Water renewal time for classification of atoll lagoons in the Tuamotu Archipelago (French Polynesia)

Abstract

The feasibility of using hydrodynamic renewal time as the basis for a classification of atoll lagoons is tested for atolls of the Tuamotu Archipelago, French Polynesia. Renewal time depends on the inflow of oceanic water through the rim of the atoll, on a daily time scale, due to wave forcing. Renewal time is computed for a large set of morphologically diverse atolls, according to significant wave height (satellite altimetry data), morphometric indicators (high-resolution satellite images), and in-situ flow measurements. Renewal times with respect to wave height are presented for a variety of atolls. Renewal times range from less than 1 day for very open and shallow atolls, to several tens of days for semi-open moderately deep atolls, and to several years for closed or very large and deep atolls. Comparisons between phytoplanktonic biomass (in the range 0.1 to 1 µg l^–1 for total chlorophyll) and renewal time (0.1 to 130 days) leads to the identification of two groups of atolls. We obtain a significant relationship between biomass and renewal time, but only for atolls with lagoon surface areas greater than 25 km².