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

Recruitment dynamics

We quantified the recruitment of barnacles, mussels, and herbivorous snails at each study site. These organisms were the most conspicuous invertebrates and all have planktonic life history stages. Barnacle recruitment was quantified in 20 randomly located, 100[cm.sup.2] quadrats at each site ( 0.5 m above MLW) that were scraped to bare rock in early spring. Immediately after settlement had ended in May, recruitment was quantified as the total number of juveniles per 100 [cm.sup.2]. Whelks were allowed access to recruitment quadrats but there was no visual evidence that they influenced early recruitment patterns (G. H. Leonard, personal observation). Recruitment of mussels was quantified using plastic mesh collectors (n = 10 collectors/site) that mimic natural settlement surfaces (see Menge 1992 for techniques). Unlike barnacles, the settlement season for mussels occurs over several months during the summer. Collectors were therefore bolted to cleared rock surfaces in the low intertidal zone ( 0.5 m above MLW) in early April and retrieved in September. Mussel recruits (length 0.5-1.0 mm) were counted under a dissecting microscope. Empty barnacle tests were used to sample Littorina littorea recruitment across sites. We quantified densities of juvenile littorines (length = 1-3 mm) in regions of similar barnacle density in the mid intertidal zone using 10 randomly located, 100[cm.sup.2] quadrats at each site.

Growth rates

Growth of all community dominants was quantified at all study sites. To quantify barnacle growth, newly settled barnacle recruits in the low intertidal zone ( 0.5 m above MLW) were thinned to prevent crowding and the basal diameter of random, solitary individuals was measured with vernier calipers from size-standardized photographs taken monthly from May-September of 1994. Differences in growth in September (after 120 d) were analyzed using a two-factor (flow type, site) nested ANOVA.

To quantify mussel growth, juveniles (30-40 mm in length) were collected from a common site and individually labeled with small plastic tags glued to the shell surface. After initial length measurements, 480 juveniles were placed in wire mesh cages (to prevent dislodgment by currents or predators) in the low intertidal zone ( 0.5 m above MLW) at all sites (n = 8 mussels/cage, 10 cages/site). Growth was calculated as change in length after 5 mo. To evaluate cage effects, we duplicated this design but removed the cages after the mussels had attached to the substrate. Data were first tested using a three-factor, nested ANOVA using flow type, site, and presence/absence of cages as factors. Caging did not affect mussel growth at any of our sites (Cage vs. Open Mussels X Site interaction, F = 1.864, P = 0.117). For clarity, data from caged mussels only will be presented here.

To quantify the growth of common mobile invertebrates, herbivorous (Littorina littorea) and carnivorous (Nucella lapillus) gastropods were collected in May 1994 from a common site and their shells individually labeled with small numbered tags and a stripe of colored paint. Gastropods were measured for total length and then released at each site (n = 400 Littorina and 200 Nucella per site, respectively). For both species, growth (change in length) was determined for all individuals recaptured after 4 mo (Littorina: High flow = 199 individuals, Low flow = 133 individuals; Nucella: High flow = 93 individuals, Low flow = 80 individuals). Data were analyzed using a two-factor (flow type, site) ANOVA. We also quantified Littorina growth by enclosing snails at low densities in 20 X 20 cm cages bolted to the rock (mesh size = 1.5 X 1.5 cm; n = 2 grazers/cage, 10 cages/site); growth was quantified as change in length after 4 mo. This provided two independent measures of grazer growth.