Autotrophic picoplankton dynamics in a Chesapeake Bay sub-estuary

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Abstract
The contribution of picoplankton (0.2 to 3.0 .mu.m) to phytoplankton biomass and production was examined in a tributary of the Chesapeake Bay, USA, during summer 1985. The water column in this tributary oscillates with the spring-neap tidal cycle (homogeneously mixed and stratified, respectively) which permits observation of the effects of water column stability on phytoplankton dynamics. Picoplankton made up 7% of the seasonal autotrophic biomass, with a peak contribution of 10 to 14% in the first half of July. Phycocyanin-rich chroococcoid cyanobacteria, visible only with fluorescence microscopy using green light excitation (510 to 560 nm), numerically dominated the picophytoplankton, being 8 .times. more abundant than the phycoerythrin-containing cyanobacteria. Together these 2 cyanobacteria types represented 51% of the picophytoplankton biomass, and exhibited a pronounced fortnightly cycle in abundance which coincided with the spring-neap tidal cycle. Picoplankton were responsible for 9% of the primary production at 293 .mu.E m-2 s-1 and 13% at 28 to 43 .mu.E m-2 s-1, averaged over the study period. Chlorophyll-specific uptake rates by the larger autotrophs (seasonal mean, 5.2 .mu.g C .mu.g chl a-1 h-1) were significantly higher than the picoplankton (seasonal mean, 2.5 .mu.g C .mu.g chl a-1 h-1) at 293 .mu.E m-2 s-1, but not at 28 to 43 .mu.E m-2 s-1. A simple model based on spring-neap, tidally-induced oscillations in mixed layer depth, and its regulation of light availability, produced cycles in cyanobacterial growth rates which could partially explain the observed cycles in abundance. These results emphasize the importance of physical processes occurring on time scales of days or weeks in regulating plankton biomass and primary production in estuarine environments.