Temporal and spatial variability in settlement and recruitment of echinoderms in kelp beds and barrens in Nova Scotia

Abstract

Settlement and recruitment of echinoderms were measured in 2 adjacent habitats: a kelp bed (Laminaria longicruris) and an echinoid (Strongylocentrotus droebachiensis)-dominated barren ground at each of 2 sites in the rocky subtidal zone (5 to 10 m depth) off Nova Scotia. Settlement was measured using artificial collectors made of 0.05 m2 panels of plastic turf that were deployed 0.2 m (low) and 2.3 m (high) above the bottom in each habitat, and sampled bi-weekly from June to November 1992 to 1994. Distinct settlement pulses of ophiuroids (Ophiopholis aculeata and Ophiura spp.), asteroids (Asterias spp.) and an echinoid (S. droebachiensis) occurred between July and September of each year at both sites. Timing of settlement differed consistently among species in relation to differences in timing of spawning and larval development. The magnitude of each pulse varied between years and species, and the year of maximum settlement differed between species, suggesting that species-specific processes regulate, settlement rather than general environmental conditions. Settlement of all species was greater at the more protected site, but between-habitat patterns were not consistent among species. Asteroids settled in greater numbers in and above kelp beds, whereas ophiuroids showed a trend toward greater settlement in the barrens. The echinoid S. droebachiensis also tended to settle more in barrens, although the difference was not statistically significant. This suggests that kelp beds do not exhibit a consistent or strong effect on settlement. Sampling settlement concurrently at different frequencies (3 to 48 d) gave different estimates of settlement, indicating a need for assessment of sampling artifacts (e.g. changes in collector quality, post-settlement morality or migration) that can occur over longer deployment intervals. For most species sampled, settlement predicted recruit density in natural populations the following year. However, the strength of the relationship varied between species, probably because of differing post-settlement processes.