Long-term variation of phytoplankton biomass and physiology in Taihu lake as observed via MODIS satellite.

Estimation of phytoplankton biomass (noted as phytoplankton carbon, Cphyto) and evaluation of phytoplankton physiology is central to the estimation of primary productivity and the carbon cycle. This issue has been widely considered in oceans but not in inland water. Here, we develop experiential and semi-analytical models, which validated by independent in situ measurement data, respectively, to derive Cphyto and phytoplankton absorption coefficient at 675 nm (aph(675)) from MODIS. The effects of nutrients and temperature on the seasonal variation of phytoplankton physiology were assessed through a novel proxy of Cphyto to aph(675) ratio (Cphyto/aph(675)) over the Lake Taihu, the third largest lake in China. Significant seasonal climatological cycles of Cphyto, aph(675) and Cphyto/aph(675) were observed in Lake Taihu, especially in Meiliang Bay and Zhushan Bay, where algal blooms occur frequently. The highest Cphyto and aph(675) values were observed in summer due to the growth of phytoplankton biomass and chlorophyll-a concentration. Lower values were observed in winter and spring, which are characterized by relatively high total nitrogen levels and low irradiance, owing to the low temperature astricts the algae growth. However, the Cphyto/aph(675) shows an opposite trend compared to Cphyto and aph(675), which have high values in winter and low values in summer. The analysis of Cphyto, aph(675) and Cphyto/aph(675) with total phosphorus (TP) levels and temperature indicates that TP are the main positive driver of the increase in Cphyto and aph(675) and negatively regulate Cphyto/aph(675). Warming promotes an increase in Cphyto and aph(675) and restricts Cphyto/aph(675) in summer. Biomass and nutrient levels are the primary drivers of the decrease of Cphyto/aph(675) in such a typical eutrophic lake. The results present some new findings compared to previous oceanic studies and expand our knowledge in the study of phytoplankton biomass and physiology in eutrophic lakes.