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1.
An enclosure experiment in the Patos Lagoon estuary, southern Brazil (32°S, 52°W) investigated the response of phytoplankton to the enhancement of mesozooplankton and fish abundance. Addition of nutrients (NO3 ? and PO4 ?) stimulated the growth of 3–20 μm diameter phytoplankton, especially the diatomCylindrotheca closterium, which, in turn, was heavily grazed by the dominant mesozooplankter, the copepodAcartia tonsa. Acartia did not consume small (2–3 μm) autotrophic flagellates and the cyanobacteriaAnabaena sp., despite their high cell number. Largest grazing ofC. closterium by the copepod occurred only after a decrease of ciliate abundance. The addition of mesozooplankton did not change the levels of primary production, but it significantly increased the phytoplankton assimilation number. Highest chlorophylla concentrations were measured in enclosures stocked with juveniles of the fishXenomelaniris brasiliensis. The large phytoplankton biomass and low mesozooplankton abundance found in this treatment indicates an, effective predator-mediated action on the phytoplankton community.  相似文献   

2.
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   

3.
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   

4.
We estimated the influence of planktonic and benthic grazing on phytoplankton in the strongly tidal, river-dominated northern San Francisco Estuary using data from an intensive study of the low salinity foodweb in 2006–2008 supplemented with long-term monitoring data. A drop in chlorophyll concentration in 1987 had previously been linked to grazing by the introduced clam Potamocorbula amurensis, but numerous changes in the estuary may be linked to the continued low chlorophyll. We asked whether phytoplankton continued to be suppressed by grazing and what proportion of the grazing was by benthic bivalves. A mass balance of phytoplankton biomass included estimates of primary production and grazing by microzooplankton, mesozooplankton, and clams. Grazing persistently exceeded net phytoplankton growth especially for larger cells, and grazing by microzooplankton often exceeded that by clams. A subsidy of phytoplankton from other regions roughly balanced the excess of grazing over growth. Thus, the influence of bivalve grazing on phytoplankton biomass can be understood only in the context of limits on phytoplankton growth, total grazing, and transport.  相似文献   

5.
Major interactions between terrestrial and marine environments in the Kara Sea occur within the estuaries of the largest Siberian rivers, the Ob and Yenisei. Mesozooplankton community plays an important role in the transformation of allochthonous organic matter. All published data on zooplankton activity in the Ob Estuary have been obtained for the period of decreased river discharge. The aim of our study was to assess zooplankton distribution and grazing under various hydrological regimes (high-low river discharge and varying wind direction) in order to better understand the mechanisms governing this process. The study was carried out along a quasi-latitudinal transect in the Ob Estuary at the beginning of August (high discharge) and end of September 2010 (decreased discharge) and end of August 2014 (high discharge and onshore winds). Zooplankton grazing was assessed with the gut fluorescent approach. Under high river discharge, zooplankton biomass was low (mean 98 mg wet weight m?3), peaks of species abundance were spatially separated, and grazing did not exceed 2% of phytoplankton biomass. Weakening river discharge at the end of September led to the formation of hydrographic fronts, and zooplankton biomass was an order of magnitude higher (mean value 947 mg wet weight m?3) with dense local aggregations with biomass reaching 3600 mg wet weight m?3. These aggregations formed a pelagic “biofilter” grazing up to 26% of phytoplankton biomass per day. The peaks of abundance of the majority of species coincided at the pronounced hydrographic front forming dense local aggregations with biomass reaching 3600 mg wet weight m?3. These aggregations formed a pelagic biofilter utilizing daily up to 26% of phytoplankton biomass.  相似文献   

6.
The Mattaponi River is part of the York River estuary in Chesapeake Bay. Our objective was to identify the organic matter (OM) sources fueling the lower food web in the tidal freshwater and oligohaline portions of the Mattaponi using the stable isotopes of carbon (C) and nitrogen (N). Over 3 years (2002–2004), we measured zooplankton densities and C and N stable isotope ratios during the spring zooplankton bloom. The river was characterized by a May–June zooplankton bloom numerically dominated by the calanoid copepod Eurytemora affinis and cladocera Bosmina freyi. Cluster analysis of the stable isotope data identified four distinct signatures within the lower food web: freshwater riverine, brackish water, benthic, and terrestrial. The stable isotope signatures of pelagic zooplankton, including E. affinis and B. freyi, were consistent with reliance on a mix of autochthonous and allochthonous OM, including OM derived from vascular plants and humic-rich sediments, whereas macroinvertebrates consistently utilized allochthonous OM. Based on a dual-isotope mixing model, reliance on autochthonous OM by pelagic zooplankton ranged from 20% to 95% of production, declining exponentially with increasing river discharge. The results imply that discharge plays an important role in regulating the energy sources utilized by pelagic zooplankton in the upper estuary. We hypothesize that this is so because during high discharge, particulate organic C loading to the upper estuary increased and phytoplankton biomass decreased, thereby decreasing phytoplankton availability to the food web.  相似文献   

7.
Florida Bay is Florida’s (USA) largest estuary and has experienced harmful picocyanobacteria blooms for nearly two decades. While nutrient loading is the most commonly cited cause of algal blooms in Florida Bay, the role of zooplankton grazing pressure in bloom occurrence has not been considered. For this study, the spatial and temporal dynamics of cyanobacteria blooms, the microbial food web, microzooplankton and mesozooplankton grazing rates of picoplankton, and the effects of nutrients on plankton groups in Florida Bay were quantified. During the study, cyanobacteria blooms (>3 × 105 cells mL−1) persisted in the eastern and central regions of Florida Bay for more than a year. Locations with elevated abundance of cyanobacteria hosted microzooplankton grazing rates on cyanobacteria that were significantly lower (p < 0.001) and less frequently detectable compared to sites without blooms. Consistent with this observation, cyanobacteria abundances were significantly correlated with ciliates and heterotrophic nanoflagellates at low cyanobacteria densities (p < 0.001) but were not correlated during bloom events. The experimental enrichment of mesozooplankton abundance during blooms yielded a significant decrease in the net growth rate of picoplankton but had the opposite effect when blooms were absent, suggesting that the cascading effect of mesozooplankton grazing on the microbial food web was also altered during blooms. While inorganic nutrient enrichment significantly increased the net growth rates of eukaryotic phytoplankton and heterotrophic bacteria, such nutrient loading had no effect on the net growth rates of cyanobacteria. Hence, this study demonstrates that low rates of zooplankton grazing and low rates of inorganic nutrient loading contribute to the persistence of cyanobacteria blooms in Florida Bay.  相似文献   

8.
Twenty-one dilution method experiments were used to measure phytoplankton growth rate, grazing rate by microzooplankton, and phytoplankton concentrations that saturate grazing in Tuggerah Lake??a large lagoon in New South Wales, Australia. Individual experiments conformed to the saturating grazing model with no evidence of a threshold phytoplankton concentration to initiate grazing. Phytoplankton concentrations that saturated grazing were highly variable between experiments and were positively correlated with chlorophyll a concentration in the lagoon. Plankton models often use a saturating grazing function that includes several constants, but constants are found to be variable from one dilution experiment to the next. Another formulation is proposed in which grazing is a quadratic function of phytoplankton growth. This enables the 21 measurements of zooplankton grazing to be fitted using only two invariant parameters. No evidence is found for saturation of microzooplankton grazing when it is calculated as a function of phytoplankton growth. When phytoplankton growth is high, about 80% of it is grazed. When phytoplankton growth is low, about 45% is grazed. Calculations illustrate that this type of grazing stabilizes the planktonic producers and grazers, as expected.  相似文献   

9.
We summarize rates of metabolism and major sources and sinks of organic carbon in the 148-k long, tidally influenced, freshwater Hudson River. The river is strongly heterotrophic, with respiration exceeding gross primary production (GPP). The P:R ration averages 0.57 (defined as the ratio of GPP to total ecosystem respiration) if only the aquatic portion of the ecosystem is considered and 0.70 if the emergent marshes are also included. Gross primary production (GPP) by photoplankton averages approximately 300 g C m?2 yr?1 and is an order of magnitude greater than that by submersed macrophytes. However, the river is deep, well mixed, and turbid, and phytoplankton spend a majority of their time in the dark. As a result, respiration by living phytoplankton is extremely high and net primary production (NPP) by phytoplankton is estimated to be only some 6% of GPP. NPP by phytoplankton and submersed macrophytes are roughly equal (approximately 20 g C m?2 yr?1 each) when averaged over the river. Emergent marshes are quite productive, but probably less than 16 g C m?2 yr?1 enters the aquatic portion of the ecosystem from these marshes. Heterotrophic respiration and secondary production in the river are driven primarily by allochthonous inputs of organic matter from terrestrial sources. Rates of metabolism vary along the river, with depth being a critical controlling factor. The P:R ratio for the aquatic portion of the ecosystem varies from 1 in the mid-river to 0.2 in the deeper waters. NPP is actually negative in the downstream waters where average depths are greater since phytoplankton respiration exceeds GPP there; the positive rates of NPP occurring upriver support a downstream advection of phytoplankton to the deeper waters where this C is largely respired away by the algae themselves. This autotrophic respiration contributes significantly to oxygen depletion in the deeper waters of the Hudson. The tidally influenced freshwater Hudson largely fits the patterns predicted by the river continuum model for larger rivers. However, we suggest that the continuum model needs to more clearly distinguish between GPP and NPP and should include the importance of autotrophic respiration by phytoplankton that are advected along a river. The organic carbon budget for the tidally influenced freshwater Hudson is balanced to within a few percent. Respiration (54%) and downstream advection into the saline estuary (41%) are the major losses of organic carbon from the ecosystem. Allochthonous inputs from nonpoint sources on land (61%) and GPP by phytoplankton (28%) are the major sources to the system. Agricultural erosion is the major source of allochthonous inputs. Since agricultural land use increased dramatically in the last century, and has fallen in this century, the carbon cycle of the tidally influenced freshwater Hudson River has probably changed markedly over time. Before human disturbance, the Hudson was probably a less heterotrophic system and may even have been autotrophic, with gross primary production exceeding ecosystem respiration.  相似文献   

10.
Responses ofSpartina alterniflora marsh to combinations of feral horse grazing, clipping, simulated trampling, and a late winter burn were studied on Cumberland Island National Seashore, Georgia. Replicated 200-m2 plots were established and sampled bimonthly from July 1983 to November 1984. Clipping and trampling each reduced peak aboveground biomass by 20% in 1983 and 50% (clipping) and 55% (trampling) in 1984. A March burn reduced peak aboveground biomass by 35% in 1984. Trampling and burning earch reduced net aboveground primary production (NAPP) by 35%, but clipping did not reduce NAPP. Standing stocks of live rhizomes were correlated with aboveground biomass and were reduced with experimental treatments. Abundance of the periwinkle snail (Littorina irrorata) was also reduced. Horse grazing had a substantial impact on standing stocks and NAPP ofSpartina, but grazing was not uniform throughout the marsh. Moderately grazed plots had NAPP reduced by 25% compared to ungrazed plots. Heavily grazed plots had extremely low NAPP, and abovegroundSpartina never exceeded 40 g m?2 dry mass compared to 360 g m?2 within exclosures.  相似文献   

11.
The Atlantic menhaden,Brevoortia tyrannus, is an abundant plankton-feeding fish that undertakes extensive seasonal migrations, moving from overwintering locations offshore south of Cape Hatteras to the mid-Atlantic Bight and New England inshore waters during spring and summer. A bioenergetic model, based on field and laboratory studies, shows that when large numbers of menhaden enter Narragansett Bay, Rhode Island, during spring and early summer, they significantly influence plankton populations through size-selective grazing and nutrient regeneration. A population biomass of 9.1×106 kg of menhaden feeding for 12 h each day in the upper bay would result, in a substantial reduction of the instantaneous growth rate of the >20-μm phytoplankton. Instantaneous growth rates of zooplankton would be negative if the same population of menhaden was present, resulting in a reduction in the biomass of zooplankton. Given the ambient phytoplankton and zooplankton populations, menhaden could achieve the seasonal growth measured in Narragansett Bay during 1976 by feeding on average about 5 h d?1. Daily nitrogen excretion rates of the 9.1×106 kg menhaden population were 56.4% of the mean standing stock of ammonia-N in the upper bay. Because menhaden travel in schools their effects are likely to be intense but strongly localized, increasing spatial heterogeneity in the ecosystem. When the fish migrate southward in the fall they transfer between 3.3% and 6.2% of the nitrogen exported annually from the bay.  相似文献   

12.
Observations of the composition and rate of input of organic matter to the sea floor were made at three locations in lower Cook Inlet, Alaska, during five cruises taken in the spring and summer of 1978. Total particulate, plant pigment, carbon, nitrogen, fecal pellet, and phytoplankton cell fluxes, inferred from sediment trap samples, were related to algal biomass and production in overlying waters. A daily average of 7.5% of the phytoplankton biomass was lost to the bottom. Of this loss, 83% was attributable to zooplankton grazing and fecal pellet production. At the three sampling sites, an average of 39 g C m?2 (range of 17–60 g C m?2, was sedimented to the bottom between May and August. This carbon flux represented an average of 12% of the total primary production measured for that time period. Kachemak Bay eastern arm of the inlet, is identified as an extremely productive embayment in which large amounts of organic matter were transferred to the sea floor.  相似文献   

13.
Data were collected in the Tagus estuary from 1999–2007 on a monthly basis and combined with published results and for several previous years between 1980 and 1995, so that a comprehensive analysis could be performed over a non-continuous 27-year period. Sampling conditions and methods were similar for all datasets. Extreme wet and dry years were observed. River flow was strongly linked to phytoplankton abundance, with the highest biomass attained in dry years. The observed range of annual median Chl a was 1.8–7.6 µg L?1 and the overall median was 3.5 µg L?1. Dissolved inorganic nitrogen (DIN) and silicate showed a clear seasonal pattern, with a maximum in winter?spring, indicating a freshwater origin. Although wastewater treatment started in 1990, no difference was detected from 1980 to the present in terms of DIN and phosphorus. The recorded seasonal pattern for biomass with highest values in late spring–summer period is comparable to other temperate tidally influenced ecosystems. In spite of interannual differences in terms of Chl a concentration or the time of the maximum Chl a occurrence, a repeatable pattern could be identified. The mean growth development time for phytoplankton was 163 days (June 12) ranging 129–206 days (May 9–July 26) during the sampling period. No obvious changes in phytoplankton community structure were observed over time: diatoms were always the dominant group, and cryptophytes were relatively abundant throughout autumn–winter. The dominant species have remained essentially the same since 1969. River inflow, light availability, and temperature were the major factors shaping phytoplankton variability patterns. The strong influence of tidal mixing on the estuarine waters appears to lower the risk of potential eutrophication in the Tagus estuary. The lack of change in nutrients and phytoplankton biomass and composition observed in this study is an important contribution towards the assessment of natural variability versus responses to man-induced inputs in this severely anthropogenically disturbed estuary.  相似文献   

14.
During 1995 the phytoplankton in the Swan River were intensively sampled to assess biomass and species composition. Continuous measurements of fluorescence, salinity, and temperature were made weekly during 40 km sampling trips along the estuary and used to map the seasonal progression of the algal biomass. Weekly measurements of primary production were made and used to model net primary production from the vertical distribution of biomass, irradiance, and phytoplankton species composition. Potential nutrient limitation was assessed with “all but one” nutrient bioassays. The results indicate a complex mixture of potentially limiting factors, which vary in time and space. Although the data sequence is short, it suggests a annual succession pattern of diatoms, chlorophytes, diatoms, and finally dinoflagellates and cryptophytes in late summer-autumn. Peak seasonal biomass was observed during January to April. Mean annual chlorophylla biomass was greatest in upstream stations (5–9), where estimates of net primary production rates averaged 1.55 g C m?2 d?1 and gross primary production was 800–1000 g C m?2 yr?1. Potential nutrient limitation was most severe from November to May, although not during January 1995. Based on bioassay results, during the period of greatest potential for nutrient limitation, nitrogen was 15 to 30 times more limiting to biomass development than phosphate. Runoff due to consistent rainfall during winter eventually breaks down stratification and flushes the estuary with low-salinity, nutrient-rich water, producing, a light-limited, nutrient-rich aquatic ecosystem. Timing and magnitude of physical forcing events, mainly rainfall, appear critical in determining the susceptibility of this ecosystem to summer and autumn algal blooms.  相似文献   

15.
Primary productivity of three size classes of phytoplankton (<5 μm, 5–22 μm, >22 μm) was measured monthly at six sites within San Francisco Bay throughout 1980. These sites in the three principal embayments were chosen to represent a range of environments, phytoplankton communities, and seasonal cycles in the estuary. Temporal variations in productivity for each size class generaly followed the seasonality of the corresponding fraction of phytoplankton biomass. The 5–22 μm size class accounted for 40 to 50% of the annual production in each embayment, but production by phytoplankton >22 μm ranged from 26% in the southern reach to 54% of total phytoplankton production in the landward embayment of the northern reach. A productivity index is derived that predicts daily productivity for each size class as a function of ambient irradiance and integrated chlorophylla in the photic zone. For the whole phytoplankton community and for each size class, this index was constant and estimated as ?0.76 g C m?2 (g chlorophylla Einstein)?1. The annual means of maximum carbon assimilation numbers were usually similar for the three size classes. Spatial and temporal variations in size-fractionated productivity are shown to be primarily due to differences in biomass rather than size-dependent carbon assimilation rates. *** DIRECT SUPPORT *** A01BY034 00005  相似文献   

16.
The effects of advection, dispersion, and biological processes on nitrogen and phytoplankton dynamics after a storm event in December 2002 are investigated in an estuary located on the northern New South Wales coast, Australia. Salinity observations for 16 d after the storm are used to estimate hydrodynamic transports for a one-dimensional box model. A biological model with nitrogen limited phytoplankton growth, mussel grazing, and a phytoplankton mortality term is forced by the calculated transports. The model captured important aspects of the temporal and spatial dynamics of the bloom. A quantitative analysis of hydrodynamic and biological processes shows that increased phytoplankton biomass due to elevated nitrogen loads after the storm was not primarily regulated by advection or dispersion in spite of an increase in river flow from <1 to 928×103 m3 d−1. Of the dissolved nitrogen that entered the surface layer of the estuary in the 16 d following the storm event, the model estimated that 28% was lost through exchange with the ocean or bottom layers, while 15% was removed by the grazing of just one mussel species,Xenostrobus securis, on phytoplankton, and 50% was lost through other biological phytoplankton loss processes.X. securis grazing remained an important loss process even when the estimated biological parameters in the model were varied by factors of ± 2. The intertidal mangrove pneumatophore habitat ofX. securis allows filtering of the upper water column from the lateral boundaries when the water column is vertically stratified, exerting top-down control on phytoplankton biomass.  相似文献   

17.
Estuaries located in the northern Gulf of Mexico are expected to experience reduced river discharge due to increasing demand for freshwater and predicted periods of declining precipitation. Changes in freshwater and nutrient input might impact estuarine higher trophic level productivity through changes in phytoplankton quantity and quality. Phytoplankton biomass and composition were examined in Apalachicola Bay, Florida during two summers of contrasting river discharge. The <20 μm autotrophs were the main component (92?±?3 %; n?=?14) of phytoplankton biomass in lower (<25 psu) salinity waters. In these lower salinity waters containing higher dissolved inorganic nutrients, phycocyanin containing cyanobacteria made the greatest contribution to phytoplankton biomass (69?±?3 %; n?=?14) followed by <20 μm eukaryotes (19?±?1 %; n?=?14), and phycoerythrin containing cyanobacteria (4?±?1 %; n?=?14). In waters with salinity from 25 to 35 psu that were located within or in close proximity to the estuary, >20 μm diatoms were an increasingly (20 to 70 %) larger component of phytoplankton biomass. Lower summer river discharges that lead to an areal contraction of lower (5–25 psu) salinity waters composed of higher phytoplankton biomass dominated by small (<20 μm) autotrophs will lead to a concomitant areal expansion of higher (>25 psu) salinity waters composed of relatively lower phytoplankton biomass and a higher percent contribution by >20 μm diatoms. A reduction in summer river discharge that leads to such a change in quantity and quality of estuarine phytoplankton available will result in a reduction in estuarine zooplankton productivity and possibly the productivity of higher trophic levels.  相似文献   

18.
The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d−1) and phytoplankton growth rates (0.70–1.62 d−1) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012  相似文献   

19.
In the tidal Potomac River, high river discharges during the spring are associated with high chlorophylla concentrations in the following in the following summer, assuming that summertime light and temperature conditions are favorable. Spring floods deliver large loads of particulate N and P to the tidal river. This particulate N and P could be mineralized by bacteria to inorganic N and P and released to the water column where it is available for phytoplankton use during summertime. However, during the study period relatively low concentrations of chlorophylla (less than 50 μg l?1 occurred in the tidal river if average monthly discharge during July or August exceeded 200 m3s?1. Discharge and other conditions combined to produce conditions favorable for nuisance levels of chlorophylla (greater than 100 μg l?1 approximately one year out of four. Chlorophylla maxima occurred in the Potomac River transition zone and estuary during late winter (dinoflagellates) and spring (diatoms). Typical seasonal peak concentrations were achieved at discharges as high as 970 m3 s?1, but sustained discharges greater than 1,100 m3 s?1 retarded development. Optimum growth conditions occurred following runoff events of 10 to 15 d duration which produced transit times to the transition zone of 7 to 10 d. Wet years with numerous moderate-sized runoff events, such as 1980, tend to produce greater biomass in the transition zone and estuary than do dry years such as 1981.  相似文献   

20.
In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, 15NH4 + and 15NO3 ? uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH4 + uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH4 + uptake ranged from 0.001 μmol N 1?1 h?1 to 4.2 μmol N 1?1 h?1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH4 + uptake was significantly related to NH4 + concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH4 + uptake ranged from 0.1 mmol N m?2 d?1 to 133 mmol N m?2 d?1 and followed the trend of apparent uptake. Annual NH4 + uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO3 ? was only 14% of maximum light uptake. Apparent NO3 ? uptake rates ranged from 0.001 μmol N 1?1 h?1 to 1.84 μmol N 1?1 h?1 with highest rates occurring in the oligohaline estuary. Apparent NO3 ? uptake was significantly related to NO3 ? concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO3 ? uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO3 ? uptake ranged from 0.1 mmol N m?2 d?1 to 53 mmol N m?2 d?1 and followed similar patterns of apparent uptake. Annual NH4 + uptake was 11.39 mol N m?2 yr?1, 10.28 mol N m?2 Yr?1, 10.93 mol N m?2 yr?1, and 7.38 mol N m?2 yr?1, and 1.84 mol N m?2 yr?1, with the 4-yr mean being 10.0. Annual NO3 ? uptake was 3.12 mol N m?2 yr?1, 3.40 mol N m?2 yr?1, 1.96 mol N m?2 yr?1, and 1.84 mol N m?2 yr?1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.  相似文献   

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