Ocean Sciences [OS]

OS53J HCC:321 Friday

Causes and Consequences of Seasonal Coastal Hypoxia/Anoxia II

Presiding:B J Eadie, NOAA Great Lakes Environmental Rresearch Laboratory; N Rabalais, Louisiana Universities Marine Consortium

OS53J-01 INVITED

Use of simple models for forecasting hypoxia

* Scavia, D (scavia@umich.edu) , University of Michigan, 444 Church St, Ann Arbor, MI 48109 United States

To be useful, hypoxia forecast models must be scaled to the spatial and temporal frames of decision makers. It is also important to understand and quantify the certainty in such models before they can be used as a basis for estimating ecological impact. We have successfully applied a very simple model formulation for predicting hypoxia extent in the Chesapeake Bay and the northern Gulf of Mexico, and will describe their basis, application, and testing. The strengths and weaknesses of simple vs more complex models will be discussed in the context of their utility for forecasts and for supporting impact assessment.

OS53J-02

Beyond Conventional Mass-Balance Oxygen Models: A Dual Budget Approach and its Consequences

* Justic, D (djusti1@lsu.edu) , Coastal Ecology Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803 United States
Quinones-Rivera, Z J (zquino1@lsu.edu) , Coastal Ecology Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803 United States
Wissel, B (bjoern.wissel@uregina.ca) , Faculty of Sciences, University of Regina, Regina, SK S4S0A2 Canada
Fry, B (bfry@lsu.edu) , Coastal Ecology Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803 United States
Rabalais, N N (nrabalais@lumcon.edu) , Louisiana Universities Marine Consortium, 8124 Hwy 56., Chauvin, LA 70344 United States
Turner, R E (euturne@lsu.edu) , Coastal Ecology Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803 United States

The inherent problem with budgets that are based on oxygen concentration measurements is that they reveal little information about the importance of individual oxygen sources and sinks. A decrease in bottom oxygen content, for example, may be a result of benthic or water-column respiration. Similarly, an increase in the oxygen content may be an effect of in situ photosynthesis, or a consequence of oxygen influx due to advection or diffusion. The synergism presents a problem in quantifying the impacts of physical and biological factors on hypoxia, because effects of biological factors are masked by physical factors, and vice-versa. This has direct implications on model parameterization and leads to the excessive use of default model parameters, i.e., parameters that were not independently verified by field studies. We developed a novel dual-budget approach aimed at quantifying oxygen sources and sinks in the Gulf of Mexico hypoxic zone. The approach is based on the partitioning of oxygen dynamics among the key biological and physical processes using oxygen isotopes, in addition to budget studies based on conventional oxygen concentration measurements. In our initial simulations, we are finding that isotopic values often reveal potential problems in model parameterization in spite of the fact that models correctly predict oxygen concentrations. This type of mistaken prediction is very useful in the overall modeling, forcing new model development.

OS53J-03

Enhancement of offshore hypoxia from cross-shelf advection processes

* DiMarco, S F (dimarco@tamu.edu) , Texas A&M University, 3146 TAMU, Department of Oceanography, College Station, TX 77843-3146 United States
Chapman, P (pchapman@lsu.edu) , CREST Office, Louisiana State University, School of the Coast and Environment, 3153 ECE Bldg, Baton Rouge, LA 70803 United States
Quigg, A (quigga@tamug.edu) , Texas A&M University at Galveston, Phytoplankton Dynamics Laboratory, Department of Marine Biology, Galveston, TX 77551 United States
Kiselkova, V (vkiselkova@ocean.tamu.edu) , Texas A&M University, 3146 TAMU, Department of Oceanography, College Station, TX 77843-3146 United States

Low near-bottom dissolved oxygen concentrations found on the Louisiana Shelf in summer are generally attributed to the combined effects of high nutrient loading, which enhances primary production, and a locally stratified water column, which inhibits the mixing down of oxygen rich water from near the surface. Both the nutrient loading and stratification are a result of discharge from the Mississippi River system. During August 2005, we observed south of Atchafalaya Bay a complex layering of inshore and offshore waters that indicate a combination of local and remote driving of observed low dissolved oxygen (DO) concentrations at near bottom and at mid-water depths. The near bottom low-DO layer is believed due to local benthic decay processes. However, the mid-water low-DO layer was found produced remotely and inshore and characterized by high nutrient concentrations, low light transmission, high fluorescence, and relatively high salinity. Total water depth in the area was about 20 m and the mid-water low-DO layer was at 10 m depth. The layering was distinct, stable and persisted for at least four days, the duration of sampling in the region. The mid-water layer had enhanced primary production relative to other layers owing to the high nutrient concentrations. Cross-shelf sampling revealed that the mid-water layer was found attached to the bottom further onshore and actively advecting hypoxic, nutrient-rich, near bottom water seaward along a density surface. The source of the nutrients is believed to be through the remineralization of organic decay products rather than direct nutrient loading from the river sources. This therefore represents a mechanism for remineralized nitrogen to contribute to primary production further offshore. The organic material will ultimately flux to the ocean bottom and provide material for microbial decay processes and presumably, under the right conditions, hypoxia. This mechanism can be of considerable importance in driving the observed seasonal hypoxia, particularly in low river discharge years, as was the case in 2005.

OS53J-04

Modeling Phosphorus Dynamics in Lake Erie and Implications for Hypolimnetic Oxygen Depletion

Beletsky, D (dima.beletsky@noaa.gov) , CILER, SNRE, University of Michigan, Ann Arbor, MI 48109 United States
* Schwab, D J (david.schwab@noaa.gov) , NOAA Great Lakes Environmental Research Laboratory, 2205 Commonwealth Blvd., Ann Arbor, MI 48105 United States
DePinto, J V , Limno-Tech, Inc., 501 Avis Drive, Ann Arbor, MI 48108 United States
Dolan, D M , University of Wisconsin - Green Bay Natural & Applied Sciences, ES-317, 2420 Nicolet Dr., Green Bay, WI 54311 United States

In the late 60's and early 70's the central basin hypolimnion of Lake Erie frequently exhibited extensive hypoxia, and occasionally anoxia, at the end of the summer. Excess phosphorus loading was thought to be the principal cause of hypolimnetic oxygen reduction. Binational phosphorus reduction strategies in the 1970's were designed based on results from mixed-reactor type models which were state-of-the-art at the time, but are relatively simple by present standards. Even so, these simple models proved successful in forecasting lake response to phosphorus abatement into the early 1990s, although incidences of hypoxia and anoxia have reoccurred since then. Interestingly, some recent measurements show that phosphorus levels might be slowly rising again. In the present study, we use results from a high resolution 3-dimensional hydrodynamic model simulation to investigate phosphorus dynamics in Lake Erie. Using daily flows from the major tributaries coupled with tributary-specific phosphorus loading estimates, we carry out a mass balance calculation for phosphorus at hydrodynamic model resolution for several years with considerably different total annual loading. In contrast to the box-model approach, a physically realistic net settling rate is used in the mass balance model. Results for total phosphorus levels are compared to box-model estimates. In addition, the results from the high resolution simulation are analyzed for significant spatial and temporal variability which would impact hypolimnetic oxygen depletion and other components of the lower food web.

OS53J-05

Upwelling-Driven Hypoxia off the Central Oregon Coast

* Chan, F (chanft@science.oregonstate.edu) , Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331 United States
Kirincich, A (akirinci@coas.oregonstate.edu) , Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331 United States
Barth, J A (barth@coas.oregonstate.edu) , Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331 United States
Lubchenco, J (lubchenco@oregonstate.edu) , Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331 United States
Menge, B A (mengeb@science.oregonstate.edu) , Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331 United States

Coastal upwelling ecosystems are marked by wind-driven transport of nutrient-enriched, but oxygen-depleted water to shallow margins. Although the influx of oxygen-depleted waters and elevated rates of export production strongly predispose upwelling shelves for oxygen deficits, the expression of hypoxia remains highly variable in time and space. In summer 2002, an exceptionally severe episode of hypoxia developed along the central Oregon coast in response to the anomalous invasion of nutrient-rich sub-arctic water into the California Current. We undertook cross-shelf cruises and deployed moored dissolved oxygen sensors in the Oregon inner-shelf during the summers of 2003, 2004, and 2005 to examine the physical and biogeochemical pathways to oxygen deficiency, and assess the potential for hypoxia recurrence in response to inter-annual variations in ocean conditions. Summertime hypoxic conditions recurred (minimum dissolved oxygen concentrations of 0.20 to 0.29 ml l$^{-1}$) across all years of observations. Cross-shelf salinity and oxygen profiles suggest that in contrast to 2002, the onset of hypoxia on subsequent years did not reflect the presence of hypoxic water masses at the shelf break and their transport to the inner-shelf. Instead, local respiration over the mid and inner-shelves was instrumental in the formation of hypoxia. Cruises conducted over daily and weekly timescales, in conjunction with moored ADCP-based current measurements and continuous dissolved oxygen time-series, also highlight strong effects of wind-driven circulation in modulating intra-seasonal variations in the severity and frequency of hypoxia events on the inner-shelf. Differences in the severity of hypoxia in regions of contrasting shelf widths reflect the influence of flow-topography interaction in structuring the sensitivity of shelf waters to onset and persistence of hypoxia.

http://www.piscoweb.org

OS53J-06

In Situ and Remote Monitoring of Hypoxia in Hood Canal: the ORCA Time-Series in Lynch Cove

* Ruef, W (wruef@u.washington.edu) , University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351 United States
Devol, A (devol@u.washington.edu) , University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351 United States
Newton, J (newton@apl.washington.edu) , University of Washington, Applied Physics Laboratory 1013 NE 40th St, Seattle, WA 98105-6698 United States
Emerson, S (emerson@u.washington.edu) , University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351 United States

We obtained high frequency measurements of chemical, physical, and biological properties throughout the water column at a fixed station in Lynch Cove, South Hood Canal using an autonomous moored profiling system. Measurements have been taken every two to six hours since January 2005 using surface meteorological sensors and a profiling underwater instrument package consisting of a Seabird CTD, dissolved oxygen electrode, and chlorophyll fluorometer. Oxygen was undersaturated at the surface in the winter, with 2 brief periods of supersaturation, corresponding to large chlorophyll blooms, occurring in February and March. This was followed by a period of prolonged undersaturation at all depths until the start of the growing season in April. Between April and October, oxygen saturation at the surface fluctuated between supersaturation and undersaturation. This fluctuation was evenly split, with undersaturated oxygen concentrations observed at the surface half the days during the summer. In the lower 5 meters, oxygen was undersaturated and hypoxic, less than 100 umol/kg ($\sim$3 mg/L), throughout the year. Maximum deep oxygen concentrations of 90 umol/kg (2.81 mg/L) were observed at depth in late March and decreased $\sim$0.5 umol/kg/day through early August to a minimum value of 25 umol/kg (0.78 mg/L). From July 5 through October the entire lower water column (8-30 meters) was hypoxic. A box model was used to evaluate the roles of sedimentary oxygen consumption, water column respiration, and advective oxygen input in maintaining the persistent hypoxia in the bottom water of Lynch Cove through out the year.