Layers of high plankton concentrations, bordered by steep gradients, here termed
plankton-rich layers (PRLs) were characterize with respect to their spatial and temporal
extend and potential importance to planktonic productivity in a shallow fjord in the
Northeastern Pacific (East Sound, Washington, USA). These naturally-forming
phytoplankton patches provided a large scale (km, days) system to test hypotheses
suggested by prior laboratory and modeling results, which indicate that estimates of
trophic and demographic rates of phytoplankton-consuming protists are enhanced by
over an order of magnitude when their foraging behaviors are accounted for (MendenDeuer
& Grünbaum, 2006) and that predator population distributions are affected by
cell-to-cell level variations in movement behaviors. At our field site, the diatom genus
Chaetoceros dominated phytoplankton biomass, whereas heterotrophic protist biomass
was dominated by thecate dinoflagellates with oligotrich ciliates and athecate
dinoflagellates at times abundant. Different processes drove phytoplankton and
heterotrophic protist distributions: phytoplankton species were found throughout the
water column (although in different concentrations), whereas heterotrophic protists were
aggregated by and large within PRLs when present. Phytoplankton prey within PRLs
uniformly exceeded dominant predators’ survival threshold, whereas prey
concentrations outside PRLs were insufficient to support growth in any but three
samples. As predicted, spatially concentrated prey and predators ability to located prey
fundamentally altered trophic dynamics within the system. PRLs would have served as
the only prey source to heterotrophic protists and led to locally much higher ingestion
rates than predicted based on average prey concentrations. Therefore, prey distribution
and more importantly predator foraging behaviors have important ramifications for
ecosystem productivity and the transfer of organic matter and energy to higher order
food-chain components