Temporal Variations in a Tropical Soft-Bottom Community, Venezuela
Keywords:Macroinfauna, soft-bottom, Caribbean, Polychaeta, multivariate analyses
The relationship between macroinfaunal density fluctuations and temporal variations of environmental parameters were studied. Physico-chemical, sediment, and biological parameters from a tropical soft-bottom community were followed monthly during a 2 1/2 years survey, and their possible relationships were elucidated using multivariate analyses. The community at the study site was composed of 37% polychaetes, 21% crustaceans (mainly amphipods), 15% nematodes, 5% oligochaetes and 22% shared among gastropods, bivalves, and other minor groups. Results showed little temporal variation for environmental and sediment variables, except for the grain-size fractions, and clear fluctuations in macroinfaunal densities. Polychaete total abundances showed two annual peaks, one during the summer months (mainly June to July, but May and July in 1991) and a second one towards the end of the year (November to December). Crustaceans, the second major group, showed only one annual peak, either early (April-May 1990 and 1991) or late summer (July-August 1989). Similar temporal density variations were also noted for the nematodes, with one annual peak during June and July (but exceptionally high values in February 1990 and 1991) and lowest values in April 1990 and January 1991. The molluscs (gastropods and bivalves) occurred with very low densities and showed no clear temporal trend. Polychaetes, the dominant group, were represented by 22 species belonging to 15 families, with spionids, paraonids, and capitellids as the families with the highest richness. Maximum species richness was obtained in October 1990 and April 1991 and minimum richness in July 1990 and January 1991, with 10 polychaete species (± 2 species) as a monthly average. In terms of abundance, 21% of the polychaetes were represented by Lumbrineris tetraura, 10% by Prionospio pygmaea, 5% by the cirratulid Tharyx sp., 5% by Magelona pettiboneae, and less than 2% for the rest of the species of this group. Lumbrineris tetraura, the dominant species, had two annual density peaks per year except in 1990, where no peak for the summer months was evident. Prionospio pygmaea had the highest densities during September-October 1989 and the lowest during April, June, and August 1991. Maximal values were also observed for Tharyx sp. in November-December 1989, and during April, June, and July of the same year for Magelona pettiboneae. Results of the ordination analyses by CCA revealed little association between faunal densities and sediment parameters suggesting that other factors may be responsible for the macroinfaunal density patterns at the study site, such as factors related to biological interactions between species or factors extrinsic to the community, related to the input of external pulses of organic material. We propose that macroinfaunal density peaks may be a consequence of a combination of the above factors at our study site.