The ecology of a high-latitude rocky intertidal community: Processes driving population dynamics in Kachemak Bay, Alaska

Abstract

The population dynamics and interactions of selected key species relative to community structure were investigated in the rocky intertidal of Kachemak Bay, southcentral Alaska (59$\sp\circ$35$\sp\prime$N, 151$\sp\circ$30$\sp\prime$W). The roles of recruitment processes and predation in regulating intertidal populations were emphasized in this investigation. Species cover was distinctly seasonal. Total cover typically exceeded 80% during the summer, especially in lower intertidal. Winter cover averaged 40-60%, with macroalgal cover varying up to six-fold between summer and winter. Barnacle recruitment varied both inter-annually and with respect to species. From 1991-1993, mean recruitment densities varied from 0.85-8.71 cm$\sp{-2}$ (range = 0-71 cm$\sp{-2}).$ In the upper intertidal, time-integrated summer recruit density of Semibalanus balanoides and Balanus glandula was 0.13 cm$\sp{-2}.$ Recruit density of S. cariosus in the low intertidal was 4.32 cm$\sp{-2}.$ In the low intertidal, recruits often saturated the surface, resulting in density-dependent mortality in two out of three years, a phenomenon which did not occur in the upper intertidal where space was never limiting. Predation was a significant source of mortality for barnacle recruits only in 1991, a poor recruitment year. However, predation by Nucella lima limited mussel (Mytilus trossulus) populations at some sites. Where N. lima density exceeded 100 m$\sp{-2},$ mussel cover was less than half that where Nucella was rare (31% vs. 72%). High densities of N. lima were estimated to remove 60-90% of mussels per season. Recruitment of the macroalga Fucus gardneri was almost 50 times greater in the presence of live barnacles than on bare rock surfaces or barnacle shells killed by heating. Recruitment in quadrats with tests of mechanically killed barnacles was intermediate. The results indicate that F. gardneri propagules are stimulated to attach by a chemical cue, probably a polypeptide, produced by barnacles. Based on population dynamics and species interactions investigated in Kachemak Bay, the mid- to low intertidal community appears to function similarly to the classical paradigm of regulation by competition and predation. The major exception is high inter-annual variability in predation relative to recruitment and competition.