Phytoplankton growth and herbivory in the subarctic Pacific: A chemotaxonomic analysis

Abstract

Chlorophyll crops in the subarctic Pacific are low and relatively constant throughout the year; however, net growth of phytoplankton occurs when natural water is enclosed in incubation containers and exposed to adequate irradiance. Pigment-based measurements of taxon-specific growth rates and taxon-specific grazing pressure were made in an attempt to understand the dynamic processes leading to the net growth of phytoplankton in bottles. Specific growth rates, determined from 14C labeling of chromatographically separated pigments showed that fucoxanthin-containing cells (diatoms) were the fastest growing microalgae; a rapid net accumulation of fucoxanthin also occurred. Independent measurements of specific growth rates, determined from Chromatographic analysis of microzooplankton dilution experiments, also showed that fucoxanthin-containing cells had the highest specific growth rates. Importantly, microzooplankton grazing rates on fucoxanthin-containing cells were only about half the specific growth rate; hence, the resultant bloom of fucoxanthin. We speculate that the lag in chlorophyll growth that has been reported previously is an artifact of subculturing; fast-growing, but dilute, fucoxanthincontaining cells do not become a significant portion of the total chlorophyll signal until late in the experiment. Other diagnostic carotenoids, such as 19’-hexanoyloxyfucoxanthin and 19’-butanoyloxyfucoxanthin, remained nearly constant through long-term incubations. However, they became actively 14C labeled and showed positive specific rates of growth. Dilution experiments showed that their specific growth rate was compensated by microzooplankton grazing. Our tentative conclusion is that in situ sinking losses and grazing activity of larger herbivores (copepods, salps) keeps large, fucoxanthin-containing diatoms in check at low resultant concentrations. When natural phytoplankton are isolated in bottles where copepods are excluded and sinking is negated, the refuge from loss processes promotes a bloom.