College - Author 1
College of Science and Mathematics
Department - Author 1
Biological Sciences Department
Degree Name - Author 1
BS in Marine Sciences
Ryan Walter, College of Science and Mathematics, Physics Department
Dissolved oxygen (DO) is an important biogeochemical factor that strongly influences nearshore coastal ecosystems. Low DO (hypoxic) events can cause physiological stressful environments for ecological and economically important species, potentially leading to mass mortalities. In order to better assess drivers of coastal hypoxia, we collected data from monthly cruises on the inner shelf and nearshore moorings inside and outside a small coastal embayment (San Luis Obispo Bay on the Central California Coast) across the full upwelling season (March to August). During the late spring and early summer, we found that the nearshore near-bottom temperature-DO (T-DO) relationship aligned with the shelf data during periods of strong upwelling winds, suggesting that the nearshore low DO waters, many of which fell below the hypoxic threshold, were driven by the direct advection of low DO subthermocline waters. This period also coincided with minimal water-column stratification and vertical DO differences across all sites in the nearshore, with phytoplankton counts dominated by diatoms. In contrast, during the late summer and early fall, the near-bottom nearshore T-DO relationship deviated significantly from the offshore T-DO relationship, with nearshore DO values much lower than those offshore for a given temperature. During these deviations, where DO also dropped below the hypoxic threshold, upwelling winds were weak to moderate, there was significant vertical temperature stratification and vertical DO differences, and phytoplankton counts were dominated by dinoflagellates. These near-bottom hypoxic events were likely driven by the decay of organic matter in the bottom layer and the strong stratification that prevented vertical mixing. These respiration-driven events were observed inside the bay, but not outside the bay, highlighting the role of the coastal embayment and this upwelling shadow system on local hypoxia development. The seasonal transition between advection-driven and respiration-driven hypoxia in the CCS is a novel finding, with important implications for assessing nearshore hypoxia risk in a changing climate. While tuned specifically for SLO Bay, these findings can be used as a baseline for similar upwelling embayments.