Spatial and temporal characteristics of plankton-rich layers in a shallow, temperate fjord

Abstract

Between June and October 2005, a CTD profiler with mounted fluorometer identified the presence and extent of plankton-rich layers (PRLs), i.e. horizontal patches of high plankton con- centrations bordered by steep gradients, in East Sound, a shallow fjord in Washington, USA. The suit- ability of this profiling approach for identifying the meter-scale plankton layers was verified through correlation analysis, which showed that in situ fluorescence was significantly correlated with all sub- sequent proxy measurements of phytoplankton abundance, including extracted chlorophyll a con- centration and plankton biomass. Species abundance and community composition within and outside the layers were analyzed during peak layer occurrence in July 2005. Layers contained up to an order of magnitude more phytoplankton biomass than surrounding waters. Furthermore, this analysis showed that (1) plankton layers were horizontally coherent, because the species composition of sam- ples from within PRLs from up to 5 stations collected on any given day were statistically indistinguish- able; (2) layers were not continuous in time, since species composition changed significantly between sampling days; and (3) layers could have formed within East Sound, since no differences were observed in species composition among samples collected at any depth. Phytoplankton biomass was dominated by the diatom genus Chaetoceros (up to 95%), whereas heterotrophic protists (5 to 200 µm) were dominated by thecate dinoflagellates (up to 80% of biomass), with oligotrich ciliates and athecate dinoflagellates at times abundant (up to 40% of biomass). Motile heterotrophic protists were significantly aggregated within phytoplankton prey layers, which confirmed predictions from prior laboratory and modeling work. Biomass of phytoplankton prey species within PRLs uniformly exceeded the dominant predator's survival threshold, whereas prey concentrations outside PRLs would not support growth in all but 3 samples. These observations suggest that PRLs may be bio- logical hot spots where trophic and demographic rates are enhanced and that biological processes could drive plankton layer formation, persistence, and decline.