Nutrient dynamics and their interaction with phytoplankton growth during autumn in Liaodong Bay, China

Abstract

Discharges of domestic sewage, industrial wastewater, and runoff of agricultural fertilizer have caused severe nutrient pollution and eutrophication in many estuaries and coastal areas around the world, especially in the Bohai Sea of China and its inner bay, Liaodong Bay. Subsequent to studies of this bay during the summer of 2013, an investigation was conducted in the autumn (dry season) to better understand the interactive effects of light, temperature, and nutrient concentrations on phytoplankton growth in the bay. Analyses of long-term data showed that dissolved inorganic nitrogen concentrations had increased roughly sixfold to eightfold since the 1980s as a result of nitrogen fertilizer use, pollution from industrial wastewater and domestic sewage, and loss of wetlands and their water purification service. In contrast, phosphate and silicate concentrations have decreased by factors of 2 and 5, respectively, during the past half century because of anthropogenic reductions of sediment loads (e.g., the construction of check dams and reservoirs and large-scale vegetation restoration projects). We found no evidence that phytoplankton growth was limited by nitrogen or phosphorus. Plots of nitrate and silicate concentrations versus salinity indicated conservative mixing in both cases. Analogous plots of ammonium and phosphate revealed a net uptake and net input, respectively. Phytoplankton uptake likely accounted for the net uptake of ammonium, and desorption of phosphorus from suspended particulate matter for the net input of phosphate. The major factors controlling phytoplankton growth in this study were light availability and temperature. Nearly all incoming photosynthetically active radiation was attenuated in the water column, and less than 1% of the light energy was absorbed by phytoplankton. Comparisons of this study with previous work in Liaodong Bay during the summer and similar case studies in contrasting regions of the world revealed a general pattern of regional maxima of phytoplankton biomass (or even harmful algal blooms) and changes of community structure associated with light availability and temperature variations in the presence of growth-rate-saturating nutrient concentrations.