Life History and Dispersal Patterns in a Dense Infaunal Polychaete Assemblage: Community Structure and Response to Disturbance

Abstract

The effects of differing life histories on the dynamics of dispersal, recruitment,and population maintenance were investigated for a dense infaunal polychaete assemblage on the Kendall—Frost mudflat in Mission Bay, California. Polychaete life history features provided the framework for investigations of small—scale dispersal mechanisms, infaunal response to disturbance, and the spatial and temporal predictability of species' abundances. Field and laboratory studies revealed that Rhynchospio arenincola Hartman, Streblospio benedicti Webster, Exogone lourei Hartman, Fabricia limnicola Hartman, and Capitella spp. shared many life history traits which limited the range of dispersal. These included small adult size, brood protection, small brood size, and planktonic larval stages which were reduced or absent. Pseudopolydora paucibranchiata Okuda, and Polydora ligni Webster exhibited initial brood protection but had larger brood sizes and longer lived larvae. Small—scale dispersal was examined by studying patterns of larval availability, recruitment into settling cartons, and colonization of defaunated sediments. The role of dispersal in response to disturbance was examined for two levels of perturbation. Small—scale disturbance, commonly generated on the mudflat by ray foraging and human digging, was studied by artificially defaunating small (0.4—m 2) sediment patches. A severe storm and consequent raw sewage spill created an episodic large—scale perturbation in the middle year of the 3—yr study. Analyses of species' responses revealed colonization ability at recruitment to be distinct from dispersal (migratory) ability. Rates and mechanisms of colonization were governed by larval development, settlement, and mobility patterns and varied with the scale of perturbation. For R. Arenincola, S. benedicti, E. Lourei, and F. Limnicola, factors such as brood protection, reduced planktonic larval phases, and postlarval movements, particularly by brooding adults, confer small—scale dispersal abilities which permit rapid colonization of disturbed patches and result in maintenance of high infaunal densities (>200,000 individuals/m 2). P. Paucibranchiata and P. Ligni possess long—lived larvae whose planktonic abundances are highly seasonal and variable from year to year. The timing of disturbance must coincide with periods of peak larval availability for successful colonization by these species. In general, the annual life cycles and flexible small—scale mobilities of most species enable persistence in the face of frequent fine—grained disturbance.