Autumn-to-spring turnover and shifts in phytoplankton community assembly signals in Shengjin Lake: The ecological context of wintering waterbird aggregation

Water temperature (WT), dissolved oxygen (DO), and electrical conductivity (EC) were measured in situ using a Hach HQ40d portable multiparameter meter. Turbidity (Turb) was measured with a Hach 2100Q turbidimeter, Secchi depth (SD) was measured using a Secchi disk, and water depth (WD) was recorded at each site. At each site, 2 L of water was collected from 0.5 m below the surface for analysis of total ammonia nitrogen (TAN), total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chl a). Chlorophyll a was determined after extraction with 90% acetone. Total phosphorus was measured using the ammonium molybdate spectrophotometric method, TN by alkaline potassium persulfate digestion followed by ultraviolet spectrophotometry, and TAN by Nessler's reagent spectrophotometry. A separate 1-L water sample was collected for phytoplankton quantification and preserved with 15 mL of Lugol's iodine solution. After sedimentation, the sample was concentrated to 30 mL, and a 0.1-mL subsample was examined and counted at 400× magnification using an Olympus BX53 light microscope. Identification was based primarily on cellular morphology, including cell structure, size, shape, and chloroplast characteristics.