Temporal variations in a tropical soft-bottom community, Venezuela

The relationship between macroinfaunal density fluctuations and temporal va riations of environmental parameters were studied. Physico-chemical, sedime nt, and biological parameters from a tropical soft-bottom community were fo llowed monthly during a 21/2 years survey, and their possible relationships were elucidated using multivariate analyses. The community at the study si te 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 environmen tal 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 M ay 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 19 89). Similar temporal density variations were also noted for the nematodes, with one annual peak during June and July (but exceptionally high values i n February 1990 and 1991) and lowest values in April 1990 and January 1991, The molluscs (gastropods and bivalves) occurred with very low densities an d showed no clear temporal trend. Polychaetes, the dominant group, were rep resented by 22 species belonging to 15 families, with spionids, paranoids, and capitellids as the families with the highest richness. Maximum species richness was obtained in October 1990 and April 1991 and minimum richness i n July 1990 and January 1991, with 10 polychaete species (+/- 2 species) as a monthly average. In terms of abundance, 21% of the polychaetes were repr esented by Lumbrineris tetraura, 10% by Prionospio pygmaea, 5% by the cirra tulid 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 gear except in 1990, where no peak for the summer months was evident. Prionospio pygmaea had the highest densities du ring September-October 1989 and the lowest during April, June, and August 1 991. Maximal values were also observed for Tharyx sp. in November-December 1989, and during April, June, and July of the same year for Magelona pettib oneae. Results of the ordination analyses by CCA revealed little associatio n between faunal densities and sediment parameters suggesting that other fa ctors may be responsible for the macroinfaunal density patterns at the stud y site, such as factors related to biological interactions between species or factors extrinsic to the community, related to the input of external pul ses of organic material. We propose that macroinfaunal density peaks may be a consequence of a combination of the above factors at our study site.