The influence of episodic events on transport of striped bass eggs to the estuarine turbidity maximum nursery area

The estuarine turbidity maximum (ETM) is an important nursery area for anadromous fish where early-life stages can be retained in high prey concentrations and favorable salinities. Episodic freshwater flow and wind events could influence the transport of striped bass (Morone saxatilis) eggs to the ETM. This hypothesis was evaluated with regression analysis of observational data and with a coupled biological-physical model of a semi-idealized upper Chesapeake Bay driven by observed wind and freshwater flow. A particle-tracking model was constructed within a numerical circulation model (Princeton Ocean Model) to simulate the transport of fish eggs in a 3-dimensional flow field. Particles with the sinking speed of striped bass eggs were released up-estuary of the salt front in both 2-d event-scale and 60-d seasonal-scale scenarios. In event scenarios, egg-like particles with observed specific gravities (densities) of striped bass eggs were transported to the optimum ETM nursery area after 2 d, the striped bass egg-stage duration. Wind events and pulses in river discharge decreased the number of egg-like particles transported to the ETM area by 20.9% and 13.2%, respectively, compared to nonevent conditions. In seasonal scenarios, particle delivery to the ETM depended upon the timing of the release of egg-like particles. The number of particles transported to the ETM area decreased when particles were released before and during wind and river pulse events. Particle delivery to the ETM area was enhanced when the salt front was moving up-estuary after river pulse events and as base river flow receded over the spawning season. Model results suggest that the timing of striped bass spawning in relation to pulsed events may have a negative (before or during events) or positive (after river flow events) effect on egg transport. Spawning after river flow events may promote early-stage survival by taking advantage of improved transport, enhanced turbidity refuge, and elevated prey production that may occur after river pulse events. In multiple regression analysis of observed data, mean spring freshwater flow rates and the number of pulsed freshwater flow events during the striped bass spawning season explained 71% of the variability in striped bass juvenile abundance in upper Chesapeake Bay from 1986 to 2002. Positive parameter estimates for these effects support the hypothesis that pulsed freshwater flow events, coupled with spawning after the events, may enhance striped bass early-stage survival. Results suggest that episodic events may have an important role in controlling fish recruitment.     

Temperature Impacts on <Emphasis Type="Italic">Eurytemora carolleeae</Emphasis> Size and Vital Rates in the Upper Chesapeake Bay in Winter

The copepod Eurytemora carolleeae dominates vernal zooplankton biomass in the Chesapeake Bay estuarine turbidity maximum (ETM) region, where it is an important prey item for larval anadromous fish. Although there have been several zooplankton studies in the Chesapeake Bay ETM focused on spring, the importance of winter zooplankton populations for establishing these vernal conditions has not been investigated. We examined the abundance, distribution, and individual sizes of E. carolleeae in winter of 2007 and 2008 and we investigated the potential impact of varying winter conditions and rising winter temperatures on Eurytemora female carbon content, egg production rate, and generation time. We found higher abundances and larger individuals in the colder 2007 than in 2008 under similar freshwater flow conditions. Empirical estimates showed that overall zooplankton productivity was higher in 2007 than in 2008. Published recruitment indices for anadromous fish including white perch and striped bass were higher in 2007 than in 2008 in the study region. Based on these findings, we hypothesize that colder conditions resulted in larger individuals and therefore increased prey biomass available to larval fish. We further hypothesize that rising winter water temperatures will negatively impact trophic transfer of primary production to copepods and ultimately to fish.     

Nursery Habitat Shifts in an Estuarine Ecosystem<Emphasis Type="Bold">:</Emphasis> Patterns of Use by Sympatric Catfish Species

The seasonal and spatial distribution (density and biomass) of five size classes of two catfish species (Cathorops spixii and Cathorops agassizii) were studied along an estuarine ecocline to test the relative importance of the nursery function of each habitat. Seasonal vs. area interactions were significant for all size classes of both species. During the early rainy season, the middle estuary is an important nursery habitat for juveniles of both species. When environmental conditions change during the late rainy season, the C. spixii primary nursery habitat shifts to the lower estuary. During this period, juveniles of C. agassizii remain in the middle estuary. Another important ecological area is the upper estuary, which becomes a breeding, spawning and hatchery area during the late dry season for both species. The nursery function of habitats shifts according to the seasonal fluctuation of salinity and dissolved oxygen, and each species responds differently to this change.     

Spatio-temporal Variability in Larval-Stage Feeding and Nutritional Sources as Factors Influencing Striped Bass (Morone saxatilis) Recruitment Success

Prey availability and feeding success affect survival of larval striped bass (Morone saxatilis) in Chesapeake Bay and contribute to the >30-fold interannual recruitment variability. Gut contents and stable isotope analyses (δ¹⁵N and δ¹³C) were conducted on striped bass larvae to evaluate sources of nutrition in 2007 and 2008, years of high and poor recruitment, respectively. Ichthyoplankton and zooplankton were surveyed in the upper Chesapeake Bay, in proximity to the estuarine turbidity maximum and associated salt front. Feeding incidence and numbers of prey per gut were similar in both years and varied in relation to the salt front. The primary prey in each year was the estuarine copepod Eurytemora affinis. Substantial consumption of the freshwater cladoceran Bosmina spp. also occurred, especially up-estuary of the salt front in 2007, demonstrating that secondary prey are important to larval diets in some years. Stable isotope analysis of yolk sac and feeding-stage larvae of striped bass revealed an ontogenetic shift from maternal stable isotope signatures to those indicative of prey source. Feeding-stage larvae from up-estuary locations had the most negative δ¹³C values, indicating a relatively high terrestrial carbon source in prey. Spatio-temporal variability in δ¹⁵N signatures of larvae followed similar trends of δ¹⁵N variability in zooplankton prey with the highest δ¹⁵N values up-estuary of the salt front and estuarine turbidity maximum. A stable isotope analysis on archived striped bass larvae collected in 1998 and 2003, years of moderate and high recruitment, respectively, expanded the documented range of isotope signatures but did not clearly distinguish effects of nutritional sources on recruitment.     

Retention of white perch and striped bass larvae: Biological-physical interactions in Chesapeake Bay estuarine turbidity maximum

Physical and biological properties of the Chesapeake Bay estuarine turbidity maximum (ETM) region may influence retention and survival of anadromous white perch (Morone americana) and striped bass larvae (Morone saxatilis). To evaluate this hypothesis we collected data in five cruises, three during May 1998 and two during May 1999, in upper Chesapeake Bay. Time series of freshwater discharge, water temperature, wind, and water level explain differences in ETM location and properties between cruises and years. During high flows in 1998, a two-layer response to wind forcing shifted the ETM up-estuary, while a high discharge event resulted in a down-estuary shift in the salt front and ETM location. In 1999, extremely low discharge rates shifted the salt front 15 km up-estuary of its position in 1998. During 1999, the ETM was less intense and apparently topographically fixed. Gradients in depth-specific abundance of ichthyoplankton were compared with salinity and TSS concentrations along the channel axis of the upper Bay. During 1998, the high flow year, most striped bass eggs (75%) and most early-stage white perch larvae (80%) were located up-estuary of the salt front. In addition, most striped bass (91%) and white perch (67%) post-yolk-sac larvae were located within 10 km of maximum turbidity readings. Total abundance of white perch larvae was lower in 1999, a low freshwater flow year, than in 1998, a high flow year. In 1999, striped bass larvae were virtually absent. White perch (1977–1999) and striped bass (1968–1999) juvenile abundances were positively correlated with spring Susquehanna River discharge. The ETM regions is an important nursery area for white perch and striped bass larvae and life-history strategies of these species appear to insure transport to and within the ETM. We hypothesize that episodic wind and discharge events may modulate larval survival within years. Between years, differences in freshwater flow may influence striped bass and white perch survival and recruitment by controlling retention of egg and early-stage in the ETM region and by affecting the overlap of temperature/salinity zones preferred by later-stage larvae with elevated productivity in the ETM.     

Regional scale climate forcing of mesozooplankton dynamics in Chesapeake Bay

A 16-yr (1985–2000) time series of calanoid copepod (Acartia tonsa andEurytemora affinis) abundance in the upper Chesapeake Bay was examined for links to winter weather variability. A synthesis of sea level pressure data revealed ten dominant, winter weather patterns. Weather patterns differed in frequency of occurrence as well as associated precipitation and temperature. The two dominant copepod species responded differently to winter weather variability.A. tonsa abundance showed little response to winter weather and did not vary in abundance during wet or dry springs.E affinis responded strongly to winter weather patterns that produced springs with high freshwater discharge and low salinities. During wet springs,E. affinis abundance increased overall and its area of dominance extended further down estuary. The different response of the two species is likely related to several factors including residence time, development time, salinity tolerance, food limitation, and life history strategy. Important fish species that rely on zo oplankton as food resources were also related to winter weather variability and spring zooplankton abundance.Morone saxatilis (striped bass) andAnchoa mitchilli (bay anchovy) juvenile indices were positively and negatively correlated toE. affinis abundance, respectively. *** DIRECT SUPPORT *** A02BY003 00004     

Short-term effects of a low dissolved oxygen event on estuarine fish assemblages following the passage of hurricane Charley

Hurricane Charley, a category 4 storm, passed directly over the Charlotte Harbor estuary and Peace River watershed on August 13, 2004. Following the storm's passage, dissolved oxygen in the Peace River fell below 1 mg l⁻¹ and hypoxic conditions (<2mgl⁻¹) extended into Charlotte Harbor. A long-term fisheries-independent monitoring program (1989 to present) was already in place in Charlotte Harbor, and sampling was intensified during the month after the storm. Changes in fish assemblages that resulted from the hypoxic event were determined using nonmetric multidimensional scaling and similarity percentages analysis. At the mouth of the Peace River and upper Charlotte Harbor, fish abundance decreased dramatically after the hurricane, and typical estuarine fish assemblages were replaced by those dominated by a few resilient estuarine and freshwater species, including the nonindigenous brown hoplo (Hoplosternum littorale) and sailfin catfish (Pterygoplichthys spp.). Fish assemblages in the estuarine portion of the Maykka River, located only a few kilometers west of the Peace River, were unaffected. The hypoxic event was short lived; dissolved oxygen and estuarine fish assemblages in the Peace River and upper Charlotte Harbor recovered within a month. The results of this study are consistent with other hurricane-related hypoxic events in the literature which have reported acute effects to estuarine systems in the short term, rapid recoveries, and long-term resilience.     

Habitat associations of estuarine species: Comparisons of intertidal mudflat, seagrass (Zostera marina), and oyster (Crassostrea gigas) habitats

The complexity of habitat structure created by aquatic vegetation is an important factor determining the diversity and composition of soft-sediment coastal communities. The introduction of estuarine organisms, such as oysters or other forms of aquaculture, that compete with existing forms of habitat structure, such as seagrass, may affect the availability of important habitat refugia and foraging resources for mobile estuarine fish and decapods. Fish and invertebrate communities were compared between adjacent patches of native seagrass (Zostera marina), nonnative cultured oyster (Crassostrea gigas), and unvegetated mudflat within a northeastern Pacific estuary. The composition of epibenthic meiofauna and small macrofaunal organisms, including known prey of fish and decapods, was significantly related to habitat type. Densities of these epifauna were significantly higher in structured habitat compared to unstructured mudflat. Benthic invertebrate densities were highest in seagrass. Since oyster aquaculture may provide a structural substitute for seagrass being associated with increased density and altered composition of fish and decapod prey resources relative to mudflat, it was hypothesized that this habitat might also alter habitat preferences of foraging fish and decapods. The species composition of fish and decapods was more strongly related to location within the estuary than to habitat, and fish and decapod species composition responded on a larger landscape scale than invertebrate assemblages. Fish and decapod species richness and the size of ecologically and commercially important species, such as Dungeness crab (Cancer magister), English sole (Parophrys vetulus), or lingcod (Ophiodon elongatus), were not significantly related to habitat type.     

Parasites and cyclopoid predators of age-0 fish in the Roanoke River,North Carolina

Altered river flow has been suggested as a cause for the low recruitment of striped bass,Morone saxatilis, in the Roanoke River (North Carolina) because of its effect on the proximity of zooplankton and larval striped bass. This results in unsuccessful feeding and subsequent starvation, which was considered to be a major mortality factor. Other mortality factors, such as parasitism and copepod predation on age-0 fish, may also be regulated to some extent by changes in river flow. The relationship of cestode plerocercoids, trematode metacercaria, mussel glochidia, and cyclopoid copepod predators with age-0 fish was evaluated in the lower Roanoke River and western Albemarle Sound from plankton net collections made in 1984 to 1986 and 1988. Plerocercoid prevalence was higher under low river flow conditions than under high flow conditions in darters (Percidae; 16.7% vs. 9.2%), minnows (Cyprinidae; 28.8% vs. 4.7%), andMorone (1.9% vs. 0%). Gut analysis of the age-0 fish revealed that copepods (source of the plerocercoids) were a major diet component ofMorone and darters but not of minnows or herring (Clupeidae). Decreases in river flow were associated with increases in copepod density (Pearson r=?0.62; p=0.0001) and plerocercoid prevalence inMorone (Pearson r=?0.29; p=0.03). The low correlation value forMorone may be quite strong considering the complexity of the variables associated with prevalence. Metacercaria were found only inMorone and minnows, and prevalence and mean intensity were less than that found for plerocercoids. Mussel glochidia prevalence was less than 0.5% for all affected taxa, an order of magnitude less that that found in other studies. The low value may indicate that the mussel population in the Roanoke River is declining. Prevalence of attacks by the predatory copepodMesocyclops edax on age-0 fish was similar to that in Chesapeake Bay, and striped bass was the primary prey. Spatial and temporal proximity of copepods and fish prey may be the key factors in regulating copepod attacks. The low prevalence of parasites and copepod predators seen in this tudy would suggest that mortality from these sources may not be a major factor in age-0 recruitment in this system. Confirmation of these conclusions would require a more controlled experimental approach.     

Long-Term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003

We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage treatment facilities and to variability in freshwater flow and nonpoint nutrient inputs to the Patuxent River estuary. We analyzed a 19-year dataset of water quality conditions, nutrient loading, and climatic forcing for three estuarine regions and also computed monthly rates of net production of dissolved O₂ and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model. Point-source loading of DIN and DIP to the estuary declined by 40–60% following upgrades to sewage treatment plants and correlated with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom-layer O₂ consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower estuary, chl-a, turbidity, and surface-layer net O₂ production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to particulate organic nitrogen. These analyses illustrate (1) the value of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems, and (3) the potential water quality impacts of altered coastal food webs.     

Contamination of the biological compartment in the Seine estuary by Cd, Cu, Pb, and Zn

A study of contamination of the biological compartment of the Seine estuary was carried out by measuring the concentrations of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) in 29 estuarine and marine species belonging to 6 phyla. Species came from three main biological zones of the estuary: the Seine channel (copepods, mysids, shrimps, and fish), the intertidal mudflats (Macoma balthica community), and the subtidal mudflats (Abra alba community). Two fish species, the bass (Dicentrarchus labrax) and the flounder (Platichthys flesus), were also selected for analyses. A comparison of metal concentrations in estuarine species of the Seine with those found in the same species collected on contaminated and non-contaminated sites showed a contamination of the estuary by Cu, Zn, and Pb. For Cd, the contamination is mainly observed in bivalves, although the concentrations observed were low and less than 2 μg g^?1 d.w. High concentrations of Cu were found in copepods, shrimps, and fish. Pb contamination was mainly found in species living in the Seine channel where the copepodEurytemora affinis shows an average concentration of 22 μg g^?1 d.w. High concentrations of Pb (>10 μg g^?1 d.w.) were found in deposit-feeders benthic invertebrates. Elevated levels of Zn were seen in all species collected in the Seine estuary, including fish and in particular small flounder. The concentrations of Cd, Cu, Pb, and Zn found in edible estuarine species (shrimp and fish) were in the same order of magnitude than those found in fish and shrimps fished along the French coast.     

Feeding ecology of o-group sea bass,Dicentrarchus labrax, in salt marshes of Mont Saint Michel Bay (France)

0-group sea bass,Dicentrarchus labrax, colonize intertidal marsh creeks of Mont Saint Michel Bay, France, on spring tides (e.g., 43% of the tides) during flood and return to coastal waters during ebb. Most arrived with empty stomachs (33%), and feed actively during their short stay in the creeks (from 1 to 2 h) where they consumed on average a minimum of 8% of their body weight. During flood tide, diet was dominated by mysids,Neomysis integer, which feed on marsh detritus. During ebb, when young sea bass left tidal marsh creeks, the majority had full stomachs (more than 98%) and diet was dominated by the most abundant marsh (including vegetated tidal flats and associated marsh creeks) resident amphipod,Orchestia gammarellus. Temporal and tidal effects on diet composition were shown to be insignificant. Foraging in vegetated flats occurs very rarely since they are only flooded by about 5% of the tides. It was shown that primary and secondary production of intertidal salt marshes play a fundamental role in the feeding of 0-group sea bass. This suggests that the well known nursery function of estuarine systems, which is usually restricted to subtidal and intertidal flats, ought to be extended to the supratidal, vegetated marshes and mainly to intertidal marsh creeks.     

Seasonal and interannual patterns of distribution and diet of bluefish within a Middle Atlantic bight estuary in relation to abiotic and biotic factors

Seasonal and interannual patterns in the spatial distribution of bluefish (Pomatomus saltatrix) within a Middle Atlantic Bight estuary were examined using multipanel gillnets fished biweekly at 14 fixeds stations in the Sandy Hook Bay-N avesink River estuary during May–November of 1998 and 1999. To characterize habitats along the estuarine gradient, we measured several abiotic and biotic variables concurrently with gillnet sampling. Juvenile (age-0 and age-1+) bluefish were captured regularly during both years along with large numbers of Atlantic menhaden (Brevoortia tyrannus), which were confirmed by diet analyses to be bluefish’s primary forage species. The date of initial appearance of age-0 bluefish and menhaden in the estuary varied between years and may have been related to interannual differences in seawater temperatures on the continental shelf during spring. Delayed estuarine arrival of prey fishes may have contributed to variability in bluefish diets between years. Within the estuary, bluefish spatial distribution were consistent across seasons and years: bluefish were most common in areas associated with high concentrations of suspended materials and the presence of menhaden. Community analyses also indicated habitat overlap between bluefish and menhaden. Spatial distribution patterns revealed the consistent occurrence of piscivorous bluefish in shallow estuarine habitats that retained suspended materials and aggregated prey fishes. Foraging success of bluefish and other estuarine piscivores may be closely linked with the availability of these productive habitat, highlighting the need for future study of biological interactions and the governing physical processes.     

An ephemeral turbidity maximum generated by resuspension of organic-rich matter in a macrotidal estuary, S.W. Wales

An ephemeral estuarine turbidity maximum (ETM) occurs at high water in the macrotidal Taf estuary (SW Wales, United Kingdom). A new mechanism of ETM formation, due to resuspension and advection of material by flood tidal currents, is observed that differs from classical mechanisms of gravitational circulation and tidal pumping. The flood tide advances across intertidal sand flats in the main body of the estuary, progressively entraining material from the rippled sands. Resuspension creates, a turbid front that has suspended sediment concentrations (SSC) of about 4,000 mg I⁻¹ by the time it reaches its landward limit which is also the landward limit of salt penetration. This turbid body constitutes the ETM. Deposition occurs at high slack water but the ETM retains SSC values up to 800 mg I⁻¹, 1–2 orders of magnitude greater than ambient SSC values in the river and estuarine waters on either side. The ETM retreats down the estuary during the ebb; some material is deposited thinly across emergent intertidal flats and some is flushed out of the estuary. A new ETM is generated by the next flood tide. Both location and SSC of the ETM scale on Q/R³ where Q is tidal range and R is river discharge. The greatest expression of the ETM occurs when a spring tide coincides with low river discharge. It does not form during high river discharge conditions and is poorly developed on neap tides. Particles in the ETM have effective densities (120–160 kg m⁻³) that are 3–4 times less than those in the main part of the estuary at high water. High chlorophyll concentrations in the ETM suggest that flocs probably originate from biological production in the estuary, including production on the intertidal sand flats.     

Interannual and long-term variation in the nearshore fish community of the mesohaline Hudson River estuary

The detection of long-term shifts in species composition and spatial structuring of aquatic communities may be obscured by high levels of interannual variation. Estuarine fish communities are likely to exhibit high levels of variation owing to the influence of riverine forcing and the importance of anadromous and transient species, whose abundances may not be locally controlled. We describe patterns of interannual variation and long-term shifts in the nearshore fish community of the mesohaline Hudson River estuary based on 21 yr of beach seine sampling conducted annually between late August and mid November. Of the 60 species encountered, the most abundant were Atlantic silversides (Menidia menidia), striped bass (Morone saxatilis), white perch (Morone americana), American shad (Alosa sapidissima), and blueback herring (Alosa aestivalis). Relationships between annual community composition and seasonal flow and temperature regimes were examined with canonical correspondence analysis. Annual variation was most closely correlated with river flows in the 3-mo period preceding fish sampling, indicating a persistent effect of environmental conditions on community structure. Despite significant interannual variation in composition, longer-term trends in community structure were observed. These included declines in catch rates of freshwater and estuarine species and a dramatic increase in the catch of Atlantic silversides, an annual marine species. Associated with these changes were declines in community diversity and increased compositional variation. These results indicate that analyses of temporal changes in community structure need to account for the multiple time scales under which forcing factors and community composition vary.     

Field and laboratory evaluation of habitat use by rainwater killifish (Lucania parva) in the St. Johns River estuary, Florida

I examined the relative importance of beds of tapegrass (Vallisneria americana) and adjacent unvegetated habitats to juvenile and adult (6–35 mm standard length) rainwater killifish (Lucania parva) over a large spatial scale within the St. Johns River estuary, Florida. Abundance of rainwater killifish did not differ between oligohaline and tidal freshwater portions of the estuary and this species was relatively rare at opposite ends of the St. Johns River estuary. The presence of rainwater killifish at a given site was determined in part by large-scale variation in environmental factors such as habitat complexity and salinity. When present at a site, rainwater killifish were found almost exclusively in structurally complex beds of tapegrass. Behavioral observations in the laboratory indicated that rainwater killifish preferred vegetated over unvegetated habitats in the absence of both potential prey and predators and that use of vegetated habitats increased further upon addition of predatory largemouth bass (Micropterus salmoides). A laboratory predation experiment indicated that survival of rainwater killifish exposed to largemouth bass was significantly higher in vegetation than over open sand. Strong preferences for structurally complex vegetation likely reflect an evolved or learned behavioral response to risk of predation and help explain habitat use of rainwater killifish in the St. Johns River estuary.     

Feeding habits of red drum (Sciaenops ocellatus) in Galveston Bay, Texas: Seasonal diet variation and predator-prey size relationships

Feeding habits, seasonal diet variation, and predator size-prey size relationships of red drum (Sciaenops ocellatus) were investigated in Galveston Bay, Texas through stomach contents analysis. A total of 598 red drum ranging from 291–763 mm total length were collected and their stomach contents analyzed during fall 1997 and spring 1998. The diet of red drum showed significant seasonal patterns, and was dominated by white shrimp (Penaeus setiferus) during fall and gulf menhaden (Brevoortia patronus) during spring. Blue crab (Callinectes sapidus) was an important component of red drum diets during both seasons. Significant differences existed between prey types consumed during fall and spring as red drum diet reflected seasonal variation in prey availability. Predictive regression equations were generated to estimate original carapace width of blue crabs from several measurements taken from carapace fragments recovered in red drum stomachs. Regressions were highly significant (r²>0.97) and increased the number of blue crabs with size information nearly three fold. Predator size-prey size relationships were determined for red drum feeding on white shrimp, gulf menhaden, and blue crab. Although regression slopes were statistically significant, prey sizes increased only slightly with increasing red drum size. Comparisons of prey sizes consumed by red drum with sizes occurring in the field indicate that red drum feed in nearshore shallow water habitats, which serve as nursery areas for many juvenile fishes and crustaceans. Our findings demonstrate that red drum feed on several prey species of commercial and recreational value and may have important effects on estuarine community structure.     

Helophyte zonation in two regulated estuarine areas in the Netherlands: Vegetation analysis and relationships with hydrological factors

The relationship between hydrological conditions and riparian helophyte vegetation was studied in two freshwater estuaries that differed in tidal regulation in order to assess the effects of large-scale hydrological regulation on the fringe vegetation. Vegetation and environmental variables were sampled for 206 sites in the Rhine-Meuse estuary (146 sites) and the IJsselmeer region (60 sites) in the Netherlands. These samples were classified into 10 vegetation types, all of which were dominated by tall helophytes. The most common vegetation types were dominated byPhragmites australis andTypha angustifolia, which formed both monospecific stands and mixtures. Tall Cyperaceae dominated three vegetation types (dominated individually bySchoenoplectus lacustris, Bolboschoenus maritimus, andSchoenoplectus tabernaemontani).Acorus calamus. Principal components analysis of the species composition of vegetation fringing open-water areas and associated environmental data revealed complex gradients incorporating differences in water depth, water-level fluctuation, were exposure, and sedimentation and/or erosion. The composition of the helophyte belts varied among the areas as the result of the differing times at which regulation occurred. Based on historical data, hydrological regulation of the estuaries has resulted in deterioration of intertidalSchoenoplectus andBolboschoenus stands, due to erosion and predation.P. australis stands have been invaded by terrestrial plant species or have been replaced byT. angustifolia. A scheme is presented of helophyte vegetation development under the influence of changes in the hydrological regime.     

Application of a multimetric fish index to assess the environmental condition of South African estuaries

A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine typologies. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity and composition, species abundance, nursery function, and trophic integrity. Metric reference conditions and scoring criteria were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good). A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively differentiate between poor and good quality sites. Applying the EFCI to estuaries in which multiple samples were taken also showed that the index is reproducible. The EFCI is both a robust and sensitive method for assessing the ecological condition of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood format for managers, policy makers, and the general public.     

Food Web Structure and Habitat Connectivity in Fish Estuarine Nurseries—Impact of River Flow

River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries. In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience.