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1.
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.  相似文献   

2.
We sampled nearshore fishes in the Sacramento-San Joaquin Delta, California, United States, during 2001 and 2003 with beach seines and gill nets. We addressed three questions. How and why did fish assemblages vary, and what local habitat features best explained the variation? Did spatial variation in assemblages reflect greater success of particular life history strategies? Did fish biomass vary among years or, across habitats? Nonmetric multidimensional scaling showed that habitat variables had more influence on fish assemblages than temporal variables. Results from both gear types indicated fish assemblages varied between Sacramento and San Joaquin River sampling sites. Results from gill net sampling were less pronounced than those from beach seine sampling. The Sacramento and San Joaquin river sites differed most notably in terms of water clarity and abundance of submerged aquatic vegetation (SAV), suggesting a link between these habitat characteristics and fish relative abundance. Among-site differences in the relative abundance of periodic and equilibrium strategist species suggested a gradient in the importance of abiotic versus biotic community structuring mechanisms. Fish biomass varied among years, but was generally higher in SAV-dominated habitats than the turbid, open habitats in which we found highest abundances of striped bassMorone saxatilis and special-status native fishes such as delta smeltHypomesus transpacificus, Chinook salmonOncorhyncus tschawytscha, and splittailPogonichthys macrolepidotus. The low abundance of special-status fishes in the comparatively productive SAV-dominated habitats suggests these species would benefit more from large-scale restoration actions that result in abiotic variability that mirrors natural river-estuary habitat than from actions that emphasize local (site-specific) productivity.  相似文献   

3.
Surface accumulations of foam and flotsam as well as sharp salinity, density, turbidity gradients and regions of acoustic scatter were characteristic of ebb-tidal fronts in Charleston Harbor, South Carolina. Surface convergence velocities at these fronts averaged 0.06 m s?1 into the front at an angle of 30° to 60° with respect to the frontal axis, indicating along-front transport during the ebb. These fronts are tidally-induced, forming on the late flood and ebb along the interfaces of water masses. Horizontal and vertical measurements of density revealed that the upper harbor fronts form along the margin of a freshwater lens produced by riverine input. The hypothesis that these frontal zones have higher densities of phytoplankton and zooplankton than adjacent water masses was tested using chlorophylla measurements and net collections. The fronts did not demonstrate any significant accumulations of phytoplankton or zooplankton during the ebb tide. The results of this study suggest that the physical characteristics of ebb-tidal estuarine fronts in Charleston Harbor are periodic in nature and may indirectly affect plankton transport in this coastal plain estuary.  相似文献   

4.
Zooplankton are an important trophic link and a key food source for many larval fish species in estuarine ecosystems. The present study documents temporal and spatial zooplankton dynamics in Suisun Bay and the Sacramento–San Joaquin Delta—the landward portion of the San Francisco Estuary (California, USA)—over a 37-year period (1972–2008). The zooplankton community experienced major changes in species composition, largely associated with direct and indirect effects of introductions of non-native bivalve and zooplankton species. A major clam invasion and many subsequent changes in zooplankton abundance and composition coincided with an extended drought and accompanying low-flow/high-salinity conditions during 1987–1994. In the downstream mesohaline region, the historically abundant calanoid copepods and rotifers have declined significantly, but their biomass has been compensated to some extent by the introduced cyclopoid Limnothoina tetraspina. The more upstream estuary has also experienced long-term declining biomass trends, particularly of cladocerans and rotifers, although calanoid copepods have increased since the early 1990s due to the introduced Pseudodiaptomus spp. In addition, mysid biomass has dropped significantly throughout the estuary. Shifts in zooplankton species composition have also been accompanied by an observed decrease in mean zooplankton size and an inferred decrease in zooplankton food quality. These changes in the biomass, size, and possibly chemical composition of the zooplankton community imply major alterations in pelagic food web processes, including a drop in prey quantity and quality for foraging fish and an increase in the importance of the microbial food web for higher trophic levels.  相似文献   

5.
The uptake and release of materials by intertidal mussel beds were directly measured in two cultivated Dutch estuaries. Generally, chlorophylla, seston, and particulate organic carbon were taken up, while ammonium, orthophosphate, and silicate were released. The observed rates were higher than values computed from organismic observations and similar to those observed for intertidal oyster reefs in South Carolina. Specific estuarine material turnover rates varied from 1 week to 38 weeks when calculated with mussel bed fluxes. The fastest turnover rates were for chlorophylla and ammonium. These results support the idea that dense assemblages of bivalves are major components in the recycling of nutrients in estuaries.  相似文献   

6.
Backwater tidal sloughs are commonly found at the landward boundary of estuaries. The Cache Slough complex is a backwater tidal region within the Upper Sacramento–San Joaquin Delta that includes two features that are relevant for resource managers: (1) relatively high abundance of the endangered fish, delta smelt (Hypomesus transpacificus), which prefers turbid water and (2) a recently flooded shallow island, Liberty Island, that is a prototype for habitat restoration. We characterized the turbidity around Liberty Island by measuring suspended-sediment flux at four locations from July 2008 through December 2010. An estuarine turbidity maximum in the backwater Cache Slough complex is created by tidal asymmetry, a limited tidal excursion, and wind-wave resuspension. During the study, there was a net export of sediment, though sediment accumulates within the region from landward tidal transport during the dry season. Sediment is continually resuspended by both wind waves and flood tide currents. The suspended-sediment mass oscillates within the region until winter freshwater flow pulses flush it seaward. The hydrodynamic characteristics within the backwater region such as low freshwater flow during the dry season, flood tide dominance, and a limited tidal excursion favor sediment retention.  相似文献   

7.
The fish assemblages of two South Carolina estuarine tide pools located in the North Edisto River were sampled from June to October 1977, and during June and July 1978, by haul seine, lift net and channel net. The nekton was dominated numerically byFundulus heteroclitus, Leiostomus xanthurus, Menidia menidia andAnchoa mitchilli. Oyster reef areas were dominated numerically byGobiosoma bosci andF. heteroclitus. The relative abundance of tide pool fishes differs from deep water areas of the North Edisto River as described from otter trawls, but is similar to that described by others for South Carolina tidal creeks.  相似文献   

8.
Abiotic factors and species introductions can alter food web timing, disrupt life cycles, and change life history expressions and the temporal scale of population dynamics in zooplankton communities. We examined physical, trophic, and zooplankton community dynamics in the San Francisco Estuary, California, a highly altered Mediterranean climate waterway, across a 43-year dataset (1972–2014). Before invasion by the suspension-feeding overbite clam (Potamocorbula amurensis) in the mid-1980s, the estuary demonstrated monomictic thermal mixing in which winter turbidity and cool temperatures contributed to seasonally low productivity, followed by a late-spring-summer clearing phase with warm water and peak phytoplankton blooms that continued into early winter. Following the clam invasion, we observed a shift in peak phytoplankton bloom timing, with peak productivity now occurring in May compared to June prior to the invasion. Peak abundance of several zooplankton taxa (Eurytemora affinis, Pseudodiaptomus, other calanoids, and non-copepods) also shifted to earlier in the season. We present the first evidence of a shift in the timing of peak abundance for zooplankton species that are key prey items of delta smelt (Hypomesus transpacificus), a federally threatened pelagic fish species. These timing shifts may have exacerbated well-documented food limitations of delta smelt due to declines in primary productivity since the invasion of the overbite clam. Future conservation efforts in the estuary should consider measures designed to restore the timing and magnitude of pre-invasion phytoplankton blooms.  相似文献   

9.
Changing light availability may be responsible for the discontinuous distribution of submersed aquatic macrophytes in the freshwater tidal Potomac River. During the 1985–1986 growing seasons, light attenuation and chlorophylla and suspended particulate material concentrations were measured in an unvegetated reach (B) and in two adjacent vegetated reaches (A and C). Light attenuation in reach B (the lower, fresh to oligohaline tidal river) was greater than that in reach A (the recently revegetated, upper, freshwater tidal river) in both years. Reach B light attenuation was greater than that in reach C (the vegetated, oligohaline to mesohaline transition zone of the Potomac Estuary) in 1985 and similar to that in reach C in 1986. In reach B, 5% of total below-surface light penetrated only an average of 1.3 m in 1985 and 1.0m in 1986, compared with 1.9 m and 1.4 m in reach A in 1985 and 1986, respectively. Water column chlorophylla concentration controlled light availability in reaches A and B in 1985, whereas both chlorophylla and suspended particulate material concentrations were highly correlated with attenuation in both reaches in 1986. Reach C light attenuation was correlated with suspended particulate material in 1986. The relationship between attenuation coefficient and Secchi depth was KPAR=1.38/Secchi depth. The spectral distribution of light at 1 m was shifted toward the red portion of the visible spectrum compared to surface light. Blue light was virtually absent at 1.0 m in reach B during July and August 1986. Tidal range is probably an important factor in determining light availability for submersed macrophyte propagule survival at the sediment-water interface in this shallow turbid system.  相似文献   

10.
The toxicPfiesteria complex are a group of dinoflagellates that have received considerable attention in recent years as causative factors in fish kill or lesion events in North Carolina estuaries and in the Pocomoke River of Chesapeake Bay. In response to the potentialPfiesteria threat, the South Carolina Task Group on Harmful Algae was formed in late 1997 and implemented programs to monitor harmful algal blooms and respond to fish kills or lesion events with particular emphasis on the Bushy Park (Cooper River, Charleston) region, a site of annually recurrent menhaden lesion events.Pfiesteria piscicida, Pfiesteria shumwayae, andCryptoperidiniopsis spp. were documented in South Carolina estuaries. Routine monitoring and fish kill or lesion event sampling consistently indicated low abundances compared to estimates from similar programs in North Carolina and Maryland that sampled areas with a history ofPfiesteria toxic activity. The finding thatPfiesteria-like organism (PLO) abundances were always low in samples collected during menhaden lesion events in Bushy Park suggested other causes for lesion progression, althoughPfiesteria spp. could not be ruled out as a factor in lesion initiation. Based on the previously demonstrated positive relationship between PLO abundance, chlorophylla, and inorganic nutrient concentrations (in laboratory experiments and North Carolina field observations), we hypothesized that the relatively low abundance ofPfiesteria spp. and other PLO (e.g.,Cryptoperidiniopsis) in South Carolina estuaries is related to the relatively low supply of phytoplankton prey, as supported by interstate comparisons in chlorophylla concentrations. Nitrate concentrations were generally much lower in South Carolina estuaries. Estuarine eutrophication may be an important consideration in explaining interstate differences in susceptibility toPfiesteria-related toxic events.  相似文献   

11.
The lower Neuse River Estuary is a temperate mesohaline system which forms the major southern tributary of Pamlico Sound, North Carolina. The crustacean zooplankton of this well-mixed system were sampled for a 20-month period from May 1988 through December 1989. A submersible pump was used to sample both the entire water column and the sediment surface. Seasonal dominants included the calanoid copepodsAcartia tonsa andParacalanus crassirostris in summer, the cyclopoid copepodOithona colcarva in fall, the cladoceranPodon polyphemoides in winter, and harpacticoid copepods in spring. Non-naupliar biomass over the study period consisted of 38.8%A. tonsa, 7.7%P. crassirostris, 21.2%O. colcarva 23.6% harpacticoid copepods, and 6.0% cladocerans. The remainder of the biomass consisted ofPseudodiaptomus coronatus and barnacle nauplii. Mean total copepod densities ranged from 600 m?3 in May 1988 to 180,000 m?3 in August 1988. Mean copepod densities for 1989 were 25,000 m?3. Maximum densities during both years occurred during summer, with subsequent descreases throughtout the year until early spring. Abundances of total copepods, and ofAcartia tonsa in particular, were significantly correlated with water temperature, but with neither chlorophylla, phytoplankton productivity, nor any of an array of other physical or chemical variables. Regression analyses using data from this investigation, and supported by results from other regional studies, indicate that water temperature is likely the single most important variable predicting zooplankton temporal abundance in North Carolina estuaries.  相似文献   

12.
Differences in phytoplankton community composition along a riverine to, freshwater tidal continuum was an important factor affecting the primary productivity and quantity of phytoplankton biomass available to the San Francisco Estuary food web downstream. The relative contribution of riverine and freshwater tidal phytoplankton was determined using measurements of primary productivity, respiration, and phytoplankton species composition along a riverine to freshwater tidal gradient in the San Joaquin River, one of two major rivers that flow into, the San Francisco Estuary. Chla-specific net primary productivity was greater in the freshwater tidal habitat and was correlated with both a higher growth efficiency and maximum growth potential compared with the river upstream. Cluster analysis indicated these differences in growth parameters were associated with differences in species composition, with greater percent diatom and green algal species biomass upstream and flagellate biomass downstream. Correlation between the chla specific net productivity and phytoplankton species composition suggested the downstream shift from riverine diatom and green algal species to flagellate species contributed to the seaward increase in net primary productivity. Environmental conditions, such as specific conductance and water transparency, may have influenced primary productivity along the riverine to freshwater tidal continuum through their effect on both species composition and growth rate. Data suggest light was not the sole controlling factor for primary productivity in this highly turbid estuary; phytoplankton growth rate did not increase when riverine plankton communities from low light conditions upstream were exposed to higher light conditions downstream. This study suggests that the availability of phytoplankton biomass to the estuarine food web may be influenced by management of both phytoplankton growth and community composition along the riverine to freshwater tidal continuum.  相似文献   

13.
During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 108 cells l?1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on 14C-labeled river assemblages, 14C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with 14C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]?1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]?1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation.  相似文献   

14.
Plant pigment decay constants were determined for four macrophytes collected from the Hudson Estuary.Typha angustifolia andScirpus fluviatilis were used as representatives of emergent aquatic vegetation (EAV), andPotamogeton sp. andVallisneria americana were used to represent submerged aquatic vegetation (SAV). Litter bags were maintained in an environmental chamber in the dark for 104 d. The fastest rate of total mass loss was in the SAVV. americana and slowest in the EAVT. angustifolia. Changes in carotenoid and chloropigment concentration resulting from microbial and meiofaunal heterotrophy in each of the macrophytes were quantified using reverse-phase, high-performance liquid chromatography (RP-HPLC) techniques. Chlorophyllc and the carotenoid, fucoxanthin, provided useful biomarkers in determining the presence of epiphytic diatom growth, which only occurred on the SAV. The highest concentrations of phaeophorbidea, commonly used as an indication of metazoan grazing, were found in the SAVV. americana. Low concentrations of phaeophorbidea in the SAVPotamogeton sp. indicate inefficient use of this SAV by meiofaunal grazers. Lutein decayed slower than all other carotenoids in both EAV and SAV. Microcosm studies such as this are necessary to further understand the mechanisms and kinetics of photosynthetic pigment transformations in natural systems.  相似文献   

15.
Macrotidal estuaries (mean tidal range >2m) generally exhibit a tolerance to pollution with nitrogen-containing nutrients despite high loadings originating from freshwater outflows. These systems, which are characterized by high tidal energy, generally exhibit lower levels of chlorophylla than systems with lower tidal energy. A comparative analysis of data from 40 microtidal and macrotidal estuaries shows that mean annual chlorophylla levels are significantly lower in systems with high tidal energy even when nitrogen concentrations are equal to nitrogen levels in the microtidal systems. Tidal range and associated processes (e.g., tidal mixing, current velocity, light penetration, and sediment resuspension) influence phytoplankton biomass in some estuaries.  相似文献   

16.
The purpose of this study was to determine the history of paleosalinity over the past 6000+ years in the Sacramento-San Joaquin Delta (the Delta), which is the innermost part of the San Francisco Estuary. We used a combination of Sr and U concentrations, δ87Sr values, and 234U/238U activity ratios (AR) in peat as proxies for tracking paleosalinity. Peat cores were collected in marshes on Browns Island, Franks Wetland, and Bacon Channel Island in the Delta. Cores were dated using 137Cs, the onset of Pb and Hg contamination from hydraulic gold mining, and 14C. A proof of concept study showed that the dominant emergent macrophyte and major component of peat in the Delta, Schoenoplectus spp., incorporates Sr and U and that the isotopic composition of these elements tracks the ambient water salinity across the Estuary. Concentrations and isotopic compositions of Sr and U in the three main water sources contributing to the Delta (seawater, Sacramento River water, and San Joaquin River water) were used to construct a three-end-member mixing model. Delta paleosalinity was determined by examining variations in the distribution of peat samples through time within the area delineated by the mixing model.The Delta has long been considered a tidal freshwater marsh region, but only peat samples from Franks Wetland and Bacon Channel Island have shown a consistently fresh signal (<0.5 ppt) through time. Therefore, the eastern Delta, which occurs upstream from Bacon Channel Island along the San Joaquin River and its tributaries, has also been fresh for this time period. Over the past 6000+ years, the salinity regime at the western boundary of the Delta (Browns Island) has alternated between fresh and oligohaline (0.5–5 ppt).  相似文献   

17.
Early in 1976 benthic studies were initiated in a 20 kilometer long portion of the Western Sacramento-San Joaquin River Estuary. Water quality determinations indicated little vertical or horizontal differences in pH, temperature, or dissolved oxygen concentration within the study area. Low river outflows allowed the encroachment of seawater into the study area, an area normally exposed to fresh or slightly brackish water. The sediment composition changed dramatically at most stations during the year, being dominated by sands early in the year but by silts and clays in late summer. The shift in sediment composition was accompanied by an increase in grease and oilland metals content. The benthic community of the study area was generally dominated by the Asiatic clam (Corbicula manilensis), Macoma balthica, oligochaetes, the amphipods Corophium stimpsoni and C. spinicorne, nematodes, and a spionid polychaete, Boccardia ligerica. These taxa comprised 98% on average of the total benthic macroinvertebrates collected at each study site. The benthic assemblages of each of the stations were generally very similar to one another. Faunal similarities and changes in benthos composition were related to substrate composition and salinity incursion. In general, the upstream-channel stations had higher abundance of benthos than the other stations in the study area. Total benthic abundance was lowest at the downstream end of the study area. Total standing crop peaked in June and was lowest in November. Our studies indicate that the most important factors controlling the size and species composition of the benthos of the study area are salinity and sediment composition.  相似文献   

18.
Zebra mussels (Dreissena polymorpha) graze on phytoplankton, and decreased phytoplankton concentrations have been associated with zebra mussels in lakes. It is not known, however, how the zebra mussel will affect phytoplankton in turbid systems such as rivers and the freshwater portions of estuaries. To determine whether zebra mussels can effectively remove phytoplankton in these turbid systems, and to determine what components of the suspended material are removed and at what rates, we conducted a series of grazing and size-selection experiments using ambient Hudson River water and its natural phytoplankton community. Zebra mussels removed both phytoplankton and total suspended weight (TSW) at comparable rates (～115 ml mussel?1 h?1). Variation in filtration rates were not correlated with TSW or chlorophylla (chla) concentration, and did not appear to depend on relative proportions of either component. Mussels removed particles with approximately equal efficiency in all particle size classes measured (0.4 μm to >40 μm). Zebra mussels appear to remove Hudson River phytoplankton effectively in the presence of suspended sediment and do so at rapid rates. Based on our measurements and unpublished estimates of the size of the population, zebra mussels filter a volume equivalent to the entire volume of the tidal freshwater portion of the Hudson River about every 2 d.  相似文献   

19.
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.  相似文献   

20.
Increased frequency and severity of droughts, as well as growing human freshwater demands, in the Apalachicola-Chattahoochee-Flint River Basin are expected to lead to a long-term decrease in freshwater discharge to Apalachicola Bay (Florida). To date, no long-term studies have assessed how river discharge variability affects the Bay’s phytoplankton community. Here a 14-year time series was used to assess the influence of hydrologic variability on the biogeochemistry and phytoplankton biomass in Apalachicola Bay. Data were collected at 10 sites in the bay along the salinity gradient and include drought and storm periods. Riverine dissolved inorganic nitrogen and phosphate inputs were correlated to river discharge, but chlorophyll a (Chl a) was similar between periods of drought and average/above-average river discharge in most of the Bay. Results suggest that the potentially negative impact of decreased riverine nutrient input on Bay phytoplankton biomass is mitigated by the nutrient buffering capacity of the estuary. Additionally, increased light availability, longer residence time, and decreased grazing pressures may allow more Chl a biomass to accumulate during drought. In contrast to droughts, tropical cyclones and subsequent increases in river discharge increased flushing and reduced light penetration, leading to reduced Chl a in the Bay. Analysis of the time series revealed that Chl a concentrations in the Bay do not directly mirror the effect of riverine nutrient input, which is masked by multiple interacting mechanisms (i.e., nutrient loading and retention, grazing, flushing, light penetration) that need to be considered when projecting the response of Bay Chl a to changes in freshwater input.  相似文献   

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