Partitioning oxygen sources and sinks in a stratified, eutrophic coastal ecosystem using stable oxygen isotopes

Abstract

Coastal hypoxia develops as a synergistic product of biological and physical factors. Based on oxygen concentration measurements alone, it is difficult to separate the effects of biological factors from physical factors, which complicates the analysis of oxygen dynamics. To address this problem we used a dual budget approach to assess the importance of oxygen sources and sinks for the Louisiana continental shelf in the northern Gulf of Mexico, which is strongly influenced by the Mississippi River and develops a large summertime zone of hypoxic (<2 mg O2 l-1) bottom waters. The dual budget approach was based on using stable oxygen isotopes (δ 18O) in combination with conventional oxygen concentration measurements. The shelf ecosystem showed strong oxygen dynamics with a wide range of oxygen saturations between 180% and almost 0% and a corresponding wide range of δ18O values from 15‰ in surface waters to 50 ‰ in bottom waters. Physical mixing primarily controlled oxygen dynamics in fall and winter, but during summer stratification, oxygen dynamics were controlled predominantly by biological processes. Model estimates indicated that during a summer 2001 shelf-wide cruise, stratified surface waters were very productive with an average calculated production/respiration (P/R) ratio of 1.12 and average gross and net primary productivities of 0.54 and 0.06 g C m -3 d-1 respectively. In bottom waters summer oxygen depletion was predominantly due to benthic respiration, accounting for about 73% of the total oxygen loss. In the most hypoxic summer waters the importance of benthic respiration declined, consistent with low oxygen conditions slowing the rate of benthic oxygen consumption. Overall, the dual budget approach yielded new estimates of productivity dynamics in surface waters and of sediment oxygen demand in bottom waters. © Inter-Research 2007.