Short-term variability of planktonic composition in subtropical waters off the Canary Islands.

Abstract

The planktonic community composition in oligotrophic waters off the Canary Islands was studied from November 2010 to June 2011 carrying out a weekly sampling and covering the productive period in these waters. The characteristic late winter bloom took place from February to April 2011, when the cooling of surface waters promoted the deepening of the mixed layer, and chlorophyll a concentration and primary production increased at the surface. The planktonic community was dominated by picoplankton, especially Prochlorococcus and heterotrophic prokaryotes, except during the productive period, when Synechococcus and picoeukaryotes dominated the picophytoplankton. During the bloom, diatoms were the major contributors to total autotrophic biomass and they were probably responsible for the highest rates of primary production. The planktonic variability was driven by bottom-up forces at a seasonal scale through the nutrient enrichment during winter. Additionally, a short-scale variability of phytoplankton biomass and productivity was observed even during stratified conditions. Short-term variability would also result from top-down processes such as feeding and grazing by nano- and microheterotrophs. In this sense, a significant correlation was found between the latter and their potential preys: bacteria and autotrophic picoeukaryotes and nanoflagellates. These results showed the need to sample at short-time scales to account for the total variability of the planktonic communities in subtropical waters. As an example, the striking and ephemeral diatom peak observed here, which would be probably not detected in a monthly sampling.