River phytoplankton: a spatially dataset
River phytoplankton data were collected as a part of the Water Pollution Control Program in the Huaihe River Basin (3055′-3636′N, 11155′-12125′E) (HRB). The basin, with a drainage area of 270,000 km2, is located between the Yangtze River and the Yellow River in China (Wang and Xia 2010). It forms a geographical separation between northern and southern China. Phytoplankton samples were collected from 217 randomly selected sites during the low-water period in May 2013 (Fig. 1). Detailed field and lab methods can be found in Zhu et al (2015). In each site, three cross-section transects were established. We measured in situenvironmental variables including water temperature (WT), pH, conductivity (Cond), turbidity and total suspend solids (TSS) using a portable HACHCDC40105. We also measured Secchi depth (SD), water depth, stream width, water velocity and elevation. Spectrophotometer (DR5000) was used to measure total phosphorus (TP), total nitrogen (TN) and chemical oxygen demand (CODMn) according to standard methods(NEPAC 2002).
For phytoplankton, a 1L sample from the 0.5 m depth below surface was collected from three cross-section transects, respectively. After complete mixing, a 1L sample was preserved with 1% Lugol’s iodine solution immediately in the field and concentrated to 50 mL after sedimentation for 48 h. After complete mixing, 0.1 mL of the concentrated sample was counted directly in a 0.1 mL counting chamber under a microscope at 400× magnification. Phytoplankton was identified according to the reference book by Hu & Wei (2006). At least 400 algal units were counted in each sample. Phytoplankton biomass was expressed as wet biomass and was estimated for individual species by assigning a geometric shape similar to the shape of each phytoplankton species (Hillebrand 1999).