Foraminiferal population densities and diversities were monitored on calcareous algae and seagrass in two Caribbean bays, one of which has been polluted by a leaking septic tank, and samples were collected monthly over a year-long period. Population densities on seagrass blades were measured using the mass of dried leaves. On other substrates, population densities were computed as the number of foraminifera per gram of detritus (diatom frustrules, sand, particulate organic matter) washed from the plants. Densities on the heads (capitulae) of the alga Penicillus capitatus in both bays were three times higher than those on either the alga Halimeda opuntia or on exposed seagrass (Syringodium filiforme, Thalassia testinudum) rhizomes and basal leaf bosses. The input of organic matter into one bay may have impacted the assemblage species composition, but it did not affect population densities. The short life span of P. capitatus, coupled with the more dense populations it supports, implies that assemblages on this alga may contribute proportionally more foraminiferal tests to the sediment than do those on other plants.
Population dynamics showed that entire foraminiferal communities on all plants fluctuated independently of one another, whether on different plants in the same bay or on the same plants in different bays. This suggests that nearshore foraminifera are patchily distributed and that the populations in the patches wax and wane independently. Population diversities fluctuated from month to month and were significantly higher in the polluted bay. However, monthly diversities and population densities were not correlated. Species dominance was greater in the windward, unpolluted bay. Differences in dominance within each bay suggest that Penicillus capitatus capitulae support a pioneering foraminiferal community, whereas long-lived seagrass rhizomes bear a climax community.