During the summer of 2011, a severe drought event occurred throughout the Zhujiang （Pearl） River Basin of South China. This decreased runoffinto 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 20ll. Results show that dissolved inorganic nitrogen concentration did not change from 2006 to 2011, whereas the proportions of NOR and NH4＋ to DIN in 2011 increased. PO4 3- concentration was lower in 2011 than in 2006, whereas there was no difference in SiO3 2- concentration between these years. Correlation coefficients of salinity with levels of NO3-, NO2-, NH4＋, PO4 3- and SiO3 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.
The effects of CO2 enrichment on various ecophysiological parameters of tropical seagrass Thalassia hemprichii （Ehrenb.） Aschers were tested. T. hemprichii, collected from a seagrass bed in Xincun Bay, Hainan island of Southern China, was cultured at 4 CO2（aq） concentrations in flow-through seawater aquaria bubbled with CO2. CO2 enrichment considerably enhanced the relative maximum electron transport rate （RETRmax） and minimum saturating irradiance （Ek） of T. hemprichii. Leaf growth rate of CO2- enriched plants was significantly higher than that in unenriched treatment. Nonstructural carbohydrates （NSC） of T. hemprichii, especially in belowground tissues, increased strongly with elevated CO2（aq）, suggesting a translocation of photosynthate from aboveground to belowground tissues. Carbon content in belowground tissues showed a similar response with NSC, while in aboveground tissues, carbon content was not affected by CO2 treatments. In contrast, with increasing CO2（aq）, nitrogen content in aboveground tissues markedly decreased, but nitrogen content in belowground was nearly constant. Carbon： nitrogen ratio in both tissues were obviously enhanced by increasing CO2（aq）. Thus, these results indicate that T. hemprichii may respond positively to CO2-induced acidification of the coastal ocean. Moreover, the CO2-stimulated improvement of photosynthesis and NSC content may partially offset negative effects of severe environmental disturbance such as underwater light reduction.
