Rocky intertidal oceanography: An association between community structure and nearshore phytoplankton concentration

Abstract

On Oregon coastal rocky shores, filter‐feeders were relatively abundant and macrophytes were relatively scarce at Strawberry Hill, whereas opposite abundance patterns occurred at Boiler Bay. To determine whether nearshore oceanographic differences were associated with these patterns, we made shore‐based measurements of nutrient and Chl a concentrations. We used a three‐level nested design to identify ecologically appropriate sampling scales: “site” (10s of km), “bench” nested within site (100s of m), and “location” nested within bench (10s of m). Nutrients varied inconsistently but Chl a was consistently higher at Strawberry Hill. For Chl a, site explained ∼70% of the variance, whereas bench and location explained <20%. For nutrients, site and bench explained most of the variance, but neither was consistently more important. The data tentatively suggest that nutrient levels are weakly related to the between‐site ecological differences. In addition to the between‐site differences, Chl a changed seasonally, with maximum levels in summer. For nutrients, temporal changes were more complex, with highest levels tending to occur in late summer and autumn. No nutrient, however, was scarce enough at either site to limit phytoplankton growth, with the possible exception of nitrate in June. These results were consistent with the hypothesis that nearshore phytoplankton standing stock, a bottom‐up factor, could underlie differences in rocky intertidal community structure.