Ecology of a Caribbean coral reef. The Porites reef-flat biotope: Part II. Plankton community with evidence for depletion

A quantitative assessment of drifting net plankton crossing a reef-flat biotope was obtained on a Caribbean coral reef. The spatial distribution and abundance of plankton were sampled to provide estimates of the removal of this potential food resource by suspension-feeding populations. Sampling was largely confined to the reef flat and adjacent waters of Laurel Cay, a flourishing coral reef present on the insular shelf off southwestern Puerto Rico. A prior study provides information on the meteorological and hydrographic characteristics of this area. Evidence for plankton accrual was found in the quantitative depletion of qualitatively similar populations sampled downstream of densely populated reef communities. Numerically, the diatom crop was reduced by 91% and zooplankton by 60% in water streaming off the reef. Significant diel and seasonal variations in plankton abundance were obcerved, as well as notable differences in volume flow, the latter closely related to the local wind regime. A time course of net plankton accrual was calculated, taking into account these various factors. During the summer season (July–August), when zooplankton was relatively abundant and water movement over the reef vigorous, the total gain from plankton reached 0.25 gC/m²/day; 75% of this occurred during a 4 h period at sunrise and sunset. Plankton retained on the reef flat in January of February and in September was around 0.1 g C/m²/day. Zooplankton biomass contributed the greatest share, exceeding that of diatoms by a factor of 10 during the day and 42 in the early evening. A mean annual accrual of 0.18 g C/m²/day is equivalent to 4 to 13% of net community metabolism.