Modeling Hypoxia in Relation to Nutrients, Climate and Ecological Controls
Session Date: June 1st 2010
Session Time: 12:51
Session Lead: Michael Kemp
Session Co-Lead(s): Ming Li, Walt Boynton and Dom DiToro
Session Abstract: The goals of this proposed session are to explore how diverse modeling approaches can help to improve understanding of how hypoxia and related dissolved oxygen distributions are controlled by variation in external nutrient loading and climatic conditions, as well as internal ecological processes. Presentations will include examples of numerical and statistical modeling studies in Chesapeake Bay and other temperate coastal ecosystems (e.g., Delaware Inland Bays, Neuse River estuary), examining system responses to key factors including nutrient management, shifts in prevailing winds and changes in stream-flow and temperature. We will also solicit presentations that illustrate interactions between bottom water oxygen, benthic animal abundance and biogeochemical processes.
Presentations:
Post-Session Review:
This session included a sequence of eight presentations dealing with diverse aspects of modeling hypoxia, eutrophication and physical circulation of estuaries. The first three talks dealt with a range of approaches for modeling eutrophication and hypoxia.
- Elizabeth North described an application of a larval transport model (LTRANS) used to examine how hypoxic bottom waters might affect recruitment success for larval oysters under varying environmental conditions and assumed larval behaviors.
- Vic Bierman illustrated his success in simulating interactions between physics and biogeochemistry in regulating relative abundance of four phytoplankton groups (diatoms, dinoflagellates, small green flagellates, cyanophytes) in the Potomac River estuary.
- Jeremy Testa and Damian Brady described recent developments in applying Dom DiToro’s sediment diagenesis model to reproduce seasonal and regional differences in sediment-water fluxes of ammonium, nitrate, silicate, phosphate and oxygen. This presentation also illustrated the power of applying a stand-alone version of this model for analyzing data on diagenetic processes and for scenario experiments.
The final five presentations described and discussed dynamics of Chesapeake Bay bottom water hypoxia, focusing on interannual variations in hypoxia (intensity and extent) over the past five decades, with particular focus on the apparent shift in hypoxia per unit nutrient loading since the early 1980s. This shift implies that, compared to decades prior to 1980, the Bay now generates more hypoxia for the same nutrient loading (Hagy et al. 2004).
- Rebecca Murphy and Bill Ball applied extensive empirical analysis and statistical modeling of Bay data to reveal distinctly different interannual trends and patterns in hypoxia between early and late summer. Early summer hypoxia has exhibited a strong increase in extent since 1984 that follows an increase in stratification strength, while late summer hypoxia extent has exhibited a leveling and slight decline in the last 15 years that is significantly correlated with changes in nitrogen loading from the watershed.
- Yun Li and Ming Li presented results from a range of ROMS simulations to explore effects of river-flow and wind on vertical mixing and longitudinal advection. Initial model experiments also reveal a tradeoff between wind and river-flow effects on hypoxia that suggest importance of factors other than physical circulation controlling inter-annual variations.
- Younjoo Lee and Walt Boynton presented their recent statistical analysis of water quality data. Their results emphasize the importance of wind direction and timing in controlling interannual variations in hypoxia, and suggest possible biological processes controlling these trends.
- Carl Cerco reviewed evolution of the Chesapeake Bay Program water quality model in relation to its ability to reproduce seasonal and interannual patterns in hypoxia. His results emphasize how model simulations have improved with development of new knowledge about processes controlling hypoxia.
- Finally, Malcolm Scully presented results of his data analysis and ROMS modelling studies suggesting that the observed shift in hypoxia per N loading is closely related to a shift in the North Atlantic Oscillation (NAO) index and associated changes in direction of prevailing summer winds in the Bay. Previously, strong southerly winds causes more efficient ventilation of bottom waters compared to more westerly winds that have dominated the summer climate since 1980. He also reported that the current NAO index from winter 2009-2010 suggests a return to pre-1980 conditions that may lead to reduced hypoxia this summer.
