Bottom Water Dissolved Oxygen Research Articles

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.

Quantifying the Impacts of Stratification and Nutrient Loading on Hypoxia in the Northern Gulf of Mexico

Stratification and nutrient loading are two primary factors leading to hypoxia in coastal systems. However, where these factors are temporally correlated, it can be difficult to isolate and quantify their individual impacts. This study provides a novel solution to this problem by determining the effect of stratification based on its spatial relationship with bottom-water dissolved oxygen (BWDO) concentration using a geostatistical regression. Ten years (1998–2007) of midsummer Gulf of Mexico BWDO measurements are modeled using stratification metrics along with trends based on spatial coordinates and bathymetry, which together explain 27–61% of the spatial variability in BWDO for individual years. Because stratification effects explain only a portion of the year-to-year variability in mean BWDO; the remaining variability is explained by other factors, with May nitrate plus nitrite river concentration the most important. Overall, 82% of the year-to-year variability in mean BWDO is explained. The results suggest that while both stratification and nutrients play important roles in determining the annual extent of midsummer hypoxia, reducing nutrient inputs alone will substantially reduce the average extent.

Influence of Physical Forcing on Bottom-Water Dissolved Oxygen within Caloosahatchee River Estuary, Florida

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary.

Reversal of eutrophication following sewage treatment upgrades in the New River Estuary, North Carolina

The New River Estuary consists of a series of broad shallow lagoons draining a catchment area of 1,436 km2, located in Onslow County, North Carolina. During the 1980s and 1990s it was considered one of the most eutrophic estuaries in the southeastern United States and sustained dense phytoplankton blooms, bottom water anoxia and hypoxia, toxic outbreaks of the dinoflagellatePfiesteria, and fish kills. High nutrient loading, especially of phosphorus (P), from municipal and military sewage treatment plants was the principal cause leading to the eutrophic conditions. Nutrient addition bioassay experiments showed that additions of nitrogen (N) but not P consistently yielded significant increases in phytoplankton production relative to controls. During 1998 the City of Jacksonville and the U.S. Marine Corps Base at Camp Lejeune completely upgraded their sewage treatment systems and achieved large improvements in nutrient removal, reducing point source inputs of N and P to the estuary by approximately 57% and 71%, respectively. The sewage treatment plant upgrades led to significant estuarine decreases in ammonium, orthophosphate, chlorophylla, and turbidity concentrations, and subsequent increases in bottom water dissolved oxygen (DO) and light penetration. The large reduction in phytoplankton biomass led to a large reduction in labile phytoplankton carbon, likely an important source of biochemical oxygen demand in this estuary. The upper estuary stations experienced increases in average bottom water DO of 0.9 to 1.4 mg l−1, representing an improvement in benthic habitat for shellfish and other organisms. The reductions in light attenuation and turbidity should also improve the habitat conditions for growth of submersed aquatic vegetation, an important habitat for fish and shellfish.

Dissolved Oxygen Saturation Controls PAH Biodegradation in Freshwater Estuary Sediments

Polycyclic aromatic hydrocarbons (PAHs) are common contaminants in terrestrial and aquatic environments and can represent a significant constituent of the carbon pool in coastal sediments. We report here the results of an 18-month seasonal study of PAH biodegradation and heterotrophic bacterial production and their controlling biogeochemical factors from 186 sediment samples taken in a tidally influenced freshwater estuary. For each sampling event, measurements were averaged from 25-45 stations covering approximately 250 km(2). There was a clear relationship between bacterial production and ambient temperature, but none between production and bottom water dissolved oxygen (DO) % saturation or PAH concentrations. In contrast with other studies, we found no effect of temperature on the biodegradation of naphthalene, phenanthrene, or fluoranthene. PAH mineralization correlated with bottom water DO saturation above 70% (r(2) > 0.99). These results suggest that the proportional utilization of PAH carbon to natural organic carbon is as much as three orders of magnitude higher during cooler months, when water temperatures are lower and DO % saturation is higher. Infusion of cooler, well-oxygenated water to the water column overlying contaminated sediments during the summer months may stimulate PAH metabolism preferentially over non-PAH organic matter.

Sediment denitrification in the Gulf of Mexico zone of hypoxia

The largest zone of anthropogenic bottom water hypoxia in the Western Hemisphere occurs seasonally in the northern Gulf of Mexico between the Mississippi River delta and the coast of eastern Texas. This zone of hypoxia reaches its greatest extent in the summer months and is a conse- quence of seasonal stratification of the water column com- bined with the decomposition of organic matter derived from accelerated rates of primary production. This enhanced pro- ductivity is driven primarily by the input of inorganic nitrogen from the Mississippi River. There are 3 likely sinks for fixed nitrogen within this zone of hypoxia: sequestration in the sediment, dispersion and dilution into the Gulf of Mexico, and denitrification. We assessed potential denitrification rates at 7 stations in the zone of hypoxia during the summer of 1999. Those data are compared with bottom water nitrate, ammo- nium and dissolved oxygen (DO) concentrations. No denitri- fication was observed in the water column. Denitrification potential rates in the surface sediments were unexpectedly low and ranged between 39.8 and 108.1 µmol m -2 h -1 . The highest rates were observed at stations with bottom water DO concentrations between 1 and 3 mg l -1 . Denitrification activity was significantly lower at stations where DO was lower than 1 mg l -1 or higher than 3 mg l -1 . Nutrient data for these sta- tions demonstrate that as anoxia is approached, the dominant species of nitrogen shifts from nitrate to ammonium. The shift in nitrogen species suggests competition between microbial populations in the sediment community. The lower denitrifi- cation rates at stations with bottom water DO <1 mg l -1 may be due to nitrate limitation or an increase in the competitive advantage of microorganisms capable of dissimilatory nitrate reduction to ammonium (DNRA). Suppression of denitrifica- tion at low DO by any mechanism will increase the residence time of bioavailable nitrogen. This trend could act as a posi- tive feedback mechanism in the formation of hypoxic bottom waters.

Estimating the spatial extent of bottom-water hypoxia and habitat degradation in a shallow estuary

Bottom-water hypoxia (≤ 2 mg l -1 dissolved oxygen (DO)) greatly modifies the benthic habitat of estuaries, depending upon spatial extent, duration, and frequency. Bottom-water hypoxia often develops under conditions of density stratification, which inhibits vertical mixing, and warm temperatures, which enhance biological oxygen demand. Long-term, mid-channel data from the Neuse River Estuary in North Carolina permitted evaluation of how stratification and temperature combined to affect DO concentrations at the bottom. Salinity stratification (∆S) and water tempera- ture (T ) explained respectively 30 and 23% of the variance in bottom-water DO concentrations. The amount of salinity stratification required to induce bottom-water hypoxia declined with increasing water temperature. About 80% of observed hydrographic profiles exhibited bottom hypoxia when ∆S exceeded 5 psu and T exceeded 20°C. Using cross-channel hydrographic surveys as verification, we derived a general set of methods to estimate the lateral extent of low-DO bottom water from mid- channel hydrographic profiles. The method involves cross-estuary and along-estuary extrapolation based on assumption of a flat oxycline. Occasional violation of this assumption resulted in modest overestimation in cross-channel extent of low DO. Application of this method produced estimates ranging from 0 to 116 km 2 of bottom area (0 to 42% of the estuarine study area) exposed to hypoxia over all sample dates in summer 1997. The maximal bottom area exposed to hypoxia corresponded closely with an independent estimate of the area (100 km 2 ) that experienced almost complete mor- tality of Macoma spp. clams, the key benthic resource for demersal fishes and crabs. Consequently, mid-channel hydrographic profiles taken along the mid-channel of the estuary can be employed to assess the spatial scale of bottom habitat degradation due to hypoxia.

Seasonal deepening of the pycnocline in a shallow shelf ecosystem and its influence on near-bottom dissolved oxygen

A 3 yr record (1992 to 1994) of dissolved oxygen (DO) concentrations from an intensive grid of 21 sampling stations in a ∼100 km 2 study area of western Massachusetts Bay (∼25 to 50 m water depth) showed a regular seasonal decline in bottom waters during stratification, but considerable spatial and temporal variability within and across years. Mean near-bottom, subpycnocline DO concentrations for the area reached 7.57, 7.85, and 6.2 mg l -1 in mid October 1992, late September 1993, and late September 1994, respectively; individual station readings were as low as 4.8 mg l -1 in 1994. Overall stratified-season rates of subpycnocline DO decline were ∼0.025 to 0.031 mg l -1 d -1 , but rates increased late in the season as the bottom layer sharply warmed to its annual temperature maximum. Concurrent with relatively lower DO concentrations in 1994, field measurements indicated high bottom-water temperatures >12°C (≥4°C above 1992 to 1993) and a deepened pycnocline just prior to overturn. To address how factors like temperature and vertical structure of the water column interact with metabolic processes to shape observed trends in DO decline and spatio-temporal variability, we used a simple model with physical and biological measurements from field monitoring as inputs. From field and model sensitivity results, we conclude that temperature and stratification strongly influence DO minima and rates of decline, and these factors interact with the bathymetric slope, as well as the topographic and depositional heterogeneity of the study area, to create subpycnocline variability in DO. With respect to lower DO in 1994, temperature contributed by accelerating both water and sediment metabolism, but a major effect was the late-season deepening of the pycnocline that enhanced the contribution of sediment respiration to DO decline by isolating a thin near-bottom water layer. In addition, dynamics of seasonal pycnocline deepening are a principal influence on interannual variability in bottom-water DO because, in contrast to the late-season effect, early in the stratified season a shallow pycnocline depth may moderate DO decline by allowing mid-water primary production to add DO to subpycnocline water.

Sediment-Water Oxygen and Nutrient Exchanges along the Longitudinal Axis of Chesapeake Bay: Seasonal Patterns, Controlling Factors and Ecological Significance

Sediment-water oxygen and nutrient (NH4 +, NO3 −+NO2 −, DON, PO4 3−, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l−1, apparently limiting SOC rates. Fluxes of NH4 + were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations ( 500 μmol N m−2 h−1) were observed. Nitrate + nitrite (NO3 −+NO2 −) exchanges were directed into sediments in areas where bottom water NO3 −+NO2 − concentrations were high (>18 μM N); sediment efflux of NO3 −+NO2 − occurred only in areas where bottom water NO3 −+NO2 − concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m−2 h−1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH4 −, PO4 3−, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.

Long‐term trends in Pamlico River Estuary nutrients, chlorophyll, dissolved oxygen, and watershed nutrient production

Trends in Pamlico River estuary ammonia nitrogen (NH4), nitrate nitrogen (NO3), phosphate phosphorus (PO4), chlorophyll a (chl a) and dissolved oxygen (DO) during the past 20–24 years were analyzed, and estimates of annual N and P production in the watershed over the past century were computed. The goal of the study was to determine whether or not the estuary is becoming more eutrophic. NO3 has decreased in the upper and middle regions of the estuary by 3–6% yr−1 since 1970, and NH4 has decreased throughout the estuary at an annual rate of 5.5–7.7% yr−1. Since 1967 PO4 has increased by 2% yr−1 in the lower two thirds of the estuary due to discharges from a phosphate mining facility. Thus the inorganic N:P ratio has decreased, which suggests that N is now potentially more limiting than in the past. In the upper estuary, chl a increased at a rate of 6.6% yr−1 since 1970, and bottom water DO decreased very slightly; neither showed trends in the middle and lower estuary regions. The weight of the evidence is that the Pamlico has not become more eutrophic during the past two decades. This finding is corroborated by the lack of a trend since 1970 in calculated N and P production from point and nonpoint sources in the watershed. Watershed nutrient production is estimated to have increased severalfold between 1880 and 1970, but appears to have stabilized after 1970, due primarily to decreased application of fertilizer on croplands.

Stratification and Bottom-Water Hypoxia in the Pamlico River Estuary

Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15°C). In July, 75% of the DO readings were <5 mg 1−1, and one-third were <1 mg 1−1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6–12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr. *** DIRECT SUPPORT *** A01BY059 00002