Seawater carbonate chemistry and calcification during a study of barrier reef flat in Moorea, French Polynesia, 1998

The relative contribution of soft bottoms to the community metabolism (primary production, respiration and net calcification) of a barrier reef flat has been investigated at Moorea (French Polynesia). Community metabolism of the sedimentary area was estimated using in situ incubations in perspex chambers, and compared with estimates of community metabolism of the whole reef flat obtained using a Lagrangian technique (Gattuso et al., 1996. Carbon flux in coral reefs. 1. Lagrangian measurement of community metabolism and resulting air-sea CO2 disequilibrium. Mar. Ecol. Prog. Ser. 145, 109-121). Net organic carbon production (E), respiration (R) and net calcification (G) of sediments were measured by seven incubations performed in triplicate at different irradiance. Respiration and environmental parameters were also measured at four randomly selected additional stations. A model of Photosynthesis-irradiance allowed to calculate oxygen (O2), organic carbon (CO2) and calcium carbonate (CaCO3) evolution from surface irradiance during a diel cycle. As chlorophyll a content of the sediment was not significantly different between stations, primary production of the sediment was considered as homogeneous for the whole lagoon. Thus, carbon production at the test station can be modelled from surface light irradiance. The modelled respiration was two times higher at the test station than the mean respiration of the barrier reef, and thus underestimated sediment contribution to excess production. Sediments cover 40-60% of the surface and accounted for 2.8-4.1% of organic carbon excess production estimated with the modelled R and 21-32% when mean R value was considered. The sedimentary CaCO3 budget was a very minor component of the whole reef budget.

Geochemistry of sediment core GeoB7920-2

The X-ray fluorescence (XRF) core scanner provides bulk-sediment chemistry data measured nondestructively at the split core sediment surface. Although this method is widely accepted, there is little known about the effects of physical properties such as density and water content on XRF core scanner data. Comparison of XRF scanner measurements from the sediment surface and dry powder samples of sediment core GeoB7920 indicates strongly reduced element intensities for the lighter elements Al and Si. We relate the lower element intensities of the measurements taken at the sediment surface to the amount of water in the sample volume analyzed by the XRF core scanner. The heavier elements K, Ca, Ti, and Fe remain relatively unaffected by the variation of any physical property within sediment core GeoB7920. Additionally, we successfully use the elemental intensity of Cl as a proxy for the seawater content in the sample volume analyzed by the XRF core scanner. This enables the establishment of a correction function for the elements Al and Si that corrects for the radiation absorption of the water content in sediment core GeoB7920 off Cape Blanc, NW Africa.

Abundances of dinoflagellate cysts in sediment trap MST-9 (Appendix 1)

To study the ecology of calcareous dinoflagellates we examined the impact of the SW and NE monsoons on cyst formation using sediment trap material, collected at 1032 m water depth, off Somalia from June 1992 to February 1993. The results do not confirm the relationship between cyst production and lower nutrient concentrations, as highest cyst fluxes were recorded during late SW monsoon under the relatively nutrient-rich and less agitated conditions of mature upwelled water. Lowest cyst fluxes were found under strongly stratified, nutrient-depleted surface waters during the inter-monsoon. Although all of the studied species seem to prefer a stratified water column, an elevated concentration of nutrients appears to be necessary to maintain high cyst production. Comparison of the mean cyst flux to the sediment trap with that into the underlying surface sediments reveals a loss of 81-96%, which can be attributed to calcite dissolution. The relatively small spheres of Thoracosphaera heimii are affected more than the cysts of the other species.