The effects of spring-neap tide on the phytoplankton community development in the Jiaozhou Bay, China

The development of the phytoplankton community was studied in the Jiaozhou Bay during the spring to neap tide in August2001, through three cruises and a 15 d continuous observation. This investigation indicates that diatom cell abundance increasedsharply following the end of a spring tide, from 9 cells/cm3 to a peak of 94 cells/cm3. The dominant species composition andabundance show a quick species sequence from spring to neap tide, and the dominant species at the start phase is Skeletomenacostatum, then changes to Chaetoceros curvisetus, finally it changes to Eucampia zodiacus. Silicate concentration increasesduring spring tide, as a result of nutrient replenishment from the watersediment interface, its initial average concentration inneap tide is 1.39 mmol/dm3 and reached the peak average concentration of 8.40 mmol/dm3 in spring tide. But the nitrogenconcentration dropped due to dilution by the low nitrogen seawater from the Huanghai Sea, its initial average concentration inneap tide is 67 mmol/dm3 and decreased to the average concentration of 54 mmol/dm3 in spring tide. The degree of siliconlimitation was decreased and phytoplankton, especially diatoms, responds immediately after nutrient replenishment in thewater column. Skeletonmea costatum, as one of the dominant species in the Jiaozhou Bay, shows a quicker response tonutrient availability than Eucampia zodiacus and Chaetoceros curvisetus. It is proposed that dominant species compositionand water column stability synchronously determine the development of phytoplankton summer blooms in the Jiaozhou bay.

Investigation of saltwater intrusion and salinity stratification in winter of 2007/2008 in the Zhujiang River Estuary in China

Saltwater intrusion is a serious environmental problem in the Zhujiang River Estuary (ZRE),which threatens the water supply of fifteen million people. The hydrological observations as well as meteorological and tidal forcing in the winter of 2007/2008 were analyzed to examine the saltwater intrusion in the ZRE.The observational results suggest that the maximum vertical difference of salinity can reach 10 in the Humen Channel during neap tide,but is very small in the Hengmen Channel. The vertically averaged salinity from time series stations during spring tide is higher than that during neap tide.A three-dimensional finite difference model was developed based on the environmental fluid dynamic code (EFDC) to study the mechanism of saltwater intrusion and salinity stratification in the ZRE.By analyzing the salt transport and the temporal variation of saltwater intrusion,the authors found that the net salt transport due to the estuarine circulation during neap tide was more than that during spring tide.This caused salt to advance more into the estuary during neap tide.However,saltwater intrusion was stronger during spring tide than that during neap tide because the spring-neap variation in salt transport was small relative to the total length of the saltwater intrusion.The physical mechanism causing this saltwater intrusion was investigated by a series of sensitivity experiments,in order to examine saltwater intrusion in response to river discharge and winds.The freshwater source was a dominant influencing factor to the saltwater intrusion and controlled salinity structure,vertical stratification and length of the saltwater intrusion.The prevailing northeast monsoon during winter could increase the saltwater intrusion in the ZRE.Though the southwest wind was uufavorable to saltwater intrusion during spring tide,it could increase stratification and saltwater intrusion during neap tide.