Seasonal variations of downward particle fluxes in front of a reef pass (Moorea Island, French Polynesia)

Abstract

Downward particle fluxes off the Taotoi Pass, a typical Polynesian lagoon pass, have been measured over an annual cycle by means of an array of three mooring lines deployed at 40 m depth on the external sandy plain seawards of the Tiahura reef. Each line was equipped with two hand-operated sediment traps located 1.5 and 10 m above the bottom. During the 1-year survey, the traps were deployed for seven periods of 2–3 weeks. One line equipped with an automated trap was further deployed at 400 m depth on the deep fore-reef slope to collect deep settling particles over two consecutive 6-month periods. For all periods investigated, fluxes on the external plain were highest in front of the pass, and decreased by half on either side (up- and downstream with respect to general circulation), indicating a net export through the pass. At all three locations, the highest downward fluxes recorded (60–100 g m^–2 d^–1) corresponded to coarse particles mostly carbonate, whereas the lower fluxes (a few g m^–2 d^–1) corresponded to smaller and lighter material mostly organic carbon. Fluxes on the deep slope were much lower than on the sandy plain (1.5–7.3 g m^–2 d^–1), but followed a broadly similar seasonal trend. Comparison of our trap data with published production values for the Tiahura reef system suggests that inorganic and organic carbon export through the pass is limited. The minimum estimates represent around 6% of total calcification but only 1% of the net production of the reef system. The qualitative and quantitative temporal evolution of downward fluxes showed a seasonally-contrasted trend, related to local hydrodynamic conditions. Fluxes were always higher during the southern summer season, and lower during winter. Carbon export appears to be ultimately controlled by climatic factors, i.e. seasonal changes in the orientation and heights of swells and waves that modulate the amount of water flowing into the lagoon, before being flushed out into the open ocean, entraining particles.