Winter–spring transition of phytoplankton chlorophyll and inorganic nutrients on Georges Bank

Abstract

Phytoplankton chlorophyll and inorganic nutrients were measured monthly during the winter–spring transition in 1997 on Georges Bank as part of the US Globec Program. We measured only chlorophyll and hydrography in January and June; nutrients were included in the February, March, April and May cruises. The winter–spring phytoplankton bloom on the Bank began early in 1997. Phytoplankton chlorophyll a concentrations were lowest in January (<1.0 μg l −1), but by February concentrations ranged between 2 and 3 μg l −1 on the western half of the top of the Bank (e.g., inside the 60 m isobath). Near-surface nitrate+nitrite concentrations in February were less than 6 μM over most of the Bank and below 4 μM on the western half. Near-surface silicate concentrations were generally less than 6 μM over most of the Bank in February, and were reduced to 2–3 μM inside the 60 m isobath; concentrations were depleted most on the western half of the top of the Bank, where they were less than 1 μM. Scotian Shelf Water intruded onto the Bank in February and remained distinct through April; near-surface nutrient concentrations in this Scotian Shelf Water in March were also low, and chlorophyll concentrations were high (4–5 μg l −1). The spring bloom was still underway in March, with increased chlorophyll concentrations spreading over most of the Bank. Silicate appeared to be the limiting nutrient beginning as early as February, with concentrations becoming depleted in March before nitrate+nitrite did. During April and May both silicate and nitrate concentrations were reduced to less than 1 μM in the near-surface waters over much of the top of the Bank; however, there were indications that silicate was being recycled on some parts of the Bank in May, which, in addition to recycled nitrogen, may have contributed to locally high chlorophyll concentrations observed in May and June. Higher near-surface nutrient concentrations also ringed the deeper edges of the Bank in late spring, where chlorophyll concentrations were generally high (greater than 5 μg l −1) but also very patchily distributed.