Abstract
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 pre- sent 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 cou- pling. We measured primary production, including dissolved (DPP) and particulate (PPP) primary production, and bacterial production (BP, 3 H-thymidine method) along a gradient. Regardless of inor- ganic 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 ΔBP:DPP ratio with increasing photon flux density. How- ever, 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 link- age 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.
Highlights
Dissolved organic matter (DOM) is produced during photosynthesis (Marañón et al 2004) and forms the principal source of new DOM in the open ocean
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
bacterial production (BP) was measured in the dark at the beginning and end of each primary production incubation in order to estimate the degree of linkage between primary production and BP
Summary
Dissolved organic matter (DOM) is produced during photosynthesis (Marañón et al 2004) and forms the principal source of new DOM in the open ocean. It is intuitive that there might be a tight relationship between the production of organic matter by autotrophic processes and the removal of organic matter by heterotrophic processes, in sites isolated from external inputs. There is a general trend of loose linkage in coastal or eutrophic sites and tight linkage in open ocean sites, at least for the temperate and polar environments examined to date (Cole et al 1988, Morán et al 2002a). Far from allochthonous DOM sources, BCD and dissolved primary production (DPP) are often tightly linked, indicating that the supply of carbon from DPP supports BCD, this
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