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Satellite estimation of chlorophyll-a (Chl-a) concentration in the Changjiang Estuary is challenging for ocean-colour retrieval algorithms. The Changjiang Estuary is characterized by suspended-sediment-dominated waters in the mouth and optically complex case 2 waters offshore of the mouth. Satellite ocean-colour products show that high-sediment loads of estuarine waters can cause overestimations or invalid flags of Chl-a concentration. A synthetic chlorophyll index (SCI) was designed for extracting chlorophyll information and for minimizing the influence of sediments on the remote-sensing reflectance spectrum. An SCI algorithm, a quadratic polynomial function of the SCI versus Chl-a concentration, was applied to the estimation of Chl-a concentration from Medium Resolution Imaging Spectrometer (MERIS) images. The overestimation of Chl-a concentration was corrected. The SCI algorithm has applications for MERIS estimation of Chl-a concentration in turbid waters with a moderate to high suspended-sediment concentration. [ABSTRACT FROM AUTHOR]

Abstract: During four surveys at spring and neap tides in July and November 2005, continuous observations were conducted at four stations adjacent to the Changjiang (Yangtze River) mouth. The observation times lasted for 26h that covered two consecutive semidiurnal cycles. Resuspension events and subsequently enhancements of suspended particulate matter (SPM) were commonly observed within a tidal cycle. Although nutrients (SiO3 2−, NO3 −, and PO4 3−) were primarily governed by salinity, their statistically significant correlations with SPM could always be extracted after partial correlation analysis. Three parameter (salinity, SPM, nutrients) regressions generally produced better results of simulating nutrient concentrations than two parameter (salinity and nutrients) regressions, although compared to the latter, the former R 2 values were elevated by no more than 13%. The partial correlation between SPM and a specific nutrient could be either positive or negative in different surveys, suggesting SPM influenced the nutrients in different ways under various conditions. The minor (albeit statistically significant) impacts of highly dynamic SPM on nutrients might be ascribed to the short duration time of resuspension events and estuarine mixing process, together with the complex nature of circulation in the Changjiang plume seawater. [Copyright &y& Elsevier]

Using a box-sampler, surface and core sediment samples were collected from the Bohai and Yellow Seas in 1998–1999. The sediments were sequentially extracted to determine six phosphorus fractions, and non-sequentially extracted for total P (TP) and inorganic P (IP), where the difference between TP and IP was assumed to be organic P (OP). It was found that more than 70% of the total P is inorganic P.The concentrations of phosphorus were high near the river mouth, reflecting the influence from the Huanghe and Changjiang sediment loads. The phosphorus levels in the Bohai Sea are higher than those in the Yellow Sea, which is related to the sediment sources and distribution of chlorophyll a in these areas. Concentrations of TP in the Bohai and Yellow Seas are comparable to the other worldwide coastal areas. Among the six forms of phosphorus, phosphorus distribution ranges were 0.8–2% for loosely sorbed P, 3–10% for iron-bound inorganic P, 4–19% for leachable organic P, 3–15% for authigenic apatite, 35–66% for detrital apatite and 14–36% for refractory organic P. Potentially bioavailable phosphorus accounts for a small amount of TP, while phosphorus load associated with eroded soil may be larger than the anthropogenic load. The regeneration of dissolved P from sediments is a slow process, which could play a role in maintaining low phosphate levels in the seas. Most of the phosphorus was regenerated in a water column, and almost all of the remaining phosphorus was buried in the sediments after its accumulation. The high values of P burial efficiencies in the study areas were related to low benthic phosphate flux, which is due to the high percent of detrital apatite that is not bioavailable and the high sediment accumulation rate. [Copyright &y& Elsevier]

The variations of nutrients in pore water of salt marsh sediment were surveyed in the middle intertidal zone of Chongming Dongtan during August 2007 to May 2008 to identify plant impact on nutrients distribution. The results show that NH4(+) -N and PO4(3-) -P concentrations are lower in pore water of Spartina alterniflora and Phragmites australis zones than in bare flat, and specially, NH4(+) -N concentrations in summer and autumn decrease by one more orders of magnitude. Compared to winter, nutrients concentrations are obviously higher during the period of plant growth, and plant biomass is clearly correlative to nitrogen and phosphorus. Vegetation growth influences nitrogen content intensively. NH4(-) -N concentrations in Spartina alterniflora and Phragmites australis zones are 44.21 and 74.38 micromol x L(-1) respectively, distinctly lower than that in bare flat and Scirpus mariquete zone (340.14 and 291.87 micromol x L(-1) respectively). Moreover, NO(x)(-) -N concentration is one to two order(s) of magnitude lower than NH4(+) -N, and its highest value exists in Phragmites australis zone (5.94 micromol x L(-1)). The results of molecule diffusive flux of nutrients in the surface sediment-overlying water interface indicate that marsh sediment is the source for SiO3(2-) -Si, NH4(+) -N and PO4(3-) -P, and the rank for NO(x)(-) -N (NO3(-) -N + NO2(-) -N), and NO(x)(-) -N flux from overlying water to sediment [16.23 micromol x (m2 x h)(-1)] is higher than NH4(+) -N flux from sediment to overlying water [15.53 micromol x (m2 x h)(-1)]. Vegetation growth accommodates nutrient structure of the estuarine ecosystem by affecting sediment-water interface mass flux and nutrient ratios in pore water and overlying water.

Two cruises were carried out in the Yangtze River Estuary and its adjacent sea (29 degrees 30' - 32 degrees 00'N, west of 123 degrees E) in July (summer) and November (autumn), 2005. A total of 345 phytoplankton species, including 43 species causing red tide, were identified. Skeletonema costatum was the dominant species. The average cell abundance was lower in July (5.48 x 10(4) cells L(-1)) than in November (2.70 x 10(5) cells L-(-1)), but the average chlorophyll a concentration was higher in July (2.34 mg x m(-3)) than in November (1.32 mg x m(-3)). The average diversity index (H) was higher in July (1.51) than in November (0.86), as was average evenness (J) (0.59 and 0.34, respectively). Spatial distribution of phytoplankton featured distinct regionality, and the seasonal variation was controlled by factors such as water source, monsoon, nutrient, suspended matter, etc., and the diurnal variation mainly correlated with tide and stratification. At the same time, an evaluation of long-term monitoring data (1996 - 2005) showed that phytoplankton community structure had been changed. Long-term unbalance of N/P ratio caused dinoflagellates to increasingly dominate the phytoplankton population, and led to an increasing frequency of red tide caused by dinoflagellates. Controlling nutrient ratios is more important than just controlling terrigenous contamination input in improving sea area environmental status.

Profiles of nutrients (i.e., NH4+, NO2-, NO3-, PO4(3-), and SiO3(2-)) in porewater of sediment were investigated monthly in the intertidal flat of Chongming Dongtan in Changjiang (Yangtze River) estuary during March 2005 to February 2006. The results indicate that concentrations of NH4+ and SiO3(2-) in porewater are always in the range of 200 micromol/L to 500 micromol/L and show different patterns of distribution profile among high, middle, and low marshes. Compared to high and middle marsh, low marsh, without vegetation coverage and of coarser grain size, generally contains higher NH4+ and lower SiO3(21) concentrations. SiO3(2-) concentrations are predominantly determined by seasonal temperature change, but variations of NH4+ are more complicated and prone to exhibit lower values in spring and summer when the vegetation above show relatively high growth rate. Concentrations of NO2- + NO3- and PO4(3-) in porewater are commonly much lower than NH4+ and SiO3(2-) by 2 to 3 orders of magnitude, however, due to factors such as bioturbation, the concentrations in top sediment may be higher than 10 micromol/L. The relatively high NO3- concentration in top sediment,together with the rapid transfer from oxygenic to anoxic environment, make it possible that significant denitrification happens, which may draw its remarkable influence on the biogeochemical cycles of nutrient elements in the land-sea interaction area in Changjiang (Yangtze River) estuary.