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1.
A nutrient mass balance for the tidal freshwater segment of the James River was used to assess sources of nutrients supporting phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to marine waters. Monthly mass balances for 2007–2010 were based on riverine inputs, local point sources (including sewer overflow events), ungauged inputs, riverine outputs, and tidal exchange. The tidal freshwater James River received exceptionally high areal loads (446 mg TN m?2 day?1 and 55 mg TP m?2 day?1) compared to other estuaries in the region and elsewhere. P inputs were principally from riverine sources (84 %) whereas point sources contributed appreciably (54 %) to high N loads. Despite high loading rates and short water residence time, areal mass retention was high (143 mg TN m?2 day?1 and 33 mg TP m?2 day?1). Retention of particulate fractions occurred during high discharge, whereas dissolved inorganic fractions were retained during low discharge when chlorophyll-a concentrations were high. On an annualized basis, P was retained more effectively (59 %) than N (32 %). P was retained by abiotic mechanisms via trapping of particulate forms, whereas N was retained through biological assimilation of dissolved inorganic forms. Results from a limited suite of stable isotope determinations suggest that DIN from point sources was preferentially retained. Combined inputs from diffuse and point sources accounted for only 20 % and 36 % (respectively) of estimated algal N and P demand, indicating that internal nutrient recycling was important to sustaining high rates of phytoplankton production in the tidal freshwater zone.  相似文献   

2.
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   

3.
The New River Estuary consists of a series of broad shallow lagoons draining a catchment area of 1,436 km2, located in Onslow County, North Carolina. During the 1980s and 1990s it was considered one of the most eutrophic estuaries in the southeastern United States and sustained dense phytoplankton blooms, bottom water anoxia and hypoxia, toxic outbreaks of the dinoflagellatePfiesteria, and fish kills. High nutrient loading, especially of phosphorus (P), from municipal and military sewage treatment plants was the principal cause leading to the eutrophic conditions. Nutrient addition bioassay experiments showed that additions of nitrogen (N) but not P consistently yielded significant increases in phytoplankton production relative to controls. During 1998 the City of Jacksonville and the U.S. Marine Corps Base at Camp Lejeune completely upgraded their sewage treatment systems and achieved large improvements in nutrient removal, reducing point source inputs of N and P to the estuary by approximately 57% and 71%, respectively. The sewage treatment plant upgrades led to significant estuarine decreases in ammonium, orthophosphate, chlorophylla, and turbidity concentrations, and subsequent increases in bottom water dissolved oxygen (DO) and light penetration. The large reduction in phytoplankton biomass led to a large reduction in labile phytoplankton carbon, likely an important source of biochemical oxygen demand in this estuary. The upper estuary stations experienced increases in average bottom water DO of 0.9 to 1.4 mg l−1, representing an improvement in benthic habitat for shellfish and other organisms. The reductions in light attenuation and turbidity should also improve the habitat conditions for growth of submersed aquatic vegetation, an important habitat for fish and shellfish.  相似文献   

4.
Numerous phytoplankton-oriented ecological studies have been conducted since 1965 in the extensive North Carolina estuarine system. Throughout a range of geomorphological estuarine types, a basic underlying pattern of phytoplankton productivity and abundance following water temperature seasonal fluctuations was observed. Overlying this solar-driven pattern was a secondary forcing mechanism consisting of a complex interaction between meteorology and hydrology, resulting in periodic winter or early spring algal blooms and productivity pulses in the lower riverine estuaries. Wet winters caused abundant nitrate to reach the lower estuaries and stimulate the blooms, whereas dry winters resulted in low winter phytoplankton abundance and primary production. Dinoflagellates (Heterocapsa triquetra, Prorocentrum minimum, Gymnodinium spp.) and various cryptomonads dominated these cool-weather estuarine blooms. Sounds were less productive than the riverine estuaries, and were dominated by diatoms such asSkeletonema costatum, Thalassiosira spp.,Melosira spp., andNitzschia spp., as were the most saline portions of riverine estuaries. Nutrient-limitation studies found that nitrogen was the principal limiting nutrient in these estuarine systems over a range of trophic states, with phosphorus occasionally co-limiting. Freshwater and oligohaline portions of large coastal plain rivers were often subject to summer blue-green algal blooms. Formation of these blooms on a year-to-year basis was also determined by meteorology and hydrology: wet winters or springs and consequent nutrient loading, coupled with low summer flow conditions and regeneration of nutrients from the sediments. Dry winters or springs resulted in less available nutrients for subsequent summer regeneration, and high flow conditions in summer flushed out the blooms. In recent years, there has been a dramatic increase in reported fish kills attributed to toxic dinoflagellate blooms, particularly in nutrient-enriched estuarine areas. This issue has become a major coastal ecological and economic concern.  相似文献   

5.
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   

6.
Copper and arsenic underwent large changes in chemical form during the development and senescence of natural phytoplankton blooms in the Patuxent River, a subestuary of Chesapeake Bay in Maryland. Arsenate was rapidly reduced to arsenite and methylated species. At a total arsenic concentration of 20 nmol l?1, arsenate reduction rates ranged from 50 amol cell?1 d?1 to >230 amol cell?1 d?1, with the rate and extent of reduction dependent upon the concentration of arsenic, the dominant phytoplankton present, the season, and the degree of decline in phosphorus concentrations during bloom development. In general, the percentage of organically-associated copper was lowest (20–40% of total copper) during periods of rapid cell growth and highest (60–100% of total copper) during periods of cell decline or periods of dominance by red tide-forming dinoflagellates, a pattern associated with periods of high release of organic compounds during either bloom senescence or dense algal blooms. The end result of biological mediation was to increase the proportion of each element present in a less toxic form, thus affecting the potential toxicity to a natural ecosystem.  相似文献   

7.
Loss of water clarity is one of the consequences of coastal eutrophication. Efforts have therefore been made to reduce external nutrient loadings of coastal waters. This paper documents improvements to water clarity between 1985 and 2008–2009 at four stations in the microtidal estuary Roskilde Fjord and find significant relationships to freshwater nutrient loadings. The paper then investigates to which extent changes in phytoplankton biomass (chlorophyll a (Chl a)), non-algal particulate organic matter (POM*), and residual attenuation in the water (K b), respectively, can account for this optical improvement. Vertical light attenuation (K d) declined, on average, by 34 %, accompanying a 71 % reduction of Chl a and an 80 % reduction of POM*. Residual attenuation declined by 26 % over the period in accordance with a measured 34 % decline of dissolved organic nitrogen. Analysis of simultaneous changes in light attenuation and Secchi depth also suggested a reduction of the scatter-to-absorption ratio over time. Considering the stronger reductions of particle concentrations than dissolved organic matter, the contribution of residual attenuation to vertical attenuation increased from 54 to 74 % in 1985 to 78 to 85 % in 2008–2009. Overall, efforts to reduce nutrient loading and improve water clarity appeared to have had a larger impact on POM* than on Chl a and colored dissolved organic matter concentrations in the estuary, which can account for the decrease in the scatter-to-absorption ratio. These optical changes lead to larger improvements of Secchi depth than of vertical light attenuation. The consequence of this is an overestimation (0.45–1.48 m) of the predicted increase of potential seagrass depth limits when based on Secchi depth rather than K d.  相似文献   

8.
Coastal upwelling in the northern California Current varies seasonally, with downwelling in winter and upwelling in summer, resulting in pronounced variability in hydrography, nutrients, phytoplankton biomass, and species composition. Winter was characterized by moderate concentrations of nitrate and silicate (averages of 10 and 18 μM, respectively) and low concentrations of chlorophyll a (Chl a). During the upwelling season, concentrations of the same nutrients ranged from near 0 μM to approximately 27 and 43 μM and Chl a 0.5?<?x?<?15 μg L?1. During autumn, upwelling weakened and nutrient concentrations were reduced, but large phytoplankton blooms continued to occur. Variations in hydrography, nutrients, and phytoplankton also occurred within the upwelling season due to alternation of the winds between northerly (active upwelling) and southerly (relaxation of upwelling), on a 5- to 10-day time scale. Eleven blooms were observed, most of which occurred near the end of active upwelling events and during relaxation of upwelling. Nonmetric multidimensional scaling ordination of species composition of the microplankton revealed four distinct communities: a winter community, early upwelling and late upwelling season communities, and an autumn community. Diatoms (Asterionellopsis glacialis, Eucampia zodiacus, and several Chaetoceros, Thalassiosira, and Pseudo-nitzschia species) dominated early in the upwelling season, averaging 80 % of the phytoplankton biomass, and dinoflagellates dominated near the end of the upwelling season, averaging 68 % of the phytoplankton biomass. Dinoflagellates formed two monospecific blooms—Prorocentrum gracile in late summer and Akashiwo sanguinea in autumn. Changes in community composition were correlated with bottom temperature and salinity (representing seasonal variability) and sea surface salinity (representing within-season event-scale variability in upwelling).  相似文献   

9.
Lake Erie is biologically the most active lake among the Great Lakes of North America, experiencing seasonal harmful algal blooms (HABs). The early detection of HABs in the Western Basin of Lake Erie (WBLE) requires a more efficient and accurate monitoring tool. Remote sensing is an efficient tool with high spatial and temporal coverage that can allow accurate and timely detection of the HABs. The WBLE is heavily influenced by the surrounding terrestrial ecosystem via rivers such as the Sandusky River and the Maumee River. As a result, the optical properties of the WBLE are influenced by multiple color producing agents (CPAs) such as phytoplankton, colored dissolved organic matter (CDOM), organic detritus, and terrigenous inorganic particles. The diversity of the CPAs and their non-linear interactions makes these waters optically complex, and the task of optical remote sensing for retrieving estimates of CPAs more challenging. Chlorophyll a, which is the primary light harvesting pigment in all phytoplankton, is used as a proxy for algal biomass. In this study, several published remote sensing algorithms and band ratio models were applied to the reflectance data from the full resolution MERIS sensor to remotely estimate chlorophyll a concentrations in the WBLE. Efficiency of the sensor and the algorithms performance were tested through a least squares regression and residual analysis. The results indicate that, among the suite of existing bio-optical models, the Simis semi-analytical algorithm provided the best model results for measures of algal biomass in the optically complex WBLE with R 2 of 0.65, RMSE 0.85 μg/l, (n = 71, P < 0.05). The superior results of this model in detecting chlorophyll a are attributed to several factors including optimizing spectral regions that are less sensitive to CDOM and the incorporation of correction factors such as absorption effects due to pure water (a w), backscatter (b b) from suspended matter and interference due to phycocyanin (δ), a major accessory pigment in the WBLE.  相似文献   

10.
Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but may be increasing in frequency and diversity. Outbreaks ofPfiesteria piscicida (Dinophyceae) were observed during 1997 in several Chesapeake Bay tributaries, while in 1998,Pfiesteria-related events were not found but massive blooms ofProrocentrum minimum (Dinophyceae) occurred. In 1999,Aureococcus anophagefferens (Pelagophyceae) developed in the coastal bays in early summer in sufficient densities to cause a brown tide. In 1997, toxicPfiesteria was responsible for fish kills at relatively low cell densities. In 1998 and 1999, the blooms ofP. minimum andA. anophagefferens were not toxic, but reached sufficiently high densities to have ecological consequences. These years differed in the amount and timing of rainfall events and resulting nutrient loading from the largely agricultural watershed. Nutrient loading to the eastern tributaries of Chesapeake Bay has been increasing over the past decade. Much of this nutrient delivery is in organic form. The sites of thePfiesteria outbreaks ranked among those with the highest organic loading of all sites monitored bay-wide. The availability of dissolved organic carbon and phosphorus were also higher at sites experiencingA. anophagefferens blooms than at those without blooms. The ability to supplement photosynthesis with grazing or organic substrates and to use a diversity of organic nutrients may play a role in the development and maintenance of these species. ForP. minimum andA. anophagefferens, urea is used preferentially over nitrate.Pfiesteria is a grazer, but also has the ability to take up nutrients directly. The timing of nutrient delivery may also be of critical importance in determining the success of certain species.  相似文献   

11.
An experimental in situ microcosm study was conducted in the tropical lagoon La Mancha (Gulf of Mexico) to determine whether or not nutrient limitation occurs and to examine the direct effect of an inorganic nutrient pulse on the phytoplankton community structure. The phytoplankton community response to the addition of four treatments with different combinations of nitrogen (N), phosphorus (P), and silica (Si) (+N-NH4 +, +P-PO4 ?, +Si-SO3, and N:P16) showed that phytoplankton was N-limited as indicated by an increase in phytoplankton biomass (i.e., chlorophyll a) (range, 8–34 mg m?3) during the dry season in two consecutive years (2006 and 2007). Picophytoplankton abundance significantly increased in the +N treatment (145.46 103 cells L?1), while microphytoplankton reached a maximum abundance (68.38 103 cells L?1) in the N:P16 treatment. Phytoplankton composition changed from a community initially dominated by dinoflagellates (e.g., Prorocentrum spp.) to another dominated by diatoms (Thalassiosira and Nitzschia longissima) in the N:P16 treatment. The +N treatment significantly increased Synechococcus sp. growth rates (1.3 divisions per day) (picocyanobacteria). Biomarker pigments measured in the experimental microcosms confirmed observed changes in phytoplankton groups. Our results reveal that La Mancha lagoon is a N-limited coastal system during the dry season and provides evidence of the temporal species successional patterns and mechanisms regulating the phytoplankton community response to nutrient enrichment pulses in this already eutrophic coastal lagoon.  相似文献   

12.
This study identified drivers of change in Barnegat Bay–Little Egg Harbor Estuary, NJ, USA over multiple long-term time periods by developing an assessment tool (an “Eutrophication Index”) capable of handling data gaps and identifying the condition of and relationships between ecosystem pressures, ecosystem state, and biotic responses. The Eutrophication Index integrates 15 indicators in 3 components: (1) water quality, (2) light availability, and (3) seagrass response. Annual quantitative assessments of condition and its consistency for three geographic segments range from 0 (highly degraded) to 100 (excellent condition). Eutrophication Index values significantly declined (p?<?0.05) by 34 and 36 % in central and south segments from 73 and 71 in the early 1990s to 48 and 45 in 2010, respectively. Ongoing declines despite periods of improvement (e.g., 1989–1992, 1996–2002, and 2006–2008) suggest these estuarine segments are currently undergoing eutrophication. The north segment had highest nutrient loading and lowest Eutrophication Index values (2010 Eutrophication Index value?=?37) but increased over time (from 14 in 1991 to 50 in 2009) in contrast to trends in central and south segments. Rapid initial declines of Eutrophication Index values with increasing loading highlight that the estuary is sensitive to loading. Ecosystem response to total nutrient loading, as described by the Index of Eutrophication, exhibited nonlinearity at loading rates of >1,200 and <5,000 kg TN km?2 year?1 and >100 and <250 kg TP km?2 year?1, values similar to responses of seagrass to nutrient loading in many ecosystems. While nutrient loading is initially a critical driver of ecosystem change, other factors, e.g., light availability and drive ecosystem condition, yield nonlinearity. Empirical evidence for switches in the driving factors of ecosystem stress adds complexity to the conceptualization of ecosystem resiliency due to feedback from multiple dynamic, nonlinear stressors.  相似文献   

13.
While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspension-feeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.  相似文献   

14.
While many coastal ecosystems previously supported dense meadows of seagrass and dense stocks of bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have contributed to the decline of these important resources. Anthropogenic nutrient loading and subsequent eutrophication has been identified by some researchers as a primary driver of these losses, but others have described potential positive effects of eutrophication on some estuarine resources. The Peconic Estuary, Long Island, NY, USA, offers a naturally occurring nutrient-loading gradient from eutrophic tidal creeks in its western reaches to mesotrophic bays in the eastern region. Over 2 years, we conducted an experiment across this gradient to examine the effects of eutrophication on the growth of estuarine species, including juvenile bivalves (northern quahogs (Mercenaria mercenaria), eastern oysters, (Crassostrea virginica), and bay scallops (Argopecten irradians)) and slipper limpet (Crepidula fornicata). Water quality and phytoplankton community biomass and composition were concurrently monitored at each site, and the effects of these variables on the growth of estuarine species were analyzed with multiple regression model. Eutrophication seemed to impact shellfish through changes in the quality of food and not the quantity since the growth rates of shellfish were more often correlated with densities of specific cell types or quality of seston rather than bulk measures of phytoplankton and organic seston. Northern quahogs and eastern oysters grew maximally within eutrophic locales, and their growth was positively correlated with high densities of autotrophic nanoflagellates and centric diatoms in these regions (p?<?0.001). The growth rates of northern quahogs were also positively correlated with relative water motion, suggesting an important role for tidal currents in delivering seston to suspension feeders. Bay scallops and slipper limpets were negatively impacted by eutrophication, growing at the slowest rate at the most eutrophic sites. Furthermore, bay scallop growth was negatively correlated with densities of dinoflagellates, which were more abundant at the most eutrophic site (p?<?0.001). These results suggest that nutrient loading can have significant but complex effects on suspension-feeding molluscs with select species (e.g., oysters and clams) benefiting from eutrophication and other species performing poorly (e.g., scallops and slipper limpets). Future management approaches that seek to restore bivalve populations will need to account for the differential effects of nutrient loading as managers target species and regions to be restored.  相似文献   

15.
Abundance of the prymnesiophyte Phaeocystis pouchetii was quantified via light microscopy at 2-week to monthly intervals in Massachusetts Bay (southern Gulf of Maine, NW Atlantic) during 1992–2012. Variability in the abundance and seasonal cycle of Phaeocystis are described and synoptic hydrographic, nutrient, and meteorological data were analyzed to identify factors that may influence Phaeocystis abundance. The maximum Phaeocystis abundance was 14?×?106 cells L?1 (10 Apr 2008). It was frequently (5 of 8 years) absent prior to year 2000, but not thereafter. Seasonally, it first appeared in February to early March, reached peak abundance in mid-April, and persisted until May or early June for a duration of 0–112 days (mean 34 days). A long-term alternation between Phaeocystis and centric diatom abundance was apparent, suggesting winter-spring selection of either Phaeocystis or centric diatoms. Phytoplankton community analysis suggested that blooms affected the rest of the phytoplankton community. Phaeocystis blooms were manifest as a substantial increase in particulate nutrients above normal levels. Phaeocystis blooms were preceded in February by a slightly elevated concentration of NO3 (9.3 vs. 6.5 μM when absent) and PO4 (0.99 vs. 0.79 μM when absent). Blooms were also preceded by elevated ratios of NO3/PO4, NO3/Si, and PO4/Si, and warmer, saltier waters reflecting reduced river discharge. The correlation with salinity and river discharge suggests that Phaeocystis bloom variability is partially determined by annually varying circulation processes that determine the degree of low nutrient, low salinity coastal water intrusion into Massachusetts Bay.  相似文献   

16.
We investigated the dissolved oxygen metabolism of the Curonian Lagoon (Baltic Sea) to assess the relative contributions of pelagic and benthic processes to the development of transient hypoxic conditions in shallow water habitats. Metabolism measurements along with the remote sensing-derived estimates of spatial variability in chlorophyll a were used to evaluate the risk of hypoxia at the whole lagoon level. Our data demonstrate that cyanobacterial blooms strongly inhibit light penetration, resulting in net heterotrophic conditions in which pelagic oxygen demand exceeds benthic oxygen demand by an order of magnitude. The combination of bloom conditions and reduced vertical mixing during calm periods resulted in oxygen depletion of bottom waters and greater sediment nutrient release. The peak of reactive P regeneration (nearly 30 μmol m?2 h?1) coincided with oxygen depletion in the water column, and resulted in a marked drop of the inorganic N:P ratio (from >40 to <5, as molar). Our results suggest a strong link between cyanobacterial blooms, pelagic respiration, hypoxia, and P regeneration, which acts as a feedback in sustaining algal blooms through internal nutrient cycling. Meteorological data and satellite-derived maps of chlorophyll a were used to show that nearly 70 % of the lagoon surface (approximately 1,000 km2) is prone to transient hypoxia development when blooms coincide with low wind speed conditions.  相似文献   

17.
We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton, drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m−2d−1 and 0.5 mmol P m−2 d−1 to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments. It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures.  相似文献   

18.
The Changjiang Estuary is one of the largest estuaries in the world, where hypoxia frequently occurs during the summer season. Recent routine surveys in the observed area found that the low dissolved oxygen (DO) in the summer bottom water was not rapidly expanding with increasing nutrient loads in Changjiang diluted waters. Based on the remote sensing data and in situ data, we examined the historic seasonal hypoxia observations for the bottom waters of the Changjiang Estuary and investigated the short- and long-term effects that runoff had on variations in DO and chlorophyll-a (Chl-a). Our analysis indicated that the recent areal variation in hypoxia was due to the changing Chl-a distribution and stratification conditions. The correspondence between hypoxia and surface Chl-a concentration showed that remotely sensed Chl-a larger than 3.0 mg L?1 was an essential condition for the formation of hypoxia off the Changjiang Estuary. The trend of Chl-a concentration was significantly impacted by the Three Gorges Dam (TGD), and the inter-annual variation of Chl-a was weakly affected by global-scale climate variability. After the TGD impoundment, the sediment loading in the Changjiang runoff and suspended sediments in Changjiang Estuary in August decreased, and the high Chl-a concentration moved landward. These shifted the hypoxia from its optimal forming conditions.  相似文献   

19.
Seasonal wind-driven upwelling along the U.S. West Coast supplies large concentrations of nitrogen to surface waters that drives high primary production. However, the influence of coastal upwelled nutrients on phytoplankton productivity in adjacent small estuaries and bays is poorly understood. This study was conducted in Drakes Estero, California, a low inflow estuary located in the Point Reyes National Seashore and the site of an oyster mariculture facility that produces 40 % of the oysters harvested in California. Measurements of nutrients, chlorophyll a, phytoplankton functional groups, and phytoplankton carbon and nitrogen uptake were made between May 2010 and June 2011. A sea-to-land gradient in nutrient concentrations was observed with elevated nitrate at the coast and higher ammonium at the landward region. Larger phytoplankton cells (>5 μm diameter) were dominant within the outer and middle Estero where phytoplankton primary productivity was fueled by nitrate and f-ratios were >0.5; the greatest primary production rates were in the middle Estero. Primary production was lowest within the inner Estero, where smaller phytoplankton cells (<5 μm) were dominant, and nitrogen uptake was dominated by ammonium. Phytoplankton blooms occurred at the outer and middle Estero and were dominated by diatoms during the spring and dry-upwelling seasons but dinoflagellates during the fall. Small flagellated algae (>2 μm) were dominant at the inner Estero where no blooms occurred. These results indicate that coastal nitrate and phytoplankton are imported into Drakes Estero and lead to periods of high new production that can support the oyster mariculture; a likely scenario also for other small estuaries and bays.  相似文献   

20.
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   

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