Type of Presentation

Oral

Session Title

General oceanography

Description

Physical and Microbial Drivers of Dissolved Oxygen Dynamics in the San Juan Islands

Comments

Dissolved oxygen (DO) and carbon dioxide (CO2) dynamics in marine ecosystems are a product of both biological and physical forces. Through photosynthesis and respiration, microbial communities are the primary biological influence on DO and CO2 variability, while processes such as mixing and air-sea interaction may dominate as physical factors. This study investigates the relative contribution of microbial-mediated biogeochemical processes and their relationship to the physical influences of oxygen and CO2 in waters exposed to both incoming Pacific Ocean intrusions and Fraser River outflow: the San Juan Channel of the San Juan Archipelago. In a year-long oceanographic monitoring study, in situ respiration rates reveal that respiration activity in incoming low dissolved oxygen and high CO2 Pacific oceanic intrusions through the Strait is relatively low (~80μg DO/L/d) and much of the oxygen consumption occurs in the estuarine outflow overlying this bottom water. Additionally, manipulative experiments indicate that this oceanic input may be more carbon-limited and respond more rapidly to additions of organic carbon than in estuarine-sourced water. Consequently, this bottom water may be more susceptible to anthropogenic nutrient loading and may enhance localized respiration, furthering influencing dissolved oxygen and CO2 concentrations in the Salish Sea. In an intensive seven-week study of the channel in fall 2015, biological oxygen consumption in excess of water column mixing was found during periods of high primary production. However, following the fall transition from coastal upwelling to downwelling, oxygen at all stations in the San Juan Channel increased, indicating the physical shift of source waters. An abrupt shift of phytoplankton genera also co-occurred with the fall transition; the chain-forming diatom Chaetoceros spp. was dominant early in autumn, but then smaller diatoms became more abundant. These observations of dissolved oxygen dynamics in the San Juan Islands may help us understand the relative impacts of biological and physical forces in other areas of the Salish Sea and the contribution of human influence on these marine ecosystems.

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Physical and microbial drivers of oxygen dynamics in the San Juan Islands

2016SSEC

Physical and Microbial Drivers of Dissolved Oxygen Dynamics in the San Juan Islands