Diatom dynamics in a long-lived mesoscale eddy in the northeast subarctic Pacific Ocean

We characterized the diatom flora and determined biogenic silica concentrations within an anticyclonic Haida eddy four times as it drifted westward from the coast of the Queen Charlotte Islands (British Columbia, Canada) into the Alaska Gyre (February 2000–September 2001). For the whole data set (eddy and surrounding waters), diatoms accounted for 6 to 91% of phytoplankton carbon (6–54% total phytoplankton abundance). The proportional contribution of diatoms to phytoplankton carbon within the eddy was higher than in the surroundings inshore of the Transition Zone between coastal and High Nitrate, Low Chlorophyll waters. As the eddy drifted away from the coast and into the Alaska Gyre over the 20-month period, the average biovolume of diatoms decreased by 2–4 times, while in the surroundings a 2-fold increase in average biovolume was observed. The highest diatom abundances were observed in June 2001, when the assemblages were dominated by small colonies of Neodenticula seminae (=Nitzschia cylindroformis) both within the eddy (at the edge and center) and in the surrounding waters. N. seminae lacked the characteristic morphological features of the type species (deck and basal ridges, solid-walled costae) and instead more closely resembled specimens observed for the first time in ~0.8 Ma in the Gulf of St. Lawrence (North Atlantic) in the same year (2001). The corresponding biogenic silica inventories were 10-fold higher in June 2001 compared to the other cruises, yet particulate organic carbon and nitrogen did not increase substantially, potentially indicating a senescing population of diatoms with high Si:C and N. Diatom diversity and evenness indices were lower in June 2001 compared to the other cruises. A combination of high retention, low colonization rates, and losses due to sinking or grazing could result in lower diatom abundances but higher diversity observed within the Haida-2000a eddy compared to the surroundings. While silicic acid concentrations may have reached low enough levels to limit diatom growth after an initial spring bloom, levels of this nutrient were not limiting in later observations. Collectively, the data underscore the importance of diatoms in phytoplankton assemblages both close to shore and in High Nitrate, Low Chlorophyll waters.