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

Hydrodynamics

We evaluated hydrodynamic and water column conditions at our study sites intermittently during the summers of 1994 and 1995 using standard oceanographic equipment. We deployed current meters (InterOcean Systems, Model S4) 0.5 m off the bottom in the shallow subtidal at each site (depth = 1-5 m) to determine mean and maximum free stream velocity during peak spring tides (sampling rate = 1 sample/min for 24 h). Bulk fluid flow was calculated by integrating the area under the velocity curve over the 24-h tidal cycle. A fluorometer (SeaTech) was deployed concurrently at the same depth and height off the bottom to quantify chlorophyll a concentration. Flux of phytoplankton (in micrograms of chlorophyll a per centimeter squared per second) was estimated by multiplying instantaneous flow velocity (in centimeters per second) by chlorophyll concentration (in micrograms of chlorophyll a per centimeter cubed) over the 24-h period. Flux was not estimated for longer time periods because flow rate was so tightly coupled to the tides. Long-term temporal variation in phytoplankton concentration and water column productivity is likely (Mann and Lazier 1991) but because of extensive mixing (Mayer et al. 1996) was likely comparable at all study sites. To quantify flow variation within the boundary layer, calcium sulfate cylinders ("chalk blocks"; n = 8 blocks/site) were bolted to the rock at MLLW for 5 d (see Sanford et al. 1994 for techniques). Chalk blocks provide a long-term measure of flow regime because they dissolve at a rate proportional to bulk water flow (Thompson and Glenn 1994).

Community patterns

Differences in community structure among sites were evaluated by random quadrat sampling across the entire intertidal zone (six equally spaced heights from 0.0 m to 3.6 m above MLW) at all sites (n = 8 replicates per height per site). In each quadrat, percent cover of primary space holders (barnacles: Semibalanus balanoides, mussels: Mytilus edulis, and algae: Chondrus crispus, Ascophyllum nodosum, and Fucus spp.) as well as bare space was quantified with a 1-[m.sup.2], 100-point grid. All mobile invertebrate molluscs (grazers: Littorina littorea, predatory whelks: Nucella lapillus) were counted. Percent cover data were arcsine square-root transformed and analyzed with a three factor, nested ANOVA to evaluate percent cover as a function of flow type and tidal height (fixed factors). Site (random factor) was nested within flow type. We also estimated predatory crab abundance (Carcinus maenas and Cancer irroratus) at all sites during high tide predation assays (see Methods: Predation losses) with 2 x 20 m transect surveys (n = 10 transects/site) using SCUBA. Abundance data were transformed as necessary and evaluated with two-factor ANOVA with site nested within flow type.

The mid and high zones of low flow sites were dominated by a 30-100% secondary cover of the canopy forming seaweed Ascophyllum nodosum (G. H. Leonard, unpublished data). In contrast, A. nodosum was generally absent at high flow sites. The role of this canopy on community dynamics in the high zone of low flow sites is likely significant and is currently under investigation. Estimates of recruitment, growth, and predation were made in the mid to low zone where A. nodosum was generally rare at both high and low flow sites.