In recent decades, increasing frequency and intensity of saltwater intrusion in the Modaomen Channel has threatened the freshwater supply in the surrounding cities of the Pearl River Estuary, and ulteriorly changed the environmental conditions of the estuarine waters. Phytoplankton biomass, primary production (PP) and species composition, as well as hydrological and chemical parameters were examined along a downstream transect in the Modaomen Channel during neap tide (NT) and spring tide (ST), when a strong saltwater intrusion event occurred in late September, 2011. A total of 46 species phytoplankton were identified, including Bacillariophyta (25 species), Dinoflagellate (14 species), Chlorophyta (4 species), Cyanophyta (2 species) and Euglenozoa (1 species). The dominant species were shifted from freshwater diatoms (e.g., Melosira granulata and Melosira granulata var. angustissima) in the upper reaches to saline water diatoms (e.g., Skeletonema costatum and Coscinodiscus sp.) in the river mouth. Generally, phytoplankton density, biomass (chl-a) and PP decreased from the upper to lower reaches along the channel, and were significantly higher in NT than those of ST. There was a shift from large-sized phytoplankton (>20 μm) in the upper reaches to relative small-sized cells (5–20 μm) in the lower reaches. Compared to NT, low discharge and flow velocity, coupled with strong easterly winds during ST specially aggravated saltwater intrusion further to the upstream (~50 km from the estuary). The intruded saltwater diluted nutrients, N/P ratios, chl-a, and phytoplankton abundances, and thereby led to a decline in PP during ST.
2À 4 ) were investigated over a complete neap tide in the estuary of Pearl River, China. Obvious saltwater intrusion was observed at the lower reaches of our survey section: surface water salinity varied from 2.00% to 17.80%, and bottom water salinity varied from 5.59% to 22.13%. The calculated stratification index indicated that the lower reaches had intense stratification during the present neap tide. Obvious dilution of NO and PO 3À 4 concentration increased with increasing water salinity. Investigation results revealed that saltwater intrusion had a significant effect on nutrient structure and probably alleviated a phosphorus-limiting situation in the estuary ecosystem. All these changes were induced to some degree by tidal currents and their associated sediment-agitation process and had a profound effect on the estuary ecosystem.
ADDITIONAL INDEX WORDS:Saltwater intrusion, stratification, nutrient structure, Modaomen.
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