FLOW-DRIVEN VARIATION IN INTERTIDAL COMMUNITY STRUCTURE IN A MAINE ESTUARY

Abstract

Understanding the factors that determine community structure remains one of the most important issues in ecology. In this paper, we examine the role of flow velocities in governing community structure in marine intertidal communities. We broaden the traditional definition of “bottom-up” forces to include the delivery of nutrients, food, and larval resources to habitats and then test the hypothesis that, by controlling these fluxes as well as mediating predator effects, flow velocities leave strong bottom-up signatures on shoreline communities. We examined this hypothesis by quantifying community structure and dynamics at high and low flow sites in a tidal estuary in Maine. High flow sites were characterized by dense barnacle and mussel cover, while low flow sites had considerable bare space. High flow sites also had greater grazer and predator densities than low flow sites. Recruitment of all common shoreline organisms with planktonic larvae was greater at high flow sites, in direct proportion to the increased fluid flux there. Flow effects on the growth of the herbivorous and carnivorous components of the food web were less predictable. High flows increased the growth of barnacles, but not mussels, and increased the growth of the carnivorous gastropods that preyed on them. In contrast, high flows decreased the accumulation of benthic diatoms, but this was unrelated to the growth rates of herbivorous gastropods. High flow sites were universally characterized by low predation intensity and per capita predation rates on all three prey species. Our results show that the strength of top-down and bottom-up forces varies with flow rate in this estuary. Consumer stress (top-down) models accurately explain patterns we saw at low flow sites, but bottom-up processes predicted from nutrient/productivity models dominate at high flow sites. High consumer pressure is the dominant structuring force at low flow sites, whereas at high flow sites predators are inhibited, and individual recruitment and growth rates become the dominant structuring forces. We suggest that hydrodynamics may commonly decouple predation and resource processes in many aquatic systems when the physical process responsible for variation in top-down forces also acts as a strong bottom-up force.