Along-estuary dissolved oxygen variability in the Coos Bay estuary, Oregon, USA

Hypoxia and anoxia are direct threats to the ecosystem and economic health of coastal communities. In estuaries, oceanic and terrestrial systems merge, making their relative influences on water properties such as dissolved oxygen (DO) difficult to tease apart. Recent studies show evidence for low DO conditions occurring on the Oregon shelf during the dry season on temporal and spatial scales unprecedented in the historic record. There is evidence now, too, showing that estuarine hypoxia, historically observed as a product mainly of terrestrial influences, is occurring due to the shifting properties of oceanic inputs. We report here on data collected in the Coos Bay estuary located on the southern Oregon coast, to quantify past and present DO levels to determine if hypoxia is occurring. We use monthly along-estuary sections of temperature, salinity, dissolved oxygen, turbidity, and fluorescence to examine the seasonal cycle of water properties. This data will be put into temporal context through analysis of historic data collected by state, federal, and private organizations. The goal is to link patterns in DO variation to the temporal and spatial circulation of water within the estuary. We show a significant along-estuary gradient in DO and discuss the variability of this gradient in time, over both seasonal and interannual scales.