Winter-to-summer phytoplankton and nutrients on Georges Bank: 1999 results

D. W. Townsend

Measurements of phytoplankton chlorophyll a, dissolved inorganic nutrients (NO3 + NO2, NH4, Si(OH)4 and PO4), dissolved organic nitrogen, and phytoplankton cell counts (only a handful to date) were made from January to June in 1999 on Georges Bank as part of the U.S. Globec broadscale surveys. Summary results follow: (1) During the period between the January and February cruises, we could see that the winter-spring phytoplankton Bloom was underway; that is: chlorophyll increased; diatom cell numbers increased; silicate concentrations were drawn down; nitrate remained relatively unchanged; the nitrogen source may have been Dissolved Organic Nitrogen, which decreased during that period. (2) February to March: the phytoplankton bloom reached its peak; we saw the high chlorophyll levels and highest diatom cell numbers; silicate was drawn down; nitrate increased slightly, perhaps as a result of new N fluxes onto the Bank (e.g. salinity increased; Dissolved Organic Nitrogen began to increase. (3) March to April: the phytoplankton bloom was declining; we saw the highest chlorophyll, but fewer diatoms; silicate was all but gone; nitrate was also depleted on the top of Bank; the DON continued to rise. (4) April to May: phytoplankton chlorophyll was low; silicate was gone; diatoms were gone; dinoflagellates were increasing; nitrate was present in surface waters only on the NE Peak; DON continued to rise. (5) May to June: phytoplankton chlorophyll was increasing; silicate was increasing in spots, suggesting regeneration, the likely reason was the warming waters; diatoms were back; DON reached highest levels (>20 M).