Biogeochemical drivers of changing hypoxia in the California Current Ecosystem

Abstract

Recent observations have revealed significant fluctuations in near-shore hypoxia in the California Current Ecosystem (CCE). These fluctuations have been linked to changes in the biogeochemical properties (e.g. oxygen and nutrient contents) of the oceanic source waters of the California Current upwelling, and projections suggest the potential for decreased oxygen and increased nutrients in the source water under climate change. We examine both the separate and combined influences of these projected changes through a sequence of perturbation experiments using a regional coupled ocean dynamics/biogeochemistry (BGC) model of the CCE. The direct effect of a projected 5% decline in source water oxygen is to expand the hypoxic area by 12.5% in winter to 22.5% in summer. This exceeds the impact of a +0.5% nitrate enrichment of source waters, which expands the hypoxic area by 6.5% to 12% via stimulation of nearshore Net Primary Productivity (NPP), increased organic matter export, and subsequent enhanced remineralization and dissolved oxygen (DO) consumption at depth. The combined effect of these perturbations consistently surpasses the sum of the individual impacts, leading to 20% to 32% more hypoxic area. The combined biogeochemical impact greatly exceeds the response resulting from a 10% strengthening in upwelling-favorable winds (+1% in hypoxic area) or the decreased oxygen solubility associated with a 2∘C ocean warming (+3%). These results emphasize the importance of improved constraints on dynamic biogeochemical changes projected along the boundaries of shelf ecosystems. While such changes are often viewed as secondary impacts of climate change relative to local warming or stratification changes, they may prove dominant drivers of coastal ecosystem change.