A 2011 drought event affecting distribution of nutrients and chlorophyll in the Zhujiang River estuary

During the summer of 2011, a severe drought event occurred throughout the Zhujiang (Pearl) River Basin of South China. This decreased runoff into the river, resulting in increased salinity and reduced suspended substance. To examine the effects of this extreme drought on the distribution of nutrients and chlorophyll, we compared two surveys from 2006 and 2011. Results show that dissolved inorganic nitrogen concentration did not change from 2006 to 2011, whereas the proportions of NO (2) (-) and NH (4) (+) to DIN in 2011 increased. PO (4) (3-) concentration was lower in 2011 than in 2006, whereas there was no difference in SiO (3) (2-) concentration between these years. Correlation coefficients of salinity with levels of NO (3) (-) , NO (2) (-) , NH (4) (+) , PO (4) (3-) and SiO (3) (2-) in 2011 were all much higher than those in 2006, suggesting greater conservation of dissolved nutrients during the extreme drought event. Furthermore, calculated amounts of regenerated nitrate and phosphorus and their proportions to observed nutrients in 2011 were much lower than in 2006, indicating that nutrient regeneration decreased during the extreme drought period. Mean concentration of chlorophyll a (Chl-a) was considerably higher in 2011 than in 2006, and a harmful algal bloom of Cochlodinium geminatum was observed in the estuary, owing to water stagnancy and lower turbidity as a consequence of drought. Therefore, the extreme drought event in 2011 changed the composition ratio of nutrients, enhanced nutrient conservative behavior, and reduced nutrient regeneration. This affected some key ecological processes in the estuary.