Evidence for the Primary Role of Phytoplankton on Nitrogen Cycle in a Subtropical Reservoir: Reflected by the Stable Isotope Ratios of Particulate Nitrogen and Total Dissolved Nitrogen

Abstract

Knowledge about the primary factor controlling stable isotope ratios of particulate nitrogen (δ15NPN) and total dissolved nitrogen (δ15NTDN) in a subtropical reservoir can improve the understanding of regional and global nitrogen cycles. Taking Lianhe Reservoir as a representative subtropical reservoir, we studied the spatial and temporal distributions of δ15NPN andδ15NTDN and their relationships with the surrounding physicochemical factors and phytoplankton. The results showed that variations in δ15NPN and δ15NTDN followed seasonal thermal cycles. The values of δ15NTDN were inversely proportional to those of δ15NPN. PCA showed that phytoplankton cell density and pH were the primary drivers of the variation of δ15NPN (45.2%). The primary factors influencing δ15NTDN were Chl a and phytoplankton cell density, which both indicated phytoplankton biomass. We also determined that the dominant species was Microcystis densa during the thermal stratification period and Staurodesmus aristiferus during the mixing period. Laboratory experiments showed that δ15NPN values in both M. densa (from 19.5 to 14.6‰) and S. aristiferus (from 19.4 to 16.0 ‰) media decreased significantly as the algal cells grew. Furthermore, the δ15NTDN values increased from 4.9 to 7.9‰ and from 4.7 to 6.9‰ in M. densa and S. aristiferus media, respectively, when the δ15NPN values decreased. These experimental results were consistent with field investigation results and indicated that variations in δ15NPN and δ15NTDN were mainly controlled by phytoplankton cell density, especially the cell density of the dominant species, in both the thermal stratification and mixing periods. The results also suggested that cell density, not phytoplankton species, was the key factor regulating the distribution of nitrogen stable isotopes. These results together indicated that phytoplankton cell density is the primary factor in the regulation of nitrogen stable isotope composition and that its influence is greater than that of other physical and chemical factors. This study provided detailed information supporting the primary role of phytoplankton in the nitrogen geochemical cycle and improved the understanding of biochemical processes in natural subtropical reservoirs.

A Kinetics Model for Predicting Microcystis Growth Based on the Synergistic Effect of Nitrogen and Phosphorus on the Growth of Microcystis densa (Cyanobacteria)

“…The specific growth rate of M. densa (μ A ) associated with the synergistic effects of N and P can be expressed in the form: (4) ...where μ A is the specific growth rate of M. densa responding to both N and P substrates. Once the concentrations of nitrogen (C N ) and phosphorus (C P ) in the water column are known, the specific growth rate of M. densa cells (μ A ) can also be calculated to determine the growth status of M. densa.…”

“…Although cells using NO 3 must induce the release of nitrate reductase and expend extra energy, NO 3 can be more readily accumulated within cells than other forms of nitrogen [11]. Comparing to NO 3 -, ammonium (NH 4 + ) can be transported directly in algal cells without additional energy, but NH 4 + concentration in natural water body is not very high, and excess cellular NH 4 + is toxic to algal cells [12]. Therefore, NO 3 is still considered the primary preferred nitrogen source for phytoplankton in the aquatic ecosystem.…”

Temporal and spatial distribution of functional nitrogen genes associated with thermal stratification in the subtropical reservoir

“…Nitrogen is the main driving factor of water eutrophication. Nitrogen pollution has become a serious environmental problem in the global aquatic ecosystem (Cai et al 2019). Nitrogen pollution in water includes endogenous pollution, atmospheric deposition, point source pollution, and non-point source pollution (Small et al 2013;Miyazako et al 2015;Li et al 2014;Taylor et al 2005).…”

Stable C and N Isotope Composition of Suspended Particulate Organic Matter in the Neva Estuary: The Role of Abiotic Factors, Productivity, and Phytoplankton Taxonomic Composition

“…A number of studies are devoted to the difference in the isotopic composition of phytoplankton, macroalgae, mangrove forests, and coastal swamps [2][3][4][5][6][7][8][9]. The important role of phytoplankton in the cycles of stable nitrogen isotopes in subtropical freshwater reservoirs and carbon in subtropical estuaries has been shown [10,11].…”