FY 2019

Spatial and temporal variability of coccolithophore blooms in the eastern Bering Sea

Ladd, C., L.B. Eisner, S.A. Salo, C.W. Mordy, and M.D. Iglesias-Rodriguez

J. Geophys. Res., 123, 9119–9136, doi: 10.1029/2018JC014302, View online (2018)

Coccolithophores are a widespread group of marine phytoplankton that produce plates of calcium carbonate that cover their cells. Large blooms of coccolithophores may significantly influence the biogeochemical properties of the ocean and atmosphere and trophic dynamics of the marine ecosystem. Because of the important implications of coccolithophore blooms, their timely monitoring and reporting is necessary for ecosystem management. To communicate with ecosystem management stakeholders, we developed an annual Coccolithophore Bloom Index (CBI) for the eastern Bering Sea shelf using satellite ocean color data. Comparisons between in situ and satellite data and the CBI (years 1997–2017) were used to examine the hypotheses regarding environmental influences on interannual bloom variability. A significant nonlinear relationship with summer stratification was found: the CBI was higher during years with either very low or very high stratification. In addition, while the blooms usually occurred over the middle shelf (50‐ to 100‐m depth), more of the bloom was located over the shallow (30–50 m) inner shelf when stratification was low. Spatial correspondence between nutrient concentrations (nitrate and ammonium) and the areal extent of the coccolithophore bloom provides tantalizing but nonconclusive evidence that nutrient availability plays a role in bloom formation and location.

Plain language summary: Intense blooms of coccolithophores, phytoplankton that occur in all of the world's oceans, have been implicated in seabird die‐offs and other effects on the food web. These blooms turn the ocean milky white and can be seen from space. Because of important implications of coccolithophore blooms, monitoring and reporting is necessary for ecosystem management. We developed a measure of spatial extent of coccolithophore blooms in the Bering Sea each September using satellite ocean color data and examined what conditions are favorable to bloom formation. The water column in this region is typically two layered during summer with warmer surface layer separated from colder deep layer. We found that the difference between the two layers (strength of stratification) was important. Coccolithophore blooms were larger during years with either very weak or very strong stratification. In addition, while blooms usually occurred over the middle shelf (50‐ to 100‐m depth), more of the bloom was located over the shallow (30–50 m) inner shelf when stratification was weak. Nutrient availability likely plays a role in bloom formation and location. Understanding the factors leading to coccolithophore blooms may allow us to forecast blooms in the future, providing important advance information for resource managers.