Dissolved inorganic phosphorus, dissolved iron, and Trichodesmium in the oligotrophic South China Sea

Abstract

Dissolved inorganic phosphorus (DIP) concentrations in the oligotrophic surface waters of the South China Sea decrease from ∼20 nM in March 2000 to ∼5 nM in July 2000, in response to seasonal water column stratification. These minimum DIP concentrations are one order of magnitude higher than those in the P‐limited, iron‐replete stratified surface waters of the western North Atlantic, suggesting that the ecosystem in the South China Sea may be limited by bioavailable nitrogen or some trace nutrient rather than DIP. Nutrient enrichment experiments using either nitrate, phosphate or both indicate that nitrogen limits the net growth of phytoplankton in the South China Sea, at least during March and July 2000. The fixed nitrogen limitation may result from the excess phosphate (N:P<16) transported into the South China Sea from the North Pacific relative to microbial population needs, or from iron control of nitrogen fixation. The iron‐limited nitrogen fixation hypothesis is supported by the observation of low population densities of Trichodesmium spp. (<48 × 103 trichomes/m3), the putative N2 fixing cyanobacterium, and with low concentrations of dissolved iron (∼0.2–0.3 nM) in the South China Sea surface water. Our results suggest that nitrogen fixation can be limited by available iron even in regions with a high rate of atmospheric dust deposition such as in the South China Sea.

Diversity and abundance of diazotrophic microorganisms in the South China Sea during intermonsoon

“…This globally important cyanobacterium was clearly a dominant diazotroph in the SCS during the study period. In previous studies in the SCS, Trichodesmium abundances were estimated to be up to 48-280 trichomes per liter Wu et al, 2003). Assuming that one trichome consists of approximately 100 cells (Capone et al, 1997) and that each cell contains one genome, the maximum Trichodesmium abundances in this study were approximately 2 Â 10 3 -6 Â 10 3 trichomes per liter, suggesting that abundances were about one order of magnitude higher than previously reported from the SCS.…”

“…On the basis of the salinity data in the study area, Mekong plume quickly turned southwards with the prevailing currents and had a very limited influence in the study area (M Voss et al, unpublished data). Trichodesmium abundances in the SCS have been proposed to reflect intrusions of the Kuroshio current , and their low abundance and low N 2 -fixation rates in the system were previously suggested to be controlled by Fe limitation (Wu et al, 2003).…”

“…The main water exchange with the Western Pacific Ocean occurs through the Luzon Strait with a sill at the 1900 m depth (Figure 1). The central gyre is permanently stratified and oligotrophic, and N limitation of phytoplankton growth has been reported (Wu et al, 2003), suggesting that the environment is favorable for nitrogen (N 2 ) fixation. Rate measurements suggest that production based on N 2 fixation contributes approximately 10% of total production, with the highest rates measured during the monsoon seasons in winter and summer and the lowest rates during intermonsoon (Voss et al, 2006).…”

Iron biogeochemistry across marine systems – progress from the past decade

“…In their contribution to this special issue, Ye et al (2009) aim to improve the understanding of the impact of dust deposition on Fe bioavailability and marine primary productivity in modeling iron speciation and biogeochemistry at TENATSO (Tropical Eastern North Atlantic Time-series Observatory). Based on recent studies on Fe speciation and the existing model for the Bermuda Atlantic Time-series Study (BATS) (Weber et al, 2007), this model aims at studying the role of dust particles in Fe removal and providing a better description of the sources and fate of organic Fe-binding ligands.…”

“…We follow the structure of the workshop topics, which were: Natural Fe fertilization (Sect. 2, articles: Ardelan et al, 2010;Chever et al, 2010;Duggen et al, 2010;Ye et al, 2009); artificial Fe fertilization (3: Bucciarelli et al, 2010;Chever et al, 2010); Fe inputs into coastal and estuarine systems (4: Breitbarth et al, 2009); Colloidal iron and organic matter (5); Linking biological processes to iron chemistry (6: Breitbarth et al, 2009;Bucciarelli et al, 2010;Hassler and Schoemann, 2009;Steigenberger et al, 2010); and Iron and Climate Change (7: Breitbarth et al, 2010;Rose et al, 2009). Each section concludes with recommendations for future research.…”