The spatial and temporal dynamics of physical variables, inorganic nutrients and phytoplankton chlorophyll a were investigated in Xiangxi Bay from 23 Feb. to 28 Apr. every six days, including one daily sampling site and one bidaily sampling site. The concentrations of nutrient variables showed ranges of 0.02-3.20 mg/L for dissolved silicate (Si); 0.06-2.40 mg/L for DIN (NH 4 N + NO 2 N + NO 3 N); 0.03-0.56 mg/L for PO 4 P and 0.22-193.37 μg/L for chlorophyll a, respectively. The concentration of chlorophyll a and inorganic nutrients were interpolated using GIS techniques. The results indicated that the spring bloom was occurred twice in space during the whole monitoring period (The first one: 26 Feb.-23 Mar.; the second one: 23 Mar.-28 Apr.). The concentration of DIN was always high in the mouth of Xiangxi Bay, and PO 4 P was high in the upstream of Xiangxi Bay during the whole bloom period. Si seems no obvious difference in space in the beginning of the spring bloom, but showed high heterogeneity in space and time with the development of spring bloom. By comparing the interpolated maps of chlorophyll a and inorganic variables, obvious consumptions of Si and DIN were found when the bloom status was serious. However, no obvious depletion of PO 4 P was found. Spatial regression analysis could explained most variation of Chl-a except at the begin of the first and second bloom. The result indicated that Si was the factor limiting Chl-a in space before achieved the max area of hypertrophic in the first and second bloom period. When Si was obviously exhausted, DIN became the factor limiting the Chl-a in space.Daily and bidaily monitoring of Site A and B, representing for high DIN: PO 4 P ratio and low DIN:PO 4 P ratio, indicated that the concentration of Si was decreased with times at both site A and B, and the dramatically drop of DIN was found in the end monitoring at site B. Multiple stepwise regression analysis indicated that Si was the most important factor affect the development of spring bloom both at site A and B in time series.
We investigated the macroinvertebrate community structure in Three-Gorges Reservoir during a three-year period after the reservoir became operational. Comparison with data of previous authors obtained before the damming showed that the benthic community changed drastically in the reservoir. Oligochaetes and chironomids dominate the present community. An apparent annual cycle in the benthic community (expressed as richness, density and community type) was found in the second year, lagging one year behind the annual cycle of transparency. After the second year, a Nais-Polypedilum community type, occurring in winter and spring with low inflow discharge and high transparency, was followed by a Limnodrilus community type that occurred in autumn and summer under contrasting conditions. Despite river regulation and damming, it appeared that macroinvertebrates in subtropical canyon-shaped reservoirs remained influenced by floods.
We evaluated a 4-year data set (July 2003 to June 2007) to assess the trophic state and its limiting factors of Three-Gorges Reservoir (TGR), China, a large subtropical reservoir. Based on Carlson-type trophic state index (TSI)(CHL), the trophic state of the system was oligotrophic (TSI(S) < 40) in most months after the reservoir became operational, although both TSI(TP) and TSI(TN) were higher than the critical value of eutrophic state (TSI(S) > 50). Using Carlson's (1991) two-dimensional approach, deviations of the TSI(S) indicated that factors other than phosphorus and nitrogen limited algal growth and that nonalgal particles affected light attenuation. These findings were further supported by the significant correlation among the values of TSI(CHL) - TSI(SD) and nonvolatile suspended solids and water residence time. The logarithmic model showed that an equivalent TSI(CHL) and TSI(SD) could be found at tau = 54 days in the TGR (Fig. 7). Accordingly, nonalgal particulates dominated light attenuation and limited algal biomass of the reservoir when tau < 54 days.
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