Flow-driven variation in intertidal community structure in a Maine estuary

Ecology, June, 1998 by George H. Leonard, Jonathan M. Levine, Paul R. Schmidt, Mark D. Bertness

We propose to broaden the traditional definition of "bottom-up" forces for these aquatic habitats to more closely match these effects. In the strict sense, bottom-up control concerns only the direct effect of nutrients on primary producers and the resulting indirect effects higher in the food web. This definition is particularly useful when there is intrinsic control of primary production in relatively closed systems such as terrestrial habitats and some lakes.

In benthic marine habitats where tides and currents are common, this strict definition of intrinsic primary productivity may not apply. Productivity of these communities may be extrinsically controlled (sensu Polis and Hurd 1996) via the input of nutrients to benthic plants (Carpenter et al. 1991). Fluid fluxes in aquatic habitats also permit both new individuals (i.e., larvae) and suspended food particles to be delivered to benthic habitats via physical transport processes. The different physical characteristics of water and air (Denny 1993) and the way physical transport processes operate in these two media may represent fundamental differences in the way bottom-up forces operate in terrestrial vs. aquatic systems. The delivery of nutrients, which drives strict "bottom-up" control, is thus intimately linked to the delivery of new individuals to the food web and resources to benthic suspension feeders that often make up the base of these food webs (Paine 1966). This suggests that a broader definition, which incorporates multiple inputs to the food web, is more appropriate in these extrinsically controlled systems.

In this paper, we evaluate the relative contribution of these broadly defined bottom-up processes with traditional top-down forces to community structure in an estuary in Maine, USA. This estuary is subject to highly predictable differences in hydrodynamics (tidal currents) that act at both the top and bottom of the food web. Although there has been great success in quantifying how small-scale variation in flow rate can influence recruitment rates and suspension feeding in laboratory flumes (e.g., Butman et al. 1988, Mullineaux and Butman 1991) and the mechanics of some suspension feeders are now well understood (e.g., Patterson 1991, Lesser et al. 1995), we know considerably less about the food-web consequences of variable flow regimes. Ours is the first study to examine the influence of local hydrodynamics on both the herbivorous and carnivorous components of an entire food web.

We examine the general hypothesis that flow regimes can play a powerful role in structuring shoreline communities. Specifically, we hypothesize that sites subjected to high fluid flow should be structured largely by bottom-up forces operating through increased delivery of larvae, suspended food, and nutrients to the benthic community. These sites are predicted to be characterized by higher recruitment and growth rates of both benthic plants and suspension feeding invertebrates but also by low consumer pressure. In contrast, we hypothesize that low flow habitats, where current-driven delivery rates are lower, should be structured by top-down processes because of greater predator mobility.