Eelgrass and Macroalgae Loss in an Oregon Estuary: Consequences for Ocean Acidification and Hypoxia

Abstract

Estuarine macrophytes are proposed to influence ocean acidification and hypoxia (OAH) via the uptake (release) of inorganic carbon (oxygen) during photosynthesis. The extent to which macrophytes mitigate OAH in estuaries depends on the interaction between variable environmental conditions and macrophyte production over space and time. To explore these complexities in detail, we considered the potential causes and consequences of intertidal eelgrass and macroalgae declines in a U.S. Pacific Northwest estuary. We compiled and analyzed a record of eelgrass (Zostera marina) and ulvoid macroalgae along with a broad suite of environmental conditions over 15 years (2004 to 2019) at 3 sites along an estuarine gradient in South Slough, Oregon. The analysis showed that declining macrophyte biomass coincided with increasing temperature (water and air), watershed disturbance, and possibly turbidity. Coincident with macrophyte loss, diel dissolved oxygen (DO) and pH variability were reduced, indicating an influence of macrophytes on water quality at an ecosystem scale. Eelgrass loss was correlated with declining gross production and respiration, which altered the diel dynamics of pH, DO, and partial pressure of carbon dioxide at some sites. Under certain conditions, there was an association between eelgrass biomass and changes in DO and pH of more than 2 mg/l and 0.3 units, respectively. We found that daytime amelioration of low DO and pH was possible at certain locations when macrophyte biomass (especially eelgrass) was high. However, our analyses suggested that the efficacy of macrophyte mitigation of OAH depends on macrophyte abundance and the volume and residence time of overlying water.