Bottom Dissolved Oxygen Concentrations Research Articles

Variations in dissolved O2 in a Chinese lake drive changes in microbial communities and impact sedimentary GDGT distributions

Glycerol dialkyl glycerol tetraethers (GDGTs) are widespread but unique membrane-spanning lipids of many Archaea and some Bacteria. However, their specific biological sources and the associated environmental controls on their distribution remain unclear, especially in lacustrine settings, hindering our understanding of these compounds and their application as environmental proxies. Here we investigated the GDGT distributions across a large bottom water dissolved oxygen (DO) gradient (0.10–7.20 mg/L) using surface sediments of Lake Yangzonghai, a warm monomictic lake in southwestern China. We show that the distributions of both branched (br) and isoprenoidal (iso) GDGTs co-vary significantly with bottom water DO concentration. The relative abundances of tetramethylated brGDGTs, 5-methyl penta- and hexamethylated brGDGTs and isoGDGT-0 show a stepwise increase as bottom water DO concentration decreases. On the other hand, the relative abundances of 6-methyl penta- and hexamethylated brGDGTs and crenarchaeol exhibit a stepwise decline with a decrease in bottom water DO concentration. Genetic data indicate these DO-induced changes in GDGT parameters are related to changes in the bacterial and archaeal communities across the oxycline in the lake. For example, the high abundance of isoGDGT-0 in low DO samples coincides with a high abundance of methanogenic archaea and Bathyarchaeota. We propose that brGDGT-producing bacteria might include a diversity of other phyla in addition to the proposed source organism acidobacteria; different groups of anaerobic and aerobic bacteria are likely to contribute to the increased abundance of 5-methyl brGDGTs and 6-methyl brGDGTs in low and high DO zones, respectively. Consequently, the MBT′ and MBT′6ME brGDGT indices display strong correlations with DO concentration. Importantly, the MBT′5ME index is not significantly influenced by changes in DO concentration, suggesting that this index might be more resilient to these impacts and more suitable to reconstruct temperature in lake systems.

Physical Controls of Summer Variations in Bottom Layer Oxygen Concentrations in the Coastal Hypoxic Region off the Northeastern Shandong Peninsula in the Yellow Sea

AbstractHypoxia has been observed worldwide and causes great damage to marine ecosystems. In the northeastern mariculture areas of the Shandong Peninsula, summer oxygen depletion and even hypoxia have been observed in the bottom layer, but the underlying dynamics are still poorly understood. In the current study, the physical controls of variations in bottom layer oxygen concentrations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations collected during 2016–2020. Over the past 5 years, the dissolved oxygen (DO) concentrations of the bottom layer have ranged from 0.8 to 13.0 mg L−1 and exhibited a robust annual cycle, with the lowest value occurring in August. Hypoxia events covering several days were observed intermittently in the summers of 2016 and 2017. Summer oxygen depletion was caused by the combined effects of ocean stratification and enhanced biogeochemical oxygen consumption. At the interannual time scale, stratification and bottom layer DO concentrations showed synchronous variations and both peaked in 2018. The underlying mechanism was related to bottom layer currents, which were created by southerly monsoon winds. Strong/weak southerly surface winds induced a strong/weak bottom shoreward advection of cold, oxygen‐rich water and resulted in strong/weak stratification and high/low DO concentrations. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia.

Occurrence of a temperate coastal flatfish, the marbled flounder Pseudopleuronectes yokohamae, at high water temperatures in a shallow bay in summer detected by acoustic telemetry

The marbled flounder Pseudopleuronectes yokohamae, a flatfish found in temperate coastal waters, is an important commercial species in the northwestern Pacific Ocean, but little is known of its distribution and movement under high water temperatures in summer. We recorded the distribution and movement of marbled flounder in shallow water in Tokyo Bay with acoustic telemetry. Seventeen wild flounder tagged with acoustic transmitters were tracked/monitored by active tracking using a research vessel and sporadic monitoring using fishing gear and a fishing boat. We located six individuals in shallow (< 30 m) water at temperatures of 19–23 °C and dissolved oxygen (DO) concentrations of 3–5 ml/l in Tokyo Bay, while bottom water temperatures and DO concentrations ranged from 19 to 24 °C and 0–7 ml/l, respectively. Our results indicated that at least 35% of the flounder remained in warm, shallow waters in summer. Multiple monitoring methods and the assistance of fishermen helped to overcome the difficulties of working in a crowded bay and increased our recovery rate of the tagged flounder.

Factors explaining the yearly changes in minimum bottom dissolved oxygen concentrations in Lake Biwa, a warm monomictic lake

Vertical profiles of dissolved oxygen (DO) and water temperature (WT) measured bi-monthly for 36 years (1980–2015) near the deepest part of a warm monomictic lake were analyzed with special reference to yearly minimum DO at bottom (DOmin). DOmin changed yearly (3.0 ± 1.2 mg l−1) and significant differences in DOmin were not observed between Period I (1980–1993; cooler and worse in water quality) and Period II (1994–2015; warmer and better in water quality). This unclear trend in DOmin was probably due to the offsetting influences between warming induced by global warming and oligotrophication attempted by local governments etc. for the study period. DOmin was positively correlated with disturbance time (timing of last cold water intrusion observed from Mar to Aug), which could be related to the start of DO depletion at bottom. Thus, the linear model using this parameter could predict yearly DOmin fairly well for the entire study period (r2 = 0.60). In addition, DOmin and time of disturbance were correlated negatively with water density at bottom in Jan and positively with water density equilibrated to air temperature (AT) in Mar. Higher lake water density after full depth mixing advances the disturbance time. In contrast, lower AT in Mar and/or higher density of influent water after Mar delays the time likely due to the larger amount of snowfall in the watershed. Further, DOmin was positively correlated with maximum wind velocity in Sep which probably induced the recovery of DO. Multiple-regression models to predict DOmin using these meteorological and water quality parameters were developed (r2 ≥ 0.38, worse performances than the model using disturbance time) to forecast future trends of DOmin through global warming and/or climate change. Significant influences of water or sediment oxygen demands on DOmin were not detected. We also discuss the applicability of the proposed models.

A potential proxy for seasonal hypoxia: LA-ICP-MS Mn/Ca ratios in benthic foraminifera from the Yangtze River Estuary

We have examined the possible use of Mn/Ca ratios in benthic foraminifera as a proxy for hypoxia, which has increasingly become a serious environmental issue in many coastal zones. We collected samples from the Yangtze River Estuary, one of the largest and most intense seasonally hypoxic zones in the world. In order to obtain high quality Mn/Ca data, unaffected by contamination, we examined two different cleaning protocols and instrument-based analysis methods. Our results showed that a relatively simple physical cleaning approach coupled to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provided excellent results on single specimens. Our observations were comparable to ICP analyses of solutions made up from dozens or even hundreds of specimens and involving complicated sample pretreatments. Using the LA methodology, we analyzed Mn/Ca ratios from living Florilus decorus (F. decorus) specimens from stations in the Yangtze River Estuary with varying bottom water dissolved oxygen (DO) concentrations. Our results showed that Mn/Ca ratios in penultimate chambers of living benthic foraminifera are sensitive to bottom water DO concentrations. We also observed significant variations of both Mn/Ca and Mg/Ca ratios from different chambers within foraminiferal shell specimens. These fluctuations were likely a response to changing ambient water DO and temperature during the specimen’s growth history. Combined use of foraminiferal Mn/Ca ratios together with other proxies could provide a powerful tool for historical reconstruction of low oxygen conditions in seasonal hypoxia areas.

Long-Term Changes in Gelatinous Zooplankton in Chesapeake Bay, USA: Environmental Controls and Interspecific Interactions

Gelatinous zooplankton (GZ) populations are sensitive to environmental perturbations, and regional changes in their abundance may be associated with degraded environmental conditions. Two time series of GZ abundances were used to analyze the population dynamics of gelatinous zooplankton in the Chesapeake Bay, USA from 1984 to 2012. Annual and interannual variations in population size and distribution of the scyphozoan medusae Chrysaora chesapeakei, Aurelia aurita, Cyanea capillata, and Rhopilema virrilli, as well as the lobate ctenophore Mnemiopsis leidyi, were compared with environmental conditions and other biological data. Scyphozoan population control by environmental factors was primarily a result of mortality and asexual reproduction by the benthic scyphistomae. C. chesapeakei was present year-round, but biovolume was highest in July–September and in salinities 9–20. M. leidyi populations were primarily controlled by C. chesapeakei predation and were most abundant in June, after waters warmed above 18 °C but before C. chesapeakei bloomed. Low bottom-water salinity was negatively correlated with summer C. chesapeakei biovolume, and low bottom dissolved oxygen concentrations delayed the timing of the peak bloom. Total GZ biovolume decreased in both time series (1984–2012), likely due to decreases in C. chesapeakei abundance. This reduction in C. chesapeakei allowed for a concurrent increase in M. leidyi and decrease in copepod abundance. Predicted future increases in spring streamflow and spring hypoxia due to global climate change would further decrease C. chesapeakei abundance, possibly allowing for future increase in M. leidyi populations.

The mechanism of bottom water DO variation in summer at the northern mouth of Isahaya Bay, Japan

Recently, bivalves have been massively killed by anoxia or hypoxia in summer at the northern part of Isahaya Bay, Japan, which constituted a major problem for fisheries. However, the mechanism behind the occurrence of hypoxic water masses is unclear. It is known that the bottom water dissolved oxygen (DO) in this area is affected by the inflow of seawater into the northern mouth of Isahaya Bay. To understand the mechanism of hypoxia, it is necessary to determine the physical processes that cause changes in the bottom DO concentrations in this area. This study shows that there is a neap-spring tidal variation in bottom DO due to a change in vertical tidal mixing, and it also suggests that the decrease in bottom DO was generated by a baroclinic flow, which is due to the internal tide, and a shear flow, which is induced by the external tide in the bottom boundary layer. In addition, our study suggests that the source of cold and hypoxic water that appears in the bottom layer at low tide is the inner area of the Ariake Sea.

Ecological thresholds of hypoxia and sedimentary H2S in coastal soft-bottom habitats: A macroinvertebrate-based assessment

Ecological thresholds of dissolved oxygen (DO) and sedimentary hydrogen sulfide (H2S) for macrozoobenthos were examined during a 30-month monitoring of two stations in a highly eutrophic canal in inner Tokyo Bay, Japan. Bottom DO and H2S concentrations fluctuated seasonally, and were significantly correlated with water and sediment temperatures. Red tide-derived phytodetritus was a major source of sediment organic matters in the canal bottom, and the sediment became highly reduced and sulfidic condition in warmer months (sedimentary H2S; up to 8.5 mM). Dominant opportunistic taxa, including polychaetes and amphipods, were eliminated under low DO and high H2S conditions (i.e., population thresholds), and devastation of community structure occurred at 2.4–3.3 mg l−1 DO and 1.8–2.7 mM H2S (i.e., community thresholds). To maintain ecosystem function in anthropogenically degraded habitats and ensure colonization by macrozoobenthos throughout the year, DO and H2S levels should be maintained below these thresholds.

A numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the Pearl River estuary

Abstract. A three-dimensional (3-D) physical–biogeochemical coupled model was applied to explore the mechanisms controlling the dissolved oxygen (DO) dynamics and bottom hypoxia during summer in the Pearl River estuary (PRE). By using the numerical oxygen tracers, we proposed a new method (namely the physical modulation method) to quantify the contributions of boundary conditions and each source and sink process occurring in local and adjacent waters to the DO conditions. A mass balance analysis of DO based on the physical modulation method indicated that the DO conditions at the bottom layer were mainly controlled by the source and sink processes, among which the sediment oxygen demand (SOD) at the water–sediment interface and the re-aeration at the air–sea interface were the two primary processes determining the spatial extent and duration of bottom hypoxia in the PRE. The SOD could cause a significant decrease in the bottom DO concentrations (averaged over July–August 2006) by over 4 mg L−1 on the shelf off the Modaomen sub-estuary, leading to the formation of a high-frequency zone of hypoxia (HFZ). However, the hypoxia that occurred in the HFZ was intermittent and distributed in a small area due to the combined effects of re-aeration and photosynthesis, which behaved as sources for DO and offset a portion of the DO consumed by SOD. The bottom DO concentrations to the west of the lower Lingdingyang Bay (i.e. the western shoal near Qi'ao Island) were also largely affected by high SOD, but there was no hypoxia occurring there because of the influence of re-aeration. Specifically, re-aeration could lead to an increase in the bottom DO concentrations by ∼ 4.8 mg L−1 to the west of the lower Lingdingyang Bay. The re-aeration led to a strong vertical DO gradient between the surface and the lower layers. As a result, the majority (∼ 89 %) of DO supplemented by re-aeration was transported to the lower layers through vertical diffusion and ∼ 28 % reached the bottom eventually. Additional numerical experiments showed that turning off re-aeration could lead to an expansion of the hypoxic area from 237 to 2203 km2 and result in persistent hypoxia (hypoxic frequency &gt; 80 %) to the west of the lower Lingdingyang Bay. Compared to re-aeration and SOD, photosynthesis and water column respiration had relatively small impacts on the DO conditions; turning off these two processes increased the hypoxic area to 591 km2. In summary, our study explicitly elucidated the interactive impacts of physical and biogeochemical processes on the DO dynamics in the PRE, which is critical to understanding hypoxia in this shallow and river-dominated estuarine system.

Improvement in the dissolved oxygen concentration and water exchange in Ofunato Bay, Japan, after the collapse of the bay‐mouth breakwater by the 2011 Tohoku Earthquake and tsunami

AbstractOyster and scallop aquaculture has flourished in Ofunato Bay, on the Sanriku Coast of Japan. Between 1967 and 2011, this bay was a highly enclosed area by a bay‐mouth breakwater, and while the breakwater provided calm conditions suitable for aquaculture, there was concern about water quality deterioration. The Tohoku Earthquake off the Pacific coast and the subsequent major tsunami on 11 March 2011 caused extensive damage to the Sanriku Coast, resulting in the collapse of the breakwater of Ofunato Bay. We analyzed the monthly hydrographic observational data that were collected between 1996 and 2015, supplemented with hydrographic observations and direct current measurements that were carried out after the earthquake. The historical hydrographic observational data showed the lowest dissolved oxygen (DO) concentrations were observed near the bottom in the bay. The average and standard deviation of the lowest DO concentrations of each monthly observation during the period before and after the earthquake were 5.81 ± 3.72 and 7.22 ± 1.49 mg L−1, respectively, and are significantly different. The oceanic water inflow and water exchange rate estimated by a box model based on the salt budget under the steady‐state estuarine circulation were also significantly different before and after the earthquake. An increase in the inflow of oceanic water and the more rapid water exchange in the bay after the earthquake induced marked improvements in the bottom DO concentration. These changes are considered to be attributable to the improved circulation of oceanic water in the bay that resulted from the collapse of the bay‐mouth breakwater.

Assessing the bio-mitigation effect of integrated multi-trophic aquaculture on marine environment by a numerical approach

With increasing concern over the aquatic environment in marine culture, the integrated multi-trophic aquaculture (IMTA) has received extensive attention in recent years. A three-dimensional numerical ocean model is developed to explore the negative impacts of aquaculture wastes and assess the bio-mitigation effect of IMTA systems on marine environments. Numerical results showed that the concentration of surface phytoplankton could be controlled by planting seaweed (a maximum reduction of 30%), and the percentage change in the improvement of bottom dissolved oxygen concentration increased to 35% at maximum due to the ingestion of organic wastes by sea cucumbers. Numerical simulations indicate that seaweeds need to be harvested in a timely manner for maximal absorption of nutrients, and the initial stocking density of sea cucumbers >3.9 individuals m−2 is preferred to further eliminate the organic wastes sinking down to the sea bottom.

Temporal and spatial dynamics of large lake hypoxia: Integrating statistical and three‐dimensional dynamic models to enhance lake management criteria

AbstractHypoxia or low bottom water dissolved oxygen (DO) is a world‐wide problem of management concern requiring an understanding and ability to monitor and predict its spatial and temporal dynamics. However, this is often made difficult in large lakes and coastal oceans because of limited spatial and temporal coverage of field observations. We used a calibrated and validated three‐dimensional ecological model of Lake Erie to extend a statistical relationship between hypoxic extent and bottom water DO concentrations to explore implications of the broader temporal and spatial development and dissipation of hypoxia. We provide the first numerical demonstration that hypoxia initiates in the nearshore, not the deep portion of the basin, and that the threshold used to define hypoxia matters in both spatial and temporal dynamics and in its sensitivity to climate. We show that existing monitoring programs likely underestimate both maximum hypoxic extent and the importance of low oxygen in the nearshore, discuss implications for ecosystem and drinking water protection, and recommend how these results could be used to efficiently and economically extend monitoring programs.

Sedimentary organic matter contents and porewater chemistry at upper bathyal depths influenced by the 2011 off the Pacific coast of Tohoku Earthquake and tsunami

Sedimentary organic matter and porewater geochemistry at upper bathyal depths off Tohoku, Japan, were investigated 1 year after the 2011 off the Pacific coast of Tohoku Earthquake. Total organic carbon (TOC) and total nitrogen (TN) concentrations, organic carbon and nitrogen isotopic compositions, dissolved oxygen (DO) concentrations, and bottom water and porewater nutrient concentrations were examined in sediment core samples collected from eight upper bathyal stations. Event deposits 1 to 7 cm thick were observed at all sampling stations, indicating that a vast area off Tohoku has been influenced by sedimentological events caused by the earthquake or subsequent tsunami. Both TOC and TN concentrations were lower in coarse-grained event deposits. Carbon and nitrogen isotopic composition, together with the C/N ratio, suggested that the event deposits, including turbidites, were composed of local sediments that had been transported relatively short distances. Nutrient fluxes across the sediment–water interface correlated with surface TOC and bottom water DO concentrations. However, event deposit thickness showed no correlation with oxygen or nutrient fluxes. The organic matter content of the event deposits was similar to that in the underlying sediments at each site, and thus no apparent effect of event deposits on nutrient fluxes was observed. Our results suggest that biogeochemical cycles in sediments of disturbed areas are affected not only by event deposit thickness, but also by the source of the event deposits.

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.

Modeled long-term changes of DIN:DIP ratio in the Changjiang River in relation to Chl-α and DO concentrations in adjacent estuary

In the past four decades (1970–2013), nitrogen and phosphorous inputs to the Changjiang River basin, mainly from human activities, have increased 3-fold and 306-fold, respectively. The riverine nutrient fluxes to the estuary have also grown exponentially. Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) fluxes of the Changjiang River increased by 338% and 574% during 1970–2013 period, and red tides and benthic hypoxia have been observed in the outflow region of the Changjiang River in the East China Sea (ECS). We assumed that time series changes in the DIN:DIP ratio from the Changjiang River could have a significant impact on Chlorophyll-α (Chl-α) concentration in the surface sea water and dissolved oxygen (DO) concentration in the bottom sea water of the Changjiang estuary. Our study showed that the DIN:DIP ratio from the Changjiang River increased from 76 to 384 between 1970 and 1985, and decreased from 255 to 149 between 1986 and 2013. The observed Chl-α concentration increased by 146% from 1992 to 2010 in the Changjiang estuary, and was negatively related to the DIN:DIP ratio in 1992–2010. Bottom sea water DO concentration decreased by 24.6% during 1992–2010 and a “low oxygen zone” (122°∼123°E, 32°∼33°N) was observed during summer since 1999. The anthropogenically enhanced nutrient inputs dominated river DIN and DIP fluxes and influenced Chl-α concentrations as well as bottom DO concentrations in the estuary. Scenarios emphasizing global collaboration and proactive environmental problem-solving may result in reductions in the river nutrient exports and in Chl-α and DO concentration in the Changjiang estuary by 2050.

Monitoring the occurrence of seasonal low‐oxygen events off the Changjiang Estuary through integration of remote sensing, buoy observations, and modeling

AbstractBottom water hypoxia occurs frequently during the summer off the Changjiang Estuary. To estimate its spatial extent and investigate how climatic variations and extreme events may affect its occurrence, we developed a regional statistical model that combines satellite and buoy observations via empirical and statistical relationships. The estimated results were validated using cruise data off the Changjiang Estuary and its adjacent areas. First, we quantified the linkage between the observed dissolved oxygen (DO) concentrations in the bottom water and the chlorophyll‐a (Chl‐a) concentrations at the surface. With the help of the model, the bottom layer DO concentrations in the region lacking in situ measurements were estimated using the observations near the buoys and the satellite‐derived Chl‐a concentrations. Comparisons between the modeled results and surveys conducted between 2006 and 2011 indicate that the error of the estimated extent of low‐oxygen bottom water was less than 26% and that the bias in the estimated minimum DO concentration was less than 0.5 mg L−1. Both buoy observations and modeled results indicate that the strength of water stratification and the amount of labile organic matter added to the bottom water are the two main factors that control the occurrence and the magnitude of seasonal low‐oxygen events off the Changjiang Estuary.

Dynamics of oxygen depletion in the nearshore of a coastal embayment of the southern Benguela upwelling system

Acquisition of high resolution time series of water column and bottom dissolved oxygen (DO) concentrations inform the dynamics of oxygen depletion in St Helena Bay in the southern Benguela upwelling system at several scales of variability. The bay is characterized by seasonally recurrent hypoxia (<1.42 ml l−1) associated with a deep pool of oxygen-depleted water and episodic anoxia (<0.02 ml l−1) driven by the nearshore (<20 m isobath) decay of red tide. Coastal wind forcing influences DO concentrations in the nearshore through its influence on bay productivity and the development of red tides; through shoreward advection of the bottom pool of oxygen-depleted water as determined by the upwelling-downwelling cycle; and through its control of water column stratification and mixing. A seasonal decline in bottom DO concentrations of ∼1.2 ml l−1 occurs with a concurrent expansion of the bottom pool of oxygen depleted water in St Helena Bay. Upwelling of this water into the nearshore causes severe drops in DO concentration (<0.2 ml l−1), particularly during end-of-season upwelling, resulting in a significant narrowing of the habitable zone. Episodic anoxia through the entire water column is caused by localized degradation of red tides within the confines of the shallow nearshore environment. Oxygenation of the nearshore is achieved by ventilation of the water column particularly with the onset of winter mixing. No notable changes are evident in comparing recent measures of bottom DO concentrations in St Helena Bay to data collected in the late 1950s and early 1960s.

Effect of Changes in Dissolved Oxygen Concentrations on the Spatial Dynamics of the Gulf Menhaden Fishery in the Northern Gulf of Mexico

AbstractDeclines in dissolved oxygen (DO) concentrations in aquatic environments can lead to conditions of hypoxia (DO ≤ 2 mg/L), which can directly and indirectly affect aquatic organisms. Direct effects include changes in growth and mortality; indirect effects include changes in distribution, movement, and interactions with other species. For mobile species, such as the pelagic filter‐feeding Gulf Menhaden Brevoortia patronus, indirect effects are more prevalent than direct effects. The northern Gulf of Mexico experiences one of the largest areas of seasonal hypoxia in the world; this area overlaps spatially and temporally with the Gulf Menhaden commercial purse‐seine fishery, which is among the largest fisheries by weight in the United States. Harvest records from the Gulf Menhaden fishery in 2006–2009 and fine‐scale spatial and temporal predictions from a physical–biogeochemical model were used with spatially varying regression models to examine the effects of bottom DO concentration, spatial location, depth, week, and year on four response variables: probability of fishing, total Gulf Menhaden catch, total fishery effort, and CPUE. We found nearshore shifts in the probability of fishing as DO concentration declined, and we detected a general westward shift in all response variables. We also found increases in CPUE as DO concentration declined in the Louisiana Bight, an area that experiences chronic, severe hypoxia. The overall effects of environmental conditions on fishing response variables appeared to be moderate. Nevertheless, movement of either Gulf Menhaden or the purse‐seine fishery in response to environmental conditions could potentially affect the susceptibility of Gulf Menhaden to harvest and could therefore influence assessment of the stock and associated stock status indicators.Received March 4, 2014; accepted June 26, 2014

Effects of bottom‐layer hypoxia on spatial distributions and community structure of mesozooplankton in a sub‐estuary of Puget Sound, Washington, U.S.A

We investigated the response of mesozooplankton to low dissolved oxygen (DO) concentrations in Hood Canal, Washington, a seasonally hypoxic sub‐estuary of Puget Sound. Depth‐stratified zooplankton net tows were conducted above and below the oxycline during day and night in midsummer (July), late summer (September), and early winter (December) 2008 using closing nets. Bottom DO concentrations declined slowly through summer. In July, DO at most stations was low but not hypoxic. By late summer, bottom waters throughout the study area were hypoxic (≤ 2 mg L−1) and were severely hypoxic (&lt; 1 mg L−1) at some stations. In winter, water‐column mixing and lower surface primary production resulted in higher DO, although sub‐pycnocline hypoxia persisted through December at some locations. Zooplankton abundance decreased from midsummer to winter. The community was dominated by gelatinous zooplankton and copepods. A shift in community structure towards a higher dominance of polychaetes and the cyclopoid copepod Oithona similis occurred over the season. Temporal and spatial patterns in zooplankton communities were primarily structured by month and bottom depth. DO played a lesser role in structuring the species composition, but vertical distributions of several taxa changed in response to hypoxia, with a greater proportion of their populations found above the oxycline during both day and night where bottom oxygen was low. However, some taxa were abundant below the oxycline, even at oxygen levels &lt; 0.5 mg L−1; these taxa may have a high tolerance to hypoxia, use low‐oxygen regions as a predation refuge, and be useful indicators of poor water‐quality conditions.

Contributions of wind and river effects on DO concentration in Tokyo Bay

This paper proposes a method of evaluating the occurrence of hypoxia or anoxia in bottom water of Tokyo Bay, which is typical of enclosed bays in Japan. In Tokyo Bay, the exchange of seawater with the ocean has previously been found to be dominantly controlled by estuarine circulation. Thus, one would expect the concentrations of dissolved oxygen (DO) in the bay head to be influenced mainly by estuarine circulation, which changes according to wind strength and river discharges. We investigated the effects of wind and river discharge on DO concentration around the bay head using a three-dimensional ecological model. To evaluate the occurrence of hypoxic or anoxic water, we developed a conceptual DO model, which was verified through the good agreement with the results from a three-dimensional ecological model. We conclude that the conceptual DO model has good potential for evaluating the factors leading to hypoxic or anoxic water around the bay head. Modeling suggested that, on average, wind effects were the dominant factor in the variation of bottom DO concentrations. However, we found that the contribution of extreme events, such as floods and strong winds exceeding 10 m s−1, was 50% or more. This suggests that extreme events play an important role in controlling the variation in DO concentration at the bottom of Tokyo Bay.