Hypoxic Estuary Research Articles

Searching for Oxygen: Dynamic Movement Responses of Juvenile Spot (Leiostomus xanthurus) in an Intermittently Hypoxic Estuary

Searching for Oxygen: Dynamic Movement Responses of Juvenile Spot (Leiostomus xanthurus) in an Intermittently Hypoxic Estuary

Zooplankton Abundance Reflects Oxygen Concentration and Dissolved Organic Matter in a Seasonally Hypoxic Estuary

Ocean deoxygenation, warming, and acidification resulting from global change and increasing nutrient inputs are major threats to marine ecosystems. Despite this, spatial and temporal patterns of oxygen availability and their impacts on marine life are understudied compared to warming and acidification, particularly in coastal tropical ecosystems. We measured the abundance of major groups of zooplankton in the context of five covarying environmental parameters [temperature, salinity, dissolved oxygen (DO), dissolved organic matter (DOM), and chlorophyll concentration] in a tropical estuary for one year. Partial least squares demonstrated that environmental conditions explained 20% of the variation in the community and found that temperature, salinity, DO, and DOM were most important (Variable Importance in Projection [VIP] > 0.8). A generalized linear model identified depth, DO, salinity, and chlorophyll as significant main effects, and temperature and DOM were also significant via two-way interactions (p < 0.05). When examined separately, the abundance of each zooplankton group was explained by a slightly different combination of environmental factors, but in all cases DO had large, significant effects, and in most cases DOM or its interactions were also significant. These results demonstrate that the seasonal cycle of hypoxia in this system significantly impacts the abundance of major zooplankton groups and likely also recruitment of benthic fauna through impacts on meroplankton and benthic-pelagic food webs.

Effects of Ferrous Iron and Hydrogen Sulfide on Nitrate Reduction in the Sediments of an Estuary Experiencing Hypoxia

Hypoxia is common feature of eutrophic estuaries and semi-enclosed seas globally. One of the key factors driving hypoxia is nitrogen pollution. To gain more insight into the effects of hypoxia on estuarine nitrogen cycling, we measured potential nitrate reduction rates at different salinities and levels of hypoxia in a eutrophic temperate microtidal estuary, the Neuse River Estuary, North Carolina, USA. We also tested the effect of hydrogen sulfide and ferrous iron additions on the nitrate reduction pathways. Overall, DNRA dominated over denitrification in this periodically hypoxic estuary and there was no correlation between the potential nitrate reduction rates, salinity, or dissolved oxygen. However, when hypoxia lasted several months, denitrification capacity was almost completely lost, and nearly all nitrate added to the sediment was reduced via DNRA. Additions of hydrogen sulfide stimulated DNRA over denitrification. Additions of ferrous iron stimulated nitrate consumption; however, the end product of nitrate consumption was not clear. Interestingly, substantial nitrous oxide formation occurred in sediments that had experienced prolonged hypoxia and were amended with nitrate. Given expanding hypoxia predicted with climate change scenarios and the increasing nitrate loads to coastal systems, coastal sediments may lose their capability to mitigate nitrogen pollution due to DNRA dominating over denitrification during extended hypoxic periods.

Species Composition and Distribution of Jellyfish in a Seasonally Hypoxic Estuary, Hood Canal, Washington

Seasonal hypoxia (≤2 mg dissolved oxygen L−1) can have detrimental effects on marine food webs. Recent studies indicate that some jellyfish can tolerate low oxygen and may have a competitive advantage over other zooplankton and fishes in those environments. We assessed community structure and distributions of cnidarian and ctenophore jellyfish in seasonally hypoxic Hood Canal, WA, USA, at four stations that differed in oxygen conditions. Jellyfish were collected in June through October 2012 and 2013 using full-water-column and discrete-depth net tows, concurrent with CTD casts to measure dissolved oxygen (DO). Overall, southern, more hypoxic, regions of Hood Canal had higher abundances and higher diversity than the northern regions, particularly during the warmer and more hypoxic year of 2013. Of fifteen species identified, the most abundant—the siphonophore Muggiaea atlantica and hydrozoan Aglantha digitale—reached peak densities > 1800 Ind m−3 and 38 Ind m−3, respectively. M. atlantica were much more abundant at the hypoxic stations, whereas A. digitale were also common in the north. Vertical distributions explored during hypoxia showed that jellyfish were mostly in the upper 10 m regardless of the oxycline depth. Moderate hypoxia seemed to have no detrimental effect on jellyfish in Hood Canal, and may have resulted in high population densities, which could influence essential fisheries and trophic energy flow.

貧酸素海域の生態系評価を目的とした内湾複合生態系モデル"ZAPPAI(雑俳)"の開発と適用 : 干潟創生、浚渫・覆砂、流入負荷削減施策に対する東京湾生態系の自律的応答と赤潮に対する耐性

貧酸素海域の生態系評価を目的とした内湾複合生態系モデル"ZAPPAI(雑俳)"の開発と適用 : 干潟創生、浚渫・覆砂、流入負荷削減施策に対する東京湾生態系の自律的応答と赤潮に対する耐性

Dissimilatory nitrate reduction to ammonium coupled to Fe(II) oxidation in sediments of a periodically hypoxic estuary

Estuarine sediments are critical for the remediation of large amounts of anthropogenic nitrogen (N) loading via production of N2 from nitrate by denitrification. However, nitrate is also recycled within sediments by dissimilatory nitrate reduction to ammonium (DNRA). Understanding the factors that influence the balance between denitrification and DNRA is thus crucial to constraining coastal N budgets. A potentially important factor is the availability of different electron donors (organic carbon, reduced iron and sulfur). Both denitrification and DNRA may be linked to ferrous iron oxidation, however the contribution of Fe(II)-fueled nitrate reduction in natural environments is practically unknown. This study investigated how nitrate-dependent Fe2+ oxidation affects the partitioning between nitrate reduction pathways using 15N-tracing methods in sediments along the salinity gradient of the periodically hypoxic Yarra River estuary, Australia. Increased dissolved Fe2+ availability resulted in significant enhancement of DNRA rates from around 10–20% total nitrate reduction in control incubations to over 40% in those with additional Fe2+, at several sites. Increases in DNRA at some locations were accompanied by reductions in denitrification. Significant correlations were observed between Fe2+ oxidation and DNRA rates, with reaction ratios corresponding to the stoichiometry of Fe2+-dependent DNRA. Our results provide experimental evidence for a direct coupling of DNRA to Fe2+ oxidation across an estuarine gradient, suggesting that Fe2+ availability may exert substantial control on the balance between retention and removal of bioavailable N. Thus, DNRA linked to Fe2+ oxidation may be of general importance to environments with Fe-rich sediments.

A Dissolved Oxygen Threshold for Shifts in Bacterial Community Structure in a Seasonally Hypoxic Estuary.

Pelagic ecosystems can become depleted of dissolved oxygen as a result of both natural processes and anthropogenic effects. As dissolved oxygen concentration decreases, energy shifts from macrofauna to microorganisms, which persist in these hypoxic zones. Oxygen-limited regions are rapidly expanding globally; however, patterns of microbial communities associated with dissolved oxygen gradients are not yet well understood. To assess the effects of decreasing dissolved oxygen on bacteria, we examined shifts in bacterial community structure over space and time in Hood Canal, Washington, USA−a glacial fjord-like water body that experiences seasonal low dissolved oxygen levels known to be detrimental to fish and other marine organisms. We found a strong negative association between bacterial richness and dissolved oxygen. Bacterial community composition across all samples was also strongly associated with the dissolved oxygen gradient, and significant changes in bacterial community composition occurred at a dissolved oxygen concentration between 5.18 and 7.12 mg O2 L-1. This threshold value of dissolved oxygen is higher than classic definitions of hypoxia (<2.0 mg O2 L-1), suggesting that changes in bacterial communities may precede the detrimental effects on ecologically and economically important macrofauna. Furthermore, bacterial taxa responsible for driving whole community changes across the oxygen gradient are commonly detected in other oxygen-stressed ecosystems, suggesting that the patterns we uncovered in Hood Canal may be relevant in other low oxygen ecosystems.

Spatial and temporal variation in nearshore macrofaunal community structure in a seasonally hypoxic estuary

Spatial and temporal variation in nearshore macrofaunal community structure in a seasonally hypoxic estuary

Increased rates of dissimilatory nitrate reduction to ammonium (DNRA) under oxic conditions in a periodically hypoxic estuary

The Yarra River Estuary is a salt wedge estuary prone to hypoxia in the bottom waters during low flow periods. Rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were quantified using 15N in relation to oxygen, nitrate and available reductants. Denitrification was the dominant nitrate reduction pathway under all oxygen conditions, however, DNRA increased from <1% under hypoxic conditions (<50μmolL−1O2) to ∼18% of total nitrate reduction under oxygen saturation in the water column in intact core incubations. Microprofiles of nitrate reduction pathways in intact cores using diffusive equilibrium in thin layer (DET) gels showed significant rates of DNRA only occurred under oxic conditions in the presence of Fe2+. Cores incubated anoxically, developed free sulfide within the porewater, had very low concentrations of Fe2+ and low rates of DNRA. Slurry incubations with varying concentrations of NO3− showed that denitrifying bacteria had a higher affinity than nitrate ammonifying bacteria with Km values of 49 and 86μmolL−1 for denitrification and DNRA, respectively, however, this could not explain the change in the rates of DNRA relative to denitrification observed. Further slurry incubations to investigate the relationship between DNRA and Fe2+ oxidation were inconclusive and complicated by very high backgrounds of sorbed (HCl extractable) Fe2+. Addition of Fe2+ to the slurry did not stimulate denitrification compared to a control (no Fe2+ addition), however, there was a significant decrease in the Fe2+ concentration over the period where DNRA occurred in the Fe2+ addition treatment, and no significant decrease in the control treatment. The ratio of DNRA to Fe2+ consumption was 15±6 and 7±3 for the Fe2+ and control treatments, respectively. We suggest reduced rates of DNRA under anoxic conditions can be explained by the binding of Fe2+ with free sulfides and the formation of FeS removing available Fe2+ for DNRA.

Movement Patterns and Distributional Shifts of Dungeness Crab (Metacarcinus magister) and English Sole (Parophrys vetulus) During Seasonal Hypoxia

Hypoxia (dissolved oxygen < 2 mg L–1) has emerged as a worldwide threat to coastal and estuarine ecosystems. Beyond direct mortality, secondary ecological impacts caused by hypoxia-driven distributional shifts may be equally important. From July–November 2009 and June–September 2010, we quantified the movement patterns of Dungeness crab (Metacarcinus magister) and English sole (Parophrys vetulus) in Hood Canal, Washington USA, a seasonally hypoxic estuary. Although highly mobile (mean cumulative distance ± SD = 11.0 ± 25.6 km, N = 60), there was little evidence of either species exhibiting large-scale directional movement out of the hypoxic region. However, Dungeness crab showed significant shifts towards shallower waters and elevation in activity in the hypoxic region, potentially increasing their vulnerability to crabbing and other indirect ecological consequences. Our findings suggest hypoxia could have a more localized impact on the mobile fauna in Hood Canal. However, more detailed information concerning the local-scale oxygen dynamics and responses of these species, such as English sole vertical movement, is essential for grasping the population and community level effects of hypoxia.

A benthic–pelagic coupled ecosystem model to estimate the hypoxic estuary including tidal flat—Model description and validation of seasonal/daily dynamics

A new ecosystem model – the Ecological Connectivity Hypoxia Model (ECOHYM) – was developed and applied to Tokyo Bay, a hypoxic estuary in Japan. ECOHYM permits study of the dynamics and mechanisms of hypoxia from two perspectives: (1) the whole estuary, composed of temporal-spatial mutual linkage of benthic–pelagic or central bay-tidal flat ecosystems (holistic approach), and (2) each biochemical and physical process, contributing to oxygen production/consumption (elemental approach). ECOHYM evaluates the oxygen–carbon–nitrogen–phosphorus-coupled cycle driven by physical and biochemical processes. The processes mechanically describe the ecosystem network consisting of benthic–pelagic and central bay–tidal flat ecosystem coupling. The model also describes the early diagenetic processes of the benthic ecosystems by setting a micro-scale vertical spatial resolution due to the drastic vertical change of benthic biological metabolism and its high contribution to total oxygen consumption. Model variables were mainly selected based on low trophic level; namely, phytoplankton, zooplankton, detritus, dissolved organic matter, NH 4–N, NO 3–N, PO 4–P, benthic algae, suspension feeders, deposit feeders (benthic faunas), dissolved oxygen (DO) and oxygen demand units (ODU, representing the stoichiometric expression of Mn 2+, Fe 2+ and S 2− oxygen demand). Detritus was divided into fast-labile, slow-labile and refractory parts. Bacterial mineralization processes were divided into oxic, suboxic and anoxic fractions. The model was applied for a time frame of 100 years under seasonal forcing functions to achieve an annual periodical steady state. The model results revealed that the seasonal dynamics were more noticeable than the daily/tidal dynamics in the central bay area. In contrast, the daily/tidal dynamics were determinant in the tidal flat area. Calculated seasonal dynamics in the central bay and daily/tidal dynamics in the tidal flat were in good agreement with the observed data. In particular, the model succeeded in reproducing the following vertical DO profiles during the hypoxic season (summer): (1) high oxygen at the sea-surface and hypoxia at the sea-bottom in the central bay area, (2) the reduced thickness of the benthic oxic layer in the central bay area, and (3) the drastic change of DO micro-profiles between daytime and nighttime of the benthic system in the tidal flat areas. Oxygen fluxes at the sediment–water interface in the model also reproduced the in situ observed values both in the central bay and tidal flat areas.

A trophic model for the Danshuei River Estuary, a hypoxic estuary in northern Taiwan

The estuary of the Danshuei River, a hypoxic subtropical estuary, receives a high rate of untreated sewage effluent. The Ecopath with Ecosim software system was used to construct a mass-balanced trophic model for the estuary, and network analysis was used to characterize the structure and matter flow in the food web. The estuary model was comprised of 16 compartments, and the trophic levels varied from 1.0 for primary producers and detritus to 3.0 for carnivorous and piscivorous fishes. The large organic nutrient loading from the upper reaches has resulted in detritivory being more important than herbivory in the food web. The food-chain length of the estuary was relatively short when compared with other tropical/subtropical coastal systems. The shortness of food-chain length in the estuary could be attributed to the low biomass of the top predators. Consequently, the trophic efficiencies declined sharply for higher trophic levels due to low fractions of flows to the top predators and then high fractions to detritus. The low biomass of the top predators in the estuary was likely subject to over-exploitation and/or hypoxic water. Summation of individual rate measurements for primary production and respiration yielded an estimate of −1791 g WW m −2 year −1, or −95 g C m −2 year −1, suggesting a heterotrophic ecosystem, which implies that more organic matter was consumed than was produced in the estuary.

Swimming behavior of juvenile anchovies (Anchoa spp.) in an episodically hypoxic estuary: implications for individual energetics and trophic dynamics

Diel swimming behaviors of juvenile anchovies (Anchoa spp.) were observed using stationary hydroacoustics and synoptic physicochemical and zooplankton profiles during four unique water quality scenarios in the Neuse River Estuary, NC, USA. Vertical distribution of fish was restricted to waters with DO greater than 2.5 mg O2 l−1, except when greater than 70% of the water column was hypoxic and a subset of fish were occupying water with 1 mg O2 l−1. We made the prediction that an individual fish would select a swim speed that would maximize net energy gain given the abundance and availability of prey in the normoxic waters. During the day, fish adopted swim speeds between 7 and 8.8 bl s−1 that were near the theoretical optimum speeds between 7.0 and 8.0 bl s−1. An exception was found during severe hypoxia, when fish were swimming at 60% above the optimum speed (observed speed = 10.6 bl s−1, expected = 6.4 bl s−1). The anchovy is a visual planktivore; therefore, we expected a diel activity pattern characteristic of a diurnal species, with quiescence at night to minimize energetic costs. Under stratified and hypoxic conditions with high fish density coupled with limited prey availability, anchovies sustained high swimming speeds at night. The sustained nighttime activity resulted in estimated daily energy expenditure over 20% greater than fish that adopted a diurnal activity pattern. We provide evidence that the sustained nighttime activity patterns are a result of foraging at night due to a lower ration achieved during the day. During severe hypoxic events, we also observed individual fish making brief forays into the hypoxic hypolimnion. These bottom waters generally contained higher prey (copepod) concentrations than the surface waters. The bay anchovy, a facultative particle forager, adopts a range of behaviors to compensate for the effects of increased conspecific density and reduced prey availability in the presence of stratification-induced hypoxia.

Predator–prey analysis of striped bass and Atlantic menhaden in upper Chesapeake Bay

AbstractBetween 1997 and 2000, an outbreak of skin lesions and observations of emaciated striped bass, Morone saxatilis (Walbaum), in upper Chesapeake Bay were attributed to a perceived shortage of its main prey, Atlantic menhaden, Brevoortia tyrannus Latrobe. Abundance estimates, Atlantic menhaden consumption per recruit analysis (modified yield‐per‐recruit), bioenergetics analysis and predator–prey theory were combined to explore whether an imbalance between striped bass and Atlantic menhaden in upper Chesapeake Bay was plausible. Reduced fishing mortality and higher size limits that underpinned the effort to restore striped bass during the 1980s and 1990s lead to more abundant and larger striped bass in upper Chesapeake Bay, increasing predatory demand for forage‐sized Atlantic menhaden, the abundance of which declined to a historic low. Nominal losses of age 0–2 Atlantic menhaden to harvest and potential striped bass predation exceeded supply after 1998. The outbreak of lesions in upper Chesapeake Bay striped bass coincided with increased variation in weight‐at‐length, decreased length‐at‐age, and decreased presence of Atlantic menhaden in diets. Decreased attack success, inferred from a 97% decline in ratios of forage‐sized Atlantic menhaden to striped bass between 1983 and 1998, would have expected to have been followed by a deterioration of striped bass nutritional state. Transmission of disease would have been aided by high density of striped bass in poor nutritional condition residing in degraded habitat (Chesapeake Bay was the most hypoxic estuary in the mid‐Atlantic region in the late 1990s). These problems suggest that striped bass exceeded their carrying capacity in Chesapeake Bay during the late 1990s.

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 chang- ing 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.

Dissolved and particulate trace metals and their partitioning in a hypoxic estuary: The Tanshui Estuary in Northern Taiwan

The distribution and partitioning of trace metals (Co, Cu, Fe, Mn, Ni, and Zn) between dissolved and particulate phases were studied in the Tanshui Estuary. The upper reach of the estuary is hypoxic and heavily polluted due to domestic and industrial discharges. The concentration ranges of dissolved and leachable particulate trace metals in the Tanshui Estuary were: Co: 0.3–6.1 nM, 1.8–18.6 mg kg−1; Cu: 5–53 nM, 22–500 mg kg−1; Fe: 388–3,364 nM, 1.08–6.67%; Mn: 57–2,914 nM, 209–1,169 mg kg−1; Ni: 7–310 nM, 6–108 mg kg−1; and Zn: 12–176 nM, 62–1,316 mg kg−1; respectively. The dissolved concentrations of the metals were 2–35 times higher than the average values of the world river water. The distributions of dissolved and particulate studied metals, except Mn, in the estuary showed scattering, which could be attributed to the discharges from many industrial wastewater disposal works located in the upper tributaries. The daily input of dissolved metals from the disposal works to the Tanshui Estuary ranged from 0.1–0.4 tons. Dissolved Mn was nearly conservative in the region with salinity higher than 10 psu, while particulate Mn decreased in the region with salinity of 10–15 psu. The concentration increased significantly seawards, corresponding with the distribution of dissolved oxygen. The distribution coefficient (KD) for Mn in the lower estuary was nearly three orders of magnitude higher than in the upper estuary. This phenomenon may be attributed to the diffusion of Mn from the anoxic sediment in the upper estuary and gradual oxidation into particulate Mn in the middle and lower estuary as the estuarine water became more oxygenated. The distribution coefficient for Cu decreased with increasing salinity. The percentages of trace metals bound by suspended particulate matter decreased in the following order: Fe>Zn, Cu>Co>Mn>Ni.