Reconciling Phytoplankton Growth Rates and Standing Stocks in the Northern Gulf of Alaska: Evidence for a Physical-Biological Mosaic (Invited)

We investigated the relationships between iron, light, and nitrogen availability in the highly productive waters of the northern Gulf of Alaska (nGOA). Even in coastal environments, low iron levels can limit phytoplankton growth. Since low nitrate concentrations tend to limit phytoplankton growth on the inner shelf of the nGOA and offshore waters are High Nitrate, Low Chlorophyll (HNLC) and thus possess low trace metal concentrations, it was hypothesized that waters of the mid-shelf would be stimulated when the near and offshore waters mixed, similar to the ‘green belt’ of the Bering Sea. Indeed, a mid-shelf chlorophyll maximum was observed; however, this mid-shelf maximum in chl was not associated with a peak in primary productivity, resulting in lower rates of chlorophyll-specific primary production compared to the inner or outer shelf waters. Data from grow out experiments suggest that low iron indeed limits primary production on the inner shelf: under both full light and darkened conditions, the addition of 4 nM iron stimulated primary productivity and resulted in higher chlorophyll (chl) standing stocks relative to controls, even without the addition of nitrogen. Iron, both in total and bioavailable forms, was highest in the inner shelf, and declined along the cross-shelf gradient (Lippiatt et al., 2010) as did the efficiency of photosynthesis as indicated by the initial slope of photosynthesis-irradiance curves. We reconcile the low chl associated with the inner shelf by invoking the importance of retention and its variability along the cross-shelf gradient. Estimate of phosphorus stress (using alkaline phosphatase activity) similarly identify differential responses on the shelf, in the cross-shelf gradient, and in the open (HNLC) waters, with more apparent stress in the gradient regions. The inner shelf is associated with the swift Alaska Coastal Current, which may dilute standing stocks, masking high primary productivity. We conclude that primary production in the nGOA is indeed influenced by iron availability (and secondarily by other factors such as light and macronutrients) and that phytoplankton standing stocks may often be decoupled from growth rates. We suggest that the phytoplankton productivity of the nGOA is best described as a mosaic controlled by a combination of physical effects (retention) and biochemical effects (light, iron, macronutrients).