Assemblages of coccolithophorids and other living microplankton off the coast of Puerto Rico during January–May 1995

Abstract

Much effort is being spent on estimating phytoplankton productivity and their role in sequestering CO2. Most of the time these estimates are based on chlorophyll a determination either from satellite spectral data or from ground truthing. No appreciable increase in microplankton standing crops occurred at stations which have high chlorophyll a contents, indicating that chlorophyll a peaks may have been associated with another group of organisms such as picoplankton which do not produce hard parts.Microplankton samples were collected biweekly from a number of depths at a fixed station off the coast of Puerto Rico between 30 January and 10 May 1995. The microplankton composition was counted and the coccolithophorid species assemblages identified. The total microplankton cell concentrations, usually ranged between 103 and 104cells/l, and were found to be consistent with previous findings from the Caribbean Sea and other ‘blue water’ subtropical regions. A spring diatom bloom did not occur at our station, although relatively higher numbers of diatoms were present in late January. Apart from a small ‘event’ in late February–early March, the diatoms (especially the centric diatoms) decreased in relative abundance and sank into deeper photic waters as spring progressed, while the relative abundance of coccolithophorids increased. During the study period, the coccolithophorid community was distributed vertically into the three classical sections of the photic zone, although the lower photic zone was not always encountered. The upper photic zone was dominated either by Emiliania huxleyi or Gephyrocapsa oceanica from January to early March, after which they declined in numbers, and Umbellosphaera irregularis, Umbellosphaera tenuis, Discosphaera tubifera, Rhabdosphaera clavigera, holococcolithophorids, and syracosphaerids then co-dominated throughout the spring months, showing monthly fluctuations in relative abundance. In the lower photic zone Florisphaera profunda was always the dominant species. However, the occasional absence or low percentage of this species and other lower photic species, probably indicates that a deepening of the nutricline occurred. The middle photic zone was characterized in late spring by peaks of E. huxleyi and centric diatoms, both of which prefer to avoid unfavourable conditions at the surface and thus their presence in this zone appears to signify a survival strategy.These results show that the Puerto Rican slope waters still exhibit seasonality even though they are considered oligotrophic and permanently stratified. There is also some evidence that material (e.g. neritic diatoms, chrysophyte statospores, pollen) from the Puerto Rican coast and mainland can be transported to the slope area at particular times of the year, probably as a result of shelf water movement in conjunction with periods of heavy monsoonal rainfall. There is also a possibility that waters originating from the Orinoco River, which reach the Puerto Rican coast in September and October each year, play some role in determining the phytoplankton composition during the winter months. It is hoped that future sampling will shed further light on the seasonality and annual cyclicity of the phytoplankton from this subtropical area.