MEPS 474:67-88 (2013)  -  DOI: https://doi.org/10.3354/meps10075

ABSTRACT: Since in situ data are sparse, we used a 1D coupled physical−biological model to investigate the complex connection between atmospheric forcing, associated physical water column structure, and primary production within the entire seasonal cycle in the central Beibu Gulf from 2000 to 2010. The results were corroborated by remotely measured sea surface temperature (SST) and surface chlorophyll concentrations in combination with in situ measurements (temperature and chlorophyll) carried out during 2 German−Chinese cruises (September/October 2009 and December 2011). Over the 11 yr period, 87% of the observed variance in daily SST was captured by the model. The model indicates that the central Beibu Gulf is generally unaffected by water masses from the South China Sea and the Beibu Gulf coastal regions. Thus, most of the primary production is locally generated. A strong stratification and a deep chlorophyll maximum develop during summer, vanishing in autumn when the stratification breaks down due to the change from the weak southwest to the stronger northeast monsoon. Deep mixing brings a higher nutrient supply to the surface waters, leading to a phytoplankton bloom which lasts on average about 6 mo. Along with the semi-annual wind system change, temporally short, strong wind events affect primary productivity in the region. Typhoon crossings can enhance primary production by up to 275% relative to the summer background chlorophyll concentration at the surface according to our modeling results. These events play an important role in primary production and the corresponding biogeochemical cycling in this oligotrophic region during the summer and the autumn low-productive periods.

KEY WORDS: Phytoplankton bloom · Model · Shelf sea · Typhoon · Tidal forcing · Remote sensing · Gulf of Tonkin