Temporal and spatial stability of bacterioplankton biomass and productivity in an atoll lagoon

Spatial and temporal variations of bacterioplankton biomass, production and growth rates were investigated 6 times from 1991 to 1994 in Tikehau lagoon (Tuamotu, French Polynesia). The water column was homogeneous from the surface to the bottom (20 m) at the reference station but some horizontal variations were detected, with biomass and production increasing from the reef-flat spillways (oceanic water inflow) to the main part of the lagoon and decreasing from the relatively eutrophic shore (near the village) to the main part of the lagoon. However, both of these perturbations were limited to the immediate vicinity of these areas. Diel fluctuations of bacterial biomass and growth rate were slight (coefficient of variation, CV < 20%) and showed no significant pattern. Day-to-day variations recorded over periods of up to 20 consecutive days appeared also to be limited (CV < 25%). A significant correlation between wind and total abundance of bacteria in the water column suggested that these day-today variations may be in part explained by wind, probably inducing resuspension of sediments. No significant pattern appeared from comparison of the 6 cruises spread over different seasons. Cruise averages differed slightly from each other, with CV for all cruises averaging 34, 10 and 41% for bacterioplankton biomass, production and growth rate, respectively. Tikehau lagoon appears to be a very stable ecosystem for bacterioplankton processes and therefore differs strongly from other reef water column systems. This untypical stability might be explained by the long residence time of water (170 d), the strong stability of the oceanic surrounding water (South Pacific Gyre), the weak seasonality of primary producers and the limited inputs from corals, bordering islands and sediments.