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1.
Microzooplankton dilution grazing experiments were conducted with water collected from Pensacola Bay, Florida (USA) on 12 dates at 2 sites. Statistically significant grazing rates were observed in 22 of 24 experiments. Grazing rates in Upper Bay and Lower Bay were similar averaging 0.54 and 0.51 d−1, respectively. Phytoplankton growth rates were also similar at the two sites, averaging 1.02 and 1.00 d−1 at Upper Bay and Lower Bay, respectively. Phytoplankton growth rates usually exceeded grazing rates by about a factor of two, though microzooplankton grazing represented a significant mortality for phytoplankton. The literature suggests a linkage between phytoplankton growth and microzooplankton grazing that spans a wide variety of aquatic environments. While individual growth and grazing rates were variable, growth frequently exceeded grazing by about two-fold. This implies that the role of microzooplankton is similar across a wide variety of aquatic systems.  相似文献   

2.
The biomass of phytoplankton, microzooplankton, copepods, and gelatinous zooplankton were measured in two tributaries of the Chesapeake Bay during the springs of consecutive dry (below average freshwater flow), wet (above average freshwater flow), and average freshwater flow years. The potential for copepod control of microzooplankton biomass in the dry and wet years was evaluated by comparing the estimated grazing rates of microzooplankton by the dominant copepod species (Acartia spp. andEurytemora affinis) to microzooplankton growth rates and by calculating the percent of daily microzooplanton standing stock removed through copepod grazing. There were significant increases in phytoplankton and copepod biomass, but not for microzooplankton biomass in the wet year as compared to the dry year. The ctenophoreMnemiopsis leidyi was present during the dry year but was absent during the sampling period of the wet and average freshwater flow years. Grazing pressure on microzooplankton was greatest in the wet year, withAcartia spp. andE. affinis ingesting 0.21–2.64 μg of microzooplankton C copepod−1 d−1 and removing up to 60% of the microzooplankton standing stock per day. In the dry year, these copepod species ingested 0.10–0.73 μg of microzooplankton C copepod−1 d−1 with a maximum daily removal of approximately 3% of the microzooplankton standing stock. Potential copepod grazing pressure was significantly less than microzooplankton growth in the dry year, but was equivalent to microzooplankton growth in the wet year, implying strong top-down control of the microzooplankton community in the wet year. These results suggest that increased grazing control of microzooplankton populations by more copepods in the wet year released top-down control of phytoplankton. Reduced microzooplankton grazing, in conjunction with increased nutrient availability, resulted in large increases in phytoplankton biomass in the wet year. Increased freshwater flow has the potential to influence trophic cascades and the partitioning of plankton production in estuarine systems.  相似文献   

3.
Phosphorus (P) species concentrations in 0–2 cm surface sediment layer were investigated monthly from November 2001 to December 2002 at the bay, channel and open sea stations in the middle Adriatic. Modified SEDEX method was used for inorganic phosphorus species determination [P in biogenic (P-FD), authigenic (P-AUT), detrital apatite (P-DET) and P adsorbed on to iron oxides and hydroxides (P–Fe)], and organic phosphorus (P-ORG). P-FD, P-AUT and P-DET concentration ranges (1.5–5.4, 0–2.7 and 0.4–3.4 μmol g−1, respectively) were similar at all stations, and showed no obvious common trend of seasonal changes. P–Fe ranged from 1.9 to 11.9 μmol g−1 with the highest values at bay station and higher seasonal oscillations than other inorganic P forms. P-ORG ranged from 0.3 to 18.7 μmol g−1 with higher concentrations at stations of fine-sized sediments and showed increased concentrations in warm part of the year at all stations. Correlation between concentrations of P–Fe in the surface sediment layer and orthophosphate sediment-water interface concentration gradients at bay and channel stations indicated to P–Fe importance in the orthophosphate benthic flux. For the bay station, linkage between sediment P-ORG and chlorophyll a concentrations, primary production and microzooplankton abundance was established, indicating a 1 month delay of sediment response to production fluctuations in the water column.  相似文献   

4.
Denitrification rates along a salinity gradient in the eutrophic Neuse River Estuary, North Carolina, were quantified using membrane inlet mass spectrometry (MIMS) within short-term batch incubations. Denitrification rates within the system were highly variable, ranging from 0 to 275 μmol N m−2 h−1. Intrasite variability increased with salinity, but no significant differences were observed across the salinity gradient. Denitrification rates were positively correlated with sediment oxygen demand at the upstream sampling site where sediment organic carbon levels were lowest. This relationship was not observed in the more saline sampling sites. Denitrification rates were highest during winter. On an annual basis, denitrification accounted for 26% of the dissolved inorganic nitrogen and 12% of the total nitrogen supplied to the system.  相似文献   

5.
Decline of native pelagic species in estuarine systems is an increasing problem, especially for native fishes in the San Francisco Estuary and Delta (SFE-D). Addressing these losses depends on understanding trophodynamics in the food web that supports threatened species. We quantified the role of microzooplankton (heterotrophic–mixotrophic protists <200 μm) in the food web of the upper SFE-D. We sampled protist plankton abundance and composition at two sites (Suisun Bay and Grizzly Bay) approximately monthly from February 2004 to August 2005 and conducted dilution experiments during spring and summer of both years in Suisun Bay. Heterotrophs dominated the protist community in Suisun Bay and Grizzly Bay, particularly in the <20 μm size range, and peaks in protistan microzooplankton biomass were associated with high phytoplankton biomass. In both years, microzooplankton grazing rates were high (0.5–0.7 day−1) during the spring and lower (~0.2 day−1) during summer. Phytoplankton growth rates peaked in April 2004 (~0.7 day−1) but were much lower (<0.1 day−1) in spring 2005, despite relatively high abundance. Thus, microzooplankton grazing consumed as much as 73% of phytoplankton standing stock during spring and ~15% of standing stock during summer of both years. Combined with earlier results, we conclude that microzooplankton can be important mediators of carbon and energy flow in the upper SFE-D and may be a “source” to the metazoan food web.  相似文献   

6.
Two different approaches to measuring phytoplankton nitrogen (N) use were compared in late summer 2004 along the main axis of Chesapeake Bay. Uptake of 15N-labeled ammonium and nitrate and dual-labeled (15N and 13C) urea and dissolved free amino acids (DFAA) were measured in surface water samples from upper, mid, and lower bay stations. Two distinct methods were used to assess the relative uptake of N substrates by phytoplankton and correct for bacterial artifacts: (1) traditional filtration using Whatman glass fiber (GF/F) filters and (2) flow cytometric (FCM) sorting of chlorophyll-containing cells. The concentration of dissolved inorganic N (DIN) decreased with distance south along the bay, whereas dissolved organic N (DON) concentrations were relatively constant. Absolute N uptake rates measured using the traditional approach exceeded those of FCM-sorted phytoplankton, thereby suggesting the possibility of bacterial “contamination.” Ammonium was the dominant N form used throughout the transect, although FCM-sorted phytoplankton relied more on urea and DFAA as the ratio of DON/DIN increased toward the bay mouth. Overall, ammonium comprised 74 ± 17%, urea 10 ± 9%, DFAA 9 ± 7%, and nitrate 7 ± 12% of total measured N uptake by phytoplankton. Results suggest that bacteria relied primarily on DFAA and ammonium for N nutrition but also used N from urea at a rate similar to that of phytoplankton, whereas bacterial nitrate uptake was insignificant. On average, phytoplankton uptake of ammonium, urea, and DFAA was overestimated by 61%, 53%, and 135%, respectively, as a result of bacterial retention on GF/F filters.  相似文献   

7.
Daily and annual integrated rates of primary productivity and community respiration were calculated using physiological parameters measured in oxygen-based photosynthesis-irradiance (P-I) incubations at 8 stations throughout central and western Long Island Sound (cwLIS) during the summer and autumn of 2002 and 2003 and the late spring of 2003. Each calculation takes into account actual variations in incident irradiance over the day and underwater irradiance and standing stock with depth. Annual peak rates, ±95% confidence interval of propagated uncertainty in each measurement, of gross primary production (GPP, 1,730±610 mmol O2 m−2 d−1), community respiration (Rc, 1,660±270 mmol O2 m−2 d−1), and net community production (NCP, 1,160±1,100 mmol O2 m−2 d−1) occurred during summer at the western end of the Sound. Lowest rates of GPP (4±11 mmol O2 m−2 d−1), Rc (−50±300 mmol O2 m−2 d−1), and NCP (−1,250±270 mmol O2 m−2 d−1) occurred during late autumn-early winter at the outer sampled stations. These large ranges in rates of GPP, Rc, and NCP throughout the photic zone of cwLIS are attributed to seasonal and spatial variability. Algal respiration (Ra) was estimated to consume an average of 5% to 52% of GPP, using a literature-based ratio of Ra:Rc. From this range, we established that the estimated Ra accounts for approximately half of GPP, and was used to estimate daily net primary production (NPP), which ranged from 2 to 870 mmol O2 m−2 d−1 throughout cwLIS during the study. Annual NPP averaged 40±8 mol O2 m−2 yr−1 for all sampled stations, which more than doubled along the main axis of the Sound, from 32±14 mol O2 m−2 yr−1 at an eastern station to 82±25 mol O2 m−2 yr−1 at the western-most station. These spatial gradients in productivity parallel nitrogen loads along the main axis of the Sound. Daily integrals of productivity were used to test and formulate a simple, robust biomass-light model for the prediction of phytoplankton production in Long Island Sound, and the slope of the relationship was consistent with reports for other systems.  相似文献   

8.
We examined the contrasting, effects of floods and droughts produced by large changes in local climatology on vegetation patterns in Nueces marsh, a semi-arid subtropical salt marsh in south Texas from 1995 to 2005. Climate variations during the study included an initial 4-yr period of moderate conditions, followed by a 2-yr interval of drought, and a recent 4-yr wet period that included large-scale floods. Variation in freshwater inflow, rainfall, and potential evapotranspiration were used in conjunction with field measurements of salinity, inorganic nitrogen, and vegetation structure collected at sites located at varying distances from Nueces Bay. Tidal creek salinities varied with Nueces Bay salinity, with strength of effect inversely related to distance from the bay. Mean (±standard deviation) pore water salinities ranged from 59±54‰ at two high, marsh stations farthest from the bay (10.1 km distant) to 30±21‰ in soil at a low marsh site closest to the bay (0.5 km distant). Mean pore water ammonium was also higher at stations most distant from the bay; nitrate + nitrite did not exhibit a high marsh to low marsh gradient. Nueces Bay salinity decreased substantially when the 10-d cumulative mean daily Nueces River flows exceeded 10 m3 s−1. During periods of low and moderate flood frequency (flows mostly below 10 m3 s−1), vegetation assemblages were dominated by stress-tolerant clonal plants. A catastrophic flood, which immersed vegetation for several weeks between July and September 2002, resulted in extensive plant mortality, but within months, unvegetated areas were rapidly colonized by the obligate annualSalicornia bigelovii. With the end of major flooding by late 2004, plant community structure began a return to pre-drought assemblages at high and middle marsh stations by summer 2005. At the low marsh station, new conditions favored clonal dominants (Spartina alterniflora andBorrichia frutescens), with the latter replacingSalicornia virginica as the dominant species. Our results support the theory that the importance of competition and abiotic stress in determining community composition are inversely related.  相似文献   

9.
Egg production of planktonic copepods, is commonly measured as a proxy for secondary production in population dynamics studies and for quantifying food limitation. Although limitation of copepod egg production by food quantity or quality is common in natural waters, it appears less common or severe in estuaries where food concentrations are often high. San Francisco Estuary, California, has unusually low concentrations of chlorophyll compared to other estuaries. We measured egg production rates of three species ofAcartia, with dominate the zooplankton biomass at salinity above 15 psu, on 36 occasions during 1999–2002. Egg production was determined by incubating up to 40 freshly collected individual copepods for 24 h in 140 ml of ambient water. Egg production was less than 10 eggs female−1 d−1 most of the year, but as high as 52 eggs female−1 d−1 during month-long spring phytoplankton blooms. Egg production was a saturating function of total chlorophyll concentration with a mean of 30 eggs female−1 d−1 above a chlorophyll concentration of 12±6 mg chl m−3. We take chlorophyll to be a proxy for total food ofAcartia, known to feed on microzooplankton as well as phytoplankton. These findings, together with long-term records of chlorophyll, concentration and earlier studies of abundance of nauplius larvae in the estuary, imply chronic food limitation ofAcartia species, with sufficient food for maximum egg production <10% of the time over the last 25 yr. These results may show the most extreme example of food limitation of copepod reproduction in any temperate estuary. They further support the idea that estuaries may provide suitable habitat forAcartia species by virtue of other factors than high food concentration.  相似文献   

10.
We used growth rates of juvenile winter flounderPseudopleuronectes americanus to assess anthropogenic influence on habitat quality at three sites in Narragansett Bay, Rhode Island. The upper bay site, Gaspee Point, had the highest population density and concentration of total nitrogen; human inputs decreased down bay. Growth rates of individually marked fish were measured in three 15-d experiments from June 8 to July 6, 1998 in 1-m2 cages placed at upper, middle, and lower bay sites. Water temperature, salinity, dissolved oxygen (DO), and benthic food were also measured. Stable isotopes of nitrogen and carbon were measured in experimental fish as possible indicators of nutrient enrichment and to identify organic carbon sources. Growth rates were 0.22–0.60 mm d−1, with the highest average at the mid-bay site. Growth was initially fastest at Gaspee Point, but dropped off as DO concentrations fell. Step-wise multiple regression indicated that location (upper, middle, or lower bay) explained most of the variability in fish growth (40%). Coefficients of other significant variables indicated that fish grew faster at lower salinities, smaller sizes, and with decreased time that DO was below 2.3 mg l−1. Benthic prey varied among sites and there was significantly less food and fewer species at Gaspee Point.Polydora cornuta was a favored food at all sites and was found in over half the stomachs. Values of δ15N in fish and sediments did not reflect differences in total nitrogen concentrations recorded near the sites. We suggest that anthropogenic influences, such as nutrients and sewage, affected habitat quality by reducing DO, which lowered fish growth rates.  相似文献   

11.
The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m2 ha−1 and 39.7 m2 ha−1 at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m2 ha−1 and 19.6 m2 ha−1 at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha−1 yr−1 and 12.0 Mg ha−1 yr−1) than in the upper estuary (3.2 Mg ha−1 yr−1 and 4.2 Mg ha−1 yr−1). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg−1 to 33.5 g kg−1, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.  相似文献   

12.
Temporal and spatial variations in phytoplankton in Asan Bay, a temperate estuary under the influence of monsoon, were investigated over an annual cycle (2004). Phytoplankton blooms started in February (>20 μg chl l−1) and continued until April (>13 μg chl l−1) during the dry season, especially in upstream regions. The percentage contribution of large phytoplankton (micro-sized) was high (78–95%) during the blooms, and diatoms such as Skeletonema costatum and Thalassiosira spp. were dominant. The precipitation and freshwater discharge from embankments peaked and supplied nutrients into the bay during the monsoon event, especially in July. Species that favor freshwater, such as Oscillatoria spp. (cyanobacteria), dominated during the monsoon period. The phytoplankton biomass was minimal in this season despite nutrient concentrations that were relatively sufficient (enriched), and this pattern differed from that in tropical estuaries affected by monsoon and in temperate estuaries where phytoplankton respond to nutrient inputs during wet seasons. The flushing time estimated from the salinity was shorter than the doubling time in Asan Bay, which suggests that exports of phytoplankton maximized by high discharge directly from embankments differentiate this bay from other estuaries in temperate and tropical regions. This implies that the change in physical properties, especially in the freshwater discharge rates, has mainly been a regulator of phytoplankton dynamics since the construction of embankments in Asan Bay.  相似文献   

13.
Deep Bay is a semienclosed bay that receives sewage from Shenzhen, a fast-growing city in China. NH4 is the main N component of the sewage (>50% of total N) in the inner bay, and a twofold increase in NH4 and PO4 concentrations is attributed to increased sewage loading over the 21-year period (1986–2006). During this time series, the maximum annual average NH4 and PO4 concentrations exceeded 500 and 39 μM, respectively. The inner bay (Stns DM1 and DM2) has a long residence time and very high nutrient loads and yet much lower phytoplankton biomass (chlorophyll (Chl) <10 μg L−1 except for Jan, July, and Aug) and few severe long-term hypoxic events (dissolved oxygen (DO) generally >2 mg L−1) than expected. Because it is shallow (~2 m), phytoplankton growth is likely limited by light due to mixing and suspended sediments, as well as by ammonium toxicity, and biomass accumulation is reduced by grazing, which may reduce the occurrence of hypoxia. Since nutrients were not limiting in the inner bay, the significant long-term increase in Chl a (0.52–0.57 μg L−1 year−1) was attributed to climatic effects in which the significant increase in rainfall (11 mm year−1) decreased salinity, increased stratification, and improved water stability. The outer bay (DM3 to DM5) has a high flushing rate (0.2 day−1), is deeper (3 to 5 m), and has summer stratification, yet there are few large algal blooms and hypoxic events since dilution by the Pearl River discharge in summer, and the invasion of coastal water in winter is likely greater than the phytoplankton growth rate. A significant long-term increase in NO3 (0.45–0.94 μM year−1) occurred in the outer bay, but no increasing trend was observed for SiO4 or PO4, and these long-term trends in NO3, PO4, and SiO4 in the outer bay agreed with those long-term trends in the Pearl River discharge. Dissolved inorganic nitrogen (DIN) has approximately doubled from 35–62 to 68–107 μM in the outer bay during the last two decades, and consequently DIN to PO4 molar ratios have also increased over twofold since there was no change in PO4. The rapid increase in salinity and DO and the decrease in nutrients and suspended solids from the inner to the outer bay suggest that the sewage effluent from the inner bay is rapidly diluted and appears to have a limited effect on the phytoplankton of the adjacent waters beyond Deep Bay. Therefore, physical processes play a key role in reducing the risk of algal blooms and hypoxic events in Deep Bay.  相似文献   

14.
We investigated trophic relationships involving microzooplankton in the low salinity zone of the San Francisco Estuary (SFE) as part of a larger effort aimed at understanding the dynamics of the food web supporting the endangered delta smelt, Hypomesus transpacificus. We performed 14 cascade experiments in which we manipulated the biomass of a copepod (Limnoithona tetraspina, Pseudodiaptomus forbesi, or Acartiella sinensis) and quantified responses of lower trophic levels including bacterioplankton, phytoplankton, and microzooplankton. Microzooplankton comprised a major food source for copepods; 9 out of 14 experiments showed removal of at least one group of microzooplankton by copepods. In contrast, the impact of copepods on phytoplankton was indirect; increased copepod biomass led to greater growth of phytoplankton in 3 of 14 experiments. Estimated clearance rates on microzooplankton were 4 mL day?1 for L. tetraspina and 2–6 mL day?1 for P. forbesi, whereas A. sinensis consumed mainly copepod nauplii. Complex trophic interactions, including omnivory, among copepods, microzooplankton, and different components of the phytoplankton likely obscured clear trends. The food web of the SFE is probably less efficient than previously thought, providing poor support to higher trophic levels; this inefficient food web is almost certainly implicated in the continuing low abundance of fishes, including the delta smelt that use the low salinity zone of the San Francisco Estuary.  相似文献   

15.
Estuaries are critical habitats for larvae and juveniles of many marine fishes, possibly because they promote high growth rates and survival rates. We investigated spatial and temporal changes in growth rate of larval bay anchovy (Anchoa mitchilli), in the middle Hudson River estuary where abundance of larvae is high. In two consecutive summer seasons, we sampled larvae at 4 sites evenly spaced over 45 km, at weekly intervals for up to a month. We examined otoliths to determine age in days and then used age-length regressions to estimate growth rate. In 1995, larval anchovy growth rates varied from 0.39 to 0.88 mm d−1 (median=0.48 mm d−1). In 1996, growth rates varied from 0.41 to 0.77 mm d−1 (median=0.55 mm d−1). In both years, we found significant spatial and temporal variation in growth rate. Larvae collected in the upper portion of Haverstraw Bay tended to grow more slowly than larvae collected in other sites. The dates on which the most rapidly growing larvae were collected varied from site to site. Neither temperature nor salinity variations explained growth rate differences. Growth rate variation, probably governed by patches of zooplankton, occurred on temporal scales of a week and spatial scales of 15 km.  相似文献   

16.
We examined relative abundance of juvenile weakfish,Cynoscion regalis, collected during 1986 and 1987 and tested for spatial differences in growth and survival within Delaware Bay. Juvenile weakfish recruit to all areas of Delaware Bay, and two cohorts were present during each year of the study. Although catch per unit effort (CPUE) varied among areas within the bay, there was a general trend of higher CPUE at lower salinities; abundance quickly declined near the end of September in all areas of the bay. Estimated growth rates from otolith increment analysis of juvenile weakfish ranged from 0.69 mm d−1 to 0.97 mm d−1. Spatial and temporal patterns in recent growth rate followed a general pattern: highest in the middle bay, lowest in the upper bay, and intermediate in the lower bay. Mortality rates were usually lowest in the low salinity region of the middle and upper bay during both years. There was no difference in mortality between cohorts in the middle bay, while in the upper bay the later-spawned fish had lower mortality and in the lower bay the early-spawned fish had lower mortality. Analysis of spatial and temporal patterns in growth and mortality suggests that there is a seasonal trade-off between habitat usage and resource availability for juvenile weakfish. The function of oligohaline and mesohaline waters as optimal nursery areas (in terms of growth and survival) changes due to the seasonally dynamic physicochemical characteristics in Delaware Bay.  相似文献   

17.
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   

18.
This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when ΔS (over 2.5 m) was <2 psu and occurred almost all the time when ΔS was >8 psu in the absence of extreme events like hurricanes. Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d−1 and the corresponding oxygen consumption as large as 7.4 g O2 m−2 d−1 fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity estimated for well mixed conditions range between 8.6–9.5 g O2 m−2 d−1 and 2.6–2.9 m2 d−1, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted to return to hypoxic conditions within a few hours to days after destratification events are terminated.  相似文献   

19.
Changes in circulation, water level, salinity, suspended sediments, and sediment flux resulted from Tropical Storm Frances and Hurricane Georges in the Vermilion-Atchafalaya Bay region during September 1998. Tropical Storm Frances made landfall near Port Aransas, Texas, 400 km west of the study area, and yet the strong and long-lived southeasterly winds resulted in the highest water levels and salinity values of the year at one station in West Cote Blanche Bay. Water levels were abnormally high across this coastal bay system, although salinity impacts varied spatially. Over 24 h, salinity increased from 5 to 20 psu at Site 1 on the east side of West Cote Blanche Bay. Abnormally high salinities were recorded in Atchafalaya Bay but not at stations in Vermilion Bay. On September 28, 1998, Hurricane Georges made landfall near Biloxi, Mississippi, 240 km east of the study area. On the west side of the storm, wind stress was from the north and maximum winds locally reached 14 m s−1. The wind forcing and physical responses of the bay system were analogous to those experienced during a winter cold-front passage. During the strong, north wind stress period, coastal water levels fell, salinity decreased, and sediment-laden bay water was transported onto the inner shelf. As the north wind stress subsided, a pulse of relatively saline water entered Vermilion Bay through Southwest Pass increasing salinity from 5 to 20 psu over a 24-h period. National Oceanic and Atmospheric Administration (NOAA)-14 reflectance imagery revealed the regional impacts of wind-wave resuspension and the bay-shelf exchange of waters. During both storm events, suspended solid concentrations increased by an order of magnitude from 75 to over 750 mg l−1. The measurements demonstrated that even remote storm systems can have marked impacts on the physical processes that affect ecological processes in shallow coastal bay systems.  相似文献   

20.
The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m−2 yr−1.  相似文献   

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