Fish larval supply to and within a lagoonal estuary: multiple sources for Barnegat Bay,New Jersey

We evaluated spatial variation in fish larval supply to a temperate, lagoon type estuary (Barnegat Bay, New Jersey) by determining species composition, size, and stage into inlets (n = 2), thoroughfares between adjacent estuaries (n = 3), and within the estuary (n = 4) in seasonal, synoptic sampling on night time flood tides during 2010–2014. Larval supply, as sampled with identical plankton nets (1 m diameter, 1 mm mesh) was dominated by post-flexion stage individuals (most 5–10 but reaching 70+ mm) from species spawned in the Atlantic Ocean from a variety of sources (e.g., Sargasso Sea, outer and inner continental shelf) and in the bay. While abundance for individual species varied among locations and years, in general, the larval composition was similar across inlets, thoroughfares, and within the bay within the same seasons. Homogenization across locations was likely the result of the tidal exchanges between the ocean, the estuary, and the adjacent locations. These exchanges provide numerous, redundant sources of larvae to this estuarine nursery. The similarity in larval supply among inlets, thoroughfares, and within the estuary indicates that the longer term study location behind Little Egg Inlet is representative for this, and probably other, estuaries along the New Jersey shore.     

Anguilla rostrata glass eel ingress into two, U.S. east coast estuaries: patterns, processes and implications for adult abundance

A time series of American eel Anguilla rostrata glass eel abundance, timing and size from Little Egg Inlet, New Jersey (16 years) and Beaufort Inlet, North Carolina (18 years) was used to provide a better understanding of ingress patterns at two, U.S. east coast estuaries. There was no evidence of synchronous declines in abundance between the two locations; however, at the Little Egg Inlet site, glass eels arrived later in the season and at significantly smaller sizes over the duration of the series. One significant linkage between sites was revealed: abundance was positively correlated with winter precipitation. Precipitation differed between sites annually and was correlated with El Niño at Beaufort Inlet and, to a lesser extent, the North Atlantic Oscillation at Little Egg Inlet. It is hypothesized that glass eels may use freshwater signals to enhance recruitment to local estuaries, thus influencing year-class strength, yet the relationship between year-class strength and adult abundance remains unresolved.     

Larval life history strategies of sub-tropical southern African estuarine brachyuran crabs and implications for tidal inlet management

Soon after hatching the larvae of many estuarine crabs migrate from estuaries to adjacent coastal waters soon, where larval development is completed before the post larval stages recruit to estuaries to settle. This study investigated the larval flux of several brachyuran crabs resident within a subtropical estuary on the east coast of South Africa. Plankton sampleswere collected over two intensive sampling periods during the spring of1997 (19 days) and the summer of 1998 (26 days). Larvae were releasedmaximally when high tide within the estuary was crepuscular, whilerecruitment occurred during nocturnal flood tides, with peak abundanceson the maximum amplitude nocturnal flood tides. The data are discussedin relation to freshwater inflow problems facing South African estuaries andthe active management of the tidal inlets of affected systems.     

Evaluation of the larval distribution and migration of the Japanese eel in the western North Pacific

The distribution of all larval stages of the Japanese eel, Anguilla japonica, were examined using historical catch records and original data in the western North Pacific (WNP) to evaluate existing information about the larval distribution and migration of this species. A total of 148 preleptocephali, 2547 leptocephali, 6 metamorphosing larvae, and 21 glass eels were collected during 37 cruises over a 52-year period (1956?C2007). Sampling effort was spatio-temporally biased in latitude/longitude among seasons with sampling effort being concentrated near the western margin of the subtropical gyre near Taiwan in the winter season and extensive effort occurring near the spawning area to the east near the seamount chain of the West Mariana Ridge in summer during the spawning season. The distribution of preleptocephali (4.2?C8.7 mm) was limited to a narrow area around 14°N, 142°E just west of the southern part of the seamount chain, while leptocephali (7.7?C62.0 mm) were widely distributed at increasing size westward in the North Equatorial Current (NEC) to the region east of Taiwan. Metamorphosing larvae (52.7?C61.2 mm) were collected only in the area 21?C26°N, 121?C129°E to the east of Taiwan, while glass eels (51.3?C61.2 mm) occurred only within or west of the Kuroshio. These distributions suggest that leptocephali begin to metamorphose within or just east of the Kuroshio, then after completion of metamorphosis the glass eels detrain from the current and migrate inshore. The relationship between catch date and body size of leptocephali suggested that the spawning season is from April to August, but further sampling is needed to eliminate possible effects of sampling bias. This analysis is consistent with the existing hypothesis that Japanese eel larvae born near the West Mariana Ridge are transported westward in the NEC and then transfer to the Kuroshio to recruit to East Asia, although more sampling effort is needed for later stage larvae in the NEC bifurcation region to help understand the larval migration in relation to the possible impacts of ocean?Catmosphere changes.     

Dietary compositions of the sparid <Emphasis Type="Italic">Acanthopagrus</Emphasis><Emphasis Type="Italic"> butcheri</Emphasis> in three normally closed and variably hypersaline estuaries differ markedly

We have compared the species composition and diversity of the diets of black bream, Acanthopagrus butcheri, in three normally closed estuaries on the central south coast of Western Australia, which vary markedly in the extents to which they become hypersaline during dry periods. Although black bream was caught seasonally in Stokes Inlet during this 3-year study, it was obtained from the Hamersley and Culham inlets only during the earlier seasons because salinities in those two estuaries subsequently rose to levels that caused massive mortalities of this sparid. Although a wide range of taxa, including macrophytes, polychaetes, molluscs, crustaceans, insects and teleosts, were ingested by A. butcheri in each estuary, the frequencies of ingestion and volumetric dietary contributions of these taxa varied greatly among the fish in these three estuaries. Thus, for example, in comparison with other estuaries, relatively greater contributions were made to the diet by polychaetes and crustaceans in Stokes Inlet, by macrophytes in Hamersley Inlet, and by insects (mainly chironomid larvae) in Culham Inlet. The relatively greater contribution of teleosts to the diets of black bream in the Hamersley and Culham inlets than in Stokes Inlet, and also differences in the main teleost species ingested in the first two estuaries, are consistent with differences in the densities of fish overall and of the main fish species in those estuaries. The diversity of the diet was far greater in Stokes Inlet than in the other two far more variably saline estuaries, presumably reflecting a greater diversity of food. The dietary compositions of black bream in upstream pools in the tributary of Culham Inlet, which offer refuge when salinities increase markedly in the main body of the estuary, differ from those in those downstream regions, further emphasising the opportunistic nature of the feeding behaviour of black bream. The dietary compositions of black bream underwent size-related changes, but the taxa contributing most to those changes varied greatly among estuaries. Size-related changes would be particularly beneficial in reducing intraspecific competition for food in the two estuaries that vary greatly in salinity and would thus be likely to contain a less diverse range of prey.     

Larval release rhythms and tidal exchange in the estuarine mudprawn, Upogebia africana

The mudprawn, Upogebia africana is common in intertidal regions of many South African estuaries. The life cycle is complex, incorporating a marine phase of development during the larval stages. Breeding peaks are in summer and first-stage larvae are released into the plankton at night. Maximum release activity and export to the marine environment follow a semi-lunar cycle synchronized to the time when high water in the estuary is crepuscular. This occurs after peak spring tidal amplitude. Estuarine reinvasion by postlarvae is also nocturnal, and maximum return occurs after neap's when low water at sea occurs around sunset. Rhythmic cycles of larval export and postlarval estuarine reinvasion are therefore asynchronous during the lunar cycle and are best explained by the timing of the change in light intensity relative to high and low water respectively. If maximum activity rhythms of Stage 1 and postlarvae are independent of tidal amplitude, then timing of maximum release and reinvasion during the lunar cycle would alter as the time of sunset shifts between solstices. Much of southern Africa experiences a semi-arid type climate and most estuaries close off from the sea for varying periods owing to sandbar development across tidal inlets. Larvae do not metamorphose if trapped in estuaries and recruitment ceases. Thus, mudprawn populations are directly affected by tidal inlet dynamics. In extreme cases populations become locally extinct if inlets remain closed for extended periods.     

Spatial and temporal analyses of water quality and phytoplankton biomass in an urbanized versus a relatively pristine salt marsh estuary

The robust growth of coastal communities in the southeastern United States is putting unique pressures on estuarine resources. Urbanization of estuarine systems may alter ecosystem function and thus affect the spatial scale and magnitude of nutrient concentrations and primary production temporally and spatially. We examined the spatial and temporal patterns of nutrient and chlorophyll a (Chl a) concentrations in two shallow well-mixed estuaries, (1) a developed estuary, Murrells Inlet (MI), South Carolina, and (2) a relatively pristine estuary, North Inlet (NI), South Carolina. The summer chlorophyll a maximum in MI was characteristically higher than in NI, which may be indicative of eutrophication. Correlations between salinity and inorganic nutrients (N and P) suggest that nutrient import from upland sources may be more pronounced in MI during stochastic precipitation events. Although inorganic nutrient concentrations between the estuaries were similar overall, during a wet period, inorganic N concentration in MI was increased to a greater extent than in NI, while only minimal increases in inorganic P were observed in both estuaries. Chlorophyll a concentrations decreased from the dry to wet period. Geographic Information System (GIS) plots of intensive spatial sampling in MI indicated spatial gradients of nutrient concentrations within this estuary that appeared to be consistent over time. These observations were investigated in more detail using regression analyses to examine the influences of coastal dilution and nutrient sources on relationships between water quality constituents. Results indicate the importance of stochastic rain events in affecting the linkages of estuarine processes to upland runoff in the urbanized estuary, MI.     

Northern pipefish,Syngnathus fuscus,occurrences over the Mid-Atlantic Bight continental shelf: evidence of seasonal migration

Synopsis Data from several areas in the northern Mid-Atlantic Bight indicate northern pipefish, Syngnathus fuscus, undergo seasonal, inshore-offshore migrations. Resident in estuaries during spring through fall, they move into nearshore continental shelf waters off Cape Cod in late September–October and off Long Island and New Jersey in November. Return to estuaries occurs in March–April. Most (> 90%) continental shelf otter trawl collections in fall (September–November) were at water temperatures of 10–15°C. Most (> 80%) spring (March–May) collections occurred at water temperatures of 3–6°C. The majority of offshore collections were within 20 km of the coast and over 90% were in depths between 10 and 20 m. Length frequency data reveal both young-of-year and older fish migrate, possibly to avoid colder estuarine water temperatures in winter.     

Tracing estuarine organic matter sources into the southern North Sea using C and N isotopic signatures

Sources and distribution of particulate organic matter in surface waters of the Humber and Thames estuaries and in the East Anglian plume in the southern North Sea were investigated in winter 2006/2007. Carbon (C) and nitrogen (N) stable isotopes provided evidence for the presence of three particulate organic matter sources; riverine plankton (δ¹³C ?30 ‰ and δ¹⁵N 7.9 ‰) identified in the Thames estuary only, marine plankton (average δ¹³C ?21.4 ‰ and δ¹⁵N 4.5 ‰) and a third source with an enriched ¹³C signature (>?16.7 ‰) and elevated C:N ratio (>12.7). Particulate organic matter with enriched ¹³C values were observed throughout the Humber estuary and at the marine end-member of the Thames estuary. While bacterial cycling of organic carbon undoubtedly takes place within these estuaries, these processes on their own are unlikely to account for the isotopic signatures seen. The ¹³C enriched organic matter source is suggested to be due to particulate organic matter input from marsh plants and seagrasses such as Spartina spp. and Zostera on the adjacent salt marshes and mudflats and/or macroalgae along the banks of the estuaries. This ¹³C enriched signal was also identified approximately 50 km offshore within the southern North Sea, in the East Anglian plume, which transports UK riverine water off-shore in a discrete plume. This plume therefore provides a mechanism to transport this estuarine derived organic matter pool offshore out of the estuaries. These results indicate that estuarine derived organic matter from marsh plants, seagrasses and/or macroalgae contributes to the southern North Sea organic matter pool and is therefore likely to contribute to winter-time shelf sea carbon and nitrogen cycles.     

Spatial and Temporal Variability in the Abundance and Size Structure of Larvae of the White Sea Herring (<Emphasis Type="Italic">Clupea pallasii marisalbi</Emphasis>) in Onega and Kandalaksha Bays of the White Sea

According to the data of ichthyological surveys conducted in Onega and Kandalaksha bays of the White Sea in June 2015, the abundance and pattern of spatial distribution of larvae of the White Sea herring Clupea pallasii marisalbi are comparable with those of 2012. Aggregations of herring larvae detected at a distance of 12?14 km from the coast in the apex part of Kandalaksha Bay are probably the result of their mass drift caused by fresh floodwater discharge. In coastal waters of the bays adjacent to the littoral part, the abundance of herring larvae above the depths less than 5 m varies considerably due to their drift under effect of alongshore and/or tidal currents. The White Sea herring larvae reach high abundance only in the inlets (Chupa, Knyazhaya, Belaya, and Maikova inlets) with the river runoff; their length increases with the distance from spawning grounds. In different years, the main bulk of herring larvae in Knyazhaya Inlet is concentrated at depths about 12–15 m at 6?8°C, or moved to the upper 5-m quasi-homogeneous layer when the water temperature at the depth was 0?1°C.     

Life history characteristics of the estuary eeltail catfish Cnidoglanis macrocephalus (Plotosidae) in eastern Australia

The biology and fisheries of cobbler Cnidoglanis macrocephalus have been examined on the Australian west coast, but not the east coast. This study identified that east coast estuary cobbler was more prevalent over vegetated habitat than bare substrata and that spawning occurred during spring and early summer. Observed longevity was 9⁺ and 20⁺ years for males and females, but there were no discernible differences between sexes or estuaries in mean total length (TL) at age. The von Bertalanffy growth function of combined sexes was L_inf?=?61.2 cm TL, k?=?0.677, t₀?=?0.007. The data demonstrate similarities and differences in life history characteristics of cobbler between coasts.

     

Molecular Phylogeography and Evolutionary History of the Estuarine Copepod, Acartia Tonsa, on the Northwest Atlantic Coast

Coastal estuaries are useful model systems to study the ecological and evolutionary responses of organisms to highly variable, discontinuous habitats. For this study, the molecular population genetic diversity of the planktonic calanoid copepod Acartia tonsa (Dana, 1849) was described based on DNA sequence variation for a 183 base-pair region of the mitochondrial 16S rRNA gene. Samples of A. tonsa were collected from four estuaries on the Atlantic coast of the USA during 1993 and 1994, one estuary on the Gulf of Mexico coast in 1994, and one site on the Pacific coast of the USA in 1994. Dispersal of A. tonsa was shown to be restricted, with significant population genetic structuring between different estuaries. For all but the closely-adjacent MA and RI samples, frequencies of haplotypes and/or length polymorphisms within one haplotype (caused by insertion/deletion mutations) revealed highly significant genetic differentiation and geographic isolation. Mt16S haplotypes of A. tonsa from Atlantic and Gulf of Mexico estuaries were assorted among four deeply-diverged clades. Haplotypes within each clade differed by <2%, while differences among clades of 10% to 14% approached those between described Acartia species (e.g., 19% to 28% among A. clausi, A. hudsonica, and A. longiremis). Atlantic and Pacific coast samples identified as A. tonsa had no haplotypes in common and genetic differences between haplotypes ranged from 18% to 29%; phylogenetic analysis supported the separation of Pacific coast A. tonsa as a distinct species. We hypothesize that the observed patterns of molecular genetic diversity and structure of A. tonsa resulted from responses to historical climatic variation, including episodic range compression and displacement, and alteration of NW Atlantic coastal and estuarine environments.     

Symposium review: long-term shifts in faunal assemblages in eastern North American estuaries: a review of a workshop held at the biennial meeting of the Coastal and Estuarine Research Federation (CERF), November 2007, Providence, Rhode Island

A workshop held at the fall 2007 meeting of the Coastal and Estuarine Research Federation addressed the topic of long-term changes in the faunal assemblages of estuaries along the east coast of North America. The workshop, organized by Rodney Rountree and Francis Juanes, brought together researchers who have examined data sets spanning greater than 20 years and who related their findings to various drivers. Changes in faunal assemblages were well-documented by most researchers and were associated with fishing pressure, eutrophication, warmer seawater temperatures, alterations of hydrology, natural environmental factors, and the interactions among these drivers.     

Adaptations to the Arctic: low-temperature development and cold tolerance in the free-living stages of a parasitic nematode from Svalbard

For nematodes with a direct life cycle, transmission is highly dependent on temperature-related development and survival of the free-living stages. Therefore, in the Arctic, where the winter lasts from October to May, nematode transmission is expected to be focused in the short summer season, yet there is strong evidence that as well as focussing egg output during winter months, the nematode parasite, Marshallagia marshalli, infects Svalbard reindeer during the Arctic winter when temperatures are persistently below freezing. To investigate the potential for development and survival of eggs and infective third-stage larvae in winter and therefore the possibility of for winter transmission, we ran a series of low-temperature laboratory experiments. These provide five key insights into the transmission and survival of the free-living stages of M. marshalli: (1) eggs hatched at temperatures as low as 2 °C, but not below 0 °C, (2) eggs were viable and developed after being exposed to sub-zero temperatures for up to 28 months, (3) infective-stage larvae survived for up to 80 days at 5 °C, (4) infective-stage larvae could survive rapid exposure to temperatures below ?30 °C, and (5) desiccation resistance may be important for long-term larval survival at low temperatures. Together, these results indicate that eggs deposited during the winter are highly tolerant of prevailing environmental conditions and have the potential for rapid development with the onset of spring. It is therefore likely that the parasite remains in the egg stage in the faeces during the winter of deposition, hatch and develop into the infective larval stage in the summer, remaining viable on the tundra until the reindeer host returns to the winter feeding grounds the following winter.     

Reproductive biology of Rioraja agassizi from the coastal southwestern Atlantic ecosystem between northern Uruguay (34°S) and northern Argentina (42°S)

A total of 552 individuals of Rioraja agassizi (257 females and 295 males) were collected by bottom-trawl during research cruises. Sexual dimorphism was observed with females are heavier than males for a given total length (TL). Using logistic regression, it was determined that TL at 50% maturity of males was 475 mm TL and of females 520 mm TL. This estimation agrees with the morphological parameters measured. Although not statistically significant, a peak in reproductive activity was observed in males during late spring and summer. The monthly variation in the gonadosomatic index and oviducal gland width, together with the largest diameter of ovarian follicles, suggests that R. agassizi females have a partially defined annual reproductive cycle with two peaks, one from November (spring) to February (summer) and another in July (winter).     

Use of Metabolic Inhibitors to Characterize Ecological Interactions in an Estuarine Microbial Food Web

Understanding microbial food web dynamics is complicated by the multitude of competitive or interdependent trophic interactions involved in material and energy flow. Metabolic inhibitors can be used to gain information on the relative importance of trophic pathways by uncoupling selected microbial components and examining the net effect on ecosystem structure and function. A eukaryotic growth inhibitor (cycloheximide), a prokaryotic growth inhibitor (antibiotic mixture), and an inhibitor of photosynthesis (DCMU) were used to examine the trophodynamics of microbial communities from the tidal creek in North Inlet, a salt marsh estuary near Georgetown, South Carolina. Natural microbial communities were collected in the spring, summer, and fall after colonization onto polyurethane foam substrates deployed in the tidal creek. Bacterial abundance and productivity, heterotrophic ciliate and flagellate abundance, and phototrophic productivity, biomass, and biovolume were measured at five time points after inhibitor additions. The trophic responses of the estuarine microbial food web to metabolic inhibitors varied with season. In the summer, a close interdependency among phototrophs, bacteria, and protozoa was indicated, and the important influence of microzooplanktonic nutrient recycling was evident (i.e., a positive feedback loop). In the fall, phototroph and bacteria interactions were competitive rather than interdependent, and grazer nutrient regeneration did not appear to be an important regulatory factor for bacterial or phototrophic activities. The results indicate a seasonal shift in microbial food web structure and function in North Inlet, from a summer community characterized by microbial loop dynamics to a more linear trophic system in the fall. This study stresses the important role of microbial loops in driving primary and secondary production in estuaries such as North Inlet that are tidally dominated by fluctuations in nutrient supply and a summer phytoplankton bloom.     

Genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S.A.

Analyses of the genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S. coast using 13 microsatellites suggest significant population differentiation between fish in North Carolina (NC) compared with South Carolina (SC) and Georgia (GA), with New River, NC, serving as an area of integration between northern and southern C. nebulosus. Although there is a significant break in gene flow between these areas, the overall pattern throughout the sampling range represents a gradient in genetic diversification with the degree of geographic separation. Latitudinal distance and estuarine density appear to be main drivers in the genetic differentiation of C. nebulosus along the south‐eastern U.S. coast. The isolation‐by‐distance gene‐flow pattern creates fine‐scale differences in the genetic composition of proximal estuaries and dictates that stocking must be confined to within 100 km of the location of broodstock collection in order to maintain the natural gradient of genetic variation along the south‐eastern U.S. coast.     

Reconstructing the range expansion and subsequent invasion of introduced European green crab along the west coast of the United States

The European green crab, Carcinus maenas, was first documented in San Francisco Bay in 1989, and has since spread north along the west coast of North America. The spread of this invasion has not been a smooth expansion, which has raised questions about the underlying causes of variation in recruitment. We modeled larval development and transport along the West Coast by employing an individual-based model that incorporated oceanographic model output of water temperature and ocean currents at fine spatial and temporal scales. The distance that larvae were advected depended primarily on the timing of larval release. However, the effect of seasonal ocean currents varied across latitude and years. Our results imply that the furthest northern transport from California occurs when larvae are released from Humboldt Bay during the fall of an El Niño year, making this a particularly risky time for invasion to Oregon and Washington estuaries. To precisely predict future spread and potential impacts of green crab, we recommend further empirical research to determine the precise timing of larval release and seasonal abundance of green crab larvae from North American west coast populations.     

Interannual variability of carbon dioxide drawdown by subantarctic surface water near New Zealand

The ocean-atmosphere flux of carbon dioxide in subantarctic surface water (SASW) east of New Zealand has been determined using data from bi-monthly cruises on a time series transect for 8 years. The 60 km long transect extends from the coast (45.770°S 170.720°E) to a station at 45.833°S 171.500°E. Sea surface temperature, salinity, nutrient concentrations and pCO₂ have been measured at a frequency of about once every 2 months from January 1998 until December 2005. Measured pCO₂ exhibits a seasonal cycle with a maximum in late winter/spring, and a minimum in late summer/autumn, a mean 356 ??atm, and an amplitude of 9 ??atm. The magnitude of ??pCO₂ (the air-sea concentration gradient) has increased over the 8 years, primarily due to the increase in atmospheric CO₂ concentration. The air-sea flux of CO₂ was determined from wind speed data and ??pCO₂. The uptake of atmospheric CO₂ by SASW in the study area changed from + 1 and +82 mmol m^?2 in 1998 and 1999 respectively (ocean as source) to ?870 and ?510 mmol m^?2 in 2004 and 2005 (ocean as sink). These values are substantially less in magnitude than the value obtained from the Takahashi et al. (Deep-Sea Res II, 2009) flux climatology.     

Distribution patterns of mesozooplankton biomass in the North Sea

During spring 1986 and winter 1987, zooplankton samples were collected over the entire North Sea by means of a multi-closing net-system. Before taxonomic treatment, wet weight estimates and carbon content conversions were carried out. From this data set, 4 962 522 tons zooplankton biomass (dry weight) were estimated for the whole North Sea during the spring survey. High biomasses (more than 100 mg C/m³) were located in areas between the Orkneys and the Shetlands, off the mouth of the Firth of Forth, the Channel and the river Rhine. Considerable zooplankton biomass was also found parallel to the Danish west coast. Furthermore, a narrow tongue of high biomass (partly greater than 200 mg C/m³) intruded from the north, between 1 °E and 4 °E, into the northern North Sea, turning to the east at 56°N, and continuing into deeper water layers to form a left turning “helix” of high biomass in the central part of the North Sea. During the winter survey the carbon content of the zooplankton stock was a factor 10 lower than in summer. Altogether, 519340 tons of zooplankton biomass (dry weight) were estimated in winter. Centres of relatively high biomass were located off the mouth of the rivers Rhine, Weser and Elbe and off the British east coast moving in a cyclic way across the Dogger Bank into the central North Sea. A further maximum of zooplankton abundance was found in the Skagerrak region. However, an intrusion of zooplankton from the shelf edge into the North Sea was not observed in winter. A qualitative analysis of species composition showed that small copepods dominated the zooplankton in the southern and eastern North Sea. The “eddy” of high biomass in the northern North Sea observed in spring, however, was mostly shaped by the large copepodCalanus finmarchicus (70–90%). The distribution of zooplankton biomass in the North Sea is discussed in relation to the hydrographic conditions and to the biology of the dominant species.