Response of Sargasso Sea phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing

The response of phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing was studied at a station southeast of Bermuda over an 18 month period. Phytoplankton growth rates and primary production were measured using the pigment-labeling method, and phytoplankton biomass was calculated from these measurements. Phytoplankton carbon biomass varied systematically over the year. Highest values were observed during the winter and spring. Seasonal variations of chlorophyll (Chl) a in the surface layer could primarily be attributed to variations in phytoplankton biomass and secondarily to photoacclimation. During the summer period, average values of carbon (C)/Chl ratios (g C g(-1) Chl) ranged from 160 at the surface to 33 at the 1.6% light level, changes attributed to photoacclimation of the phytoplankton, consistent with the observation that phytoplankton biomass did not vary as a function of depth. Phytoplankton growth rates in the surface layer did not vary systematically over the year, ranging from 0.15 to 0.45 day(-1), in spite of seasonally varying concentrations of nitrate. Growth rates varied as a function of depth from average values of 0.3 day(-1) in the surface layer to <0.1 day(-1) at the 1.6% light level. Thus, the primary response of the phytoplankton community to nutrient enrichment during the winter period was an increase in phytoplankton biomass rather than an increase in growth rates. A simple nutrient-phytoplankton-zooplankton model was used to explore this phenomenon. The model demonstrated that the observed response of the phytoplankton to nutrient enrichment is only possible when phytoplankton growth is not severely limited by nutrients.