Growth rates of coastal phytoplankton from time-series measurements with a submersible flow cytometer

Abstract

Our understanding of the dynamics of phytoplankton communities has been limited by the space and timescales associated with traditional monitoring approaches. To overcome some of these limitations, we have developed a submersible flow cytometer (FlowCytobot) that is designed for extended autonomous monitoring of phytoplankton abundance, cell size, and pigmentation. FlowCytobot was moored on the seafloor from late July to October 2001 at the Long-term Environmental Observatory study site off the coast of New Jersey, and water samples from 5 m depth were pumped continuously through the instrument. Analysis of cells' optical properties revealed distinct populations of Synechococcus and cryptophytes, as well as an assemblage of other pico- and nanophytoplankton of mixed taxonomy. For each of these groups, dramatic variations in cell concentration were observed within the sampling period. Diel variations in cell scattering, which are indicative of changes in cell size, were consistent with patterns of cell growth during the light period and cell division late in the day. We developed a size-structured matrix population model that accommodates simultaneous growth and division and then used the model and size distribution data from FlowCytobot to estimate daily specific growth rates for Synechococcus; these estimates are independent of cell concentration and do not include mortality. The results show that a dramatic autumn decline in the concentration of Synechococcus can be attributed to a decrease in the specific growth rate rather than to effects of physical transport processes or trophic interactions.