Showing papers on "Pelagic zone published in 1996"

Organism life cycles, predation, and the structure of marine pelagic ecosystems

Abstract: This paper explores the notion that the theoretical basis for contemporary research concerning the structure and function of marine pelagic ecosystems is self-limiting. While some findings such as the microbial food web have extended our knowledge of the biological components of the upper water column and their relationships to fluxes of materials and energy, they have not advanced our understanding of why specific pelagic forms occur in time and space, and why only some attain dominant status and contribute the bulk of biogenic fluxes emanating from the mixed layer. It is argued here that a major impediment to improved conceptual models is the historic focus on resource-driven or 'bottom-up' factors as being the dominant variables structuring planktonic ecosystems. Evidence is presented that predation or 'top-down' trophic effects may be equally important in specifying the occurrence of particular taxa, the biomass within adjacent trophic levels, and the morphology of dominant herbivores and carnivores. It is suggested that key species, because of unique combinations of life history strategies, metabolic demands, and physiological performance, may exert a dominant role in the extent to which predatory interactions cascade through pelagic food webs. There is considerable evidence of evolution of predation avoidance strategies among phytoplankton and zooplankton. It is proposed that future research might profitably be directed toward the question of how the pelagic environment selects for life histories and morphologies of organisms under conditions when resource availability and predation are both significant structural buttresses. Methodological approaches should include detailed studies of dominant key taxa from different environments, with the goal of identifying the critical aspects of life history, behavior, or morphology which account for their success.

Potential Abiotic and Biotic Impacts of Zebra Mussels on the Inland Waters of North America

Abstract: SYNOPSIS. The expansion of zebra mussel distribution into inland waterways of North America portends significant abiotic and biotic changes mediated either directly or indirectly by Dreissena . Dreissena fouls a wide array of submerged substrates including rock surfaces, macrophytes, native molluscs, canal and dock walls, and watercraft and motor outdrives. Fouling of water intake pipes and associated installations can severely impair water delivery to hydroelectric, municipal and industrial users, necessitating proactive or reactive control measures. Mussels increase water clarity by removing suspended clay, silt, bacteria, phytoplankton, and small zooplankton. Clear water phases associated with Dreissena grazing may exceed in magnitude and duration those generated by zooplankton grazing. Enhanced water clarity increases light transmittance and growth of benthic plants. Some benthic invertebrates {e.g., unionid molluscs) are adversely affected by Dreissena , whereas others, including amphipod crustaceans, exploit structure associated with or wastes generated by zebra mussels. Dreissena is exploited by a host of predators, most notably waterfowl, fish and crayfish. Waterfowl predators that consume contaminated Dreissena have elevated concentrations of organic pesticides and polychlorinated biphenyl compounds. Invasion of shallow lakes and ponds by Dreissena may divert production and biomass from pelagic to benthic foodwebs, shifting ecosystems to an alternative state.

Evolution patterns of radiolaria and organic matter variations: A new approach to identify sea-level changes in mid-Cretaceous pelagic environments

Abstract: The relation between sea-level changes, plankton productivity, and evolution, as well as the occurrence of anoxic sediments, provides an interesting avenue of paleooceanographic research. In this context, we examined mid-Cretaceous radiolarian faunas, carbonate isotopic and organic matter type data of Deep Sea Drilling Project and Ocean Drilling Program cores from the North Atlantic, and samples from outcrops of the western Tethys from central and northern Italy. Former studies indicate that an expansion of the oxygen minimum zone caused plankton extinctions at the Cenomanian-Turonian boundary. An expanded oxygen minimum zone would destroy deeper habitats of planktic foraminifera, causing the extinction of deeper dwelling forms. Although this model is well established for the Cenomanian-Turonian boundary, not much is known about the causes of extinctions and radiations during the entire mid-Cretaceous (Aptian-Turonian). We demonstrate that the dimension of the oxygen minimum zone, which depends on the relative sea level and the corresponding nutrient supply, causes the complex pattern of evolution and radiation of planktic protozoa and the sedimentation of black shales in the mid-Cretaceous. This new depositional model allows correlation of micropaleontologic data and different types of black shales in the pelagic realm within a sequence stratigraphic framework.

Cephalopods as prey. iii. cetaceans

Abstract: Over 80% of odontocete species and two balaen whale species include cephalopods in their diet regularly. In 28 odontocetes they comprise the main food. Predominently cephalopod-eating species are found in the Physeteridae, Ziphiidae, Phocaenidae and Delphinidae. By far the most important of the 28 families of cephalopods represented in the diet of cetaceans are the oceanic Ommastrephidae, Histioteuthidae and the Cranchiidae, with the neritic Loliginidae assuming most importance on the continental shelves. Onychoteuthidae and Gonatidae assume greater importance in polar regions and the North Pacific. The other 22 families form a reservoir from which various cetaceans eat opportunistically and as their sizes permit. There are probably less than 60 cephalopod species regularly in the diet of cetaceans. Species composition of the food varies regionally, seasonally and annually. Locally, the greatest difference is found between cetaceans that live in oceanic water and continental shelf water. There is a positive correlation between the size of the prey and both the size of pelagic feeding cetacean species and the growth stage within a species. This leads to some partitioning of the food and less competition. Broad estimates show that the biomass of oceanic cephalopods consumed annually by the largest odontocete, Physeter catodon, may be over twice the biomass of fish caught by man. Regional estimates show that consumption by cetaceans of little known cephalopod species may greatly exceed the local catches of commercial fish.

What the pelagic stages of coral reef fishes are doing out in blue water: Daytime field observations of larval behavioural capabilities

Abstract: Late pelagic stages of coral reef fishes captured with light-traps were individually released during daylight by SCUBA divers in open water, 20-35 m deep, in the Great Barrier Reef Lagoon at three sites > 1 km from the reefs of Lizard Island. Observations in situ on 111 individuals of 11 families, but primarily Apogonidae, Chaetodontidae and Pomacentridae, constitute the first data of their kind. Most fish showed no overt reaction to the divers. Some individuals of some taxa of three families settled quickly to the bottom. Acceptable observations on swimming were made on 66 larvae. Individuals selected a wide range of depths, but when grouped by family, mean depths chosen by individuals were: apogonids, 6.5 (± 1.5, 95% CI) m; pomacentrids, 7.7 (± 1.5) m; and chaetodontids, 9.3 (± 1.3) m. Rough estimates of speed of up to 30 cm s-1 varied among taxa. Swimming directions of 59 of the 66 larvae were non-random. Mean directions differed among sites and were offshore at all of them. Most larvae swam offshore regardless of the side of the island where they were released. The late pelagic stages of coral reef fishes are strong swimmers capable of active horizontal and vertical movement. They swim directionally, can apparently detect reefs >1 km away, and orientate relative to those reefs. A taxonomic component is evident in many of these behaviours.

The importance of macrophyte bed size for cladoceran composition and horizontal migration in a shallow lake

Abstract: Cladoceran composition and did horizontal migration were studied in 2,10 and 25 m diam- eter macrophyte exclosures established in the littoral zone of shallow Lake Stigsholm, Denmark. The exclosures were protected from waterfowl grazing, but open to fish. The macrophyte community com- prised Polamogeton pectinatus, Potamogeton pusillus and Callitriche hermaphroditica. Cladocerans were sampled randomly every third hour inside and outside the macrophyte exclosures during a 24 h period. In the 2 m exclosure, the pelagic species Ceriodaphnia spp. and Bosmina spp. dominated during the day, mean density being as high as 3430 indiv. h1. During the night, density decreased to 10-20% of the daytime density, thus indicating diel horizontal migration. In the 10 and 25 m exclosures, the daytime mean density of Ceriodaphnia spp. was 865 and 202 indiv. h1, respectively, and did not decrease at night. In contrast to the pelagic species, the density of macrophyte-as sociated species tended to be higher in the 10 and 25 m exclosure than in the 2 m exclosure. In the daytime, Eurycer- cus lamellatus density in the 2,10 and 25 m macrophyte exclosures was 7,28 and 16 indiv. H, respec- tively, while that of Simocephalus vetulus was 11, 171 and 92 indiv. I"1, respectively. There was no day-night difference in the density of macrophyte-associated species. We conclude that cladoceran community composition varies with macrophyte bed size, and that the edge zone between the bed and open water is an important daytime refuge for potentially migrating pelagic cladocerans.

Ecosystem‐specific patterns in the relationship between zooplankton and POM or microplankton del13C

Abstract: Measurements of plankton δ 13 C from 28 temperate lakes show that zooplankton are depleted in 13 C relative to smaller planktonic size fractions. A broad literature survey indicates that this is a general pattern in lakes but not in marine and estuarine plankton communities, where zooplankton are generally enriched in 13 C relative to particulate organic matter (POM) or micro-plankton. Marine plankton thus conform to the assumptions that pelagic food webs are essentially driven by phytoplankton and that POM or small planktonic size classes largely reflect algal carbon, which is transferred to zooplankton with a slight enrichment in 13 C. The plankton of lakes and possibly of estuaries, however, do not conform to this expectation. We show that there is a continuum in the δ 13 C differences between POM and zooplankton from open ocean to coastal areas and estuaries and that a similar pattern occurs from unproductive to highly productive lakes. These differences probably reflect both the degree in POM dilution by nonalgal sources of organic carbon and depth-related changes in the isotopic signature of phytoplankton in lakes and suggest systematic differences in organic carbon flow pathways among aquatic ecosystems.

Ocean warming and long-term change in pelagic bird abundance within the California current system

Abstract: As a result of repeated sampling of pelagic bird abundance over 3 X lo5 km2 of open ocean 4 times a year for 8 yr we report that seabird abundance within the California Current system has declined by 40\"0 over the penod 1987 to 1994 This decline has accompanied a concurrent, long-term increase in sea surface temperature The decline In overall b ~ r d abundance is largely but not entirely, a consequence of the 90% decline of sooty shearwaters Puff~nus grlseus, the numeilcally dominant species of the California Current Seabirds of the offshore waters we sampled showed a different pattern from s e a b ~ r d s of the shelf and slope waters Leach s storm-petrels Oceanodroma leucorl~oa, the commonest species offshore, significantly ~ n c l e a s e d uring 1987 to 1994, while sooty shearwaters and other inshore specles declined Thus the clearest pattein that emerges from our data is one of gradual but persistent changes in abundance that transpire at time scales longer than 1 yr Nevertheless we did find evidence of change at shorter time scales (weeks and months) that may relate to the El Nino episode of 1992 to 1993 P~onounced positive anomalies of abundance of brown pelicans Pelecanus ocaden ta~ i s and Heeiman S gulls Larus heerman! In fall 1991, and black Oceanod~onia melania and least 0 in~c roson~a storm-petrels in late summer 1992 likely reflect northward d~spe r sa l following reproductive failure in the Gulf of California

Neritic and pelagic carbonate sedimentation in the marine environment: ignorance is not bliss

Abstract: Synthesis of available data allows us to define general patterns of late Quaternary carbonate production and sedimentation in the global ocean. During high stands of sea level, the neritic and pelagic environments appear to sequester approximately similar amounts of carbonate, whereas during low stands of sea level the decreased neritic zone produces and accumulates approximately an order of magnitude less carbonate. Assuming that global accumulation of deep-sea carbonates remains more or less constant during glacially induced changes in sea level, the ocean becomes depleted with respect to calcium carbonate during high stands and recharges during low stands. Before we can achieve a better understanding of the global carbonate system, however, we need a better understanding of key environments and processes: (a) production and accumulation on continental shelves both as potential sinks (accumulation) and as sources (export to the deep sea); (b) a better measure of pelagic carbonate production; and (c) late Quaternary (late Pleistocene and Holocene) mass accumulation rates in the deep sea.

Avoidance by Daphnia magna of fish and macrophytes: Chemical cues and predator-mediated use of macrophyte habitat

Abstract: Recent biomanipulation studies suggest that macrophytes are an important refuge from fish predation for large pelagic zooplankton. We conducted two laboratory experiments that tested the behavioral responses of Daphnia magna to a macrophyte (Myriophyllum exalbescens L.) and a sunfish (Lepomis cyanellus Rafinesque) and whether responses were chemically (for fish) or structurally (for macrophytes) mediated. In the first experiment, we measured Daphnia response to four treatments in separate 38-liter tanks. In controls without macrophytes and fish, ∼ 15% ofthe daphnids were found in the central zone (∼ 50% of the tank area); the others were found around the tank perimeter (especially in the corners). With macrophytes present, 80% of the daphnids were found in the central zone (unvegetated in all treatments). When fish or fish odor alone were present 35% and ∼45%, respectively, of the Daphnia occupied the central zone. Thus, chemically mediated avoidance of Lepomis caused Daphnia to increase its occupation of macrophytes. In the second experiment, we tested whether the repellent effect of Myriophyllum resulted from structural characteristics of the macrophyte; the results suggest that both chemical and structural cues contributed to Daphnia avoidance of the macrophyte. Overall, our results are consistent with the suggestion that large pelagic zooplankton may use macrophytes as a refuge in shallow lakes where vertical migration is restricted.

Catch rates, selectivity and yields of reef fishing

Abstract: Fishing on tropical reefs is the preserve mainly of small-scale artisanal fishers, where increases in human power, rather than machine power, are used to generate large volumes of reef fish landings. The main forms of fishing gears deployed on reefs and in reef lagoons are handlines, traps, gill nets, seine nets and spears. Trawls can be deployed on the soft-bottom substrata adjacent to coral reefs but they catch mainly non-reef-associated species. The selectivity of nets, hooks, traps and spears is reviewed using, as far as possible, examples from reef fisheries. Although there is substantial variation, all gears catch a relatively wide range of species and selectivity is primarily size-based. However, species selectivity also results from the interaction of fish behaviour and gear characteristics. Reef fishes are also captured alive for restaurants and aquaria, and small pelagic fishes in coral reef lagoons are captured live for bait for pole-and-line tuna fishing. Observed yields from tropical reef fisheries range from around 0.2 tkm-2 year-1 in Papua New Guinea to over 40 tkm-2 year-1 from American Samoa. Yields in excess of 5 t km-2 year-1 are probably sustainable in the long term, although the upper limit for sustainable harvests from reef fisheries has yet to be accurately determined and will, in any case, vary among areas.

Transport mechanisms of larval and pelagic juvenile bluefish (Pomatomus saltatrix) from South Atlantic Bight spawning grounds to Middle Atlantic Bight nursery habitats

Abstract: In this study we examined the mechanisms by which Pomatomus saltatrix (Pisces: Pomatomidae) larvae and pelagic juveniles are transported from South Atlantic Bight spawning grounds to Middle Atlantic Bight estuarine nursery habitats. Data on larval and pelagic juvenile distributions, estuarine juvenile recruitment, hydrography, wind speed and direction and satellite-derived, sea surface temperature were used to examine potential larval transport mechanisms. On the basis of these analyses, a scenario for northward transport of P. saltatrix was developed. Gulf Stream-associated flow moves P. saltatrix larvae northeastward from their South Atlantic Bight spawning grounds. Larval transport from the Gulf Stream to the Middle Atlantic Bight shelf edge occurs in warm-core ring streamers, but some more developed individuals may swim across. Finally, P. saltatrix pelagic juveniles actively swim across the Middle Atlantic Bight shelf, a behavior initiated when the surface shelf-slope temperature front dissipates in late spring. This scenario predicts that the number of South Atlantic Bight-spawned P. saltatrix juveniles entering estuaries (i.e. recruitment) is determined in part by warm-core ring streamer activity. The timing of recruitment, however, is determined almost entirely by the timing of the dissipation of the surface shelf-slope temperature front.

Carbon budget of sea‐ice algae in spring: Evidence of a significant transfer to zooplankton grazers

Abstract: The fate of ice-bottom algae, before and after release from the first-year sea ice into the water column, was assessed during the period of ice-algal growth and decline in Resolute Passage (Canadian Arctic). During spring 1992 (from April to June), algae in the bottom ice layer and those suspended and sinking in the upper water column (top 15 m) were sampled approximately every 4 days. Ice-bottom chlorophyll a reached a maximum concentration of 160 mg m−2 in mid-May, after which it decreased to lower values. In the water column, chlorophyll a concentrations were low until the period of ice-algal decline (∼0.1 mg m−3), with most biomass in the 65% of total export) and that ice-algae were ingested by under-ice grazers after release from the ice. These results stress the importance of ice algae for pelagic consumers during the early stages of ice melt and show that the transfer of ice algae to higher trophic levels extends beyond the period of maximum algal production in the ice bottom.

Cephalopods as prey. IV. Fishes

Abstract: Predatory fishes that consume cephalopods have broad spectrum diets that include other groups, such as fishes and crustaceans. Extremely few fish predators feed on cephalopods to the virtual exclusion of other prey, although the tawny nurse shark Nebrius ferrugineus and the sicklefin weasel shark Hemigaleus microstoma may be exceptions, and some deep-living spiny dogfish may feed largely on cephalopods when they are available. Cephalopods are rarely taken in estuaries but they become more important prey on continental shelves and squids may be more dominant prey during their spawning aggregations. Cephalopods generally become more important components of the diets of larger predatory fishes, such as sharks, that inhabit the continental slope and rise. They may be important to pelagic fishes, such as swordfish and tunas in some parts of the oceans. Cephalopods are rarely taken by benthic Antarctic fishes but they are important prey of some pelagic fishes in the Arctic. Abundance and size of potential prey is critical, and the behaviour of both predator and potential prey is decisive, in prey choice. Factors such as prey switching with growth, between areas and at different times, have major influences on the diets of predators. There are extremely few studies that obtain rigorous enough data to allow quantitative analysis of the significance of cephalopods (or other prey) in the diets of predatory fishes and even the most comprehensive studies are not predictive because findings relate only to the time period of each study. Nevertheless, cephalopods are important components of most marine food webs and, furthermore, may play an indirect role in facilitating prey capture to secondary predators, and in providing rejecta to benthic scavengers.

Pelagic-benthic coupling: Profundal benthic community response to spring diatom deposition in mesotrophic Lake Erken

Abstract: Phytodetrital sedimentation, bacterial activity in sediments, and benthic macro- and meiofauna abundance were measured to calculate a carbon budget for pelagic-benthic coupling in Lake Erken. Bacterial activity in surficial sediment was limited by tempera

Energetic and life-history patterns of deep-sea benthic, benthopelagic and seamount-associated fish

Abstract: A review of energy use and the life histories of deep-water demersal fishes suggests that there are two primary groups or guilds; those that live dispersed over the sea floor and those that aggregate in association with topographic features like seamounts. Dispersed deep-sea fishes typically have a body plan designed for slow cruising or ‘sit and wait’ predation, and are characterized by very low energy stores and metabolic rates. Scaled for body size, the metabolism of these fishes was comparable to that of bathypelagic fishes. On the other hand, aggregatory deep-water species are characterized by robust morphology and strong locomotory ability to maintain themselves in environments characterized by strong, variable currents. Their flesh has high protein and lipid but low water content. The metabolic rate of orange roughy, an aggregating deep-water species, was substantially higher than that of dispersed deep-water fishes and was comparable to that of haddock, a shelf demersal species. However, although the estimated ration of orange roughy was higher than that of dispersed demersal deep-water species, its growth rate was comparable and its growth efficiency was far lower due to its high metabolic costs. Large deep-water dispersed fish species are characterized by late maturity and an extended reproductive period, but these characteristics are less pronounced than in deep seamount-associated species, which may live in excess of 100 years.

Cephalopods and mesoscale oceanography at the Antarctic Polar Front:satellite tracked predators locate pelagic trophic interactions

Abstract: Predator data and exploratory fishing in the Scotia Sea have revealed the presence of cephalopod stocks in the Antarctic Polar Frontal Zone (PFZ) This is a vast, remote region where large epipelagic cephalopods aggregate into highly mobile schools making them difficult to locate and sample We used satellite tagged predators and shipboard acoustics for coarse and fine scale location of cephalopod concentrations, and sampled them with commercial and scientific nets to determine the relationship between cephalopod distribution and mesoscale oceanographic features at the PFZ Satellite tags were attached to 9 grey-headed albatrosses Diomedea chrysostoma, breeding at Bird Island, South Georgia, to monitor foraging at sea in January-March 1994 A foraging area at the PFZ, north of South Georgia, was located, an acoustic survey undertaken and a fixed station established where acoustic targets were found A net survey was carried out with a commercial pelagic trawl, a rectangular midwater trawl 25 m2 (RMT25), a horizontal multiple plankton sampler and a neuston net Acoustic layers were targeted and the RMT25 sampled 200 m layers to 1000 m in daylight and darkness Cephalopods were simultaneously recovered from food samples fed to D chrysostoma chicks at Bird Island Two CTD transects, approximately normal to the major current flow, were undertaken across the PFZ and remote-sensed sea-surface temperature images from NOAA polar orbiting satellites were obtained aboard ship The pelagic trawl sampled a cephalopod community that closely resembled that exploited by D chrysostoma The largest and most conspicuous species was the ommastrephid squid Martialia hyadesi which is the most important cephalopod prey species Net-sampled M hyadesi had been feeding on crustaceans and mesopelagic fish The cephalopod community was sampled in a feature, interpreted as a warm core ring, in an area characterised by mesoscale features associated with the bathymetry of the northern end of the Northeast Georgia Rise and near a gap in the Falkland Ridge The association of these mesoscale features with the bathymetry suggests that they may be predictable foraging locations for the cephalopods and their predators

Reconstructing the past density of planktivorous fish and trophic structure from sedimentary zooplankton fossils: a surface sediment calibration data set from shallow lakes

Abstract: 1. As quantitative information on historical changes in fish community structure is difficult to obtain directly from fish remains in lake sediments, transfer function for planktivorous fish abundance has been developed based on zooplankton remains in surface sediment (upper 1 cm). The transfer function was derived using weighted average regression and calibration against contemporary data on planktivorous fish catch per unit effort (PF-CPUE) in multiple mesh size gill nets. Zooplankton remains were chosen because zooplankton community structure in lakes is highly sensitive to changes in fish predation pressure. The calibration data set consisted of thirty lakes differing in PF-CPUE (range 18–369 fish net–1), epilimnion total phosphorus (range 0.025–1.28 mg P l–1) and submerged macrophyte coverage (0–57%). 2. Correlation of log-transformed PF-CPUE, total phosphorus and submerged macrophyte coverage v the percentage abundance in the sediment of the dominant cladocerans and rotifers revealed that the typical pelagic species correlated most highly to PF-CPUE, while the littoral species correlated most highly to submerged macrophyte coverage. Consequently, only pelagic species were taken into consideration when establishing the fish transfer function. 3. Canonical correspondence analysis (CCA) revealed that the pelagic zooplankton assemblage was highly significantly related to PF-CPUE (axis 1), whereas the influence of total phosphorus and submerged macrophyte coverage was insignificant. Predicted PF-CPUE based on weighted average regression without (WA) and with (WA(tol)) downweighting of zooplankton species tolerance correlated significantly with the observed values (r2 = 0.64 and 0.60 and RMSE = 0.54 and 0.56, respectively). A marginally better relationship (r2 = 0.67) was obtained using WA maximum likelihood estimated optima and tolerance. 4. It is now possible, quantitatively, to reconstruct the historical development in planktivorous fish abundance based on zooplankton fossil records. As good relationships exist between contemporary PF-CPUE data and indicators such as the zooplankton/phytoplankton biomass ratio, Secchi depth and the maximum depth distribution of submerged macrophytes, it is now also possible to derive information on past changes in lake water quality and trophic structure. It will probably prove possible further to improve the transfer function by including other invertebrate remains, e.g. chironomids, Chaoborus, snails, etc., and its scope could be widened by including deeper lakes, more oligotrophic lakes, more acidic lakes and lakes with extensive submerged macrophyte coverage (in the latter case to enable use of the information in the fossil record on plant-associated cladocerans).

Dietary changes of seabirds indicate shifts in pelagic food webs

Abstract: Long-term monitoring of prey deliveries to chicks by gannets and guillemots have provided useful indices of annual variability in the timing of the inshore movements, relative abundance, sizes and reproductive conditions of pelagic fishes and cephalopods in the Northwest Atlantic. From 1977 through 1994, prey harvests by gannets tended to shift from warm-water, long-distance migrants (mackerel, squid, saury) to cold-water residents (capel in). Annual variation in prey harvests by gannets and guillemots indicated 3- to 4-week delays in the inshore migration, reproductive maturation and spawning by capelin during cold sea surface events in the 1990s. Cold surface water anomalies appear to have influenced pelagic food webs by inhibiting the movements of migrant, warm-water pelagic fishes and cephalopods into inshore regions in the Northwest Atlantic. These oceanographic influences may have amplified the effects of the overfishing of shortfinned squid in the region in the late 1970s/early 1980s. We c...

Pelagic production and biomass in the Adriatic Sea

Abstract: SUMMARY: On the basis of the available data on plankton production and biomass in the Adriatic Sea, three different areas may be delineated: a - Open waters of the central and southern Adriatic, characterized by an “oceanic” community. Primary production rates are low. Microphytoplankton and especially diatoms are abundant; microzooplankton is dominated by tintinnids; herbivorous copepods are dominant throughout the year and carnivorous species are also important. The plankton communities show high diversity and great stability. A “top down” control appeared to regulate the energy fluxes. b Open waters of the northern basin, characterized by neritic associations with moderate primary production rates and biomass. The species originating from the southern Adriatic and the Mediterranean are present throughout the year. A marked west-to-east gradient of production and biomass is present during the stratified period. The models of phytoplankton control appear to shift from “bottom up” to “top down” control, following the western-eastern gradient and changing seasonally. c - A coastal zone characterized by a neritic community of low diversity and high primary production and biomass. Nanoplankton primary production generally prevails. This production appear to be used mainly by ciliates other than tintinnids during the stratified period. Nanoplankton also represents an important food source for the abundant filter feeders. During the persistence of the frontal system, separating coastal from off shore waters, terrigenous inputs stimulate high primary production which is confined to inshore areas. Zooplankton grazing is not sufficient to control the produced biomass and transfer to offshore is restricted, resulting in eutrophication

The plant life of the pelagic

Abstract: (1996). The plant life of the pelagic. SIL Proceedings, 1922-2010: Vol. 26, No. 1, pp. 97-113.

Algal recruitment from lake sediments in relation to grazing, sinking, and dominance patterns in the phytoplankton community

Abstract: I assessed the importance of algal recruitment from the sediment surface to the water relative to other population variables such as grazing, growth, and sinking. In four la’w-productivity lake basins, which were stratified throughout the study, 32% of the identified algal taxonomic groups exhibited recruitment, whereas the other 68% spent their entire lives in the pelagic habitat. For most species, recruitment from sediment to water occurred primarily at shallow depths (~8 m) and often increased total phytoplankton abundance by 1 O-50% per day. Grazing was the main loss process, reducing algal abundance by usually between 10 and 50% per day; sinking rates were usually one order of magnitude lower. Recruitment was highest in shallow water (< 10 m deep); when maximum recruitment occurred below the thermocline, no relation was found between recruitment and abundance in the euphotic zone, suggesting that the thermocline functioned as a physical barrier for algal vertical migration, In three of the lakebasins, recruiting algae dominated more often than expected by chance (P < 0.05), suggesting a competitive advantage with the adaptation to perform recruitment. Successful recruitment may have a considerable impact on dominance patterns in the phytoplankton community.

Vertical structure of small metazoan plankton, especially noncalanoid copepods. I. Deep Arabian Sea

Abstract: The abundance and vertical distribution of micro-metazoans sampled with fine nets of 0.05 mm mesh size were studied at three stations in the Arabian Sea during the intermonsoon period (April/ May 1987) and down to 1850 m depth. In the epipelagic zone (0-100 m). values of biomass and meta- zoan abundance tended to be higher than those reported for other tropical oceanic areas. In the meso- pelagic zone, which is characterized by an extreme oxygen deficiency between 100 and 1000 m depth, the abundance of metazoan taxa and species numbers of non-calanoid copepods were largely reduced. However, intermediate abundance maxima occurred within this zone, which were dominated by specif- ic metazoan taxa (copepods. appendicularians) and species of non-calanoids (Oncaea sp. C). In the bathypelagic zone below 1050 m, the species diversity of the dominant copepod family Oncaeidae increased substantially. Two-thirds of a total of 69 oncaeid species recovered were confined to this layer. As most of them were small in size and occurred in low abundance only, the increase in total oncaeid density and/or plankton biomass was less conspicuous. Dominant Oncaea species in the bathy- pelagic zone were O.longipes and O.brodskii. The results are compared with published data from the Arabian Sea and other tropical oceanic areas with and without an extreme mesopelagic oxygen mini- mum zone Possible causes of the intermediate abundance maxima within the oxygen deficiency zone are discussed.

Relationships between sea surface temperature, salinity, and pelagic fish distribution off northern Chile

Abstract: Geographic Information System techniques were used in the analysis of the distribution of three pelagic resources: anchovy, sardine, and jack mackerel. The data were derived from hydroacoustic estimations obtained during four seasonal cruises in 1993‐1994 in the region oV northern Chile. The spatial distribution of these resources was analysed relative to the surface temperature and salinity distributions, the latter two variables being measured concurrently during these cruises. Results indicated the occurrence of a stratified distribution of the three species together with a high variability in the distribution pattern between cruises. The distribution of the three species was associated with the occurrence and intensity of thermal and haline fronts. ? 1996 International Council for the Exploration of the Sea

Long-term trends in ten non-target North Sea fish species

Abstract: Catch data on 10 non-target fish species from the International Bottom Trawl Survey during the years 1970-1993 are analysed for changes in distribution and abundance by size class. Trends in catch rates of spurdog, starry ray, bib, poor cod, four-bearded rockling grey gurnard, bullrout, long rough dab, dab, and lemon sole have been compared using correlation and cluster analysis with indices describing different aspects of the North Sea ecosystem, including biomass of pelagic, demersal and industrial species, temperature, eutrophication, and beam trawl effort. Most species appear to have increased over the period. However, the statistical analysis does not provide a plausible explanation of the factors responsible for the observed changes.

Estuarine Ichthyoplankton Ecology in Relation to Hydrology and Zooplankton Dynamics in a Salt-wedge Estuary

Abstract: The ichthyoplankton assemblage of the Hopkins River estuary, Victoria, was dominated by estuarine taxa that included demersal (goby, gudgeon) and pelagic (black bream, estuary perch, anchovy) species. The two seasonal peaks in fish larvae abundance were related to (1) the spring/summer zooplankton increase that occurred after flooding of the estuary and was comprised predominantly of copepod nauplii, thereby providing a good prey field for very young fish larvae, and (2) the autumn zooplankton maxima, which would provide a wide selection of copepod stages and meroplankton and promote dietary partitioning and flexibility among older larval stages. The two peaks in fish larvae abundance occurred well before and well after the attainment of maximum water temperature in the estuary. Goby, gudgeon, estuary perch and anchovy larvae were in the plankton over an extended period coinciding with the more stable conditions of salt-wedge presence and maximum zooplankton densities. The spawning of black bream and anchovies in the estuary was clearly related to physical conditions, such as salinity and water temperature, and habitat-although spawning of both species also occurred at times of high concentrations of potential prey organisms for their larvae. Adults of all the above fish species appear to have evolved spawning strategies that are adapted to the average hydrological and biological conditions in the estuary that would lead to the enhanced survival of their larvae. With the exception of the black bream, it appears that a ubiquitous and prolonged rather than a synchronous and confined spawning strategy is more widely used by estuarine-spawning fishes in the Hopkins River estuary. This type of spawning strategy, combined with the generally high density of food available to fish larvae in estuaries, suggests that the match-mismatch hypothesis of Cushing may be less relevant in the estuarine than in the marine environment.

Spatial organization of pelagic fish: echogram structure, spatio-temporal condition, and biomass in Senegalese waters

Abstract: Understanding the spatial distribution of fish schools is expected to enable future improvement in the reliability of acoustic abundance estimates as well as catch data. The analysis of echograms will provide detailed morphological descriptions of fish aggregations together with characteristics of their habitat. The data set studied here is a series of comparable echograms taken during monitoring surveys and showing important fluctuations in biomass. Echoes are coded into diVerent consistent morphological types. First, we study how the occurrence of these types varies with diel cycle, sea-bottom depth, and fish density by computing a chi-square table. Second, characteristics of inter-school spatial structure are analysed by computing indicator variograms. Third, location of high-density values within the areas where schools are present is studied by computing indicator cross-variograms. A descriptive model of habitat occupation is proposed where some parameters are related to biomass level and others are not. ? 1996 International Council for the Exploration of the Sea