You are here:
Contiguous Low Oxygen Waters between the Continental Shelf Hypoxia Zone and Nearshore Coastal Waters of Louisiana, USA: Interpreting 30 Years of Profiling Data and Three-Dimensional Ecosystem Modeling
Citation:
Jarvis, B., Rick Greene, Y. Wan, J. Lehrter, L. Lowe, AND D. Ko. Contiguous Low Oxygen Waters between the Continental Shelf Hypoxia Zone and Nearshore Coastal Waters of Louisiana, USA: Interpreting 30 Years of Profiling Data and Three-Dimensional Ecosystem Modeling. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, , 11, (2021). https://doi.org/10.1021/acs.est.0c05973
Impact/Purpose:
Bottom water hypoxia on the Louisiana Continental Shelf has been a long term recurring seasonal event. Despite significant efforts to resolve the causes and extent of hypoxia since measurement began in 1985, the relationship between localized nearshore water quality and widespread shelf hypoxia remains difficult to assess over the timeframe during which hypoxia occurs. In this study we combined an analysis of 30 years of hydrographic data from multiple research programs with a three-dimensionsional complex ecosystem model to quantify the frequency, temporal and spatial extent of hypoxia within Louisiana nearshore coastal waters. We further assessed whether hypoxia in nearshore waters was contiguous with the LCS hypoxic water mass and the factors controlling it.
Description:
The multidecadal expansion of northern Gulf of Mexico continental shelf hypoxia is a striking example of the adverse effects of anthropogenic nutrient enrichment on coastal oceans. Increased nutrient inputs and widespread shelf hypoxia have resulted in numerous dissolved oxygen (DO) water quality problems in nearshore coastal waters of Louisiana. A large hydrographic dataset compiled from research programs spanning 30 years and the three-dimensional hydrodynamic-biogeochemical model CGEM (Coastal Generalized Ecosystem Model) were integrated to explore the interconnections of low DO waters across the continental shelf to nearshore coastal waters of Louisiana. Cross-shelf vertical profiles showed contiguous low DO bottom waters extending from the shelf to coastal waters nearly every year in the 30+ year time series, which were concurrent with strong cross-shelf pycnoclines. A threshold Brunt−Vaisa ̈ lä ̈frequency of 40 cycles h−1 was critical to maintaining the cross-shelf subpycnocline layers and facilitating the formation of a contiguous low DO water mass. Field observations and model simulations identified periods of wind-driven bottom water upwelling lasting between several days to several weeks, resulting in both physical advection of oxygen-depleted offshore waters to the nearshore and enhanced nearshore stratification. Both the upwelling of low DO bottom waters and in situ respiration were of sufficient temporal and spatial extent to drive DO below Louisiana’s DO water quality criteria. Basin-wide nutrient management strategies aimed at reducing nutrient inputs and shelf hypoxia remain essential to improving the nearshore coastal water quality across the northern Gulf of Mexico.
