Hypoxia In Estuaries Research Articles

Simulating implications of fish behavioral response for managing hypoxia in estuaries with spatial dissolved oxygen variability

Hypoxia, or low dissolved oxygen (DO), is a widespread water quality problem affecting estuaries and coastal waters around the world. Water quality criteria for DO have been established for every estuary in the US and are an important part of the regulatory response to nutrient pollution and associated anthropogenic eutrophication. Experimental studies examining effects of low DO exposure have been to quantify outcomes based on hypoxia effects observed in individuals, such as increased mortality or growth impairment. Although laboratory exposure tests provide useful benchmarks for policy development, most of those considered in policy development did not consider behavioral responses to low DO. However, experimental research has shown that behavioral responses occur, and that behavior modifies exposure to low DO conditions. Here we begin development of a spatially explicit individual based model (SEIBM) intended to project behavioral outcomes of exposure to spatially variable hypoxia in estuaries. Our goal is to consider the responsiveness of an SEIBM to both different behavioral hypotheses, as well as realistic spatial patterns in hypoxia. A sensitivity analysis was used to explore responsiveness based on two movement strategies: avoidance and behavioral switching. We tested the sensitivity of a suite of movement parameters to changes in spatial patterns representative of an index estuary. The sensitivity analysis demonstrated that model responses to changes in movement strategies include biologically meaningful changes in site occupancy and movement distance centered on individual behavior near a normoxic–hypoxic boundary. Further, the model demonstrated important sensitivity to realistic changes in movement parameters, including the size and shape of the individual neighborhood describing knowledge useful for movement decisions. These results support the utility of the developed SEIBM for exploring behavioral responses of fish to hypoxia in estuaries. The sensitivity analysis also demonstrates parameter values that must be set based on empirical data and are sensitive to data quality. These results will be used to further develop the model and to plan field and laboratory studies to support model parametrization. The end goal is a model framework that can inform policy decisions regarding hypoxia resulting from anthropogenic nutrient loading in estuaries.

Marsh archive reveals human population history and future implications for estuarine health in Long Island Sound

Coastal marshes are efficient ecosystems providing a multitude of benefits for invertebrates, birds, fish and humans alike. Yet despite these benefits, wetlands are threatened by anthropogenic inputs such as human wastewater which contain high levels of nitrogen (N). Increased nitrogen loads cause eutrophication and hypoxia in estuaries leading to further degradation of these valuable ecosystems that are already stressed by sea level rise and climate change. Policies to protect wetlands via wastewater treatments are reactive rather than proactive and a growing body of research shows that characteristics associated with population health and economic activity can be identified in wastewater. Analysis of a 2-m salt marsh sediment core reveals δN15 signatures indicative of human population rise and connects human impact to ecosystem health. Using key X-ray fluorescence (XRF), pollen, sediment and nitrogen signatures along the core, a robust chronology was produced dating back to 1700. This result was coupled with population data to observe the relationship between δN15 levels and population over three centuries. There is a statistically significant positive correlation between δN15 and population. Other external factors such as federal government policies (regulating clean water) show a clear reduction in this association but the use of synthetic nitrogen fertilizer masks the strength of this relationship. Further research to refine the relationship between population and δN15 could be beneficial in predicting nitrogen loads as human population grows, which in turn would create a proactive system to protect our coastal ecosystems.

Spatiotemporal Characteristics of Dissolved Oxygen and Control Mechanism of Hypoxia (Low Oxygen) in the Watershed-Coastal System in Fujian Province

Dissolved oxygen is a key parameter to measure water environment quality and ecosystem health. Currently, the problem of hypoxia (low oxygen) is prominent in coastal areas in China, but there is a lack of research on the spatiotemporal characteristics of dissolved oxygen and the control mechanism of hypoxia in the watershed-coastal system. Based on the data of 135 surface water (including estuaries) and 66 coastal water monitoring sites in Fujian Province from 2011 to 2020, this study analyzed the spatiotemporal variation pattern of dissolved oxygen at seasonal and interannual time scales. The data of hypoxia (10% quantile, corresponding to 67% saturation) were selected to study the characteristics and control mechanism of hypoxia in four types of water bodies (i.e., rivers, reservoirs, estuaries, and coastal waters) using mathematical statistics and a random forest model. The results showed that the dissolved oxygen saturation was the highest in the coast[(98.2±10.2)%] and the lowest in the estuary[(79.2±17.9)%]. Compared with that in the 12th Five-Year Plan (2011-2015), the frequency of hypoxia detection in rivers and reservoirs in the 13th Five-Year Plan (2016-2020) was significantly reduced, but the change in estuaries was not significant. Counting the points with hypoxia detection, the multi-year average hypoxia detection frequency of rivers and reservoirs was highest in autumn, and the frequency of estuaries was highest in summer. Hypoxia in reservoirs and estuaries was the most prominent but with different mechanisms. Specifically, hypoxia in reservoir reaches was related to summer runoff carrying large amounts of organic matter input, stratification leading to continuous oxygen depletion in the bottom water, and vertical mixing or discharge through dams in autumn, whereas hypoxia in estuaries was associated with strong pollution inputs and reductive materials. Systematic management and regionalized control mechanisms need to be established to further strengthen watershed-coastal pollution abatement to help mitigate eutrophication and hypoxia problems.

Distinct Features of Sedimentary Archaeal Communities in Hypoxia and Non-Hypoxia Regions off the Changjiang River Estuary.

ABSTRACTWater hypoxia (DO < 2 mg/L) is a growing global environmental concern that has the potential to significantly influence not only the aquatic ecosystem but also the benthic sedimentary ecosystem. The Changjiang River Estuary hypoxia, classified as one of the world's largest seasonal hypoxic water basins, has been reported to be expanding rapidly in recent decades. However, the microbial community dynamics and responses to this water hypoxia are still unclear. In this study, we examined the abundance, community composition, and distribution of sedimentary archaea, one important component of microbial communities in the Changjiang River Estuary and the East China Sea (ECS). Our results indicated that Thaumarchaeota and Bathyarchaeota were predominant archaeal groups in these research areas, with their 16S rRNA gene abundance ranged from 8.55 × 106 to 7.51 × 108 and 3.18 × 105 to 1.11 × 108 copies/g, respectively. The sedimentary archaeal community was mainly influenced by DO, together with the concentration of ammonium, nitrate, and sulfide. In addition, distinct differences in the archaeal community's composition, abundance, and driving factors were discovered between samples from hypoxia and non-hypoxia stations. Furtherly, microbial networks suggest various microbes leading the different activities in hypoxic and normoxic environments. Bathyarchaeota and Thermoprofundales were “key stone” archaeal members of the low-DO network, whereas Thaumarchaeota constituted a significant component of the high-DO network. Our results provide a clear picture of the sedimentary archaeal community in coastal hypoxia zones and indicates potential distinctions of archaea in hypoxia and non-hypoxia environments, including ecological niches and metabolic functions.IMPORTANCE In this study, the sedimentary archaeal community composition and abundance were detailed revealed and quantified based on 16S rRNA genes off the Changjiang River Estuary. We found that the community composition was distinct between hypoxia and non-hypoxia regions, while Thaumarchaeota and Bathyarchaeota dominated in non-hypoxia and hypoxia samples, respectively. In hypoxia regions, the sedimentary archaea were mainly affected by salinity, ammonium, and nitrate, whereas total organic carbon, total nitrogen, and sulfide were major influencing factors in non-hypoxia regions. The distinct microbial network may suggest the niche difference of archaeal community under various oxygen level.

Modeling the role of riverine organic matter in hypoxia formation within the coastal transition zone off the Pearl River Estuary

AbstractGlobally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river‐delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical‐biogeochemical model and observations to investigate how OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to hypoxia along the plume path is a combined result of the wind‐driven eastward shelf current and the OMterr‐released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream.

Continuous Monitoring Reveals Drivers of Dissolved Oxygen Variability in a Small California Estuary

Estuarine ecosystem diversity and function can be degraded by low oxygen concentrations. Understanding the spatial and temporal patterns of dissolved oxygen (DO) variation and the factors that predict decreases in DO is thus essential to inform estuarine management. We investigated DO variability and its drivers in Elkhorn Slough, a shallow, well-mixed estuary affected by high nutrient loading and with serious eutrophication problems. Long-term (2001–2012), high-resolution (15 min) time series of DO, water level, winds, and solar radiation from two fully tidal sites in the estuary showed that hypoxia events close to the bottom are common in the summer at the more upstream estuarine station. These events can occur in any lunar phase (spring to neap), at any time of the day, and both on sunny or cloudy days. They are, however, short-lived (lasting in average 40 min) and mainly driven by momentary low turbulent diffusion around slack tides (both at high and low water). Tidal advective transport explains up to 52.1% of the daily DO variability, and the water volume (or DO reservoir) contained in the estuary was not sufficient to avoid hypoxia in the estuary. Solar radiation was responsible for a positively correlated DO daily cycle but caused a decreased in the averaged DO in the summer at the inner station. Wind-driven upwelling reduced the average DO at the more oceanic station during spring. The approach we employed, using robust techniques to remove suspect data due to sensor drift combined with an array of statistical techniques, including spectral, harmonic, and coherence spectrum analysis, can serve as a model for analyses of long-term water quality datasets in other systems. Investigations such as ours can inform coastal management by identifying key drivers of hypoxia in estuaries.

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

Increased flux of carbon and nutrients from human activities in river basins were linked to acidification and deepwater hypoxia in estuaries and coastal areas worldwide. Annual loads (1995–2011) of suspended particulate matter (SPM), dissolved organic carbon (DOC), total nitrogen (TN) and total phosphorus (TP) were assessed at the Lake Ontario inlet of the St. Lawrence River (SLR) (7110 m3 s−1) and its estuarine outlet at Québec City (12,090 m3 s−1). Internal loads from the Ottawa River (1950 m3 s−1), seventeen other tributaries, urban wastewaters, atmospheric deposition and erosion were also estimated. Erosion (65% of SPM, 29% of TP), inflow from Lake Ontario (42% of DOC, 47% of TN) and Ottawa River (28% of DOC) contributed important flux to the estuary. Loads from other tributaries (20 and 27% of TN and TP at Quebec City) largely exceeded municipal sources (6% of exported TN and TP) and require future remediation. Aquatic plants fixed 277,000 t of C, 49,000 t of N and 7000 t of P (May–Sept.), delaying the nutrient flux to the estuary and turning the SLR into a nutrient sink over summers of lowest discharge. Degradation of exported organic C could consume 5.4–7.1 million t O2 year−1 in the estuary whereas SLR flux of N and P represent 31–47% and 7–14% of total annual estuarine flux, respectively. Carbon and Nitrogen flux from freshwaters partly explain the decline in pH and oxygen concentrations in deep estuarine waters thus highlighting the need to reduce diffuse sources of nutrients in the entire watershed.

Application of unstructured grid finite-volume coastal ocean model (FVCOM) to the Yangtze River hypoxic zone

Purpose To observe the relations between three important factors resulting in estuarial Hypoxia phenomena of Yangtze River – wind, river flow, and stratification. The purpose of the paper is to analyze the possible reasons why hypoxia occurs in late spring and disappears in October. Design/methodology/approach To overcome the difficulty in examining the role of physical and biological factors affecting hypoxia based on measurement alone, finite-volume community ocean model (FVCOM) was introduced into the investigation. Observed freshwater flux data from Yangtze River, monthly averaged wind speed data, and other observed data were input into the model; the accuracy of which was validated with various kinds of data. The authors used the trajectories of Lagrangian particles from Yangtze River to study the regions of strong riverine influence under different wind forcing conditions and compared the simulation results with former observations. Findings Trade wind is a significant factor to influence the forming and receding of hypoxia across the Yangtze River. Originality/value Using FVCOM to investigate estuary hypoxia is more economical and effective.

Forecasting future estuarine hypoxia using a wavelet based neural network model

Ecosystem based modeling and predictions of hypoxia in estuaries and their adjacent coastal areas have become increasingly of interest to researchers and coastal zone managers. Although progress has been made in modeling oxygen dynamics and short-term predictions, there is still a lack of long-term forecasts that incorporate multiple inputs including climatological effects such as El Niño-Southern Oscillation (ENSO) events. In this study, we first develop a hypoxic volume index (HVI) using 26-years of hypoxic volume (<62.5 μm g l−1) measurements from the main-stem of the Chesapeake Bay. Then a cross-wavelet analysis is used to identify and weight input parameters in order to build a neural network model of future hypoxic volume. The time-forward dynamic model uses cross-bay winds along with the Oceanic Niño Index (ONI), and Susquehanna River flow indexes to predict a hypoxic volume index over the next several years. Wavelet analysis indicates an anti-phase relationship between southwesterly winds and hypoxic volume index, and an 18-month phase lag between Susquehanna River index and hypoxic volume index. The neural network model results yield R-values of 0.99, and 0.91 for training, and validation and an R2 of 0.68 for predictions illustrating the usefulness and promise of these types of models for long-term predictions of hypoxic volume. Model results could be used as a climatologically based hypoxic volume baseline for comparing actual hypoxic volume response to nutrient load reductions.

Temporal distribution of bacterial community structure in the Changjiang Estuary hypoxia area and the adjacent East China Sea

Bacterial community structure and the effects of environmental factors on the microbial community distribution were investigated in the Changjiang Estuary hypoxia area and its adjacent area in the East China Sea (ECS) in June, August and October, 2006. Profiles of bacterial communities were generated by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes followed by DNA sequence analysis. The dominant bacterial groups were affiliated to Gammaproteobacteria, Cytophaga–Flavobacteria–Bacteroides (CFB), Deltaproteobacteria, Cyanobacteria and Firmicutes, which were mostly from the marine seawater ecosystem. Effects of environmental factors on the bacterial community distribution were analyzed by the ordination technique of canonical correspondence analysis (CCA). The environmental factors significantly influencing bacterial community structure were different in the three months; dissolved organic carbon (DOC) and temperature in June and nitrite in August. No environmental variables displayed significant influence on the bacterial community at the 5% level in October. The seasonal environmental heterogeneity in the Changjiang Estuary and the adjacent ECS, such as seasonal hydrodynamic conditions and riverine input of nutrients, might be the reason for the difference in the key environmental factors determining the bacterial community in the three months.

Temporal distribution of the archaeal community in the Changjiang Estuary hypoxia area and the adjacent East China Sea as determined by denaturing gradient gel electrophoresis and multivariate analysis

The archaeal community and the effects of environmental factors on microbial community distribution were investigated at five sampling sites in the Changjiang Estuary hypoxia area and the adjacent East China Sea in June, August, and October 2006. Profiles of the archaeal communities were generated by denaturing gradient gel electrophoresis of 16S rRNA genes followed by DNA sequence analysis, and the results were analyzed by multivariate statistical analysis. Denaturing gradient gel electrophoresis band patterns were analyzed by cluster analysis to assess temporal changes in the genetic diversity of the archaeal communities. Most of the October samples grouped together separately from those of June and August. Analysis of DNA sequences revealed that the dominant archaeal groups in the Changjiang Estuary hypoxia area and the adjacent East China Sea were affiliated with Euryarchaeota (mainly marine group II) and Crenarchaeota. The effects of environmental factors on the archaeal community distribution were analyzed by the ordination technique of canonical correspondence analysis. Salinity had a significant effect on the archaeal community composition.

Diel hypoxia in marsh creeks impairs the reproductive capacity of estuarine fish populations

Seasonal hypoxia in estuaries and continuous hypoxia in the laboratory significantly reduce the reproductive capacity of estuarine fish, but the impacts of diel hypoxia are unknown. This field study quantifies reproductive capacity in populations of gulf killifish (Fundulus grandis) sam- pled from sites with different dissolved oxygen cycles. Condition factor, gonadosomatic index (GSI), steroid sex hormones, and vitellogenin were measured in F. grandis captured in Weeks Bay (WB), Alabama (July 2004 and 2005) and Pensacola Bay (PB), Florida (July 2005). Compared to a control site with little or no diel hypoxia, testes and ovaries were significantly smaller under both moderate (WB: 2.61 mg l -1 , 0.6 h d -1 ; PB: 2.41 mg l -1 , 1.5 h d -1 ) and severe (0.93 mg l -1 , 3.4 h d -1 ) diel hypoxia. Male 11-ketotestosterone (11KT) concentrations were significantly lower under moderate hypoxia, while both testosterone (T) and 11KT concentrations were significantly reduced under severe diel hypoxia. In females, T concentrations were similar regardless of the occurrence or severity of hypoxia, but estradiol-17β (E2) concentrations were lower under severe diel hypoxia. Since T is unchanged, but its products 11KT and E2 are significantly reduced, hypoxia may affect reproduction by inhibiting spe- cific steroidogenic enzymes in the gonad. The association between diel hypoxia and lower reproduc- tive capacity was consistent across estuaries. Hypoxia-related changes in gulf killifish populations could affect trophic structure in marshes and bays since F. grandis move carbon from the upper marsh surface to the lower marsh, as increased biomass and as prey for juvenile piscivores.

Redox environment effect on redox sensitive elements in surface sediments of the Changjiang Estuary hypoxia zone

The grain size and element (including redox sensitive elements and terrigenous elements) concentration of surface sediments from the Changjiang Estuary hypoxia zone and its adjacent sea area were measured in this research. Based on the obtained data, the hypoxic environment’s influence on the distribution of elements in surface sediments was further studied. We believe that the “redox environment effect” greatly influences the distribution of the RSE, which reveals the “patchy enrichment pattern” offshore in the hypoxia zone, while the distribution of the terrigenous elements which shows the “stripped enrichment pattern” near shore is mainly affected by “granularity effects”. Due to the existence of the hypoxia zone of the Changjiang Estuary, the distribution of the RSE such as Mo, Cd and V in the study area exhibits the characteristics of “redox environment effects”.

Changes in mitochondrial gene and protein expression in grass shrimp, Palaemonetes pugio, exposed to chronic hypoxia

Spatial and temporal increases of hypoxia in estuaries are of major environmental concern. Since mitochondria consume most of the oxygen in the cell, we examined the potential role of mitochondrial gene and protein expression in adaptation to chronic hypoxia in the grass shrimp Palaemonetes pugio. Grass shrimp were exposed to DO levels slightly above and below the critical pO 2, 1.8 mg/L, for P. pugio, and hypoxia-induced alterations in gene expression were screened using custom cDNA macroarrays. Mitochondrial gene expression was not affected by exposure to moderate hypoxia (2.5 mg/L DO). However, chronic exposure to severe hypoxia (1.5 mg/L DO) for 7 days resulted in an increase of transcription of genes present in the mitochondrial genome (including 16S rRNA and Ccox 1), together with up-regulation of genes involved in Fe/heme metabolism. This pattern was completely reversed by day 14, when a significant down-regulation of these genes was observed. Separating mitochondrial proteins in two dimensions by IEF and reverse phase chromatography, followed by LC/MS/MS of differentially expressed proteins, showed cytochrome c oxidase subunit 2, encoded by Ccox 2, was down-regulated after 12 d exposure to severe hypoxia. It appears therefore that decreases in mitochondrial Ccox gene transcription result in decreased mitochondrial Ccox protein synthesis. These results suggest that mitochondrial genes and proteins show promise as molecular indicators of exposure to hypoxia.

VIBRIO AND PHYTOPLANKTON DYNAMICS DURING THE SUMMER OF 2004 IN A EUTROPHYING ESTUARY

Bacterioplankton response to eutrophication is a critical part of the transition from phytoplankton blooms to bottom‐water hypoxia in estuaries. This topic is of particular concern when endemic bacterial populations, such as Vibrio sp., may pose a health risk to local human populations using estuaries for recreation and food. The purpose of this study was to observe the dynamics of both Vibrio and phytoplankton in the Neuse River Estuary (NRE) in North Carolina, USA, during the summer of 2004. Vibrio concentrations, particle suspension characteristics, and chlorophyll a were measured in surface water collected at a series of stations along the estuarine gradient. Consistent with previous studies, a strong positive relationship between Vibrio concentration and salinity was found. Both Vibrio sp. and phytoplankton concentrations increased moving downstream, but beyond the salt front, phytoplankton declined while Vibrio sp. concentrations continued to increase. In surface waters, a large portion of suspensions was comprised of phytoplankton cells. The fraction of Vibrio cells attached to phytoplankton increased with phytoplankton concentration and decreased with increasing salinity. These observations of Vibrio dynamics in the NRE provide evidence that populations interact with phytoplankton populations in surface waters. This information may prove critical for models of estuarine bacterial dynamics in response to eutrophication.

Upwelling-driven nearshore hypoxia signals ecosystem and oceanographic changes in the northeast Pacific.

Seasonal development of dissolved-oxygen deficits (hypoxia) represents an acute system-level perturbation to ecological dynamics and fishery sustainability in coastal ecosystems around the globe. Whereas anthropogenic nutrient loading has increased the frequency and severity of hypoxia in estuaries and semi-enclosed seas, the occurrence of hypoxia in open-coast upwelling systems reflects ocean conditions that control the delivery of oxygen-poor and nutrient-rich deep water onto continental shelves. Upwelling systems support a large proportion of the world's fisheries, therefore understanding the links between changes in ocean climate, upwelling-driven hypoxia and ecological perturbations is critical. Here we report on the unprecedented development of severe inner-shelf (<70 m) hypoxia and resultant mass die-offs of fish and invertebrates within the California Current System. In 2002, cross-shelf transects revealed the development of abnormally low dissolved-oxygen levels as a response to anomalously strong flow of subarctic water into the California Current System. Our findings highlight the sensitivity of inner-shelf ecosystems to variation in ocean conditions, and the potential impacts of climate change on marine communities.