Spatio-temporal variations of primary productivity and phytoplankton community composition in the Nakdong River, Korea

Abstract

Primary productivity and phytoplankton community composition, which are sensitive to environmental changes reflect the nutritional status of aquatic environments. Anthropogenic influences, such as excessive nutrient inputs originated from agriculture and livestock and increased residence time due to construction of weirs, accelerate algal blooms and deteriorate water quality. In particular, the proliferation of cyanobacteria which produce toxins is of global concern in recent years. This study was conducted 1) to identify environmental factors influencing primary production; 2) to determine variations in phytoplankton community composition; and 3) to evaluate distributions of cyanobacteria during summer (July and August). The water samples were collected from the Nakdong River once or twice a month from April to October 2018. Primary productivity was measured through in-situ incubation experiment using 13C tracer. The particulate organic carbon of the water samples was analyzed by elemental analyzer coupled with isotope ratio mass spectrometry (EA-IRMS). The CHEMTAX program based on pigment analysis by high-performance liquid chromatography (HPLC) was used to estimate the relative contribution of phytoplankton classes. Phycocyanin was measured by spectrophotometer. Primary production was higher in spring (1129 ± 1136 mg C m-2 d-1) and summer (1062 ± 814.2 mg C m-2 d-1) than in autumn (180 ± 220 mg C m-2 d-1). The result of principal component analysis (PCA) indicated that the environmental factors influencing primary production were different according to season. The major factor controlling primary production was light intensity. The contribution of phytoplankton communities calculated by the CHEMTAX program showed good agreement with those of microscopy. Diatoms (70% ± 30%) were dominant in spring, with increasing contributions of chlorophytes (27% ± 13%) and cyanobacteria (34% ± 26%) in summer. Phyocyanin highly correlated (r = 0.844, p < 0.01) with contribution of cyanobacteria estimated by the CHEMTAX program, implying a useful indicator for cyanobacterial biomass.