In the Northern Gulf of Alaska (NGA), rising sea surface temperatures and enhanced freshwater inputs are causing surface waters to become increasingly stratified, which isolates phytoplankton from cooler, nutrient-rich waters directly below. Strong stratification has been shown to limit growth of phytoplankton in some coastal areas of the NGA, but we do not have a continuous record of in-situ phytoplankton productivity measurements that can support this idea of reduced growth due to increasing stratification, UNTIL NOW!
Oxygen (O2) is the currency of life. Marine algae produce it during photosynthesis, and the rest of the animal community consume it during respiration. Therefore, by measuring changes in O2 within the sunlit surface waters, we can estimate the net amount of carbon being produced or consumed by the marine community! There’s just one problem, it’s not just biological processes that alter O2 content. Oxygen from the atmosphere can ‘bubble’ into the surface waters, and waters below the surface that are generally lower in O2 can be mixed upward. Fortunately, these ‘mixing-related’ (or ‘physical’) O2 changes are accompanied by other gases, including N2, which has almost no biological interactions in the Gulf of Alaska. Therefore, by measuring O2 + N2 together, we can remove the ‘physical’ components and look specifically at biological O2 production. This type of approach, where we formulate an O2 ‘budget’, is typically called mass-balance.
In July 2020, we deployed two sensors just below the surface on the GEO, the Gulf of Alaska Ecosystem Observatory (hyperlink to: https://nga.lternet.edu/research/gulf-of-alaska-ecosystem-observatory-geo/), a remote monitoring station located on the edge of the continental shelf. The first measures O2 concentration, and the other measures total gas pressure from which we can derive N2 concentration. Both sensors are measuring every 15 minutes for the entire year, providing high-resolution measurements over time (i.e. a time-series)!
In July 2021, we will recover these sensors, download the data, and deploy a new set that will measure for another year. In year 1, we expect to see changes in biological production due to various events like the spring bloom, coastal eddies, and storms. We can also calculate the net annual carbon production, which provides valuable information regarding the carbon available for export into deep waters, a critical process that regulates the amount of carbon dioxide in our atmospheres. Over time, we hope to establish an understanding of long-term trends in biological productivity.
For now, we just hope our sensors will survive the long, cold, rough winter ahead!
Check back for the verdict in July 2021!