QUANTIFYING DECADAL-SCALE COMPOSITIONAL CHANGES IN SEAGRASS-ASSOCIATED MOLLUSCAN ASSEMBLAGES USING MULTI-YEAR CENSUS DATA FROM TWO SITES AROUND ST. CROIX, USVI

Comparisons of living and subfossil assemblages are useful for assessing environmental change. Time-averaged death assemblages tend to “smooth out” small-scale faunal changes associated with short-lived events such as storms and, therefore, provide effective historical baselines. The stability of death assemblages on decadal time-scales has not been fully elucidated, however. Here, we utilize multiyear faunal censuses taken from two locations along the coast of St. Croix, USVI to determine: a) whether a significant decadal-scale decline documented previously in the abundance of a once-prolific gastropod, Cerithium litteratum, persists to the present day; and b) the comparative nature of taxon-specific temporal variations between the two study sites.

Previous molluscan census data were available from dense seagrass beds for 1980 and 2002 from Smuggler’s Cove, a lagoon on the northeastern coast of St. Croix, and for 1989 from Salt River Bay, an embayment on the north-central coast. In June 2011, samples were collected from seagrass beds at both sites. Each of the historic samples were compared to the 2011 samples using NMDS ordination and correlation techniques. In Smuggler’s Cove, ordination revealed that samples from 1980 and 2002 were compositionally different from one another, and from those collected in 2011. Much of this variation can be explained by a depletion of the gastropod C. litteratum in the 2002 death assemblage. The 2011 samples also exhibit an increase in the gastropod Modulus modulus relative to 1980 and 2002. By contrast, the 2011 samples from Salt River Bay plot directly among the 1989 samples in ordination space, suggesting that the two intervals are compositionally similar.

The major species present in Smuggler’s Cove persist through time, but rank orders change significantly. We point to the decline in the density of C. litteratum in both the life and death assemblages from 1989 to 2002 as an example. While the causes of these sub-decadal shifts are uncertain, climatological variations may be at play. Temperature and precipitation are important constraints on larval development, and an unusual spike in precipitation in 1979 on St. Croix may have played a role in the C. litteratum “spike” observed in 1980.