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

METHODS

Study sites and experimental design

We examined the influence of flow on shoreline community structure at six sites in the Damariscotta River, a 23 km long, tidal estuary in Maine. Freshwater input to this system is very low (McAlice 1977) and water column stability is governed largely by temperature (McAlice 1979). Wave heights are generally [less than]0.5 m but horizontal currents are strong and driven largely by tides. Although patterns of salinity suggest typical estuarine circulation patterns (Mann and Lazier 1991; G. H. Leonard, personal observation), the center of the estuary is highly mixed vertically and horizontally, especially during the summer when freshwater input is lowest (Mayer et al. 1996). This is evidenced by vertical isotherms (G. H. Leonard, unpublished data) and the low variation in dissolved nutrients ([approximately]2.5 [micro]mol/L nitrate and nitrite) and chlorophyll a ([approximately]2.0-3.0 [[micro]gram]/L) concentrations at the scale of kilometers (Mayer et al. 1996). The summer temperature range in the center of the estuary is 10-15 [degrees] C while the salinity range is 3234[per thousand] (Mayer et al. 1996; G. H. Leonard, unpublished data). The geomorphology of the Damariscotta River also varies on small spatial scales such that some habitats experience strong tidal currents while those only a few hundred meters away experience very little tidal flow. This results in nearby benthic habitats that often experience radically different flow regimes yet similar water masses. We chose three high and three low flow sites in the center of the estuary to represent these extreme differences in hydrodynamics [ILLUSTRATION FOR FIGURE 1 OMITTED]. High flow sites were located at constrictions in the estuary while low flow sites were nearby shorelines of similar topography but not subject to strong tidal currents.

We compared these six sites for differences in (1) hydrodynamics, (2) community pattern, (3) recruitment and growth rates of the dominant species, and (4) predation intensity on dominant prey. Our approach was not strictly experimental, where resources and predators are simultaneously manipulated and the resulting effects on community structure measured (e.g., Posey et al. 1995). In situ manipulation of flow regime, although possible, has generally been unsatisfactory (e.g., Judge et al. 1992) and is impractical when examining community responses. As a first step in evaluating community responses to variable flow regimes, our approach has been to quantify bottom-up effects (recruitment and growth rates) while simultaneously measuring predation losses for dominant prey. Although there has been a call for strictly experimental approaches to evaluating top-down and bottom-up community control (Hunter and Price 1992, Osenberg and Mittelbach 1996, Persson et al. 1996), we believe a comparative approach across sites of naturally varying hydrodynamic characteristics can provide strong evidence for the relative importance of these processes in natural assemblages.