prey.

Marine protected areas and ocean basin management

Abstract: 1. All reserve designs must be guided by an understanding of natural history and habitat variability. 2. Differences in scale and predictability set aside highly dynamic pelagic systems from terrestrial and nearshore ecosystems, where wildlife reserves were first implemented. Yet, as in static systems, many pelagic species use predictable habitats to breed and forage. Marine protected areas (MPAs) could be designed to protect these foraging and breeding aggregations. 3. Understanding the physical mechanisms that influence the formation and persistence of these aggregations is essential in order to define and implement pelagic protected areas. We classify pelagic habitats according to their dynamics and predictability into three categories: static, persistent and ephemeral features. 4. While traditional designs are effective in static habitats, many important pelagic habitats are neither fixed nor predictable. Thus, pelagic protected areas will require dynamic boundaries and extensive buffers. 5. In addition, the protection of far-ranging pelagic vertebrates will require dynamic MPAs defined by the extent and location of large-scale oceanographic features. 6. Recent technological advances and our ability to implement large-scale conservation actions will facilitate the implementation of pelagic protected areas. 7. The establishment of pelagic MPAs should include enforcement, research and monitoring programmes to evaluate design effectiveness. 8. Ultimately, society will need a holistic management scheme for entire ocean basins. Such overarching management will rely on many innovative tools, including the judicious use of pelagic MPAs. Copyright © 2000 John Wiley & Sons, Ltd.

Sediment mineralization, nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium in an estuarine fjord with sea cage trout farms

Abstract: Sediment oxygen consumption, nutrient fluxes across the sediment-water interface, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) were measured in sediment below the cages of a rainbow trout farm in the estuary of Horsens Fjord, Denmark, and compared to that in the sediment at reference stations elsewhere in the fjord Denitrification and DNRA were measured by the isotope pairing technique and a 15N technique, respectively The measurements are the first published data of in sjtu activities in sediment below marine fish farms Oxygen consumption by the sediment and effluxes of NH,' and pod3were markedly stimulated by organic matter loading from the overlying fish cages Denitrification of nitrate from the water column (D,) was only of significance during autumn and winter when runoff from land resulted in high water colun~n itrate concentrations In contrast, coupled nitrification-denitrification (D,) occurred in the sediment throughout the summer, with the rates in the sediment below the fish cages surprisingly being of the same magnitude as those at the reference stations Competition for inorganic nitrogen from benthic microalgae probably inhibited the activity of both nitrifiers and denitrifiers a t the reference stations, while a mat of Beggiatoa spp on the sediment surface below the fish cages in late summer indicated the presence of sulfide in the uppermost layers of the sediment, which almost ceased D, While DNRA was absent in sediment unaffected by the trout farms, it was of quantitative importance in the reduced sediment below the fish cages, DNRA activity being up to 7-fold greater than denitrification activity The ecological consequence of this shift in the relative importance of the processes in response to organic matter loading was a reduced nitrogen removal by denitrification and an increased efflux of NH,' to the water column, resulting in stimulation of pelagic primary production and increased nitrogen retention by the ecosystem The excess nitrogen input to the fjord from the trout farms corresponded to approximately 12 % of the total nitrogen load from land during the summer months Only 01 % of this additional nitrogen input was removed by denitrification in the underlying sediments during the fish production season In general, shallow estuarine ecosystems hosting fish farms may therefore face increasing eutrophication problems due to the direct supply of nutrients in the fish food and to shifts in the dominant microbial processes in the sediment below the fish cages

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.

Recruitment in flatfish, with special emphasis on North Atlantic species: progress made by the Flatfish Symposia

Abstract: Van der Veer, H. W., Berghahn, R., Miller, J. M., and Rijnsdorp, A. D. 2000. Recruitment in flatfish, with special emphasis on North Atlantic species: Progress made by the Flatfish Symposia. – ICES Journal of Marine Science, 57: 202–215. In summarizing the main results on recruitment that emerged from the series of Flatfish Symposia, two aspects were distinguished: mean level and interannual variability. Recruitment to a stock appears to be related to the quantity of juvenile nursery habitats, suggesting that either larval supply or the carrying capacity of the nurseries is the limiting factor. However, available information on growth of 0-group flatfish suggests that the carrying capacity of nursery areas is never reached. Variability in year-class strength is generated during the pelagic egg and larval stage, probably by variations in the hydrodynamic circulation and in the mortality rates of eggs and larvae. Density-dependent processes seem to occur only during the juvenile stages, particularly in respect of growth. However, no impact on recruitment variability has been found. Density-dependent mortality during the phase shortly after settlement dampens the interannual recruitment variability. There is no evidence of densitydependent effects in the adult phase at present, but these may have been important at lower levels of exploitation. The importance of the factors determining recruitment vary not only among species, but also over the species’ range. It is suggested that damping processes can only occur in the demersal stage, implying that variability in year-class strength can only decrease in fish species with a demersal stage. If true, ultimate variability in recruitment in fish species will be related to the relative duration of the pelagic and demersal stages. 2000 International Council for the Exploration of the Sea

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.

A Trophic Flow Model of the Caeté Mangrove Estuary (North Brazil) with Considerations for the Sustainable Use of its Resources

Abstract: The Caete Estuary lies within the world's second largest mangrove region, 200 km south-east of the Amazon delta. It has an extension of about 220 km 2 and is subjected to a considerable human impact through intensive harvest of mangrove crabs ( Ucides cordatus ) and logging of mangroves. In order to integrate available information on biomass, catches, food spectrum and dynamics of the main species populations of the system, a trophic steady state model of 19 compartments was constructed using the ECOPATH II software (Christensen & Pauly, 1992). Ninety-nine percent of total system biomass is made up by mangroves ( Rhizophora mangle , Avicennia germinans and Laguncularia racemosa ), which are assumed to cover about 45% of the total area and contribute about 60% to the system's primary production. The remaining biomass (132 g m −2 ) is distributed between the pelagic and benthic domains in proportions of 10% and 90% respectively. Through litter fall, mangroves inject the main primary food source into the system, which is either consumed directly by herbivores (principally land crabs, Ucides cordatus ) or, when already metabolized by bacteria, by detritivors (principally fiddler crabs, Uca spp.). These two groups are prominent in terms of biomass (80 g and 14·5 g m −2 ), and food intake (1120 g m −2 yr −1 and 1378 g m −2 yr −1 respectively). According to the model estimates, energy flow through the fish and shrimp compartments is of relatively low importance for the energy cycling within the system, a finding which is contrary to the situation in other mangrove estuaries reported in the literature. The dominance of mangrove epibenthos is attributed to the fact that a large part of the system's production remains within the mangrove forest as material export to the estuary is restricted to spring tides, when the forest is completely indundated. This is also the reason for the low abundance of suspension feeders, which are restricted to a small belt along the Caete River and the small creeks which are watered daily. Phytoplankton, temporarily refloating benthic diatoms, neritic zooplankton and small pelagic fish dominate the (low) pelagic biomass. Total system throughput (10 559 g m −2 yr −1 ) and mean transfer efficiency between trophic levels (9·8%) calculated by the model fit well into the range reported for other tropical coastal ecosystems. The very high gross efficiency of the fishery (catch/net primary production) of 8·6% and its low trophic level (2·1) is explained by a high harvesting rate of mangroves and the fact that the main animal resource in the system are the mangrove crabs ( Ucides cordatus ), which feed at the first trophic level. The model was balanced asuming a turnover rate for the land crabs of P / B =0·25 ( P / B : production per unit of biomass) which is possibly too high. If this value was replaced by a (possibly more realistic) lower value, the model would not balance, suggesting a situation in which more biomass is being harvested than produced, which hints to an overexploitation of this resource A ranking of the various system components in terms of their contribution to the system function (ascendency sensu Ulanowicz, 1997) revealed that detritus and associated bacteria contribute 34%, mangroves 19%, fiddler crabs 13%, phytoplankton and microphytobenthos 10%, mangrove crabs 10%, and the remaining 14 groups 14% to the total ascendency. Summary statistics of the model are given and compared with those of other coastal ecosystems.

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.

Growth, morphological variation and ontogenetic niche shifts in perch (Perca fluviatilis) in relation to resource availability.

Abstract: Despite the common occurrence of ontogenetic niche shifts, their consequences for morphological adaptations have been little studied. To address this question, we studied morphological adaptations related to ontogenetic niche shifts in Eurasian perch (Perca fluviatilis) in eight lakes that varied in density of benthic resources and planktivorous fish biomass. Perch start to feed on pelagic zooplankton, then shift to benthic resources at intermediate sizes, and finally, when large enough, mainly feed on fish. These three functional niches over ontogeny are expected to set constraints on the morphology and size-specific growth of perch. The growth of perch was negatively related to planktivorous fish biomass in the zooplanktivorous niche, but positively related to planktivorous fish biomass in the piscivorous niche. The number of gill rakers of perch was negatively related to the biomass of planktivorous fish, providing evidence for the occurrence of character displacement as a consequence of competition in the zooplanktivorous niche. Perch in lakes with low densities of predator-sensitive macroinvertebrates had greater body height measurements and a larger mouth early during ontogeny. This pattern is suggested to be a result of a selection for increased efficiency in the benthic niche when the availability of benthic resources is low. Perch in lakes with a high biomass of planktivorous fishes had fusiform body morphology, a thicker tail and a larger mouth then the average piscivorous perch. The different responses of perch morphology to variation in the availability of benthic resources compared to variation in planktivore biomass are suggested to be partly because the availability of the former resource to a larger extent is set by abiotic conditions (humic content). We suggest that the key factors affecting size-specific growth and body morphology of perch in the system studied are the availability of resources in the benthivorous and piscivorous niches. We also provide evidence for morphological trade-offs, especially between the benthivorous and the piscivorous ontogenetic niches.

Pelagic species diversity, biogeography, and evolution

Abstract: Pelagic (open-ocean) species have enormous population sizes and broad, even global, distributions. These characteristics should damp rates of speciation in allopatric and vicariant evolutionary models since dispersal should swamp diverging populations and prevent divergence. Yet the fossil record suggests that rates of evolutionary turnover in pelagic organisms are often quite rapid, comparable to rates observed in much more highly fragmented terrestrial and shallow-marine environments. Furthermore, genetic and ecological studies increasingly suggest that species diversity is considerably higher in the pelagic realm than inferred from many morphological taxonomies. Zoogeographic evidence suggests that ranges of many pelagic groups are much more limited by their ability to maintain viable populations than by any inability to disperse past tectonic and hydrographic barriers to population exchange. Freely dispersing pelagic taxa resemble airborne spores or wind-dispersed seeds that can drift almost ...

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.

Marine catchment basin effects versus impacts of fisheries on semi-enclosed seas

Abstract: Synchronous anthropogenic effects on marine coastal systems, particularly since World War II, make it difficult to separate effects of fishing from terrestrial inputs, especially those caused by nutrient runoff. Natural enrichment is vital to productive fisheries, but over the long term anthropogenic nutrient impacts generally predominate over fishery effects in semi-enclosed seas and affect biodiversity and susceptibility to fishing. Such impacts on marine fisheries beyond natural levels of outflow are referred to here as marine catchment basin (MCB) effects. They require coordinated actions within the catchment area to control them, since fisheries management actions alone are unlikely to be effective in rectifying ecosystem impacts. Net nutrient inflows may be positive or negative, depending on intensity and degree of retention by the receiving marine system and may promote ecosystem change and lead to ecological dominance by exotic species. Initially positive effects of enrichment may disguise the urgent need for coordinated environmental and fishery management measures in semi-enclosed seas. Fisheries impacts are serious, but may be secondary and synergistic, but potentially catastrophic under hypoxic conditions since eutrophic processes make demersal ecosystems particularly sensitive to disturbance of bottom habitats. Hence, fishing with bottom gear may impact bottom fauna and demersal resources within or above organic sediments affected by eutrophic processes and hypoxia. These effects show up as changes in the ratio of pelagic to demersal landings, and support broad-brush comparative studies of areas subject to different levels of environmental impact.

Hares and Tortoises: Interactions of Fast and Slow Variablesin Ecosystems

Abstract: Components with diverse turnover times establish the tempo of ecosystem dynamics. The gradient of turnover times within an ecosystem is more than a useful device for understanding; it has practical value as well (O’Neill and others 1986). When initiating a study of a particular ecosystem process, ecologists know that the context is set by other processes with longer turnover times, while the mechanisms derive from another set of processes with shorter turnover times. With the help of the template formed by context, process, and mechanism, the investigator formulates hypotheses, chooses approaches and methods, and prioritizes the research objectives. Skilled modelers know that the essential dynamics of a given ecosystem phenomenon can be captured by including the key processes with longer and shorter turnover times (slower or faster turnover rates). Slow processes are treated as parameters, whereas fast processes may be solved at equilibrium. The range of turnover times in ecosystem components spans at least 12 orders of magnitude. At one extreme, the turnover times of limiting nutrients in pelagic ecosystems are less than a second; at the other, the weathering of rock formations can take millions of years. Organismic turnover times range from about a day for microbes to centuries for old-growth trees. Disparities in turnover times make ecological legacies possible. Extensively weathered landscapes may have phosphorus-limited vegetation, whereas glacially reworked landscapes support nitrogen-limited vegetation. Dead trees in forests support insects and birds or serve as nurse logs for extended periods of time. Large, infrequent disturbances may structure an ecosystem for centuries, and the effects of human land use can persist in ecosystems long after the activity has ceased (Turner and Dale 1998). Ecosystem dynamics are history-dependent because of the coupling of events across a range of cycling times. “Big effects from small causes” (Ricker 1963) are

Fisheries in the Mediterranean

Abstract: The aim of this paper is to give a description of the Mediterranean fisheries, and its level of exploitation and to address the main questions dealing with its management. The Mediterranean is a semi-enclosed marine area with generally narrow continental shelves. The primary production of the Mediterranean is among the lowest in the world (26-50g C m-2 y-1). The Mediterranean fisheries can be broken down into three main categories: small scale fisheries, trawling and seining fisheries, which operated on demersal, small pelagic and large pelagic resources. After a general description of the state of the resources in the different areas of the Mediterranean it is concluded that (a) the overall pictures from the western to the eastern Mediterranean are not considerably different, (b) the total landings in the Mediterranean have been increased the last decades, and (c) from the perspective of stock assessment, the very few available time series data show stable yield levels. In general fisheries management in the Mediterranean is at a rela- tively early stage of development, judging by the criteria of North Atlantic fisheries. Quota systems are generally not applied, mesh-size regulations usually are set at low levels relative to scientific advice, and effort limitation is not usually applied or, if it is, is not always based on a formal resource assessment. The conservation/management measures applied by the Mediterranean countries can be broadly separated into two major categories: those aiming to keep the fishing effort under control and those aiming to make the exploitation pattern more rational. The most acute problems in the management of the Mediterranean resources are the multispecificity of the catches and the lack of reliable official statistics.

Pelagic fishes and the cod recruitment dilemma in the Northwest Atlantic

Abstract: Like most other stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic, cod in the southern Gulf of St. Lawrence declined to low abundance in the early 1990s. Recovery has been slow in con...

Hydrographic mesoscale structures and Poleward Current as a determinant of hake (Merluccius merluccius) recruitment in southern Bay of Biscay

Abstract: The relationship between the recruitment of European hake (Merluccius merluccius) and environmental conditions in southern Bay of Biscay is examined. The historical series of autumn bottom trawl surveys carried out in Galicia and Cantabrian Sea waters from 1983 show that the processes of hake recruitment lead to well-defined patches of juveniles, found in localized areas of the continental shelf. These concentrations vary in density according to the strength of the year-class, although they remain generally stable in size and spatial location. The size of the patches, estimated using basic geostatistical techniques, is found to be from 20 to 35 km in diameter. The existence of spatial correlation is assumed by computing variograms, and the year-onyear repetition of the spatial patterns is shown to be a way of linking them to environmental conditions. In the eastern, progressively narrowing, shelf of the Cantabrian Sea, years of massive inflow of the eastward shelf-edge current produce low recruitment indices, due to larvae and pre-recruits being transported away from spawning areas to the open ocean. Under these conditions, high mortality is expected because of the difficulties juveniles have in finding the shelf grounds. On the other hand, the transport of larvae within anticyclonic mesoscale structures moving towards the recruitment areas will be an aid to recruitment. These eddies displace westward according to the condition of potential vorticity conservation. When orographic features, such as big capes, occur in their drift path their eastern edges are held back. This situation causes patches of recruits to be located east of the main capes of the western Cantabrian Sea. The pattern of feeding of juvenile hake includes vertical migrations searching for small pelagic fish. Upward motions of nutrient-rich deeper water masses were found close to the recruitment areas, stemming from variations of the vorticity field by mesoscale eddies. The resulting enhanced primary production seems to affect the distribution and size of hake recruit concentrations. 2000 International Council for the Exploration of the Sea

Long-term variation of the Bay of Brest ecosystem: benthic-pelagic coupling revisited

Abstract: Observations over the last 20 yr of the development of pelagic and benthic communities of a western European coastal ecosystem (Bay of Brest) provide complementary hypotheses to the 'silicic acid pump' hypothesis. An increase in nitrogen inputs, independent of silicic acid levels, has lowered the Si:N molar ratios during the last 20 yr in river discharges. Since 1981, maximum concen- trations of chlorophyll a during the first spring bloom of the annual cycle have decreased, in contrast to the subsequent spring and summer blooms. Concomitantly, extensive spreading of an exotic gas- tropod Crepidula fornicata has modified the trophic structure of benthic communities by increasing suspension-feeder biomass. The following hypotheses on ecosystem functioning are made: (1) the decrease of chlorophyll biomass during the first spring bloom results from silicic acid limitation and increased suspension feeder activity, (2) benthic filtration and biodeposition activities enhance bio- genic silica retention at the sediment-water interface, and (3) recycling of trapped biogenic silica maintains diatom populations by providing silicic acid in summer and reduces primary production seasonality. These hypotheses suggest that benthic organisms control the export rate of biogenic silica towards the open-water ocean and thus the specific composition of secondary phytoplankton blooms in the Bay.

Benthic and pelagic dives: a new foraging behaviour in rockhopper penguins

Abstract: The pattern and characteristics of diving of 55 daily foraging trips performed by 16 female southern rockhopper penguins Eudyptes chrysocome filholi were studied in coastal waters of Kerguelen Archipelago during the guard stage. Diving patterns and dive profiles indicated that birds used 2 foraging behaviours. First, they performed typical pelagic dives, as previously decribed for other penguin species. Second, they also performed series of consecutive square-wave dives reach- ing similar maximum depths, with no deeper dives within the series, all criteria which characterized benthic dives. Two groups of benthic and pelagic dives were subsequently selected to compare their parameters. In agreement with optimization concepts in foraging theory, rockhopper penguins max- imize bottom time (= feeding time) of benthic dives through an increase in both descent and ascent rates, thus minimizing transit time between the sea surface and the bottom. Regardless of dive depth, bottom time was longer and diving efficiency higher in benthic dives than in pelagic ones. Penguins were also more active during benthic dives, as indicated by higher numbers of depth and light wig- gles at the bottom of these dives. Bathymetry and dive depth indicate that penguins were able to reach about 80% of the sea floor surrounding the colony. Abrupt changes in dive depth within series of benthic dives were identical in height to the thickness of lava flows, the main geological features of the landscape, strongly suggesting that birds followed the bottom topography at a fine scale. Dietary analysis showed that rockhopper penguins fed upon benthic prey (a few fish and the mysid Mysidetes morbihanensis) and pelagic organisms, including the major item Euphausia vallentini. There was a positive linear relationship between the mass of food brought ashore and an index of the proportion of benthic dives during the daily trips, thus emphasizing the importance for rockhopper penguins living in a coastal marine environment of feeding on pelagic migrators trapped at or near the sea floor during the day.

Seasonal changes in the diets and relative abundances of perch and roach in the littoral and pelagic zones of a large lake

Abstract: In the Enonselka and Laitialanselka basins of Lake Vesijarvi, perch Perca fluviatilis and roach Rutilus rutilus were abundant in the littoral and in the pelagic zones throughout the summer In the littoral zone, roach was always more numerous than perch, while perch dominated in the open water Intraspecific diet overlap values were higher than interspecific values In the pelagic zone, perch <155 mm fed mainly on the cladoceran Leptodora kindtii, while small bosminids were most important food items for roach Large perch were piscivorous, feeding mainly on smelt Osmerus eperlanus In the littoral zone small perch foraged on zooplankton and chironomid larvae and large perch on chironomids and fish (small perch) Small roach fed mainly on bosminids and detritus, while for roach <185 mm macrophytes (Elodea Canadensis, Lemna trisulca) were also of importance Detritus was more common in the food of roach in Laitialanselka than in Enonselka The slower growth rate of roach in Laitialanselka compared with Enonselka was probably connected with this However, considering the latitude of the lake, the growth rate of both roach and perch was relatively fast in both basins The results indicated that in a large lake both perch and roach are able to utilize effectively the different habitats and diverse food resources By segregation in food resource utilization they are able to co-exist in large quantities, at the same time maintaining a relatively fast growth rate

Subsurface phytoplankton blooms fuel pelagic production in the North Sea

Abstract: The seasonal phytoplankton biomass distribution pattern in stratified temperate marine waters is traditionally depicted as consisting of spring and autumn blooms. The energy source support- ing pelagic summer production is believed to be the spring bloom. However, the spring bloom dis- appears relatively quickly from the water column and a large proportion of the material sedimenting to the bottom following the spring bloom is often comprised of intact phytoplankton cells. Thus, it is easy to argue that the spring bloom is fueling the energy demands of the benthos, but more difficult to argue convincingly that energy fixed during the spring bloom is fueling the pelagic production occurring during summer months. We argue here that periodic phytoplankton blooms are occurring during the summer in the North Sea at depths of >25 m and that the accumulated new production (sensu (Dugdale and Goering, Limnol. Oceanogr., 12, 196-206, 1967)) occurring in these blooms may be greater than that occurring in the spring bloom in the same regions. Thus, such blooms may explain apparent discrepancies in production yields between different temperate marine systems.

A revised nitrogen budget for the Arabian Sea

Abstract: Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N2O and ammonia, deep-sea N burial, and N inputs by rivers and marginal seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.

Fishing effects on spatial distribution and trophic guild structure of the fish community in the Georges Bank region

Abstract: We examined the changes in the spatial distribution and trophic guild structure of the Georges Bank fish community associated with intense exploitation over the last 30 years. As heavily exploited species declined in abundance, their spatial ranges and degree of overlap with other species declined. The converse was true for weakly exploited species whose populations have increased. Despite dramatic alterations in species composition, the trophic guild structure was consistent over time. There are five major trophic guilds on Georges Bank across gradients from benthic to pelagic feeders and fish to invertebrate prey. Exploitation has strongly impacted the piscivores in this system and has modified species interactions through alterations in species composition.

Viral Density and Virus-to-Bacterium Ratio in Deep-Sea Sediments of the Eastern Mediterranean

Abstract: Viruses are now recognized as a key component in pelagic systems, but their role in marine sediment has yet to be assessed. In this study bacterial and viral densities were determined at nine deep-sea stations selected from three main sites (i.e., the Sporades Basin, the Cretan Sea, and the Ierapetra Trench at depths of 1,232, 1,840, and 4,235 m, respectively) of the Eastern Mediterranean. The three areas were characterized by different phytopigment and biopolymeric carbon concentrations and by changes in the protein and carbohydrate pools. A gradient of increasing trophic conditions was observed from the Sporades Basin (North Aegean) to the Ierapetra Trench (South Aegean). Viral densities (ranging from 1 × 109 to 2 × 109 viruses ml of sediment−1) were significantly correlated to bacterial densities (n = 9, r2 = 0.647) and reached values up to 3 orders of magnitude higher than those generally reported for the water column. However, the virus-to-bacterium density ratio in deep-sea sediments was about 1 order of magnitude lower (range of 2 to 5, with a modal value of 2.6) than in pelagic environments. Virus density decreased vertically with depth in sediment cores at all stations and was below detection limits at the 10-cm depth of the abyssal sediments of the Ierapetra Trench. Virus density in the sediment apparently reflected a gradient of particle fluxes and trophic conditions, displaying the highest values in the Sporades Basin. The low virus-to-bacterium ratios and their inverse relationship with station depth suggest that the role played by viruses in controlling deep-sea benthic bacterial assemblages and biogeochemical cycles is less relevant than in pelagic systems.

Barriers to gene flow in Embiotoca jacksoni, a marine fish lacking a pelagic larval stage.

Abstract: Marine species generally show high dispersal capabilities, which should be accompanied by high levels of gene flow and low speciation rates. However, studies that focused on the relationship between dispersal and gene flow in marine fishes have been inconclusive. This study focuses on the black surfperch, Embiotoca jacksoni, a temperate reef fish that lacks a pelagic larval stage and lives on almost continuous reefs along the California and Baja California coasts. Mitochondrial control-region sequences from 240 individuals were obtained, and phylogeographic patterns were analyzed. A major phylogeographic break was found at Santa Monica Bay, a sandy expanse that prevents adult dispersal. Deep water separating the southern California Channel Islands was also found to be a major barrier to gene flow. Minor phylogeographic breaks were also detected in the Big Sur/Morro Bay and in the Punta Eugenia/Guerrero Negro regions, but none in the Point Conception region. Gene flow levels in E. jacksoni were fo...

Exploitation of distant Antarctic waters and close shelf-break waters by white-chinned petrels rearing chicks

Abstract: The foraging ecology of white-chinned petrels Procellaria aequinoctialis rearing chicks was examined at the Crozet Islands in the Southern Ocean, based on satellite tracking, diet and provisioning studies. White-chinned petrels from the Crozet Islands exploit a wide variety of marine environments ranging from sub-tropical waters to the limit of pack-ice at the edge of the Antarctic continent. This capability was made possible by the use of a 2-fold strategy whereby adults exploit alternatively distant oceanic waters and neritic slope waters in the vicinity of the breeding grounds. On average the birds conducted a long foraging trip followed by 2.2 short trips. During trips of long duration over oceanic waters, birds tended to commute mainly to cold, deep Antarctic waters where most foraging activity took place. They commuted from and returned to Crozet at high speeds (mean 31 and 34 km h -1 respectively) with a mean foraging range of 1868 km (maximum 2421 km). In Antarctic waters, the white-chinned petrel appears to feed mainly on pelagic fishes and on Antarctic krill Euphausia superba and concentrates its efforts in waters with sea-surface temperatures of 2°C. During short trips the birds commuted to the Crozet shelf break, where they fed mainly on fish. Diet samples delivered to chicks after short trips indicate that adults relied at least in part on food made available by longliners as baits and discards. Despite its small size compared to albatrosses, the white-chinned petrel from Crozet appears to be a particularly wide-ranging species and an opportunistic feeder in terms of the marine environment exploited, which explains its wide distribution in the Southern Ocean. This study highlights the particular importance of Antarctic waters for this sub-Antarctic species during the chick-rearing period, i.e. in summer when the retreat of the pack-ice makes abundant resources such as Antarctic krill available.