The Changjiang (Yangtze River) ranks the 3rd longest and 5th biggest rivers by discharge volume in the world. Thus, the Changjiang Estuary, as one of the largest typical riverine-marine systems, presents freshwater-seawater mutual interactions with the complicated distributions of salinity and other physical-chemical properties (e.g., river discharge, tide, turbidity, nutrients, and sedimentation). Phytoplankton community in the estuary has been investigated extensively, due to its importance in the estuarine ecosystem. However, most studies focused on the areas outside of the estuary mouth or around the mouth so far, less refer to the inner estuary (tidal freshwater reaches). In the present study, we collected the phytoplankton samples in the tidal freshwater estuary of Changjiang using a 76 μm plankton net in June and August 2009, in order to explore the distribution pattern of phytoplankton community and its relationship with the extremely changed environmental factors. Besides, we examined the long-term changes in phytoplankton community in the Changjiang Estuary combined with the historical data.
Based on the two cruises, a total of 239 species (included the varietas and forma) belong to 6 phyla and 99 genera were identified, including 123 diatom, 42 chlorophyte, 42 cyanobacteria, 19 dinoflagellate, 9 euglenophyte, and 4 xanthophyte species. Cyanobacteria absolutely dominated in the tidal freshwater zone and followed by the diatoms, taken together contributed above 95% in total abundance. The dominants mainly consisted of cyanobacterial taxa (e.g., Oscillatoria spp., Microcystis spp., Anabaena flos-aquae, Coelosphaerium kuetzingiarum, and Phorimidium spp.),. although including two diatom species (i.e., Skeletonema spp. and Aulacoseira granulata), The freshwater species were principally located in inner estuary with the salinity below 0.5, while the marine species were mostly found outside of the river mouth with the salinity above 13. However, the brackish species dominated around the river mouth under the stress of extreme salinity variation. The simultaneous-observed inflow velocity of runoff increased from 38 900 (in June) to 44 350 m³/s (in August) at Datong resulted in dissolved inorganic nitrogen (from 113.87 μmol/L to 135.45 μmol/L) and silicate (from 93.84 μmol/L to 117.19 μmol/L) concentrations amplification, especially the dissolved inorganic phosphorus, which increased from 1.77 μmol/L to 3.28 μmol/L. Driven by the increasing temperature (from 23.18 ℃ to 28.15 ℃) and nutrient levels, the algal density in August (347.75×10⁴ cells/m³) was much higher than that in June (204.19×10⁴ cells/m³), although no significant difference between them was found. According to the non-metric multidimensional scaling and analysis of similarity, phytoplankton community presented significant spatio-temporal differences in the tidal freshwater zone. The Pearson correlation showed that: in June, both the cyanobacteria and phytoplankton abundance were significantly (P<0.05) and positively related to the temperature but negatively correlated with the salinity; in August, the cyanobacteria abundance was significantly (P<0.05) and positively related to the temperature and nutrients but negatively correlated with the salinity, while the diatom and phytoplankton did not significantly associated with the environmental factors. Comparing with the data since 1980s, the net-phytoplankton abundance increased evidently and the dominant species changed from the diatoms (i.e., Skeletonema, Melosira, and Coscinodiscus) to cyanobacteria (e.g., Oscillatoria, Microcystis, and Anabaena) during wet season.