Experimental dynamics of nitrate uptake by winter phytoplankton assemblages

Abstract

The scope for the development of winter blooms depends on phytoplankton response time to transient calm periods and on grazing pressure. We examined the dynamics of nitrate uptake by winter phytoplankton in experimental microcosms, as a proxy for growth dynamics. Natural phytoplankton assemblages from one coastal and one shelf station off the Cantabrian Coast were exposed to "natural" (simulated in situ conditions) and "high" (resembling spring conditions) light regimes, and the time required for uptake of 10%, 50% and 90% of the available nitrate was determined. All three values decreased significantly (up to 5-fold) from January to March at the coastal station, but remained nearly constant at the offshore station. The length of calm weather periods, estimated from wind speed data, does not change significantly during winter. In early winter, these periods only suffice for activation of nitrate uptake. However, windows of opportunity during late winter would be long enough for uptake of 50% or even 90% of the available nitrate, in particular for coastal assemblages. Observed nitrate uptake dynamics displayed timescales similar to those of microzooplankton or appendicularian grazers, but shorter than those of copepods. Therefore, microzooplankton may exert a tight control on phytoplankton growth dynamics during winter. © The Author 2010. Published by Oxford University Press. All rights reserved.