NUTRIENT-PHYTOPLANKTON RELATIONSHIPS IN NARRAGANSETT BAY DURING THE 1974 SUMMER BLOOM

ABSTRACT Nutrient and 14C uptake by natural phytoplankton populations was measured. Concentrated populations were incubated with uptake saturating concentrations of nitrate, silicate and phosphate, and nutrient uptake was then monitored at 30 min intervals over an eight hour period to yield Vmax, V, ρN and ρsi. Changes in the amount of particulate matter and absolute rates of uptake were related to in situ levels of phytoplankton biomass and dissolved nutrients, and replenishment (generation) times (R) estimated. Independent estimates of R were derived from calculations of particulate nitrogen (Np) and silica production (Sip) based on 14C uptake and appropriate elemental ratios. Forty-two percent of the annual carbon production of 308 g m−2 occurred during July and August when in situ nutrient levels were very low. The hourly uptake rates for nitrate (ρN) ranged from 0.118 to 0.136; ρsi was 0.007 to 0.150 μM. The daily N supply for the water column would have to be replenished one to 12 times daily to support Np and 2.5 to 17 days for Sip. It would take from 2 to 5 days for the nitrogen excretion rates of the zooplankton (> 153 μM.) and benthos to supply the daily phytoplankton nitrogen needs.

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