Nekton community change along estuarine salinity gradients: Can salinity zones be defined?

Organisms tend to inhabit predictable portions of estuaries along salinity gradients between the ocean inlets (salinity > 35 psu) and the freshwater tributaries (salinity = 0). Previous studies have suggested that the continuous change in biological community structure along this gradient is relatively rapid at certain salinities. This is the basis for estuarine salinity zonation schemes similar to the classic Venice System (i.e., 0–0.5, 0.5–5, 5–18, 18–30, 30–40, > 40). An extensive database (n > 16,000 samples) of frequency of occurrence of nekton was used to assess evidence for estuarine salinity zones in two southwest Florida estuaries: Tampa Bay and Charlotte Harbor. Rapid change in nekton community structure occurred at each end of the estuarine salinity gradient, with comparatively slow (but steady) change in between. There was little strong evidence for estuarine salinity zones at anything other than low salinities (0.1–1). As previously suggested by other authors, estuaries may be regarded as ecoclines, because they form areas of relatively slow but progressive ecological change. The ends of the estuarine salinity gradient appear to be ecotones (areas of rapid change) at the interfaces with adjacent freshwater and marine habitats. This study highlights the rapid change that occurs in nekton community structure at low salinities, which is of relevance to those managing freshwater inflow to estuaries.     

A Before–After–Control–Impact Analysis of the Effects of a Mississippi River Freshwater Diversion on Estuarine Nekton in Louisiana, USA

Since 1991, the Caernarvon Freshwater Diversion has been reintroducing Mississippi River water into a previously hydrologically isolated estuary in an effort to restore wetlands. To determine the effect of freshwater inflow on estuarine nekton community structure, a Before?CAfter?CControl?CImpact study design was applied. As a result of the opening, salinities in the impact area decreased, and the nekton community structure in the estuary changed significantly. Species of economical or ecological importance either increased in biomass or exhibited no response to the opening of the diversion. Higher abundances of small fish were observed in the area receiving freshwater flow, which is an indication that the area serves as a refuge from large marine predators. Because a salinity gradient was established, as opposed to a uniform but lower salinity regime, aquatic habitat was available to nekton species from a wide spectrum of salinity tolerances.     

Small-scale spatial variation of macrobenthic community structure

Examination of small-scale spatial variation in essential to understanding the relationships between environmental factors and benthic community structure in estuaries. A sampling experiment was performed in October 1993 to measure infauna association with sediment composition and salinity gradients in Nueces Bay, Texas, USA. The bay was partitioned into four salinity zones and three sediment types. Higher densities of macrofaua, were found in sediments with greater sand content and in areas with higher salinity. High diversity was also associated with high homogeneous salinity (31–33‰) and greater sand content. Macrofauna biomass and diversity were positively correlated with bottom salinity, porewater salinity, and bottom dissolved inorganic nitrogen (DIN). Furthermore, species dominance shifted along the estuarine gradient.Streblospio benedicti dominated at lower salinity, but,Mediomatsus ambiseta andMulinia lateralis were the dominant species at higher salinity. Statistical analyses revealed significant correlations for sediment characteristics (i.e., increased fine sediments, water content, and total organic carbon) with decreased total abundance and diversity. Increased salinity and DIN were correlated with increased total biomass, diversity, and macrofauma community structure. These physico-chemical variables are regulated by freshwater inflow, so inflow is an important factor influencing macrofauna community structure by indirectly influencing the physico-chemical environment.     

Importance of water quality to nekton habitat use in a North Carolina Branch estuary

Nekton abundance and water quality were examined over 8 yr (1986–1993) in Isaac Creek, a small (2.5 km long), shallow (1–2 m), estuarine creek draining to Adams Creek (Neuse River system), North Carolina, United States. Water quality and nekton were sampled at 8 to 12 stations at 2–3 wk intervals from April to October (76 dates). The nekton assemblage, sampled by trawl, included 42 taxa but was dominated by 11 species (7 fish and 4 decapod crustaceans). Nekton and water quality (temperature, salinity, dissolved oxygen [DO], salinity gradient) data were grouped and analyzed by three (upper, middle, and lower) creek zones to determine if estimated abundance was correlated with water quality. Potentially stressful water quality conditions for salinity (<5 ppt), temperature (>30°C in morning), and DO (<2 mg 1⁻¹) mainly occurred in the upper and middle zones. The most frequent occurrence of potentially stressful conditions for salinity was in the spring and for dissolved oxygen and temperature in middle to late summer. The frequency of potentially stressful conditions increased during a 3-yr period following timber harvest of a large portion of the watershed. Canonical correspondence analysis (CCA) suggested little correlation between the top 11 species and water quality and indicated an assemblage with regular seasonal changes. Comparison of nekton use of the middle and upper zones of the creek for 3-yr pre- and post-harvest periods showed an increase in proportion of nekton caught in those zones, despite the higher frequency of potentially stressful water quality conditions. This observation suggests that a complex set of factors, including water quality, influence the pattern of nekton use in Isaac Creek.     

Is Salinity Variability a Benthic Disturbance in Estuaries?

Freshwater inflow is a driver of the functioning of estuaries, and average salinity is usually measured to identify the effects of inflow in salinity-zone habitats. However, salinity variability could act as a disturbance by producing unstable habitats, leading to the question: is salinity variance an indicator of benthic disturbance, and therefore a driver of community stability? The macrofauna communities of five estuaries that lie in a climatic gradient on the Texas coastline were analyzed using a 26-year data set. Comparisons within and between estuaries with different inflow regimes were used as a natural experiment to simulate press disturbance events (i.e., climatic inflow) and pulse disturbance (i.e., floods) in maintaining community stability. Salinity average and variance was compared with benthic community diversity, evenness, and species richness. Salinity variance was more correlated to benthic diversity for each estuarine system (r?=??0.6610; p?=?0.0015) than average salinity (r?=?0.3818; p?=?0.0967). As salinity variance decreased (i.e., stability increased), diversity levels of benthic communities increased, and areas with mgore freshwater inflow displayed lower levels of benthic diversity. These findings advance a component of the general theory of diversity maintenance that persistent stressors, such as salinity variability, can influence diversity.     

Nekton Density Patterns in Tidal Ponds and Adjacent Wetlands Related to Pond Size and Salinity

We examined the distribution of nekton across the marsh landscape using a 1-m² drop sampler to compare nekton densities across three different salinity zones (intermediate, brackish, saline), three pond sizes (diameter <40 m = small, ∼250–300 m = medium, >750 m = large), and two habitat types (pond, adjacent marsh) in the Barataria Bay Estuary, Louisiana. Nekton assemblages of ponds and the adjacent marsh appeared to be structured by the responses of individual species to the estuarine salinity gradient at the landscape scale and to pond habitat attributes locally. Our results indicate that ponds in the brackish and saline zones are more important nursery areas for most fishery species than ponds in the intermediate zone. Medium and large ponds supported higher densities of most species than small ponds. Most species of nekton were associated with vegetation structure, and individuals of these species were either concentrated among plant stems at the marsh edge or within submerged aquatic vegetation in ponds.     

Seasonal changes in the nekton community of the Suwannee River estuary and the potential impacts of freshwater withdrawal

In Florida, issues related to alterations of estuarine salinity caused by freshwater withdrawal have recently gained increasing attention. We examined nekton community structure in the Suwannee River estuary (1997–2000) and investigated the relationship between environmental factors and the abundance of fisheries resources. We compared nekton community structure and environmental factors seasonally and annually using multidimensional scaling (MDS) ordination and cluster analysis and observed a strong seasonal pattern. This pattern was consistent among years and closely paralleled those for temperature and river discharge. Representative species for cold seasons includedLeiostomus xanthurus andLagodon rhomboides, and those for warm seasons includedMembras martinica andAnchoa hepsetus. Species that contributed most to the dissimilarity in community structures between wet and dry seasons were abundant and generally preferred lower salinity (e.g.,L. xanthurus, Eucinostomus spp., andMenidia spp.). A period of low freshwater inflow during the latter portion of our study coincided with both decreases and increases in the abundances of some dominant and some economically important species. We have established a baseline which will assist in measuring the effects of long-term changes in freshwater input on the nekton communities of the Suwannee River estuary, but our ability to predict these effects is still limited.     

Seasonal and spatial distribution of planktonic crustacea in the lower Saint John River,a multibasin estuary in New Brunswick,Canada

Five stations on the lower Saint John River, a complex multibasin estuary, were sampled semiquantitatively for zooplankton at biweekly intervals for one year, and qualitatively over a 4-year period. Planktonic Crustacea were dominated by the true estuarine copepods,Acartia tonsa andEurytemora affinis and the euryhaline marine copepodsOithona similis andPseudocalanus minutus. Atypical estuarine forms, confined to a lower fiord-like basin with salinity of 20‰, were the amphipod,Parathemisto abyssorum and the mysidErythrops erythrophthalma. River flows were highly variable from year to year. Certain basins function as lakes in some years and estuaries in other years, causing extreme zooplankton community fluctuations, and succession patterns dependent on salinity rather than season. On occasion freshwater zooplankters maintained viable populations at unusually high salinities (ca. 5‰). Vertical and horizontal distributions of zooplankters indicate that the estuary in fact comprises two systems: a true estuary in the upper reaches and the surface waters at the lower end, and a fiord in a subsidiary basin in the lower end.     

The Influence of Freshwater on Nekton Community Structure in Hydrologically Distinct Basins in Northeastern Florida Bay, FL, USA

Natural patterns of freshwater delivery to the Florida Bay estuary have been disrupted by flood-control and water-supply projects. Restoration efforts are likely to alter salinity regimes and patterns of nekton distribution and abundance. Spatial and seasonal community structure differences were analyzed for small-bodied and large-bodied nekton collected by fisheries-independent monitoring from 2006 through 2009 in the northeastern basins of Florida Bay. The small-bodied nekton community was dominated by resident fish that may be indicators of ecosystem health because they spend their lives within the bay and are not directly influenced by human harvest; the large-bodied nekton community was dominated by transient and, in some cases, economically important species. Differences in community structure revealed a gradient in similarity that was associated with freshwater influence, as determined by salinity variability over the study period. These observed changes associated with salinity regimes within and between basins underscore the importance of monitoring communities before and after alterations in freshwater inflow.     

Axial zonation patterns of subtidal macrozoobenthos in the Gamtoos estuary,South Africa

The distribution of macroinfauna was quantified in subtidal, soft-bottom habitats, extending from the estuarine mouth to the tidal head of the Gamtoos—a small, shallow, temperate estuary situated on the south coast of South Africa. Sampling covered the full salinity gradient from fresh to marine waters, and all sediment types from marine sands to fluvial silts. A total of 35 taxa was recorded, of which 22 occurred throughout the year. Species richness and diversity declined from the seawater-dominated mouth region toward the fresh water section at the tidal head of the estuary. Sediment type generally bore no clear relation to biotic diversity. A marked drop in salinity between winter and summer sample series (Δ 0.2‰ to 24‰) coincided with a reduction of mean macrofaunal density by 70%, a more seaward relocation, and a compression of axial ranges of most taxa. Numerical classification and ordination of faunistically similar regions and of co-occurring species delineated four habitat zones along the longitudinal axis of the estuary which harbour four distinct macrofaunal assemblages: 1) A tidal inlet area with salinities close to seawater; clean, coarse, marine sands, rich in CaCO₃ harbour a stenohaline fauna normally found on adjacent, marine sandy beaches. 2) In the lower reaches, where fine, fluvial silts of high organic content prevail, euryhaline polychaetes dominate the macrozoobenthic community; bottom salinities in this zone seldom dropped below 25‰ 3) The middle reaches, characterized by oligohaline- to polyhaline waters, stretch over sandy sediments of intermediate carbonate, silt, and organic fractions; the fauna comprises typical estuarine forms, which occurred throughout most of the estuary except at its seaward and landward limits. 4) The upper reaches encompass the limnetic waters near the tidal head of the estuary with sediments in this zone being composed mostly of coarse, clean sands, low in CaCO₃; the macrobenthos in this region is dominated by taxa of freshwater origin, which generally do not penetrate seaward beyond the oligohaline waters, and by exceptionally euryhaline estuarine species. Salinity appears as the main factor in controlling faunal assemblages at both extremes of the estuarine gradient (i.e., tidal inlet and head), whereas sediment type delineates between communities in the mesohaline to polyhaline reaches. Axial (i.e., from tidal inlet to tidal head of the estuary) zonation patterns of macroinfauna broadly matched those of mesozooplankton and fishes, supporting the notion of a general structure underlying species distribution patterns in the Gamtoos estuary.     

Paleoenvironmental setting and description of an estuarine oyster reef in the Eocene of Patagonia,southern Argentina

A middle Eocene Crassostrea sp. reef near Río Turbio, southwestern Patagonia (Argentina), represents the earliest record of an oyster reef associated with estuarine facies in the southern hemisphere, and also one of the few known worldwide occurring in Paleogene rocks. The reef grew in an outer estuary environment subject to periodic changes in salinity and may have reached a maturing phase. The Río Turbio reef – by its dimensions, geometry, and substrate lithology– would have been located in a tidal channel convergence area. This reef provides new evidence suggesting that estuaries served as refuges for Crassostrea populations allowing them to disperse into fully marine environments many times throughout the Cenozoic.     

Differential effects of salinity changes on two estuarine fishes,Leiostomus xanthurus andMicropogonias undulatus

Salinity fluctuation has been proposed as an important determinant of estuarine fish distribution. To test this idea, we compared distribution, behavioral preference and physiological sensitivity of two juvenile estuarine fishes, spot (Leiostomus xanthurus) and croaker (Micropogonias undulatus), with respect to salinity change. In field collections, spot: croaker ratios were positively correlated with salinity variation. Subsequent behavioral observations revealed that croaker tend to cross a 10‰ salinity gradient less often than spot. We proposed that energetic costs of salinity adaptation may be higher for croaker, resulting in the observed avoidance behavior. Oxygen consumption rates over rapid salinity fluctuations showed no significant differences in metabolic response between species, although there was some indication that sensitivity changes with fish size. Apparently, juvenile spot and croaker are well-equipped to withstand extreme changes in salinity. We conclude that environmental factors correlated with salinity change may be responsible for distribution differences between these two abundant species.     

Exploring the Effects of Shoreline Development on Fringing Salt Marshes Using Nekton,Benthic Invertebrate,and Vegetation Metrics

Fringing marshes are important but often overlooked components of estuarine systems. Due to their relatively small size and large edge to area ratio, they are particularly vulnerable to impacts from adjacent upland development. Because current shoreland zoning policies aim to limit activities in upland buffer zones directly next to coastal habitats, we tested for relationships between the extent of development in a 100-m buffer adjacent to fringing salt marshes and the structure of marsh plants, benthic invertebrates, and nekton communities. We also wanted to determine useful metrics for monitoring fringing marshes that are exposed to shoreline development. We sampled 18 fringing salt marshes in two estuaries along the coast of southern Maine. The percent of shoreline developed in 100-m buffers around each site ranged from 0 to 91 %. Several variables correlated with the percent of shoreline developed, including one plant diversity metric (Evenness), two nekton metrics (Fundulus heteroclitus %biomass and Carcinus maenas %biomass), and several benthic invertebrate metrics (nematode and insect/dipteran larvae densities in the high marsh zone) (p?<?0.05). Carcinus maenas, a recent invader to the area, comprised 30–97 % of the nekton biomass collected at the 18 sites and was inversely correlated with Fundulus %biomass. None of these biotic metrics correlated with the other abiotic marsh attributes we measured, including porewater salinity, marsh site width, and distance of the site to the mouth of the river. In all, between 25 and 48 % of the variance in the individual metrics we identified was accounted for by the extent of development in the 100-m buffer zone. Results from this study add to our understanding of fringing salt marshes and the impacts of shoreline development to these habitats and point to metrics that may be useful in monitoring these impacts.     

Nekton use of subtidal oyster shell habitat in a Southeastern U.S. estuary

Subtidal accumulations of oyster shell have been largely overlooked as essential habitat for estuarine nekton. In southeastern U.S. estuaries, where oyster reef development is mostly confined to the intertidal zone, eastern oyster (Crassostrea virginica) shell covered bottoms are often the only significant source of hard subtidal structure. We characterized and quantified nekton use of submerged shell rubble bottoms, and compared it to use of intertidal reefs and other subtidal bottoms in the North Inlet estuary, South Carolina. Replicate trays (0.8 m²) filled with shell rubble were deployed in shallow salt marsh creeks, and were retrieved after soak times of 1 to 25 days from May 1998 to March 2000. Thirty six species of fishes, representing 21 families, were identified from the 455 tray collections. Water temperature, salinity, soak time and the presence of a shell substrate all affected the catch of fishes in the trays. Catches during the warmer months were two to five times greater than those during the winter. Fishes were present in 98% of the trays with an overall average of 5.7 fish m^?2. The assemblage was numerically dominated by small resident species including naked goby (Gobiosoma bose), oyster toadfish (Opsanus tau), and crested blenny (Hypleurochilus geminatus). Transient species accounted for 23% of all individuals and 62% of the total biomass due to the presence of relatively large sheepshead (Archosargus probatocephalus) and black sea bass (Centropristis striata). Both the transient and resident species displayed distinct periods of recruitment and rapid growth from April to October. Lower abundances of juvenile gobies and blennies during 1998 were attributed to long periods of depressed salinity caused by high rainfall associated with El Niño conditions in spring. Crabs and shrimps, which were often more abundant than the fishes, accounted for comparable biomass in the tray collections. In comparisons of subtidal tray and trawl catches, trays yielded 10 to 1,000 fold higher densities of some demersal fish groups. Comparisons of intertidal and subtidal gear catches indicated that many species remain in the subtidal shell bottom at all stages of the tide. This study suggests that subtidal shell bottom may be essential fish habitat for juvenile seabass, groupers, and snappers and that it may be the primary habitat for a diverse assemblage of ecologically important resident fishes and crustaceans. Given the high levels of nekton use and the areal extent of oyster shell bottoms in eastern U.S. and Gulf estuaries, increased attention to protection and restoration of these areas appears justified.     

Ocean distribution of dungeness crab megalopae and recruitment patterns to estuaries in Southern Washington State

We investigated the distribution of meroplankton and water properties off southern Washington and simultaneously measured time series of larval abundance and water properties in two adjacent estuaries, Grays Harbor and Willapa Bay. The cruise period, in late May 1999, coincided with large variation in the alongshore wind stress that caused dynamic change in the position of the Columbia River plume, coastal upelling and downwelling, and offshore phytoplankton production. In the coastal ocean, meroplankton groups responded differently to this wind event and the associated advection of water masses. Dungeness crab (Cancer magister) megalopae were largely indifferent to the wide salinity variation, and were found throughout the surveyed area in both plume and recently upwelled waters. Megalopae of kelp crab (Pugettia producta) and hermit crab (Pagurus spp). were more abundant in upwelled water and low numbers were caught in the plume water. Barnacle cyprids appeared to track the advective transport suggesting that they may be more passively dispersed. Within the estuaries, hydrography responded rapidly and synchronously to variation in wind stress. Intrusions of both plume and newly upwelled waters were detected at estuarine sites, depending on the type of water present at the coast, indicating a tight link between the estuaries and the coastal ocean in this region. A 90-d record ofC. magister megalopae abundance was made at 3 estuarine sites using light traps. The bulk of theC. magister recruitment was limited to a relatively brief period in late May through June. Within this window, megalopae occurred in distinct pulses of 3–5 d interspaced with periods of low or zero abundance.C. magister megalopae recruited to the estuaries over a wide range of wind forcing, and were transported into the estuary within varied water types. There were no periodic patterns indicative of spring-neap tidal variations in the abundance time series. Abundance was only weakly cross-correlated between the adjacent Grays Harbor and Willapa Bay estuaries, which contrasts with the more synchronous estuarine-coastal linkages measured for water properties. These results suggest the interaction of larval aggregation size in the ocean with estuary-ocean exchange processes likely controls patterns of estuarine recruitment.     

The ichthyofauna and abiotic hydrological environment of the Ria de Aveiro,Portugal

The abiotic hydrological environment and the community dynamics of the fish fauna were investigated in the Ria de Aveiro, an estuarine coastal lagoon system (43 km²), which has both marine and fluvial influences. Abiotic hydrological and fish community parameters were recorded routinely during 12 months at ten stations. Temperature ranged between 9.5°C and 26.0°C, salinity between 0.0‰ and 32.0‰, dissolved oxygen between 0.8 mg 1^?1 and 15.4 mg 1^?1, pH between 6.1 and 9.4, and transparency between 4.4% and 100.0%. No significant differences were observed in temperature and dissolved oxygen among stations, or in seasonal variation in transparency. Nineteen thousand thirty-one fish specimens comprising 55 species were sampled. Abundance, biomass, and species richness were highest in summer and late winter at stations near the lagoon entrance. Sedentary species were most numerous, marine migratory species had the highest biomass, and the category “occasional species” had the highest number of species. Atherinidae, Mugilidae, and Gobiidae were the most important families. Eight species represented about 80% of the total fish abundance and biomass but only six species occurred in all the months and at all the stations. It was concluded that the Ria de Aveiro, with high seasonal and spatial abiotic variation, has a very rich and representative fish community compared with temperate and tropical estuaries and estuarine coastal lagoons around the world.     

The impact of a severe drought on the vegetation of a subtropical estuary

In order to document the effect of the recent drought and the resulting marine intrusion event on plant-community shifts in a Louisiana estuary, we analyzed two vegetation data sets collected in Barataria estuary in 1997 and 2000 and compared community shifts to surface salinity changes at four points along the estuarine gradient within the study area. We used the major vegetation types identified in our previous research of larger data sets and tested the use of a simple vegetation classification technique. This vegetation classification technique is based primarily on the dominant and co-dominant species, and secondarily on the number of taxa observed. To distinguish vegetation types with similar dominant species but different associated species, the vegetation classification technique used a salinity score derived from the species composition. Surface water salinity increases were reflected by a change in species composition in the mesohaline to fresh marshes. The largest species composition shift observed was the shift from oligohaline wiregrass (species rich vegetation type dominated bySpartina patens) to mesohaline wiregrass (vegetation type dominated byS. patens with few other species). Shifts in vegetation composition may have been enhanced by the presence of the major dominant species at a low abundance in other vegetation types. The vegetation classification technique used could classify over 95% of the stations. This vegetation classification technique provides a simple method to classify Louisiana's coastal vegetation based on plant species composition.     

Nekton Community Structure Varies in Response to Coastal Urbanization Near Mangrove Tidal Tributaries

To assess the potential influence of coastal development on estuarine-habitat quality, we characterized land use and the intensity of land development surrounding small tidal tributaries in Tampa Bay. Based on this characterization, we classified tributaries as undeveloped, industrial, urban, or man-made (i.e., mosquito-control ditches). Over one third (37 %) of the tributaries have been heavily developed based on landscape development intensity (LDI) index values >5.0, while fewer than one third (28 %) remain relatively undeveloped (LDI?<?3.0). We then examined the nekton community from 11 tributaries in watersheds representing the four defined land-use classes. Whereas mean nekton density was independent of land use, species richness and nekton-community structure were significantly different between urban and non-urban (i.e., undeveloped, industrial, man-made) tributaries. In urban creeks, the community was species-poor and dominated by high densities of poeciliid fishes, Poecilia latipinna and Gambusia holbrooki, while typically dominant estuarine taxa including Menidia spp., Fundulus grandis, and Adinia xenica were in low abundance and palaemonid grass shrimp were nearly absent. Densities of economically important taxa in urban creeks were only half that observed in five of the six undeveloped or industrial creeks, but were similar to those observed in mosquito ditches suggesting that habitat quality in urban and mosquito-ditch tributaries is suboptimal compared to undeveloped tidal creeks. Furthermore, five of nine common taxa were rarely collected in urban creeks. Our results suggest that urban development in coastal areas has the potential to alter the quality of habitat for nekton in small tidal tributaries as reflected by variation in the nekton community.     

Tissue nitrogen and carbon variations in New England estuarineAscophyllum nodosum (L.) Le Jolis populations (Fucales,Phaeophyta)

The seasonal patterns ofAscophyllum nodosum (L.) Le Jolis tissue composition (N, C and ash) and ambient inorganic N were measured for 18 months at seven sites throughout the Great Bay Estuary System, New Hampshire-Maine, U.S.A. Overall, the seasonal cycle of tissue N coincided with ambient dissolved inorganic N, with low values in the late spring and summer and highest values in late fall and winter. even so, a pronounced lag occurred in the spring when ambient nutrients decreased, and the levels of tissue N remained high. The seasonal patterns of tissue N were very similar throughout the estuary, although the spring maxima were conspicuously greater at inner than outer estuarine sites. No conspicuous seasonal trend was apparent for tissue C, and the C/N ratios merely reflected variation in N. The percent ash values were extremely variable, and they did not reflect a simple salinity gradient within the Estuary. Two examples of the potential significance ofAscophyllum to the nitrogen pools in northern New England estuaries are discussed.     

Fish biomass estimates from the littoral zone of an estuarine coastal lake

The fish community of Swartvlei, a southern African coastal lake, was sampled using gill and lift nets. A primary aim of the research was to determine the fish biomass of the lake shelf region as well as the distribution patterns of marine species which utilize this estuarine system as a nursery and/or foraging area. The catch per unit effort of fishes was more than three times higher in the littoral zone when compared to the pelagic, slope, or profundal zones. Both habitat diversity and available food resources were maximal in the shelf region. The biomass of the littoral fish community during 1980 was estimated at 12.4 g m² wet weight, with detritivorous species contributing 3.2 g m^?2, zoobenthos consumers 2.8 gm², piscivorous species 2.3 g m², herbivorous/epifaunal consumers 2.7 g m^?2, and zooplanktivorous species 1.4 g m². Published information on fish atanding stock estimates from various estuaries and coastal habitats around the world is collated and compared. It is concluded that estuarine fish biomasses do not excced those of productive freshwater or marine environments and that further research is necessary to determine the size and variability of estuarine fish stocks in relation to other ecosystems.