Bottom Water Dissolved Oxygen Research Articles

Photosynthetic Production Determines Bottom Water Oxygen Variations in the Upwelling Coastal South China Sea Over Recent Decades

Dissolved oxygen (DO) in seawater is fundamental to marine ecosystem health. How DO in coastal upwelling areas responds to upwelling intensity under climate change is of particular interest and vital importance, because these productive regions account for a large fraction of global fishery production and marine biodiversity. The Yuedong upwelling (YDU) in the coastal northern South China Sea can be served as a study case to explore long-term responses of DO to upwelling and climate due to minor influence of riverine input. Here, bottom water DO conditions were recovered by sedimentary C28Δ22/Δ5,22 ratios of steroids in three short cores, with lower ratio value indicating higher DO concentration. The ratio records showed oscillations in varying degrees and exhibited no clear trends before ∼1980s, after which, however, there occurred a persistent decreasing trend or basically remained at lower values. Thus, inferred DO variations by the C28Δ22/Δ5,22 ratio records are not compatible with regional YDU-involved physical processes under climate change, such as southwesterly wind-induced onshore advection of reduced-oxygenated source waters from outer shelf and oceanic warming that would rather lead to less oxygenation in bottom waters in recent decades. Intriguingly, the alcohol records of n-C20:1/C28Δ5,22 and br-C15/C28Δ5,22 ratios, indicative of the relative strengths between biogeochemical oxygen consumption (i.e., by zooplankton and microbes) and photosynthetic oxygen production (i.e., by phytoplankton), changed almost in parallel with the C28Δ22/Δ5,22 records in three cores. Accordingly, we propose that net photosynthetic oxygen production outweighs source water– and warming-induced increasing deoxygenation in the study area. This study may suggest an important biogeochemical mechanism in determining bottom water DO dynamics in shallow coastal upwelling regions with minor contribution of riverine input.

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.

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.

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.

Space-Time Geostatistical Assessment of Hypoxia in the Northern Gulf of Mexico.

Nearly every summer, a large hypoxic zone forms in the northern Gulf of Mexico. Research on the causes and consequences of hypoxia requires reliable estimates of hypoxic extent, which can vary at submonthly time scales due to hydro-meteorological variability. Here, we use an innovative space-time geostatistical model and data collected by multiple research organizations to estimate bottom-water dissolved oxygen (BWDO) concentrations and hypoxic area across summers from 1985 to 2016. We find that 27% of variability in BWDO is explained by deterministic trends with location, depth, and date, while correlated stochasticity accounts for 62% of observational variance within a range of 185 km and 28 days. Space-time modeling reduces uncertainty in estimated hypoxic area by 30% when compared to a spatial-only model, and results provide new insights into the temporal variability of hypoxia. For years with shelf-wide cruises in multiple months, hypoxia is most severe in July in 59% of years, 29% in August, and 12% in June. Also, midsummer cruise estimates of hypoxic area are only modestly correlated with summer-wide (June-August) average estimates ( r2 = 0.5), suggesting midsummer cruises are not necessarily reflective of seasonal hypoxic severity. Furthermore, summer-wide estimates are more strongly correlated with nutrient loading than midsummer estimates.

Vertical Distribution of Macrobenthos in Hypoxia-Affected Sediments of the Northern Gulf of Mexico: Applying Functional Metrics

Continuing trends of declining bottom-water dissolved oxygen (BWDO) in coastal oceans are a threat to marine organisms, especially the benthos, and in the northern Gulf of Mexico (nGOM), the hypoxic area is expected to expand. Within-sediment vertical functional metric responses of macrobenthos were examined in four depth strata at four sites with different histories of exposure to seasonal hypoxia. The sites H7, D5, E4, and A6 between 30 and 39-m water depth represented a historical oxygen stress gradient from 5.4 to 2.1 mg L−1 BWDO (mid-summer 11-year mean), from which macrobenthos were sampled in spring and late summer 2009 and in mid-summer in 2010. High abundance together with high biomass and production potential within upper strata under low BWDO stress supports the primacy of a numerically driven response in this system. Total abundance decreased along the BWDO stress gradient, and the lowest abundance coincided at the highest stress (MaxStress) site together with the highest proportion of opportunists, the smallest mean size, and the shortest turnover time. Accordingly, the relative abundance of opportunists within the upper two strata increased while surface deposit feeders decreased with greater hypoxic stress. Notwithstanding the proportion of opportunists, functional metrics generally decreased across the BWDO stress gradient within the upper strata, and metrics generally varied inversely between upper and lower strata. The findings underscore how vertical benthos distribution patterns reflect the BWDO exposure regime and how secondary production varies as an indicator of trophic transfer potential and organic matter processing within this system.

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.

Dynamics of Sulfate-Reducing Bacteria Community Structure in Surface Sediment of a Seasonally Hypoxic Enclosed Bay.

We herein report on the dynamics of a sulfate-reducing bacteria (SRB) community structure in the surface sediment of a seasonally hypoxic enclosed bay for two consecutive years (2012 and 2013). The uppermost (0–5 mm) and subsurface (5–10 mm) layers of sediment were examined with a terminal-restriction fragment length polymorphism (T-RFLP) analysis based on the dissimilatory sulfite reductase (dsrA) gene. The SRB community significantly differed between the two sediment layers over the sampling period. This difference was mainly attributed to operational taxonomic units (OTUs) that were unique to either of the sediment layers. However, nearly 70% of total OTUs were shared between the two layers, with a few predominating. Therefore, no significant shift was observed in the SRB community structure under varying dissolved oxygen (DO) conditions in bottom water overlying the sediment surface. An additional analysis of 16S rRNA gene amplicon sequences, conducted for three uppermost sediment samples (July, August, and September in 2012), revealed that Desulfococcus, a member of SRB with high tolerance to oxygen, was the predominant Deltaproteobacteria across the uppermost sediment samples. Based on the predominance of shared OTUs across the SRB community in the sediment (0–10 mm) regardless of bottom-water DO, some SRB that are physiologically tolerant of a wide range of DO conditions may have dominated and masked changes in responsive SRB to DO concentrations. These results suggest that the SRB community structure in the enclosed bay became stable under repeated cycles of seasonal hypoxia, but may be compromised if the severity of hypoxia increases in the future.

The evolution of deepwater dissolved oxygen in the northern South China Sea since 400 ka

The dissolved oxygen (DO) content of the ocean provides potential evidence and clues for the waxing and waning of marine productivity, ocean circulation, global climate change, and the evolution of ecological communities. On the basis of the analysis of the Benthic Foraminiferal Oxygen Index (BFOI), the percentage of epifaunal benthic foraminifers, and the redox-sensitive trace elements (Mo/Al) in Core MD12-3432, we reconstruct the evolution of deep water DO content in the northern South China Sea (SCS) during the past 400ka and discuss the mechanisms of variable DO content. The results show that the changes of BFOI, Mo/Al, and the percentage of epifaunal benthic foraminifer present a similar pattern, which may reflect the variations of the DO content in seawater since 400ka. Both variations in BFOI and Mo/Al indicate that the DO content was high during most time of marine isotope stage (MIS) 11 to mid-MIS 6, then decreased in late-MIS 6, and remained stable in MIS 5. It increased in MIS 4 and decreased until MIS 2, and then rose again. The non-synchronous variations between bottom water DO and benthic foraminiferal δ18O indicate that the DO content is not controlled by glacial-interglacial cyclic environmental variations. Comparing the biomarker productivity index with the DO content of water, we find that the DO content is affected mainly by productivity. Bulimina, Uvigerina and Chilostomella oolina are high productivity species. The accumulation rates of Bulimina and Uvigerina are higher during 61–15kyr interval, but the accumulation rate of C. oolina is higher during 168–130kyr. The results infer that different intensity of primary productivity and DO during these two periods. The DO and phytoplankton total (PT) show the opposite relation, which also indicates that the DO is affected by primary productivity, and the combined action of productivity and DO decides the growth of Bulimina, Uvigerina, and C. oolina. Besides the influences of productivity, the change of DO in the ocean is also affected by oceanic circulation.

Composition and vertical distribution of metazoan meiofauna assemblages on the continental shelf off central Chile

A quantitative study of metazoan meiofauna was carried out in Valparaiso Bay (33°S 71°W) which is affected by seasonal hypoxia in central Chile. The contents of bottom water, dissolved oxygen (BWDO), organic carbon, chloroplast pigments and composition of stable carbon isotope (d13C) in the sediment were used as a measure of the contribution of primary production in the water column, which accumulates in the sediment. Meiofauna abundances in the three sampling stations (80-140 m depth) ranged from 2.218 ± 643 to 1.592 ± 148 ind 10 cm-2. Nine upper metazoan meiofauna groups were recorded, with nematodes as the dominant group, contributing with more than 95% of total abundances. The abundance vertical distribution was concentrated in the first layers of sediment in most groups except Acari and nauplii larvae. Canonical correspondence analysis revealed significant correlations (P &lt; 0.05) between the meiofauna abundance and organic content, depth and redox potential from sediments. These results represent a first approach to understanding the ecology of meiofaunal assemblages in the Valparaiso Bay and may be useful as a baseline for future comparisons and descriptions of the ENSO (El Niño Southern Oscillation) and seasonal variations of these unknown benthic communities.

Macrobenthos and megabenthos responses to long-term, large-scale hypoxia on the Louisiana continental shelf

The macrobenthos and megabenthos responses to long-term, recurring hypoxia on the Louisiana continental shelf were compared at four locations with different historical (2000–2010) episodes of annual exposure to bottom-water hypoxia. Measurements of abundance, biomass, species diversity, and community composition of the two size classes of benthos suggested that the macrobenthic response is driven chiefly by tolerance to hypoxia, whereas the megabenthic response was affected by the ability to migrate and the availability/unavailability of macrobenthos prey at the sediment surface. The site exposed to the historically lowest average bottom-water dissolved oxygen (BWDO) concentration exhibited the lowest species diversity for macrobenthos and the highest species diversity for megabenthos, exemplifying the differential effects of hypoxia on different size classes. The high diversity and smaller average size of the megabenthos at the lowest DO site was due to high abundance of invertebrates and a preponderance of small, less vagile fishes that appeared to remain in the area after larger dominant sciaenids had presumably emigrated. The average size and the depth of habitation in the sediment of macrobenthos prey may have also influenced the abundance and biomass of megabenthos foragers.

Evaluation of mercury cycling and hypolimnetic oxygenation in mercury-impacted seasonally stratified reservoirs in the Guadalupe River watershed, California

Surface water reservoirs trap inorganic mercury delivered from their watersheds, create conditions that convert inorganic mercury to highly toxic methylmercury (MeHg), and host sportfish in which MeHg bioaccumulates. The Santa Clara Valley Water District (District) actively manages and monitors four mercury-impaired reservoirs that help to serve communities in South San Francisco Bay, California. The Guadalupe River watershed, which contains three of those reservoirs, also includes the New Almaden mercury-mining district, the largest historic mercury producer in North America. Monthly vertical profiles of field measurements and grab samples in years 2011-2013 portray annual cycling of density stratification, dissolved oxygen (DO), and MeHg. Monitoring results highlight the role that hypolimnetic hypoxia plays in MeHg distribution in the water column, as well as the consistent, tight coupling between MeHg in ecological compartments (water, zooplankton and bass) across the four reservoirs. Following the 2011-2013 monitoring period, the District designed and installed hypolimnetic oxygenation systems (HOS) in the four reservoirs in an effort to repress MeHg buildup in bottom waters and attain regulatory targets for MeHg in water and fish tissue. Initial HOS operation in Calero Reservoir in 2014 enhanced bottom water DO and depressed hypolimnetic buildup of MeHg, but did not substantially decrease mercury levels in zooplankton or small fish. This article is protected by copyright. All rights reserved.

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.

Assessing biophysical controls on Gulf of Mexico hypoxia through probabilistic modeling.

A mechanistic model was developed to predict midsummer bottom-water dissolved oxygen (BWDO) concentration and hypoxic area on the Louisiana shelf of the northern Gulf of Mexico, USA (1985-2011). Because of its parsimonious formulation, the model possesses many of the benefits of simpler, more empirical models, in that it is computationally efficient and can rigorously account for uncertainty through Bayesian inference. At the same time, the model incorporates important biophysical processes such that its parameterization can be informed by field-measured biological and physical rates. The model is used to explore how freshwater flow, nutrient load, benthic oxygen demand, and wind velocity affect hypoxia on the western and eastern sections of the shelf, delineated by the Atchafalaya River outfall. The model explains over 70% of the variability in BWDO on both shelf sections, and outperforms linear regression models developed from the same input variables. Model results suggest that physical factors (i.e., wind and flow) control a larger portion of the year-to-year variability in hypoxia than previously thought, especially on the western shelf, though seasonal nutrient loads remain an important driver of hypoxia, as well. Unlike several previous Gulf hypoxia modeling studies, results do not indicate a temporal shift in the system's propensity for hypoxia formation (i.e., no regime change). Results do indicate that benthic oxygen demand is a substantial BWDO sink, and a better understanding of the long-term dynamics of this sink is required to better predict how the size of the hypoxic zone will respond to proposed reductions in nutrient loading.

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.

Living and dead benthic foraminifera assemblages in the Bohai and northern Yellow Seas: Seasonal distributions and paleoenvironmental implications

Benthic foraminifera fossil shells are particularly useful in reconstructing water depth, temperature, the exported flux of organic carbon to the sea floor, and the level of bottom-water oxygenation in paleoenvironments. In this study, we investigated the living (stained) and dead (thanatocoenoses) benthic foraminifera assemblages collected in surface sediment samples from the Bohai Sea (BS) and the northern Yellow Sea (NYS). The samples were collected from different seasons, i.e. 50 samples in May (spring) and 46 samples in November (autumn). Our benthic foraminifera fauna analysis of these samples shows four living assemblages in May and three in November, indicating a sensitive response to seasonal environmental changes where the fauna live in the modern setting of the BS and NYS. Our redundancy analysis (RDA) of the living assemblages and their corresponding environmental parameters indicates that in spring, the abundances of Astrononion tasmanensis, Nonionella stella and Bulimina sp. have positive correlations with increased water depth, density and salinity, but are negatively correlated with increased FLOUR (chlorophyll). The abundances of Buccella frigid and Verneuilinulla advena are positively correlated with higher levels of dissolved oxygen (DO) and negatively correlated with increases in temperature. The abundances of Cribrononion subincertum are positively correlated with increased turbidity (TURB) and temperature, but negatively correlated with increased DO. In autumn, there is a positive correlation between the abundances of V. advena and increased salinity. Protelphidium tuberculatum, Ammonia beccarii (vars.) abundances, however, have negative correlations with salinity. During the autumn, the benthic foraminifera species P. tuberculatum was noticeably expanded and the dominance areas occupied by Elphidium magellanicum and C. subincertum were significantly reduced in the BS; along the southern coast of the Liaodong Peninsula, the dominant Buccella frigida in the spring was replaced by V. advena in autumn. The seasonal shifting of benthic foraminifera species are local responses to the combined changes in bottom water temperature, salinity, TURB and DO in the BS and NYS, and also to the more intensified autumn/winter intrusion of the Yellow Sea Warm Current (YSWC). The dead assemblages of benthic foraminifera, which are used to reconstruct paleoenvironments, exhibited considerable destruction of agglutinated assemblages. We found that such species as V. advena, Trochammina sp., Ammoscalaria sp. and Polskiammina asiatica are missing from the dead assemblages, possibly due to postmortem taphonomic processes in the BS and NYS. Considerable destruction of agglutinated species in dead assemblages may slightly bias our estimates of paleoenvironmental parameters when they are based on benthic foraminifera fossil shells.

Modern distribution of ostracodes and other limnological indicators in southern Lake Malawi: implications for paleocological studies

This modern distribution study from the southwest arm of Lake Malawi quantitatively relates variables of the lake environment to surficial assemblages of ostracodes and other paleoenvironmental indicators (molluscs, Botryococcus, fish, and charcoal) from 102 sites, across a gradient of littoral to shallow profundal conditions. The goal of this research is to use the resultant relationships to help quantify paleoecological interpretations of the fossil record from sediment cores. Site locations varied by depth (1–60 m) and adjacent shoreline environment. Thirty-three ostracode species are identified from 54 sites including four new, undescribed species of Cypridopsinae (2) and Limnocythere (2). Ostracodes are extremely abundant between 1 and 25 m water depth, but are rare to absent between 30 and 60 m. This disappearance is probably taphonomically controlled, with carbonate dissolution in the death assemblage since abundant live ostracodes have been found in the lake at greater than 30 m depth, where bottom sediments lack calcium carbonate. Constrained correspondence analysis (CCA) of ostracode species abundance suggests depth and dissolved oxygen (DO) content to be the primary environmental variables affecting their distribution. Additional CCA models using all biological indicators suggest limnologic variables correlated with depth (e.g., bottom water temperature and DO) and adjacent shoreline environment were most significant.

Infaunal macrobenthos of the oxygen minimum zone on the Indian western continental shelf

AbstractThe Arabian Sea is characterized by a mid‐depth layer of reduced dissolved oxygen (DO) concentration or oxygen minimum zone (OMZ ‐DO concentration &lt;0.5 ml·l−1) at ~150–1000 m depth. This OMZ results from the flux of labile organic matter coupled with limited intermediate depth water ventilation. Generally, benthic animals in the OMZ have morphological and physiological adaptations that maximize oxygen uptake in the limited oxygen availability. Characteristics of OMZ benthos have been described from only a few localities in the Arabian Sea. We measured the bottom water DO and studied the characteristics of infaunal macrobenthos of the Indian western continental shelf by collecting samples at 50, 100 and 200 m in depth from 7° to 22° N. The DO values observed at 200 m (0.0005–0.24 ml·l−1) indicated that this area is lying within an OMZ. Five major taxa, namely Platyhelminthes, Sipunculoidea, Echiuroidea, Echinodermata and Cephalochordata were absent from the samples collected from this OMZ. In general, declines in total macrobenthic density and biomass and polychaete species richness and diversity were observed in this OMZ compared with the shallower depths above it. Community analyses of polychaetes revealed the dominance of species belonging to families Spionidae, Cirratulidae and Paraonidae in this OMZ. Low oxygen condition was more pronounced in the northern continental shelf edge (≤0.03 ml·l−1), where the majority of spionids including Prionospio pinnata and cirratulids were absent; whereas amphipod, isopod and bivalve communities were not impacted.

Design and testing of a novel hypolimnetic oxygenation system to improve water quality in Lake Bard, California

Hypolimnetic oxygenation, the engineered addition of oxygen gas to the bottom of lakes and reservoirs, can improve water quality by repressing the accumulation of nutrients, metals, and toxic compounds in bottom waters. This study designed and tested an oxygenation system in Lake Bard, California. Hypolimnetic oxygen demand, a key design parameter, was estimated through the combination of sediment–water chamber incubations and numerical analysis of water column dissolved oxygen (DO) profiles. Chamber incubations showed that increased water velocity near the sediment–water interface substantially increased the rate of sediment oxygen uptake, and this observation was incorporated into system sizing. The chamber study also confirmed that maintenance of oxygenated conditions repressed sediment release of phosphate, ammonia, manganese, and sulfide. Phosphorus and manganese release rates were 6–8 mg/m2/d and 2–5 mg/m2/d under anoxic conditions, respectively, but negligible or negative under oxic conditions. The novel hypolimnetic oxygenation system used a waste product, oxygen-rich off-gas from an ozone contactor at a nearby water treatment plant, to sustainably improve source water quality. The system was tested over a 2 week period in June 2004. Oxygen addition averaged 380 kg/d and increased bottom water DO from 1–2 to 5–6 mg/L with a concurrent drop in water column phosphate and iron, and a delay in sediment release of ammonia. Manganese, with its slow oxidation kinetics, remained in bottom waters during the oxygenation test, indicating that the oxygenation system needs to be turned on earlier in the season to better control manganese accumulation in bottom waters.