Particulate material as an indicator of pearl-oyster excess in the Takapoto lagoon (Tuamotu, French Polynesia)

The aquatic environment of black-lipped pearl oysters (Pinctada margaritifera) in the Takapoto lagoon was studied to determine the cause of their recent diseases, Both N and P limitations were observed, and the oligotrophy was known from previous reports. The low N concentration appeared to affect the volumes of the various picoplankton compartments more than their numbers, whereas the P concentration affected phytoplankton, estimated as chlorophyll a. The values for the resulting biomasses of phytoplankton (3.8 mu g C 1(-1)) and bacteria (2.8 mu g C 1(-1)) were amongst the lowest reported values, as were the respective production rates (2.2-3.5 mu g C 1(-1) reported and 1.22 mu g C 1(-1) current work). Although unexpectedly low, the values for biomass and production of bacteria are consistent with the close relationship observed between phytoplankton and bacteria. Waste products from the reared P. margaritifera stock enhanced the growth rates of phytoplankton and picoplankton as shown when incubating in unpoisoned sediment traps, even though the concentrations of both populations were lower than those achieved in the surrounding wafer, Oyster filtering affected not only phytoplankton, but also smaller-sized plankton, including flagellates and the viable heterotrophic bacteria, but had a significant impact on total counts of bacteria only in the 0-7 m layer, The density of the pearl oysters appeared to exceed the nutritional potential of the lagoon. Owing to the oligotrophic conditions, phytoplankton and even bacteria were unable to sustain their grazing. The 0-7 m depth appeared to be more favourable to oyster growth than the bottom, despite the higher amount of pigments collected in bottom traps. The suspended material in the upper layer was less degraded and showed a higher potential for microbial multiplication. The lack of exchange between the intermediate layer of 18 m and the upper and lower levels suggested that during our study, the suspended material at the bottom did not originate directly from the 0-7 m level owing to the particular circulation of water in the lagoon.