labs_title

Retrospective analysis of midsummer hypoxic area and volume in the northern Gulf of Mexico, 1985-2011

D.R. Obenour, D. Scavia, N.N. Rabalais, R.E. Turner and A.M. Michalak

Many coastal ecosystems are subject to an environmental condition known as hypoxia, where oxygen levels are too low to sustain desirable aquatic life. Robust estimates of hypoxic extent are important for assessing the overall severity of hypoxia, and for determining its causes and consequences. Here, we develop a probabilistic model to quantify both the area and volume of hypoxia along the Louisiana-Texas coast. Our results suggest that the largest midsummer hypoxic volume occurred in 2008 (140 km3) when the hypoxic area covered approximately 22,000 km2 (roughly the size of New Jersey) and averaged 6.2 m in thickness.


Figure: Probability of midsummer hypoxia, as derived from geostatistical model.

Abstract

Robust estimates of hypoxic extent (both area and volume) are important for assessing the impacts of low dissolved oxygen on aquatic ecosystems at large spatial scales. Such estimates are also important for calibrating models linking hypoxia to causal factors, such as nutrient loading and stratification, and for informing management decisions. In this study, we develop a rigorous geostatistical modeling framework to estimate the hypoxic extent in the northern Gulf of Mexico from data collected during midsummer, quasi-synoptic monitoring cruises (1985–2011). Instead of a traditional interpolation-based approach, we use a simulation-based approach that yields more robust extent estimates and quantified uncertainty. The modeling framework also makes use of covariate information (i.e., trend variables such as depth and spatial position), to reduce estimation uncertainty. Furthermore, adjustments are made to account for observational bias resulting from the use of different sampling instruments in different years. Our results suggest an increasing trend in hypoxic layer thickness (p = 0.05) from 1985 to 2011, but less than significant increases in volume (p = 0.12) and area (p = 0.42). The uncertainties in the extent estimates vary with sampling network coverage and instrument type, and generally decrease over the study period.

Obenour, D.R., D. Scavia, N.N. Rabalais, R.E. Turner, A.M. Michalak (2013) "Retrospective analysis of midsummer hypoxic area and volume in the northern Gulf of Mexico, 1985-2011", Environmental Science & Technology, 47 (17), 9808–9815,doi:10.1021/es400983g.