Indirect influence of the summer monsoon on chlorophyll–total phosphorus models in reservoirs: a case study

This study examines how the Asian monsoon influences relationships between chlorophyll-a (Chl) and total phosphorus (TP), by means of a case study on Taechung Reservoir, Korea. A log–log regression model of Chl vs. TP was found to be quadratic relations, with Chl approaching an asymptote above 50 μg l−1 TP. This is similar to the pattern previously reported for temperate lakes in Europe and North America. However, viewing the data by season, a highly significant linear model was obtained during the pre- and post-monsoon periods (R2≥0.83), but not during the summer monsoon (R2=0.004). In the monsoon period, samples >50 μg l−1 TP (23% of the total observations) had fourfold lower Chl yields per unit TP (average=0.17) compared to samples from the same TP range in the other seasons (average=0.76) and were strongly correlated (r=0.97, n=17) to non-volatile suspended solids (NVSS). Yield of Chl per unit TP also increased significantly (P<0.05) in the mid- and down-lake zones, compared to the headwater zone. Residual analysis indicated that a water residence time (WRT) of <8 days in the headwater zone had a significant antagonistic effect on Chl, perhaps due to light limitation caused by inorganic suspended solids, which exceeded 10 mg l−1 during the monsoon. The result indicates that the seasonal monsoon can indirectly influence phytoplankton responses to nutrients, and should be taken into consideration in developing predictive eutrophication models in regions that experience this type of seasonal pattern.