Summertime Hypoxia Research Articles

Enhanced oxygen consumption results in summertime hypoxia in Mikawa Bay, Japan.

In Mikawa Bay, where hypoxia occurs in the bottom layer during summer, six shipboard observations were conducted from the mouth to the head of the bay from May to August 2014 to investigate the spatiotemporal variation in the bottom layer oxygen consumption rate (OCR). The OCR was determined from the dark incubation of sample waters using an optical oxygen sensor, which showed a range of 5.7-38.3mmolm-3days-1. A high OCR was observed at the inner-most station, where higher concentrations of nutrients and chlorophyll a (Chl a) than at the other stations were found, and bottom hypoxic water appeared during the observation period after late June. These OCRs can deplete the oxygen dissolved in water within a week. The OCR showed a highly significant positive correlation with particulate organic carbon concentrations in the bottom water. Considering the relatively low carbon-to-nitrogen mole ratio (~ 6.4-7.6) and high carbon isotope ratio (between approximately - 20.2 and - 18.8‰) of particulate organic matter at the stations, the supply of fresh organic matter produced in the bay as opposed to the land may have affected the OCR by acting as a substrate for microbial aerobic respiration. High temporal resolution data from two automated observation buoys near the bay mouth and the inner area captured increases in Chl a at both sites in response to typhoon events, along with the subsequent appearance of bottom hypoxic water at the inner site and its expansion at the mouth. This supports our hypothesis that enhanced organic matter production due to nutrient input to the surface layer through vertical mixing would increase the bottom OCR, resulting in hypoxia. The apparent oxygen decline in the bottom layer from the buoy data was consistent with incubation-based OCRs during the observation period. Therefore, it is essential to model the OCR in numerical simulations of hypoxia, to which the variability characteristics that we revealed made significant contributions.

Effects of Biophysical Processes on Diel-Cycling Hypoxia in a Subtropical Estuary

In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m3 s-1) and low (< 175 m3 s-1) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m-3), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.

Habitat use and activity patterns of female Deacon Rockfish (Sebastes diaconus) at seasonal scales and in response to episodic hypoxia

We combined a high-resolution acoustic telemetry array with presence/absence receivers to conduct a preliminary study of the seasonal movements, activity patterns, and habitat associations of the newly described Deacon Rockfish (Sebastes diaconus). Eleven mature female Deacon Rockfish were tagged and monitored during an 11-month period, at a nearshore rocky reef off Seal Rock, Oregon, USA, an area of recurring seasonal hypoxia (defined as dissolved oxygen concentration [DO] < 2 mg l−1). Two tags were detected leaving the study area by day 35, indicating predation or emigration. Three tags became inactive within the array, indicating tag loss or fish death. Six “resident” fish inhabited the array for 246–326 days. Resident fish exhibited high site fidelity, small home ranges (mean 95% KDE = 4907 m2), and consistent activity patterns for the duration of the summertime high-resolution array (5 months), except during seasonal hypoxia. Resident fish were strongly diurnal in summer, with high levels of daytime activity above the bottom in relatively rugose habitat, followed by nighttime rest periods in deeper, less rugose habitat. During summertime hypoxia, resident fish exhibited less daytime activity during daytime hours with no rest periods at night, inhabited shallower water depths, and moved well away from their core activity areas on long, erratic forays. During the winter, diel patterns were less evident with higher activity levels at night (than in the summer) and lower activity levels in the day (than in the summer). We propose that some Deacon Rockfish continuously inhabit nearshore reefs throughout the year, but that daily/seasonal movement patterns, seasonally occurring hypoxia, and prey preferences for planktonic organisms influence relocation.

Effects of Physical Forcing on Summertime Hypoxia and Oxygen Dynamics in the Pearl River Estuary

A validated hydrodynamic-biogeochemical model was applied to investigate the effects of physical forcing (i.e., river discharge, winds, and tides) on the summertime dissolved oxygen (DO) dynamics and hypoxia (DO &lt; 3 mg L−1) in the Pearl River estuary (PRE), based on a suite of model sensitivity experiments. Compared with the base model run in 2006 (a wet year), the simulated hypoxic area in the moderate year (with 75% of river discharge of the base run) and the dry year scenario (with 50% of river discharge of the base run) was reduced by ~30% and ~60%, respectively. This is because under the lower river discharge levels, less particulate organic matter was delivered to the estuary that subsequently alleviated the oxygen demand at the water–sediment interface, and in the meantime, the water stratification strength was decreased, which facilitated the vertical diffusion of DO. Regarding the effect of winds, the highly varying and intermittent strong winds had a significant impact on the replenishment of bottom DO by disrupting water stratification and thus inhibiting the development of hypoxia. Sensitivity experiments showed that the hypoxic area and volume were both remarkably increased in the low wind scenario (with a bottom hypoxic zone extending from the Modaomen sub-estuary to the western shoal in Lingdingyang Bay), whereas hypoxia was almost absent in the strong wind scenario. The DO budget indicated that winds altered the bottom DO mostly by affecting the DO flux due to vertical diffusion and horizontal advection, and had a limited influence on the DO consumption processes. Moreover, the DO concentration exhibited remarkable fluctuations over the spring-neap tidal cycles due to the significant differences in vertical diffusion. The results of a tide-sensitivity experiment indicated that without tide forcing, most of the shallow areas (average water depth &lt; 5 m) in the PRE experienced severe and persistent hypoxia. The tides mainly enhanced mixing in the shallow areas, which led to higher vertical diffusion and enhanced replenishment of bottom DO.

Emergence of Summertime Hypoxia and Concurrent Carbonate Mineral Suppression in the Central Bohai Sea, China

AbstractThe Bohai Sea to the west of the North Yellow Sea in the Northwest Pacific is a semienclosed shallow‐water marginal sea on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism‐induced summertime dissolved oxygen (DO) decline and acidification, eight field surveys were conducted between August 2011 and September 2017, investigating seasonal and interannual variations in DO and carbonate system parameters in the central Bohai Sea. The historically lowest Bohai Sea DO was recorded at 66 μmol O2/kg in subsurface waters in early September 2015. By comparing our data set with literature data obtained in the 1980s, 1990s, and 2000s, the emergence history of summertime hypoxia in this marginal sea was shown. Associated with the seasonal DO shortage, the metabolism‐enhanced CO2 accumulation in subsurface waters in summer resulted in critical aragonite saturation states of close to or even less than 1.5, threatening calcifiers with potentially severe consequences for valuable shellfish fisheries. Based on model results combining the aerobic respiration (as indicated by apparent oxygen utilization) and the given wintertime/springtime atmospheric CO2 level of the time, this study suggests that combined stresses of the seasonal DO shortage and CO2 acidification capable of threatening Bohai Sea ecosystems could develop within decades.

Characteristics of the Carbonate System in a Semiarid Estuary that Experiences Summertime Hypoxia

In oceanic environments, two sources of CO2 have been found to contribute to acidification of stratified water bodies, i.e., CO2 invasion due to anthropogenic atmospheric CO2 increase and respiration-produced CO2 from organic matter remineralization. Acidification caused by these CO2 sources has been observed frequently in numerous environments spanning from open continental shelves to enclosed estuaries. Here, we report observations on carbonate system dynamics in a relatively well-buffered lagoonal estuary, Corpus Christi Bay (CCB), in a semiarid subtropical region that is influenced by summertime hypoxia as well as strong evaporation and seagrass vegetation in the vicinity. While the relationship between dissolved oxygen (DO) and pH in the bottom waters of CCB was positive as in other coastal and estuarine environments prone to hypoxia, the slope was significantly less than in other systems. We attribute the high buffering capacity in CCB to the presence of abundant seagrass meadows adjacent to CCB and strong evaporation-produced density flow that delivers low CO2 waters to the bottom of CCB. Thus, despite the occurrence of hypoxia, neither bottom water carbonate saturation state with respect to aragonite (Ωarg) nor CO2 partial pressure (pCO2) reached critical levels, i.e., undersaturation (i.e., Ωarag 1000 µatm), respectively.

Chemical oceanography of the Arabian Gulf

Most of the available information about basin-wide chemical processes in the Arabian Gulf is based on observations made during the expeditions of the Research Vessels Meteor (1965), Atlantis II (1977) and Umitaka-Maru (1993–1994) with the ongoing national programs in shallow territorial waters largely focusing on local environmental issues. Despite the acute paucity of data, it is clear that the Gulf's biogeochemistry and ecosystems are experiencing massive changes resulting from human activities. The two key components of these changes are a reduction in river runoff that has caused a large increase in salinity in the northern part of the Gulf, and an increase in nutrient loading largely through sewage discharge from the sprawling urban centers along the coasts. The change from a pristine, oligotrophic state to a human-perturbed, eutrophic system is not so clear from the long-term nutrient and chlorophyll trends, presumably because of rapid uptake of nutrients by the phytoplankton and consumption of the photosynthesized organic matter by the heterotrophs. However, eutrophication has led to large increases in the frequency and intensity of phytoplankton blooms since the 1990s. It may also account for the high total organic carbon (TOC) concentration in the water column and the recent development of summer-time hypoxia in near-bottom waters of the central Gulf, which have important implications for the future evolution of the oxygen minimum zone of the North-Western Indian Ocean. The high atmospheric dust deposition maintains high dissolved trace metal concentrations in seawater. There are several intriguing and hitherto unresolved aspects of nutrient budgets in the Arabian Gulf, mostly arising from the differences between nutrient concentrations in the inflowing and outflowing currents, highlighting the unknown roles and strengths of the local sources and sinks. However, even though the latter are being modified by human activities, the N:P concentrations in the deeper part of the Gulf vary in Redfieldian proportion. An interesting feature of nitrogen biogeochemistry in the Gulf is the accumulation of high nitrite in the euphotic zone. The relative contributions of nitrification and assimilatory nitrate reduction by phytoplankton to the nitrite build up are not known. Dissolved nitrous oxide concentrations are slightly above saturation. The extremely high thermohaline variability and biogenic carbonate production within the Gulf also make inorganic carbon cycling in the Gulf quite unusual. However, this remains one of the least studied topics of research in the Gulf. Another subject of disproportionately high significance is benthic biogeochemistry due to shallow depths and high temperatures within the Gulf. Since the small-sized Gulf is shared by eight countries, active regional cooperation is essential to understand its processes and come up with measures to address the serious threats faced by its biogeochemistry and ecosystems from human activities.

Impact of Seabed Resuspension on Oxygen and Nitrogen Dynamics in the Northern Gulf of Mexico: A Numerical Modeling Study

AbstractResuspension affects water quality in coastal environments by entraining seabed organic matter into the water column, which can increase remineralization, alter seabed fluxes, decrease water clarity, and affect oxygen and nutrient dynamics. Nearly all numerical models of water column biogeochemistry, however, simplify seabed and bottom boundary layer processes and neglect resuspension. Here we implemented HydroBioSed, a coupled hydrodynamic‐sediment transport‐biogeochemical model to examine the role of resuspension in regulating oxygen and nitrogen dynamics on timescales of a day to a month. The model was implemented for the northern Gulf of Mexico, where the extent of summertime hypoxia is sensitive to seabed and bottom boundary layer processes. Results indicated that particulate organic matter remineralization in the bottom water column increased by an order of magnitude during resuspension events. This increased sediment oxygen consumption and ammonium production, which were defined as the sum of seabed fluxes of oxygen and ammonium, plus oxygen consumption and ammonium production in the water column due to resuspended organic matter. The increases in remineralization impacted biogeochemical dynamics to a greater extent than resuspension‐induced seabed fluxes and oxidation of reduced chemical species. The effect of resuspension on bottom water biogeochemistry increased with particulate organic matter availability, which was modulated by sediment transport patterns. Overall, when averaged over the shelf and on timescales of a month in the numerical model, cycles of erosion and deposition accounted for about two thirds of sediment oxygen consumption and almost all of the sediment ammonium production.

Origin and Dynamics of Dissolved Organic Matter in a Mariculture Area Suffering From Summertime Hypoxia and Acidification

Based on six cruises from March to September in 2016, we investigated monthly distributions of dissolved organic matter (DOM) and ancillary water chemistry parameters in a mariculture area in the Northern Yellow Sea, where summertime hypoxia and seawater acidification were observed. The most severe oxygen depletion (hypoxia covered approximately one third of the aquaculture area) and the largest pH decrease (8.07 ± 0.05 in surface layer vs. 7.66 ± 0.07 in bottom layer) were revealed in August. Concentration of dissolved organic carbon (DOC) and the absorption properties of chromophoric DOM (CDOM) were used to characterize DOM. Results showed that DOM was mainly originated from marine in situ processes. In March, a DOM pool with the lowest DOC concentration of 2.53 ± 0.28 mgC L-1 and nearly uniform optical characteristics were presented in the well-mixed water column. In August, however, DOC increased to 4.33 ± 0.35 mgC L-1 in the surface layer and 4.10 ± 0.30 mgC L-1 in the bottom layer. Two nonlinear relationships between the absorption coefficient at 355 nm (aCDOM(355)) and the absorption spectral slope over 275–295nm (S275-295) were revealed. According to modelling results, the nonlinear relationships were mostly caused by the conservative mixing of a refractory CDOM pool with a freshly-produced CDOM pool. Apparent oxygen utilization in August was positively related to DOC but not to aCDOM(355) and S275-295, presumably due to multiple sources of CDOM in bottom waters. The absorption properties of CDOM respond largely to varied pH, however, they would be less affected by ocean acidification since the latter process leads to a limited pH decline.

Physical dynamics structures and oxygen budget of summer hypoxia in the Pearl River Estuary

AbstractA summertime hypoxia sporadically occurred in the lower Pearl River Estuary (PRE) for more than three decades. Although its mechanism has already been extensively studied, the topic on why seasonal hypoxia is persistent in patchy waters is still an open question. Here, we presented the investigation of physical dynamics structures and dissolved oxygen (DO) processes for controlling the spatial distribution and maintenance of coastal hypoxia. Field observations were conducted in the 2015 summer in the PRE and adjacent shelf sea. High river discharge forms intense haloclines in the river plume, while salinity intrusion of shelf benthic waters results in a notable pycnocline at the top of salt wedge. A mid‐depth transitional layer with the weakest mixing over water column functions as a barrier for DO vertical exchange between river plume and shelf salt wedge. A benthic hypoxia in the 2015 summer appears at the overlapping zone between river plume and shelf salt wedge. Based on physical and biological processes, a DO budget for the hypoxic system was established. The DO advection by gravitational circulation from shelf benthic waters is roughly balanced by bacterial respiration in water column. The DO diffusion from river plume to benthic hypoxia is completely inhibited by the barrier layer. The patchy distribution of benthic hypoxia for the 30‐yr period in the PRE can be satisfactorily predicted by the numerical simulations of the overlapping zones between river plume and shelf salt wedge. These findings will have an important implication for predicting and mitigating coastal hypoxia.Physical structures and processes of DO dynamics were investigated to understand the spatial distribution and maintenance of coastal hypoxia. Summertime hypoxia appear near the head of shelf salinity intrusion, where a mid‐depth barrier layer inhibits the vertical exchange between river plume and shelf salt wedge. DO advection by gravitational circulation from DO‐rich shelf benthic waters is roughly balanced by bacterial respiration in water column. The spatial distribution of coastal hypoxia can be well predicted by the overlapping zone between river plume and shelf salt wedge.

Evidence of persistent, recurring summertime hypoxia in Green Bay, Lake Michigan

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (<5 mg L−1) affect nearly 70% of the 38 stations sampled representing an area of ~500–600 km2. Stratification typically lasts 2+ months, from late June to early September, and some stations exhibit bottom water hypoxia (<2 mg L−1) at a frequency of nearly 25% when sampled during this period. A monitoring program initiated in 1986 by the Green Bay Metropolitan Sewerage District has provided a 23 year, recreational season record (May–September) of continuous (15 min interval) in situ bottom water oxygen and temperature measurements at the Entrance Light station of the Green Bay navigational channel. The duration of the hypoxic season ranges from 2 weeks to over 3 months at this shallow 7 m offshore site. This variability likely results from a combination of thermal stratification, oxygen consumption in deeper waters of the bay, and physical forcing mechanisms that drive cool, oxygen depleted, bottom waters on a southerly trajectory across this sensor. These data suggest the duration of hypoxic conditions may have increased during the stratified season in recent years. Hypoxia in the bay would also appear to be sensitive to relatively small changes in these forces, particularly changes in organic carbon loading and the duration of stratification.

Long-Term Phosphorus Loading and Springtime Temperatures Explain Interannual Variability of Hypoxia in a Large Temperate Lake.

Anthropogenic eutrophication has led to the increased occurrence of hypoxia in inland and coastal waters around the globe. While low dissolved oxygen conditions are known to be driven primarily by nutrient loading and water column stratification, the relative importance of these factors and their associated time scales are not well understood. Here, we explore these questions for Lake Erie, a large temperate lake that experiences widespread annual summertime hypoxia. We leverage a three-decade data set of summertime hypoxic extent (1985-2015) and examine the role of seasonal and long-term nutrient loading, as well as hydrometeorological conditions. We find that a linear combination of decadal total phosphorus loading from tributaries and springtime air temperatures explains a high proportion of the interannual variability in average summertime hypoxic extent (R2 = 0.71). This result suggests that the lake responds primarily to long-term variations in phosphorus inputs, rather than springtime or annual loading as previously assumed, which is consistent with internal phosphorus loading from lake sediments likely being an important contributing mechanism. This result also demonstrates that springtime temperatures have a substantial impact on summertime hypoxia, likely by impacting the timing of onset of thermal stratification. These findings imply that management strategies based on reducing tributary phosphorus loading would take several years to reap full benefits, and that projected future increases in temperatures are likely to exacerbate hypoxia in Lake Erie and other temperate lakes.

Physical Processes Contributing to Localized, Seasonal Hypoxic Conditions in the Bottom Waters of Smithtown Bay, Long Island Sound, New York

ABSTRACT Swanson, R.L.; Bauer, C.L.; Wilson, R.E.; Rose, P.S., and O'Connell, C., 2016. Physical processes contributing to localized, seasonal hypoxic conditions in the bottom waters of Smithtown Bay, Long Island Sound, New York. Summertime hypoxia regularly occurs in the bottom waters of Smithtown Bay, Long Island Sound, New York. Hypoxia is plaguing many coastal estuaries and can be detrimental to aquatic organisms. A generally accepted model for the cause of coastal hypoxia in temperate regions is seen in the introduction of anthropogenic nutrients, which fuels excessive primary production, leading to oxygen depletion exacerbated by seasonal water-column stratification. No major point sources of anthropogenic nitrogen discharge directly into the bay. Groundwater nitrogen discharge is somewhat more than double that of a small sewage treatment plant. Despite great efforts to reduce nitrogen loading into Long Island Sound, summertime hypoxia is a continuing occurrence in Smithtown Bay. Hydrographic cruise...

Understanding the flushing capability of Bellingham Bay and its implication on bottom water hypoxia

In this study, an unstructured-grid finite-volume coastal ocean model (FVCOM) was used to simulate hydrodynamic circulation and assess the flushing capability in Bellingham Bay, Washington, USA. The model was reasonably calibrated against field observations for water level, velocity and salinity, and was further used to calculate water residence time in the study site. The model results suggest that, despite the large tidal ranges (∼4 m during spring tide), tidal currents are relatively weak in Bellingham Bay with surface currents generally below 0.5 m/s. The local water residence time in Bellingham Bay varies from near zero to as long as 15 days, depending on the location and river flow conditions. In general, Bellingham Bay is a well-flushed coastal embayment affected by freshwater discharge, tides, wind, and density-driven circulation. The basin-wide global residence time ranges from 5 to 7 days. The model results also provide useful information on possible causes of the emerging summertime hypoxia problem in the north central region of Bellingham Bay. Model results suggest that the formation of hypoxia in bottom water results from an enhanced oxygen consumption rate in the oceanic bottom water inflow with low dissolved oxygen by organic matters accumulated at the regions characterized with relatively long residence time in summer months.

Evidence for Directional Wind Response in Controlling Inter-annual Variations in Duration and Areal Extent of Summertime Hypoxia in Western Long Island Sound

Observations of summertime hypoxia duration and hypoxia areal extent in western Long Island Sound from 1991 to 2010 are correlated with local wind forcing. Results show a strong dependence on wind direction consistent with straining due to axial winds. The analysis of current data from moored ADCPs in the western Sound shows that the dominant mode of response is that of axial currents to axial winds. Estimates for the Wedderburn number (W) are relatively low ranging from 0.75 to 2.5 for prevailing winds, putting the western Sound in a regime more dominated by wind straining than by wind mixing. Estimates for the Kelvin number (Ke) range from approximately 1 to 6 in this diverging channel suggesting the importance of rotation; in the wide part of the basin, current observations show that significant vertically sheared lateral currents develop consistent with the tilting of planetary vortex lines by wind-driven axial current shear. Lateral straining, however, offsets longitudinal straining in the wide part of the basin supporting the hypothesis that simple longitudinal straining associated with axial winds exerts the most important control on the development summertime hypoxia in the constricted part of the western Sound.

Evaluation of 224Ra as a tracer for submarine groundwater discharge in Long Island Sound (NY)

The approach to quantify submarine groundwater discharge using Ra isotopes generally involves developing a Ra mass balance in an estuary, bay or lagoon. In this work we present a 224Ra mass balance used to evaluate the importance of the submarine groundwater discharge (SGD) in Long Island Sound (NY, US), the third most important estuary in US, located between Long Island and Connecticut that is usually affected by summertime hypoxia in the western basin. Three surveys were conducted between April 2009 and August 2010 where 25 water stations were sampled for Ra isotopes, oxygen and Mn. Stations were oriented along 4 transects: one axial extending from the western to the eastern Sound and three longitudinal transects in the western, central and eastern Sound. The inventory of 224Ra in the water column in summer was circa 2 times greater than in winter, suggesting an increased 224Ra flux to the Sound in summer. A mass balance for 224Ra was constructed considering tidal exchange, inputs from rivers, desorption from resuspended particles, diffusive fluxes (including bioirrigation) from bottom sediments and radioactive decay in the water column. Fluxes of 224Ra from bottom sediments were measured by incubating cores under oxic conditions in a continuous flow mode such that the overlying water was circulated through a Mn-oxide fiber to maintain a constant activity of 224Ra. Fluxes from muddy sediments (comprising ∼67% of the Sound bottom) ranged from 127 to 312dpmm−2d−1 and were ∼60dpmm−2d−1 in sandy sediments (33% of the Sound). Incubations under hypoxic conditions showed variable fluxes depending on reduction and mobilization of Mn. The 224Ra mass balance shows a net input of Ra to the Sound of 106±50×1012dpmy−1 in spring and 244±112×1012dpmy−1 in the summer that is attributed to SGD. Elevated 224Ra values were observed near shore and in the pore fluids of the coarse beach sands along the Long Island and Connecticut coasts, suggesting that SGD driven by tidal recirculation through the beach face is a major source of 224Ra to the Sound. Seasonal variation in this source seems unlikely, and the calculated 224Ra SGD fluxes for spring and summer overlap within the uncertainties. Nevertheless we conclude that variations in the 224Ra water column inventories could be produced by seasonal changes in bioirrigation and/or redox cycling of Mn as well as sediment resuspension and desorption of 224Ra from resuspended particles, and that our mass balance underestimates these terms, particularly in the summer. 224Ra fluxes from sediments in estuaries, especially those with significant areas of muddy sediments and seasonal hypoxia, are important and should be well constrained in future uses of this isotope as a tracer for SGD.

Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading

A previously observed shift in the relationship between Chesapeake Bay hypoxia and nitrogen loading has pressing implications on the efficacy of nutrient management. Detailed temporal analyses of long-term hypoxia, nitrogen loads, and stratification were conducted to reveal different within-summer trends and understand more clearly the relative role of physical conditions. Evaluation of a 60-year record of hypoxic volumes demonstrated significant increases in early summer hypoxia, but a slight decrease in late summer hypoxia. The early summer hypoxia trend is related to an increase in Bay stratification strength during June from 1985 to 2009, while the late summer hypoxia trend matches the recently decreasing nitrogen loads. Additional results show how the duration of summertime hypoxia is significantly related to nitrogen loading, and how large-scale climatic forces may be responsible for the early summer increases. Thus, despite intra-summer differences in primary controls on hypoxia, continuing nutrient reduction remains critically important for achieving improvements in Bay water quality.

Hypoxia in the Pearl River Estuary, the South China Sea, in July 1999

The Pearl River Estuary empties into the South China Sea in southeastern China, and hypoxia is one of the major environmental problems here during summer. It can be caused by coastline, topography, hydrodynamics, tidal dynamics and biological processes (mainly by stratification and oxidation reaction). Since ecological processes are closely related to dynamic processes, we revise and link together both a physical and a biological model to establish a coupled hydrodynamic-ecological numerical model to study the summertime hypoxia. The model simulates physical and biological processes and reproduces the observed hypoxia in the bottom layer in two regions located in west shallow water and around Nei Lingding Island. A low-salinity plume forms after the freshwater drifts offshore. Strong stratification exists in the water body shoreward where the vertical mixing is weak. The salinity front and stratification cause a poor exchange in the vertical direction and greatly influence biological processes for nutrients, phytoplankton, and detritus, which induce the hypoxia occurrence in the bottom layer of the Pearl River Estuary. On the other hand, in two troughs the tide diminishes the strength of stratification and prohibits the large-scale growth of the hypoxia. The physical processes related to topography, run off and tidal mixing regulate the distribution of biological factors and the occurrence of hypoxia.

Ecological network analyses and their use for establishing reference domain in functional assessment of an estuary

We developed a framework to use ecological network analysis for functional assessment of large aquatic ecosystems in the context of ecosystem-based management. We established a reference domain for the Neuse River Estuary, North Carolina, USA, from changes over time. Four reference network models of the trophic structure of the estuary during early and late summers of 1997 and 1998 were constructed and analyzed. The estuary has experienced various symptoms of eutrophication during the past 20 years, including summer-time hypoxia and fish kills. The networks were used to quantify indices of nominal trophic dynamics and their variation. The ratio of biomass of nekton to that of macrobenthos, derived from network construction, was used to index severity of eutrophication and to promote accessibility of ecological network analysis to environmental management. The ratio increased from early to late summer, and network metrics demonstrated a variety of responses in association with that change. Some variables from network analysis, especially related to consumers, reflected some but not all of this change. Others reflected the most severe increase in the ratio in late summer 1997 when hypoxia was most extensive. We evaluated uncertainty and the modulating effects of hierarchy by comparing variation of input biomasses with integrative response variables. Relative variation in input variables was generally greater than that of the integrative response variables as predicted by hierarchy theory. Ecological network analysis has previously served as support for ecosystem-based management of large aquatic systems with some success. However, its use can be enhanced by making it more accessible to environmental managers and policy makers. Ways to do this include promoting simple metrics from network construction and explicitly associating network analysis to concepts familiar to the management community, such as functional assessment and reference.

A carbon, nitrogen, and sulfur elemental and isotopic study in dated sediment cores from the Louisiana Shelf

Three sediment cores were collected off the Mississippi River delta on the Louisiana Shelf at sites that are variably influenced by recurring, summer-time water-column hypoxia and fluvial loadings. The cores, with established chronology, were analyzed for their respective carbon, nitrogen, and sulfur elemental and isotopic composition to examine variable organic matter inputs, and to assess the sediment record for possible evidence of hypoxic events. Sediment from site MRJ03-3, which is located close to the Mississippi Canyon and generally not influenced by summer-time hypoxia, is typical of marine sediment in that it contains mostly marine algae and fine-grained material from the erosion of terrestrial C4 plants. Sediment from site MRJ03-2, located closer to the mouth of the Mississippi River and at the periphery of the hypoxic zone (annual recurrence of summer-time hypoxia >50%), is similar in composition to core MRJ03-3, but exhibits more isotopic and elemental variability down-core, suggesting that this site is more directly influenced by river discharge. Site MRJ03-5 is located in an area of recurring hypoxia (annual recurrence >75%), and is isotopically and elementally distinct from the other two cores. The carbon and nitrogen isotopic composition of this core prior to 1960 is similar to average particulate organic matter from the lower Mississippi River, and approaches the composition of C3 plants. This site likely receives a greater input of local terrestrial organic matter to the sediment. After 1960 and to the present, a gradual shift to higher values of δ13C and δ15N and lower C:N ratios suggests that algal input to these shelf sediments increased as a result of increased productivity and hypoxia. The values of C:S and δ34S reflect site-specific processes that may be influenced by the higher likelihood of recurring seasonal hypoxia. In particular, the temporal variations in the C:S and δ34S down-core are likely caused by changes in the rate of sulfate reduction, and hence the degree of hypoxia in the overlying water column. Based principally on the down-core C:N and C:S ratios and δ13C and δ34S profiles, sites MRJ03-3 and MRJ03-2 generally reflect more marine organic matter inputs, while site MRJ03-5 appears to be more influenced by terrestrial deposition.