Pattern and Process in Tidepool Algae: Factors Influencing Seasonality and Distribution

Abstract

The distributions and seasonal changes of 6 algal taxa commonly found in tidepools in Washington state [USA] are examined. Distinct seasonal patterns were found in 5 taxa: Collinsiella tuberculata and benthic diatoms are most abundant in the winter; Rhodomela larix in high pools is most abundant in the winter while that in low pools peaks in the summer; and Prionitis spp. and articulated corallines fluctuate less regularly but suffer losses in the fall and winter. Cladophora sp. shows an aperiodic pattern, with large reductions in abundance occurring unpredictably throughout the year. Although such seasonal patterns are usually ascribed to changing temperature and light regimes, data suggest that herbivores play a key role. Seasonal fluctuations in the abundance of herbivorous mollusks are implicated in some of these algal patterns, especially those of Collinsiella, high-pool Rhodomela and diatoms. The fall and winter losses of the more herbivore-resistant Prionitis and articulated corallines are probably due to increased wave drag acting on thalli that have grown unstably large. Cladophora is also affected by waves; it grows in large mats, loosely attached to the substratum, which slough off when they become thick. Physical extremes of temperature and light may set the upper limits of these algae but have a lesser role in seasonal patterns than previously assumed. The factors excluding certain intertidal algae from pools are also examined. Herbivores play a role in this pattern as well; algae transplanted into pools remain healthy and may grow, but are rapidly consumed by herbivorous mollusks and crustaceans. Possible refuges from herbivory of many of the common tidepool algae are discussed.