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1.
Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction. 相似文献
2.
We examined the processes influencing summer hypoxia in the mainstem portion of Chesapeake Bay. The analysis was based on the Chesapeake Bay Monitoring Program data collected between 1985 and 2007. Self-organizing map (SOM) analysis indicates that bottom water dissolved oxygen (DO) starts to be depleted in the upper mesohaline area during late spring, and hypoxia expands down-estuary by early summer. The seasonal hypoxia in the bay appears to be related to multiple variables, (e.g., river discharge, nutrient loading, stratification, phytoplankton biomass, and wind condition), but most of them are intercorrelated. The winter–spring Susquehanna River flow contributes to not only spring–summer buoyancy effects on estuarine circulation dynamics but also nutrient loading from the land-promoting phytoplankton growth. In addition, we found that summer hypoxia is significantly correlated with the late winter–spring (February–April) northeasterly–southwesterly (NE–SW) wind. Based on winter–spring (January–May) conditions, a predictive tool was developed to forecast summer (June–August) hypoxia using river discharge and NE–SW wind. We hypothesized that the late winter–spring wind pattern may affect the transport of spring bloom biomass to the western shoal or the deep channel of the bay that either alleviates or increases the summer hypoxic volume in the midbay region, respectively. To examine this hypothesis, residual flow fields were analyzed using a hydrodynamic ocean model (Regional Ocean Modeling System; ROMS) between 2000 and 2003, two hydrologically similar years but years with different wind conditions during the spring bloom period. Simulation model results suggest that relatively larger amounts of organic matter could be transported into the deep channel in 2003 (severe hypoxia; frequent northeasterly wind) than 2000 (moderate hypoxia; frequent southwesterly wind). 相似文献
3.
The distribution and intensity of hypoxia (low dissolved oxygen) in estuaries is increasing worldwide due to cultural eutrophication.
This study quantifies the strength of associations between the duration of diel-cycling severe hypoxia (≤2 mg O2 l−1) in bottom water (∼15 cm above bottom) of a shallow (<2 m) coastal lagoon estuary (Delaware, USA) and abiotic environmental
variables (water temperature, insolation, tide, streamflow, and wind) and predicts the duration of severe hypoxia given different
combinations of these variables. The intensity and spatial extent and dynamics of diel-cycling severe hypoxia events were
defined. Vertical variability in dissolved oxygen (DO) concentration during the daytime was also determined. During the summers
of 2001–2005, bottom DO data were collected for periods of weeks to months at multiple sites using automated sondes. Multiple
linear regression (MLR) and regression tree analysis (RTA) were used to determine the relative importance of the environmental
variables in predicting the number of hours of severe hypoxia per day. Key findings of the study were that severe hypoxia
events of minutes to hours in duration occurred frequently in all four tributaries sampled, primarily between 0200 and 1000
hours. Severe hypoxia duration and diel-cycling amplitudes of DO concentration increased in the up-tributary direction. Hierarchically,
the duration of severe hypoxia was influenced mostly by the mean daily water temperature, then by preceding days’ total insolation,
percentage of morning hours (02:00 to 10:00 a.m.) ebb tide, and daily streamflow. Collectively, the variables examined by the MLR and the RTA approaches accounted for 62%
and 65% of the variability in the duration of severe hypoxia, respectively. RTA demonstrated that daily mean water temperature
above 26.3°C and previous day’s total insolation below 13.6 kW m−2 were associated with the longest lasting severe hypoxic events (9.56 h). The environmental variables and combinations of
conditions that modulate or augment diel-cycling hypoxia presented in this paper enhance understanding of this widespread
and growing phenomenon and provide additional insight regarding the extent to which it can impact food webs in very shallow
estuarine waters that often serve as nursery habitat. 相似文献
4.
Alternation of factors limiting phytoplankton production in the Cape Fear River Estuary 总被引：1，自引：0，他引：1
Michael A. Mallin Lawrence B. Cahoon Matthew R. McIver Douglas C. Parsons G. Christopher Shank 《Estuaries and Coasts》1999,22(4):825-836
Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and 14C uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient-rich oligohaline station, significant responses to nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (P) and N were occasionally limiting in spring with some N+P co-limitation. Light was apparently limiting during fall and winter when the water was turid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation. 相似文献
5.
Rikke Jeppesen Miguel Rodriguez Jenna Rinde John Haskins Brent Hughes Laura Mehner Kerstin Wasson 《Estuaries and Coasts》2018,41(1):89-98
Human land use activities around estuaries can result in high levels of eutrophication. At Elkhorn Slough estuary, a highly eutrophic California estuary, we investigated the effects of impaired water quality on two stress-tolerant estuarine species, a common fish, the staghorn sculpin, Leptocottus armatus and a foundational invertebrate, the Olympia oyster, Ostrea lurida. We caged the two indicator species at six wetlands with different levels of water quality impairment, four of which had restricted tidal flow. We also recorded water quality parameters simultaneously at all sites using YSI sondes, and sampled nutrients and chlorophyll-a monthly, building on the National Estuarine Research Reserve System-wide Monitoring Program. We found that the monitored environmental variables predicted ecological responses by the indicator species. In particular, we found that the duration and severity of hypoxia were negatively correlated with fish survival and oyster growth. Further, our results corroborate previous studies that artificial tidal restriction leads to increased hypoxia stress. We conclude that large diurnal fluctuations in dissolved oxygen and extended nighttime hypoxia can have lethal and sub-lethal effects even on stress-tolerant organisms in the estuary. While laboratory experiments have often shown such effects, it is relatively rare to demonstrate negative effects of oxygen variation with in situ experiments, which provide stakeholders with concrete evidence for impaired water quality at local wetlands. Tidally restricted sites, which experience the largest fluctuations in dissolved oxygen and longest periods of hypoxia, harbor conditions harmful to vertebrates and invertebrates in the estuary. Reversing the anthropogenically induced low oxygen levels, by restoring more natural tidal exchange and by decreasing agricultural runoff, could improve the survival and growth of important estuarine organisms. 相似文献
6.
To fill in the large existing data gap, this study presents results of a comprehensive data set from Kuwait Bay (KB), showing the horizontal and vertical distribution of its prominent hydrodynamic variables (i.e., water temperature, seawater salinity, seawater density) and water quality variables (i.e., chlorophyll-a concentration, turbidity, dissolved oxygen (DO) concentration, DO saturation, apparent oxygen utilization, photosynthetic active radiation). Field measurements were carried out between 11 September 2014 and 17 August 2015 covering number of monitoring stations in the entire bay. The results revealed significant seasonal variations and apparent three-dimensional features of the measured variables exemplifying the necessity of not considering the bay as a well-mixed water body in the future oceanographic studies anytime of the year. However, well-mixed conditions (with the Brunt-Vaisala frequency of about 0.0002 s?1) were existent only in the winter. Bay’s water column began stratifying in late spring, and these were significantly intensified (with the Brunt-Vaisala frequency of about 0.0008 s?1) during the summer and early autumn. The stratification, together with the increase oxygen demand for decomposition processes and decrease in DO solubility in summer, led to the formation of a lower DO water mass (daytime DO concentration <4.2 mg l?1) at the lower layers. Results also suggested that the water of the KB is more transparent (indicated by lower turbidity) compared to the adjacent sea water. Measurement of light intensity along the water column indicated that the light extincts rapidly (i.e., the light reduced to approximately 10 % of the surface values at water depth of 2–6 m) in KB and many times absent at the water near the seabed. This study also suggested that the KB can be classified at a year-round negative, hypersaline, inverse, and hyperpycnal estuary. 相似文献
7.
Although seasonal hypoxia is a well-studied phenomenon in many coastal systems, most previous studies have only focused on variability and controls on low-oxygen water masses during warm months when hypoxia is most extensive. Surprisingly, little attention has been given to investigations of what controls the development of hypoxic water in the months leading up to seasonal oxygen minima in temperate ecosystems. Thus, we investigated aspects of winter–spring oxygen depletion using a 25-year time series (1985–2009) by computing rates of water column O2 depletion and the timing of hypoxia onset for bottom waters of Chesapeake Bay. On average, hypoxia (O2 <62.5 μM) initiated in the northernmost region of the deep, central channel in early May and extended southward over ensuing months; however, the range of hypoxia onset dates spanned >50 days (April 6 to May 31 in the upper Bay). O2 depletion rates were consistently highest in the upper Bay, and elevated Susquehanna River flow resulted in more rapid O2 depletion and earlier hypoxia onset. Winter–spring chlorophyll a concentration in the bottom water was highly correlated with interannual variability in hypoxia onset dates and water column O2 depletion rates in the upper and middle Bay, while stratification strength was a more significant driver in the timing of lower Bay hypoxia onset. Hypoxia started earlier in 2012 (April 6) than previously recorded, which may be related to unique climatic and biological conditions in the winter–spring of 2012, including the potential carryover of organic matter delivered to the system during a tropical storm in September 2011. In general, mid-to-late summer hypoxic volumes were not correlated to winter–spring O2 depletion rates and onset, suggesting that the maintenance of summer hypoxia is controlled more by summer algal production and physical forcing than winter-spring processes. This study provides a novel synthesis of O2 depletion rates and hypoxia onset dates for Chesapeake Bay, revealing controls on the phenology of hypoxia development in this estuary. 相似文献
8.
Jonathan H. Sharp Kohei Yoshiyama Alexander E. Parker Matthew C. Schwartz Susan E. Curless Allison Y. Beauregard Justin E. Ossolinski Allen R. Davis 《Estuaries and Coasts》2009,32(6):1023-1043
The Delaware River and Bay Estuary is one of the major urbanized estuaries of the world. The 100-km long tidal river portion
of the estuary suffered from major summer hypoxia in the past due to municipal and industrial inputs in the urban region;
the estuary has seen remarkable water quality improvements from recent municipal sewage treatment upgrades. However, the estuary
still has extremely high nutrient loading, which appears to not have much adverse impact. Since the biogeochemistry of the
estuary has been relatively similar for the past two decades, our multiple year research database is used in this review paper
to address broad spatial and seasonal patterns of conditions in the tidal river and 120 km long saline bay. Dissolved oxygen
concentrations show impact from allochthonous urban inputs and meteorological forcing as well as biological influences. Nutrient
concentrations, although high, do not stimulate excessive algal biomass due to light and multiple nutrient element limitations.
Since the bay does not have strong persistent summer stratification, there is little potential for bottom water hypoxia. Elevated
chlorophyll concentrations do not exert much influence on light attenuation since resuspended bottom inorganic sediments dominate
the turbidity. Dissolved inorganic carbon and dissolved and particulate organic carbon distributions show significant variability
from watershed inputs and lesser impact from urban inputs and biological processes. Ratios of dissolved and particulate carbon,
nitrogen, and phosphorus help to understand watershed and urban inputs as well as autochthonous biological influences. Owing
to the relatively simple geometry of the system and localized anthropogenic inputs as well as a broad spatial and seasonal
database, it is possible to develop these biogeochemical trends and correlations for the Delaware Estuary. We suggest that
this biogeochemical perspective allows a revised evaluation of estuarine eutrophication that should have generic value for
understanding other estuarine and coastal waters. 相似文献
9.
Material transfer between estuaries and the nearshore zone has long been of interest, but information on the processes affecting Pacific Northwest estuaries has lagged behind other areas. The west coast of the U.S. is a region of seasonally variable upwelling that results in enhanced phytoplankton production in the nearshore zone. We examined estuarine-nearshore links over time by measuring physical oceanographic variables and chlorophylla concentration from an anchor station in South Slough, Oregon. Data was collected during 24-h cruises conducted at approximately weekly intervals during summer 1996 and spring 1997. The results demonstrate that the physical oceanography of this estuarine site was strongly influenced by the coastal ocean. Marine water reached the estuarine site on every sampled tide, and chlorophylla was clearly advected into the estuary with this ocean water. In contrast, phytoplankton concentrations were comparatively reduced in the estuarine water. There were, however, large fluctuations in the import of chlorophyll over the course of the summer. These variations likely reflect upwelling-generated phytoplankton production in the coastal ocean and subsequent cross-shelf transport to the estuary. Suspension feeding organisms in South Slough likely depend on the advection of this coastally-derived phytoplankton. The large allochthonous chlorophyll input measured for this system appears dissimilar from most estuaries studied to date. Previous investigations have focused on the outwelling and inwelling of materials in estuaries. We must now consider the influence of coastal upwelling and downwelling processes on estuarine material exchange. 相似文献
10.
Seasonal dynamics of dissolved trace metals (Cd, Co, Cu, Ni and Zn) and its relationship with redox conditions and phytoplankton
activity has been studied in the Scheldt estuary, during nine surveys carried out between May 1995 and June 1996. Seasonal
profiles of dissolved trace metals and general estuarine water quality variables are compared, to identify the geochemical
and biological processes responsible for the observed trace metal distributions. In keeping with previous studies, the behavior
of dissolved Cd, Cu, and Zn can be explained by the presence of anoxic headwaters and the restoration of dissolved oxygen
within the estuary. In the river water, the concentration of dissolved Cu and Zn is generally low, except during winter when
dissolved oxygen is present in the water column, although highly undersaturated. Mobilization of particle-bound Cd, Cu, and
Zn occurs as dissolved oxygen increases with increasing salinity, possibly because of oxidation of metal sulfides in the suspended
matter. The geochemistry of dissolved Co is also related to the redox conditions but in an opposite way. Dissolved Co is mobilized
in the anoxic upper estuary, along with the reduction in Mn (hydro) oxides, and subsequently coprecipitated with Mn (hydro)
oxides when dissolved oxygen is restored. Conservative behavior is observed for dissolved Ni within the estuary. In the middle
estuary, Cd and Zn are readsorbed during phytoplankton blooms, as suggested by the low concentrations of these metals during
the most productive periods in spring and early summer. The removal may be caused by direct biological uptake and/or increased
adsorption to suspended matter because of the pH increase associated with algae blooms. In the lower estuary, chemical gradients
are much weaker and dilution with seawater is the dominant process. 相似文献
11.
12.
Jian Shen Taiping Wang Julie Herman Pam Mason Gretchen L. Arnold 《Estuaries and Coasts》2008,31(4):652-663
Two distinct hypoxic patterns were revealed from high-frequency dissolved oxygen (DO) data collected from North Branch of Onancock Creek, a shallow coastal estuary of the Chesapeake Bay, from July to October 2004. Diurnal hypoxia developed associated with large DO swings during fair weather and hypoxia/anoxia developed for prolonged 2–5-day periods following rainfall events. A simplified diagnostic DO-algae model was used to investigate DO dynamics in the creek. The model results show that the modeling approach enables important features of the DO dynamics in the creek to be captured and analyzed. Large anthropogenic inputs of nutrients to the creek stimulated macroalgae blooms in the embayment. High DO production resulted in supersaturated DO in daytime, whereas DO was depleted at night as the high respiration overwhelmed the DO supply, leading to hypoxia. Unlike deep-water environments, in this shallow-water system, biological processes dominate DO variations. High macroalgae biomass interacting with low light and high temperature trigger the development of prolonged hypoxic/anoxic postrainfall events. 相似文献
13.
The temporal and spatial distributions of salinity, dissolved oxygen, suspended particulate material (SPM), and dissolved nutrients were determined during 1983 in the Choptank River, an estuarine tributary of Chesapeake Bay. During winter and spring freshets, the middle estuary was strongly stratified with changes in salinity of up to 5‰ occurring over 1 m depth intervals. Periodically, the lower estuary was stratified due to the intrusion of higher salinity water from the main channel of Chesapeake Bay. During summer this intrusion caused minimum oxygen and maximum NH4 + concentrations at the mouth of the Choptank River estuary. Highest concentrations of SPM, particulate carbon (PC), particulate nitrogen (PN), total nitrogen (TN), total phosphorous (TP) and dissolved inorganic nitrogen (DIN) occurred in the upper estuary during the early spring freshet. In contrast, minimum soluble reactive phosphate (SRP) concentrations were highest in the upper estuary in summer when freshwater discharge was low. In spring, PC:PN ratios were >13, indicating a strong influence by allochthonous plant detritus on PC and PN concentrations. However, high concentrations of PC and PN in fall coincided with maximum chlorophyll a concentrations and PC:PN ratios were <8, indicating in situ productivity controlled PC and PN levels. During late spring and summer, DIN concentrations decreased from >100 to <10 μg-at l?1, resulting mainly from the nonconservative behavior of NO3 ?, which dominated the DIN pool. Atomic ratios of both the inorganic and total forms of N and P exceeded 100 in spring, but by summer, ratios decreased to <5 and <15, respectively. The seasonal and spatial changes in both absolute concentrations and ratios of N and P reflect the strong influence of allochthonous inputs on nutrient distributions in spring, followed by the effects of internal processes in summer and fall. 相似文献
14.
The pattern and influence of low dissolved oxygen in the Patuxent River,a seasonally hypoxic estuary
Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River,
a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in
bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the
influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous
predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models,
to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may
affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline
Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation
in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and
trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of
the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make
the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model
simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings
may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in
space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to
levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated. 相似文献
15.
Jianyu Chen Delu Pan Mingliang Liu Zhihua Mao Qiankun Zhu Ninghua Chen Xiaoyu Zhang Bangyi Tao 《Estuaries and Coasts》2017,40(4):1055-1065
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
16.
Kenneth A. Rose Sean Creekmore Peter Thomas J. Kevin Craig Md Saydur Rahman Rachael Miller Neilan 《Estuaries and Coasts》2018,41(1):233-254
We developed a spatially explicit, individual-based model to analyze how hypoxia effects on reproduction, growth, and mortality of Atlantic croaker in the northwestern Gulf of Mexico lead to population-level responses. The model follows the hourly growth, mortality, reproduction, and movement of individuals on a 300 × 800 spatial grid of 1-km2 cells for 140 years. Chlorophyll-a concentration, water temperature, and dissolved oxygen (DO) were specified daily for each grid cell and repeated for each year of the simulation. A bioenergetics model was used to represent growth, mortality was assumed stage- and age-dependent, and the movement behavior of juveniles and adults was modeled based on temperature and avoidance of low DO. Hypoxia effects were imposed using exposure effect submodels that converted time-varying exposures to low DO to reduced hourly growth, increased hourly mortality, and reduced annual fecundity. Results showed that 100 years of either mild or intermediate hypoxia produced small reductions in population abundance, while repeated severe hypoxia caused a 19% reduction in long-term population abundance. Relatively few individuals were exposed to low DO each hour, but many individuals experienced some exposure. The response was dominated by a 5% average reduction in annual fecundity of individuals. Under conditions of random sequences of mild, intermediate, and severe hypoxia years occurring in proportion to their historical frequency, the model predicted a 10% decrease in the long-term population abundance of croaker. A companion paper substitutes hourly DO values from a three-dimensional water quality model for the idealized hypoxia and results in a more realistic population reduction of about 25%. 相似文献
17.
Disturbances influence ecological communities over a wide range of scales. We investigated the effects of localized hypoxic
disturbances on an estuarine fish assemblage at several spatial (m2 and 10s km2) and temporal (days, seasons, years) scales in a multivariate framework (temperature, salinity, depth, dissolved oxygen).
We examined whether seasons, years) scales in a multivariate framework (temperature, salinity, depth, dissolved oxygen). We
examined whether there were consistent changes in fish and crustacean estuarine assemblage characteristics along environmental
gradients and whether these relationships were altered by hypoxic disturbances. We also investigated at what scale dissolved
oxygen concentration may be influencing the structure of motile estuarine assemblages and whether the size of the hypoxic
zone altered its effects on the estuarine assemblage. Hypoxic disturbances altered fish and crustacean assemblages along the
depth gradients that were present during well-oxygenated periods. Species diversity, richness, and catch rates were lower
in hypoxic patches than in oxygenated areas. Dissolved oxygen concentration remained an important explanatory variable for
patch-level assemblage dissimilarity, species richness, and diversity when data were aggregated across seasons. When we examined
the data at a larger scale, by aggregating information across the study area, we did not detect influences of hypoxia on assemblage
structure. Fish moved out of local hypoxic zones, but remained within the estuary even in years with extensive hypoxia. There
was no effect of size of the hypoxic distrubance on whether organisms responded to hypoxia or on diversity or richness of
the study site. These results suggest that these local disturbances play an important role in structuring motile species assemblages
at a patch-level within an estuary, but regional factors such as recruitment and migration are important in influencing species
assemblages for the entire estuary over months and years. 相似文献
18.
Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15°C). In July, 75% of the DO readings were <5 mg 1?1, and one-third were <1 mg 1?1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6–12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr. *** DIRECT SUPPORT *** A01BY059 00002 相似文献
19.
Gotzon Basterretxea Antoni Jordi Marly Carolina Martínez-Soto Antonio Tovar-Sánchez 《Estuaries and Coasts》2017,40(5):1247-1262
To investigate to what extent episodic physical processes regulate nutrient availability and phytoplankton assemblages of the Mahon estuary (Minorca Island), we carried out an intensive field study during 2010–2011. During the study period, environmental conditions spanned from intense stratification to a continuous mixing and from lack of riverine inflow to intense runoff. Our data reveals a sequence of biogeochemical states of the estuary that result from the interplay between runoff, other non-periodic forcings (winds, sea level oscillations), and variations in water renewal. Seasonal runoff was revealed as a major driver of winter circulation and of the influx of inorganic nutrients, in particular nitrate. However, because of the combination between runoff and flushing time, the effects of floodwater events on phytoplankton are short-lived (days). Conversely, during summer, when freshwater influx declines, water renewal relies on pulsed atmospheric forcing that may be of local or remote origin. As depicted from the low nitrate concentrations (<1 μM) and enhanced ammonium (>1 μM), this change in circulation and external loads carries nutrient assimilation within the estuary head and forces the use of remnant nutrients through regenerating pathways to sustain an enhanced phytoplankton biomass at the lower estuary. Episodic variability represented between 52 and 65% of the annual chlorophyll variance. Despite the fact that episodic pulses represented intense departures from base biogeochemical state of the estuary, at time scale larger than weeks, the phytoplankton community composition and dynamics was largely regulated by the integrated effect of these episodes and other environmental drivers associated with seasonality rather than by individual storm events only. Our results suggest that even though the system presents good recovery capacity to individual storm episodes, it may be more vulnerable to increased nutrient fluxes during summer, as well as to changes in episode timing and frequency. 相似文献
20.
A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O2: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O2 concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO3 −/I− redox couple quickly responded to suboxic conditions in the water column by a marked increase in I− concentrations and corresponding depletion of IO3 −. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn2+ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h−1. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments. 相似文献