Phytoplankton-bacterioplankton coupling in a subtropical South Pacific coral reef lagoon

Tropical coastal ecosystems are often ignored in carbon cycling budgets even though it is now recognised that these ecosystems can be important in terms of coastal carbon cycling. We present results from a series of incubations conducted during a 10 d cruise in the southwest lagoon of New Caledonia, southwestern Pacific, aimed at examining phytoplankton-bacterioplankton coupling. We measured primary production, including dissolved (DPP) and particulate (PPP) primary production, and bacterial production (Bp, H-3-thymidine method) along a gradient. Regardless of inorganic nutrient concentration, we failed to see a tight coupling over the short term (4 h incubation) between net DPP and BP. A negative relationship between BP and the photosynthetic response to light was observed, leading to a decreased Delta BP:DPP ratio with increasing photon flux density. However, when all the data were compiled, BP was correlated with DPP and total primary production. Assuming bacterial growth efficiencies from 1 to 10%, bacterial carbon demand (BCD) was higher than the supply of DPP in the most oligotrophic sites (up to 400 % of DPP). This implies a weaker linkage between bacteria and phytoplankton than previously observed in other oligotrophic sites and requires external subsidies of dissolved organic carbon. In the coastal sites, characterised by higher inorganic and organic matter concentrations, DPP was estimated as being sufficient to support BCD, and bacteria and phytoplankton are considered to be strongly linked. These results were in contrast to previously published results for temperate and polar coastal regions and highlight the differences of these often understudied tropical systems.