OS46J-01
Megatransect: Mapping low dissolved oxygen, zooplankton and fish abundance over a 24 hour period in the Neuse River, USA
The Neuse River estuary in North Carolina, USA was the focus of an intensive, 24 hour sampling effort in August of 2004. The goal was to produce highly resolved spatial maps of dissolved oxygen, zooplankton biomass and fish biomass. From one vessel, we measured environmental variables, zooplankton biomass and fish biomass. Environmental variables and zooplankton biomass were measured using a towed body (ACROBAT) mounted with a CTD and an optical plankton counter; fish biomass was measured using a hull mounted acoustic profiler. A second vessel measured zooplankton abundance using plankton traps and net tows. The Neuse River was strongly stratified with dissolved oxygen concentrations < 2.0 mg L$^{-1}$ in the bottom layer. Zooplankton biomass was distributed evenly throughout the water column during the day transect and more concentrated in the bottom waters during the night transect. Zooplankton biomass was observed to be elevated in hypoxic water, particularly during the night transect. The presence of abundant A. tonsa in plankton traps from these locations supported the optical plankton counter measurements. Fish biomass was low during the day transect, when fish were presumably present in the shoals. Fish biomass was elevated during the night transect and more evenly distributed throughout the water column. Fish biomass was reduced in areas of hypoxic water, despite the high concentration of zooplankton prey in these areas. We conclude from this study that zooplankton are using hypoxic water as a refuge from fish predation in the Neuse River estuary. The persistence of low dissolved oxygen conditions likely leads to alteration of food web dynamics.
OS46J-02
Dissimilatory Nitrate Reduction to Ammonium (DNRA) as a "link" for N and a "sink" for oxygen in shallow, hypoxic, coastal environments
Dissimilatory nitrate reduction to ammonium (DNRA) is a heterotrophic reaction, which can compete with denitrification and assimilatory uptake in controlling the fate of nitrate in coastal ecosystems. DNRA is potentially an important process because it maintains nitrogen (N) in the system as ammonium, a biologically available form, and is a component of sediment oxygen consumption (SOC). The oxygen end-product of DNRA is H2O, rather than O2, so DNRA can serve as a "sink" for the nitrate-oxygen derived from nitrification. Thus, DNRA can affect the fates of both N and O in coastal hypoxic regions. Knowledge of the occurrence and rates of DNRA is needed to develop an understanding of the importance of this process to hypoxia development. Potential DNRA rates were observed in N-dynamics experiments conducted on intact cores from diverse shallow, eutrophic sites to evaluate occurrence and potential rates. After 15N-labeled NO3- was added to inflow water under steady-state conditions, concentrations and atom percent 15N of ammonium were measured by high performance liquid chromatography to calculate DNRA rates as 15NH4+ production. Potential DNRA rates ranged from zero to 100 micro-mol m-2 h-1 and were correlated positively with salinity in Texas Estuaries. DNRA rates ranged from 11 to 170 micro-mol m-2 h-1 and were correlated (p < 0.001) with sediment oxygen consumption [SOC] rates in Florida Bay under high, but relatively constant, salinity conditions (37 to 41 in August 2004 and 33 to 45 in January 2005). DNRA rates were at times equal to or higher than denitrification rates under high-salinity conditions. DNRA was observed in some freshwater environments (e.g. Lake Erie and its Old Woman Creek tributary, and Lake Taihu, China), but rates were low relative to denitrification, except for a polluted Lake Taihu tributary where DNRA rates were high (250 micro-mol m-2 h-1) and comparable to denitrification rates after 15NO3 additions (390 micro-mol m-2 h-1). DNRA may be a significant oxygen sink in coastal systems when redox layers are thin, because ammonium can diffuse into oxic zones and be nitrified. Thus, nitrification, physical diffusion, and DNRA may work in combination as an oxygen pump, intensifying oxygen removal rates at the sediment-water interface in hypoxic regions.
OS46J-03
Rates of Delivery Of Carbon and Nutrients to the Hypoxic Region of Lake Erie
The central basin (CB) hypolimnion of Lake Erie generally exhibits hypoxia, and occasionally anoxia, at the end of the summer, with equilibrium saturation of O2 re-established during the late winter - early spring when vigorous mixing occurs within the isothermal water column. In an attempt to eliminate the low DO, phosphorus loads to the lake were reduced through regulation and lake TP declined until about 1995. A cause of current concern is that recent monitoring data imply that lake phosphorus concentrations are increasing from the low values in the mid-1990s. We deployed sediment traps in the hypolimnion of the central and eastern basin (recipient of transport from the CB) to measure the rates of delivery of mass, organic matter and nutrients. CB fluxes ranged from 0.8 to 37 g/m2/d over 9 day collection intervals, with peaks in the fall-early winter, declining to a minimum after ice covers the lake. Higher fluxes reappear after ice out in the spring. In the CB, trap-measured OC flux is greater than OC sediment accumulation and respiration, implying localized sediment resuspension.
OS46J-04
Phytoplankton Assemblages and Coastal Hypoxia in the Gulf of Mexico
The Mississippi and Atchafalaya rivers discharge nutrient rich freshwater onto the northern Gulf of Mexico shelf causing seasonal bottom-water hypoxia. We studied the phytoplankton pigment composition and estimated major phytoplankton groups as shown by HPLC pigments from 2001 through 2004 on a transects near the Mississippi River (monthly) and one off the Atchafalaya River (bimonthly). Average chlorophyll a concentrations were similar on both transects ranging 4.1 to 5.5 μg L\-1 in 2001-2003 and higher in 2004 with 6.7 μgL\-1 to 8.5μg L\-1. For specific station/time data, concentrations reached 50 μg L\-1 routinely and up to 290 μg L\-1. The flux of carbon from these blooms\-senescent cells, fecal pellets, and aggregates\-contributes the organic matter that fuels seasonal hypoxia. Shifts in nutrient levels and ratios affect the composition of the phytoplankton community and, in turn, the amount of carbon reaching the seabed. Diatoms (fucoxanthin) were the major contributors to chlorophyll a near both rivers, and the major source of carbon flux in winter and spring from inshore to 30 km offshore. In July 2004 chlorophyll a concentrations from cyanobacteria (zeaxanthin) were the highest in a 4\-year period (over 40 μg L\-1 nearshore in both transects. Chlorophyll a concentrations attributed to dinoflagellates (peridinin) were mostly observed in spring at nearshore stations on both transects. These algae are less likely to sink or be grazed, but the high biomass observed at nearshore stations is sufficient to lead to carbon flux and subsequent hypoxia.
OS46J-05
Sediments Tell the History of Coastal Eutrophication and Hypoxia
Management strategies concerning coastal water quality are often faced with a lack of critical knowledge of how the system has changed, at what rate and in relation to what factors. The trends needed to develop valid management scenarios can be determined from sediment paleoindicators of eutrophication and oxygen conditions. Sediment analyses from uniformly accumulated sediments in estuaries and on the continental shelf document recent eutrophication in many coastal waters. The evidence consists of increased accumulation of diatom remains, shifts in diatom assemblages, shifts in naturally occurring isotopes, and increasing accumulation of marine-origin carbon in the sediments. Surrogates for oxygen conditions that indicate worsening oxygen stress include glauconite abundance, benthic Foraminifera, ostracods, algal chloropigments, and elemental shifts. The changes are more apparent in areas of chronic hypoxia and are coincident with increasing nutrient loads from adjacent watersheds and airsheds. The temporal shifts in sediment indicators of eutrophication and worsening hypoxic conditions are robust across similar coastal waters globally, and represent different time lines of landscape alterations and increased nutrient loads.
OS46J-06
The Effect of Shelf Waves on Bottom Oxygen in the Northern Gulf of Mexico
A combination of {\it in situ} sampling and numerical modeling is used to investigate the effects of mesoscale circulation patterns and stratification on the erosion of hypoxia on the Louisiana shelf. In the Gulf of Mexico, hypoxia (dissolved oxygen concentration below 1.4 ml/L) usually occurs from late spring through late summer. Fresh water input from the Mississippi and Atchafalaya Rivers, high surface sea temperatures and weak winds during summer maintain stability of the water column inhibiting vertical mixing. A series of seven oceanographic cruises was conducted during spring-summer of 2004-2005 along the Texas-Louisiana coast. Field measurements show evidence of a large-amplitude, shelf-scale wave in the distribution of physical properties in the water column. Vertical displacements of isopycnals as large as 10 m in 20 m total depth with estimated along shelf wavelength about 50 km were observed during late summer in 2004 and throughout 2005. Observations indicate that dissolved oxygen concentrations are increased at the bottom where the break down in stratification occurs; as stratification weakens, it allows for vertical mixing to ventilate the lower layers of the water column. This wave is also associated with alternating onshore/offshore current transport, and may enhance the dispersion of the lower layer of hypoxic water.
OS46J-07
Environmental Factors Influencing Seasonal Hypoxia in Narragansett Bay, Rhode Island
A series of automated water column time-series measurement systems established in Narragansett Bay, Rhode Island from 2001 to 2005 show that hypoxic events occur intermittently in the upper reaches of the bay. A pervious study of two buoy sites from July through September 2001 revealed that the occurrence of episodic surface phytoplankton blooms followed by subsurface hypoxic events were linked to the periodic weak neap tidal cycles that occur alternately with stronger neap tidal cycles. This study analyzed five years of near surface and near bottom temperature, salinity, dissolved oxygen, pH, chlorophyll fluorescence and pressure data for the presence of hypoxia during the weak neap tides. In addition to weak neap tides, freshwater input, winds, air temperature and solar radiation were examined for their influence on oxygen concentrations in the upper bay. Water column stratification was greater and persisted for long periods in 2001 and 2003 than in other years (average delta sigma-t 1.4 kg/m$^{3}$ and 3.2 kg/m$^{3}$ respectively). In both years, a heavy runoff event occurred early in the summer and river flow into the Bay was equal to or greater than the 10-year average. In 2002, the river flow into the Bay was less than the 10-year average, however air temperatures were much warmer than the 9-year average. As a result, severe hypoxia was observed during the weak neap tidal periods during these three years. No major hypoxic events occurred in 2004 or 2005 due to better mixing (average delta sigma-t less than 0.9 kg/m$^{3}$). Multi-year time-series observations have enhanced the understanding of the importance of river flow, winds and tides on the development and break down of stratification and the influence of stratification on the extent and duration of hypoxic events in Narragansett Bay.
OS46J-08
Benthic response to near bottom dissolved oxygen using sediment profile camera measures
The United States Environmental Protection Agency and other environmental authorities regulate concentrations of dissolved oxygen (DO) to control nutrient-related eutrophication in estuarine and coastal waters. Benthic faunal communities integrate the impacts of low DO over time, and can be rapidly assessed using benthic imaging. The goal of this study was to quantify the relationships between near-bottom dissolved oxygen (DO) and benthic response derived from imaging with a sediment profile camera. We monitored three stations in Narragansett Bay (Rhode Island, USA) for DO and other water quality parameters 15-20 cm above the sediment surface at 15 minute intervals between July and November 2002, and regularly sampled these stations with a sediment profile camera throughout this time period. These soft-bottomed stations encompassed several DO environments. We tested for relationships between near-bottom DO and Nilsson and Rosenberg's Benthic Habitat Quality (BHQ) index, the apparent Redox Potential Discontinuity (aRPD) depth, and various faunal features that can be identified in sediment profile images. Camera measures were examined against a variety of methods for characterizing DO, including the mean DO, and the percent of time a location remained under various DO thresholds calculated over time periods from 1 day to 49 days. The best relationship (highest r2) between near-bottom DO and BHQ was found when DO was evaluated as the percent of time a site remained under a hypoxic threshold of 2.6 mg/L over the 28 day time period preceding each camera deployment. We found that, over a range of benthic environments, BHQ was successful at identifying stations that had experienced relatively high or low DO over the preceding four weeks. When mean BHQ was 1.0 or less in our data set, DO had been at or below 2.6 mg/L at least 60% of the time during the 28 days prior to sampling. When the BHQ mean was 5.0 or more in our data set, DO had been above 2.6 mg/L at least 80% of the time during the 28 day time period prior to sampling. In the intermediate values (1 < mean BHQ < 5), our sediment profile data showed more variability with DO, but DO had been at or below 2.6 mg/L no more than 50% of the time during the 28 day time period prior to sampling. We conclude that the sediment profile camera measures of benthic condition correlate to DO in areas where low DO is the primary stressor, integrate DO over ecologically relevant time scales, and enable sampling over spatial scales that are meaningful for mapping by virtue of rapid deployment and analysis. We submit that sediment profile camera imagery is a useful assessment and mapping tool for environmental managers interested in benthic condition and in first-order quantitative estimates of near-bottom DO regimes in areas where low DO is the primary benthic stressor.
OS46J-09
Fine-Scale Vertical Distribution of Acoustic Biomass in Hypoxic Waters on the Gulf of Mexico Shelf
The northwestern Gulf of Mexico shelf experiences one of the largest known seasonal hypoxic zones due to freshwater and nutrient discharge from the Mississippi-Atchafalaya River system. Avoidance of hypoxic bottom water by mobile fishes through both vertical and horizontal shifts in distribution may lead to a variety of indirect effects that potentially impact growth and mortality. We used hydroacoustics and paired bottom trawls to investigate fish distribution and a Seabird CTD to measure temperature, salinity, and dissolved oxygen in and around the hypoxic zone on the Louisiana shelf during July 2004. While trawl biomass decreased in hypoxic waters, acoustic biomass was more concentrated above the bottom low oxygen layer. Acoustic biomass was greater in normoxic waters at night when compared to day. In addition, the largest biomass from both trawl and acoustic sampling occurred near hypoxic edges. Our results indicate that hypoxia influences the vertical and horizontal distribution of species. Vertical shifts in fish distribution may have impacts on trophic interactions above the bottom low oxygen layer.
OS46J-10
Nutrient and Freshwater Distribution on the Louisiana Shelf in Summer 2004
We describe three cruises during April, June and August 2004 to show the vertical and horizontal distributions of freshwater and nutrient concentrations and their relation with bottom dissolved oxygen concentration in the hypoxic zone of the Louisiana Shelf. Hypoxia on the shelf is initiated and sustained by the combined effects of nutrient loading by the Mississippi River system and stratification of the water column enhanced by weak upwelling favorable winds during early summer. Property distributions reflect differences in the physical and biochemical processes across the shelf as well as seasonal changes in discharge. Off the Atchafalaya and west of Terrebonne, fresh water was nutrient poor as nutrients were consumed rapidly on entering the shelf. However, near the bottom, nutrient concentrations were high when dissolved oxygen concentrations were low, indicating remineralization in the bottom waters and sediments. The resuspension of organic material, in addition to surface advection, was fueling the hypoxia far from the river sources. Distributions indicate that high nutrient values were rarely seen in the surface waters of the western study region south of the Atchafalaya. Transmission increased with depth, indicating increase in particulate organic matter (settled diatoms). The density stratification was also strongest in this region. During August, winds changed to downwelling favorable, establishing a coastal jet, which contributed to the breakdown of stratification and the mixing of oxygen-poor bottom waters. This study is a part of a NOAA funded research project titled `Mechanisms Controlling Hypoxia on the Louisiana Shelf`.
OS46J-11
Combined Effects of Food Quality and Hypoxia on Some Life History Parameters of {\it Acartia tonsa} Dana (Copepoda: Calanoida).
Survival, egg hatch, egg production rates (E$_{r}$), and ingestion rates were determined for adult {\it Acartia tonsa} and their offspring in a series of laboratory experiments. Experiments followed a 2$^2$ factorial design, which tested a low- or high-carbohydrate-rich diet in a low- and high-oxygen environment. Survival and egg hatch varied between experiments, but were generally high. In most cases, E$_{r}$ were higher for the high-oxygen treatments. E$_{r}$ were also strongly correlated with ingestion rates. Although total protein and carbohydrate content for each diet was similar between experiments, the diet producing the highest E$_{r}$ varied between experiments. Higher E$_{r}$ occurred for the offspring compared to the parent generation in two experiments, which is hypothesized to be the result of a constant food environment. Results reflect the importance of determining not only the relationship of diet biochemical composition and egg production, but also why certain diets are selectively ingested over others. E$_{r}$ of {\it A. tonsa} under certain sublethal levels of DO may not be significantly reduced compared to E$_{r}$ at high-oxygen levels if an adequate diet is available.
OS46J-12
Sustained Hypoxia off Southwest Louisiana During 2005
While hypoxia is usually found mainly over the Louisiana shelf between the Atchafalaya and Mississippi Rivers, it may be found further west and extend onto the Texas shelf. During 2005, hypoxia was established west of the Atchafalaya (92°-93°W) in March following high freshwater discharge. This patch persisted through the summer and expanded to the west (94°W) during July and August even though hurricanes disrupted hypoxia on the shelf east of 92°W. In July, the area affected by hypoxia was associated with a potentially harmful red tide caused by Gymnodinium sanguineum. We present data on the physical, chemical and current regimes in this region prior to the impacts of Hurricanes Katrina and Rita in late August and September respectively.
OS46J-13
Methodology for Robust Multiple Regression Modeling and Example Applications: Mapping Phycocyanin in Lake Erie from LANDSAT TM and SeaWiFS Data
Multiple regression models have been considered unsuitable for mapping ocean features since an attempt to use one such model to predict El Nino occurrences two decades ago. For a given application, however, the model will most likely be robust if it meets three criteria (Vincent, 2000): 1. The input image data are preprocessed to reduce atmospheric and illumination differences and to reduce geometric errors near sites where water samples were collected. 2. The best multiple regression model is restricted to the one with the highest R2(adjusted) that also passes the Durbin-Watson test for auto-correlation. 3. That best model is successfully tested with a withheld data set collected at a time different from the data set used to create the model. One such best model (out of hundreds of combinations of ratios tested by the statistical software) was created for mapping phycocyanin, the pigment that has the most nearly unique association with cyanobacteria, in the Western Basin of Lake Erie and its tributaries (Vincent et al, 2004). The phycocyanin content (PC) model, calculated from LANDSAT TM data of 30 water sample areas on July 1, 2000, is a linear combination 6 dark-object-corrected spectral ratios. It was tested on data from another date, September 27, 2000, displaying rms error departures of predicted from actual measurements for 20 sample areas on those dates of 2 μg/liter (18.2 percent of the PC range for that date). A similar PC model was created from the same two data sets in the Western Basin of Lake Erie for SeaWiFS data (Dash and Vincent, 2005). Though the errors were slightly greater for the 1.1-km sized pixels of SeaWiFS data (compared to 30-m pixels for LANDSAT TM) and the spectral bands are different for the two satellites, both PC models mapped very similar patterns in the Western Basin of Lake Erie for the same dates (July 1 and September 27, 2000). Both models were mapping the same thing, PC content (or some surrogate highly correlated with phycocyanin content) on a withheld data set. Dash, Padmanava, and R. K. Vincent, Computer Animation of Cyanobacteria Blooms in Lake Erie from July-October, 2003, As Mapped from SeaWiFS Data with a New Phycocyanin Algorithm, Submitted to the Proceedings of the 16th Pecora Conference on Global Priorities in Land Remote Sensing, Sioux Falls, SD, October 23-27, 2005. Vincent, R.K., Forecasts of Monthly-Averaged Daily Temperature Highs in Bowling Green, Ohio from Monthly Sea Surface Temperature Anomalies in the Eastern Pacific Ocean During the Previous Year, Photogrammetric Engineering and Remote Sensing, V. 66, No. 8, pp. 1001-1009, 2000. Vincent, R.K., X. Qin, R. M. L. McKay, J.Miner, K. Czajkowski, J. Savino, and T. Bridgeman, Phycocyanin Detection from LANDSAT TM Data for Mapping Cyanobacterial Blooms in Lake Erie, Remote Sensing of Environment, Vol. 89, No. 3, pp 381-392, 2004.
OS46J-14
CDOM Dynamics Along the Louisiana Coast in Relation to Hypoxia Using Field and Satellite Ocean Color Data
An extensive set of spectral CDOM (Colored Dissolved Organic Matter) absorption measurements obtained during four one-week cruises in March, May, July and August of 2005 along the Louisiana coast are examined in relation to water-column oxygen and chlorophyll concentrations, productivity and salinity. River discharge which started high in 2005 was by August a near record low and was reflected in lower CDOM absorption associated with decrease in freshwater along the coast. While CDOM absorption linkages to oxygen concentrations have yet to be examined, preliminary results indicate CDOM - salinity relationships to be conservative along the Louisiana coast. With freshwater plumes and stratification being important factors controlling hypoxia in the near bottom waters, CDOM absorption could also provide additional insights on the occurrence of hypoxia in the study area. A previously developed regional CDOM algorithm for SeaWiFS indicates the possibility of retrieving CDOM using the Oceansat-1 OCM satellite sensor data. Based on a larger data set and satellite CDOM absorption estimates we present an assessment of the CDOM dynamics in a complex river dominated coastal environment.
OS46J-15
Hypoxia in Hood Canal, Washington: controlling factors and current status
Hood Canal, a fjord-like sub-basin of Puget Sound, Washington State, USA, is a long, deep, narrow, productive estuary with strong seawater density stratification and slow circulation. These are conditions conducive to seasonally low oxygen concentrations, which have been observed in records dating back to the 1950's. However, in recent years, especially since the mid-1990's, the frequency, duration, and spatial extent of the hypoxia has increased. Major biota kills occurred during both 2002 and 2003, though kills have been reported historically as far back as the 1920's. Of concern is that the dissolved oxygen inventories of the deep waters in the southern portion of the canal (where the hypoxia is strongest) measured during the 2000's are among the lowest, with 2004 the absolute lowest, on record compared to the data from the 1950's, 60's, and 90's. Causes for this severe and seemingly deteriorating condition could be many and potentially include human-mediated loading of nitrogen or organics, changes in river flow delivery that could affect circulation, changes in oceanic water properties, and local weather forcing. We present the evidence for some of these contributing factors. A modeling study will address the quantitative balance of these factors in driving the observed hypoxia.
http://www.hoodcanal.washington.edu
OS46J-16
Mechanism(s) of 15N Enrichment in Eutrophication-Impacted Estuaries
Anthropogenic nitrogen loading is the common cause of eutrophication in many aquatic systems, particularly in coastal estuaries and embayments. Excess nitrogen over-stimulates biological production leading to reductions in both water column clarity and subsurface oxygenation. Almost as a general observation, natural 15N/14N ratio (d15N) has been found to increase with degree of eutrophication over a wide range in aquatic systems. Whether a single mechanism or set of mechanisms is responsible is not known. Such knowledge, though, is critical to making quantitative use of d15N data to diagnose and monitor N-stimulated eutrophication. In Long Island Sound (LIS), eutrophication has been manifest as seasonal development of hypoxia, particularly at its poorly-flushed western end, during summertime vertical stratification. We have found relatively elevated d15N for the entire LIS, correspondingly increasing from east to west by several per mil. Dated sediment cores indicate that the onset of eutrophication and increasing d15N began with the urbanization of the LIS watershed in the mid-1800's. This 15N enrichment can be caused by 1) higher d15N in the anthropogenic N source particularly sewage, 2) biological removal of nitrogen in the rivers flowing into LIS, and/or 3) denitrification removal of nitrate in LIS proper under hypoxic conditions. We are conducting a study of the isotopic composition of N sources to LIS as well as a seasonal study of the LIS water column to distinguish between these possible mechanism. So far, elevated d15N in riverine inputs suggests mechanism \#2 as important. Mechanism \#3 is not likely to be as important as originally thought since in the summer subsurface, O2 does not appear to become low enough and nitrate concentration not high enough to fuel significant denitrification.
OS46J-17
Lakewide Survey of Phytoplankton Community Composition and Health in Lake Erie Surface Waters Using Fluorimetric Applications: June, July, and September 2005
Recent technological advances in fluorimetry enable aquatic scientists to establish qualitative and quantitative assessments of the phytoplankton community at high levels of resolutions over large geographic scales. Thus, the resolution of past efforts to measure floristic diversity and phytoplankton productivity over the entire region of Lake Erie does not match our current knowledge of the scale and frequency of physical forcing in this system. Efforts underway to understand hypoxia and anoxia in Lake Erie require basin scale measurements of phytoplankton biomass as contributors to oxygen demand in hypolimnetic waters. Our objectives were to obtain the first lake-wide measurements of phytoplankton quantity (plankton size class, taxonomic grouping) and health (photosynthetic efficiency) in Lake Erie using advanced instrumentation; correlate phytoplankton size class, major phytoplankton divisions, and phytoplankton health to water quality parameters, and to analyze the variability in phytoplankton community indices (photosynthetic efficiency, composition) to characterize the dominant frequencies of variation for comparison with the dominant scales of physical forcing energy as revealed by parallel studies of hydrodynamics. We hypothesized that gradients in phytoplankton community composition exist in near-shore/offshore transects, and that photosynthetic efficiency varies with season (photosynthetic efficiency in pelagic surface waters will be reduced in late summer due to loss of nutrients from surface waters). In June and September, water was sampled from a depth of 1-1.5 m using a Teflon-lined polyethylene hose maintained at depth using a steel fish at a speed of 8-11 knots (R/V Lake Guardian) and sent to the lab using a trace metal clean pumping system; sampling in July was through an in-hull pump at a 1 m depth (CCGS Limnos). Water was mixed in a 9.5 L ferrybox; water flow rates were adjusted to vessel speed to maintain a spatial resolution of 0.5 km. Phytoplankton community composition was assessed using a FluoroProbe fluorimeter (Series 7, bbe Moldaenke), designed to provide an integrated spectral algae class analysis. The FluoroProbe measured major phytoplankton divisions (Chlorophyta, Cyanophyta, Heterokontophyta-Dinophyta, and Cryptophyta) in addition to water temperature; the FluoroProbe cannot spectrally differentiate the Heterokontophyta and Dinophyta. Phytoplankton health was assessed by measurements of photosynthetic efficiency (Fv/Fm) using a fast repetition rate fluorimeter (FRRF; MkI, Chelsea Instruments). Colored dissolved organic matter (CDOM) was determined fluorimetrically (Turner Designs, model 10-AU). All data were collected into georeferenced time stamped bins suitable for analysis by GIS software. During the June and September cruises, water was sampled periodically for total chlorophyll-a and dissolved (<0.4 μm) iron by tapping water from the inflow prior to the ferrybox. Nearshore-offshore transects and west-east transects show strong gradients in water quality and phytoplankton biomass and composition. Seasonal differences in phytoplankton community composition were apparent. Community composition will be discussed in relation to measured photosynthetic health and water quality parameters.
OS46J-18
A Multiproxy Approach to Extending Historical Record of Low-Oxygen Bottom Waters and Ecosystem Changes on the LA Continental Shelf
Studies of the benthic faunal assemblages and the organic carbon concentrations (TOC) in several sediment cores from the LA shelf indicate that low-oxygen bottom water conditions have increased in severity since the 1950's in association with an increase in the use of commercial fertilizer in the Mississippi Basin. Analysis of a long sediment core has recognized that low-oxygen events of similar magnitude to those of the last 50 years have occurred periodically from 1650 - 1950 AD. The most recent record of low-oxygen bottom-water events shows a distinct relationship between low-oxygen faunal assemblages, total organic carbon content (TOC) of bulk sediments and carbon isotopic indicators of predominately algal organic matter. Older low-oxygen events are characterized by variable coupling of benthic fauna and TOC and are associated with changes in the dominant source of organic matter from algal to terrestrial and/or bacterial sources. These findings indicate that multiple mechanisms are responsible for the development of low-oxygen bottom water conditions on the LA shelf. We are expanding the multi-proxy approach to reconstruct ecologic changes associated with the modern hypoxic and historical low-oxygen events on the LA shelf by utilizing molecular organic geochemical techniques. Analysis of marine derived sterols, fatty acids, and alkanes in concert with foraminiferal assemblage analyses are providing insight into the response of primary and secondary producer communities to the onset, expansion and increasing severity of oxygen depletion events in modern times and the geologic past. Molecular analyses of bacterially derived hopanols are providing insights into the role of microbial recycling in the development and propagation of oxygen-depletion events. The results of this study are expanding the instrumental and historical record and processes involved in the development of low-oxygen conditions on the LA shelf and will be critical in evaluating and predicting current and future trends in ecosystem structure and function as anthropogenic influences and natural climatic conditions evolve.
OS46J-19
Mesozooplankton Interactions with the Hypoxic Hypolimnion of Lake Erie
Undulating tows of a plankton survey system (consisting of an optical plankton counter, CTD, PAR sensor, oxygen sensor, and fluorometer) used simultaneously with acoustics for fish mapped the distribution of mesozooplankton relative to oxygen concentration and other physical and biotic variables in the central basin of Lake Erie. Both long transects covering great horizontal distances (30-50 km; sampled day and night) and shorter transects (5 km) sampled at 4 h intervals allowed us to examine broad regions of space, yet define impacts on diel vertical migration of zooplankton and interactions with fishes. Species abundance and composition of zooplankton at different depths were determined using a large volume diaphragm pump. Generally oxygen decreased with time throughout the summer, and during late summer very low oxygen concentrations (below 1 mg/L) were attained over broad areas. In contrast to some systems where zooplankton use the hypoxic area as a refuge from fish predation, Lake Erie zooplankton avoided very hypoxic waters. Preliminary experiments showed that zooplankton fed at lower rates and had increased mortality in hypoxic hypolimnetic water relative to aerated controls.
OS46J-20
Local Mixing Processes in the Hypoxic Zone off Louisiana
A large set of vertical profiles of temperature microstructure data was collected during a 24-hour on station experiment in ~20 m water depth in the hypoxic region off Louisiana in August 2005. Estimates of turbulence kinetic energy dissipation (TKED) rates show, in addition to near-surface mixing, significant and persistent mixing activity in the vicinity of the pycnocline region with TKED rates in the general range of 1e-8 to 1e-7 W/kg, but higher at times. Near bottom mixing events, although in general weaker and less frequent than in the pycnocline region, were still significant and TKED rates in excess of 1e-7 W/kg were not uncommon. A unique observed feature was a persistent layer, 3-5 m thick, of higher dissolved oxygen (DO) concentrations than in the layers above and below it. This layer, presumably advected from oxygenated offshore waters, may act as a source of DO to other parts of the water column through the local mixing processes. A one-dimensional turbulence model, forced with observed surface meteorology from nearby stations, is currently used to better understand the mechanisms driving the observed mixing processes. Initial modeling results and the potential for vertical mixing of DO as well as nutrients, based on both observations and simulations, will be presented.
OS46J-21
Consequences of Tidal Forcing on the Distribution of Trace Metals in the Redox Transition Zone of the Chesapeake Bay.
The seasonal hypoxia and anoxia observed in the Chesapeake Bay may be attributed to changes in hydrologic behavior as well as inputs of sediments, nutrients, and pollutants that increase the organic carbon content. The most pronounced oxygen depletion is noted in late July and August as a result of eutrophication coupled with a strong salinity stratification of the water column. The objective of this study was to investigate the effect of tidal cycling on trace metals in the water column during the peak anoxic seasons. CTD casts and in situ voltammetric microeletrode profiling of dissolved O $_{2}$, Fe(II), Mn(II), \Sigma H$_{2}$S at high spatial resolution indicates that the suboxic zone shifts vertically by ~5m during a tidal cycle. Discrete water samples were collected simultaneously and analyzed by ICP-MS to demonstrate the effect of tidal forcing on the depth profiles of manganese, uranium, arsenic, lead, chromium, cobalt, nickel, copper, barium, zinc and cadmium. Manganese and iron cycles are directly affected by tidal-related variations in the oxygen and dissolved sulfide profiles. Oxygen penetrates the anoxic zone at ebb tide and oxidizes Mn(II) and Fe(II), while dissolved sulfide diffuses into the chemocline at flood tide to reduce Mn- and Fe-oxides with subsequent release of dissolved trace metals. Trace metals are generally low in concentration throughout the water column but seem to accumulate below the redox interface. Total dissolved manganese, uranium, arsenic, and chromium increase with depth over all tidal cycles and follow a nutrient-type profile, while total dissolved barium decreases with depth suggesting precipitation of a mineral phase. Overall, particulate and dissolved trace metal data suggest adsorption and precipitation occurs in the anoxic zone and that the effect of tidal forcing on the distribution of trace metals near the oxic- anoxic interface is regulated by Mn- and Fe- oxide scavenging and sulfide intrusion.