Long-term trends in nitrogen and phosphorus concentrations in the Hii River as influenced by atmospheric deposition from East Asia

Abstract

Deposition of atmospheric nutrients is known to alter oligotrophic ecosystems such as the open ocean, but the role of such nutrients in the further deterioration of eutrophic aquatic ecosystems is largely unknown. The Hii River watershed in Japan encompasses lagoons that have been eutrophic since 1980s. This study examined the atmospheric deposition of nitrogen and phosphorus in the Hii River watershed and the nitrogen and phosphorus concentrations in the Hii River over an 11-yr period. Total nitrogen (TN) concentrations of both precipitation and river water were significantly higher in cold months (November-March) than in warm months (April-October). Most of the TN was nitrate, which suggests that the TN source was atmospheric nitrogen from East Asia transported by seasonal winds. In contrast, total phosphorus (TP) concentrations of both precipitation and river water were significantly higher in warm months. Over time, the TN concentration in the river water showed a decreasing trend although the trend was not significant, while the TP concentration increased significantly. This was attributed to an increase in the atmospheric deposition of TP originating from East Asia since 2000. The increase in the deposition of atmospheric phosphorus might also have increased phosphorus leakage from the soils. In this work, the TN : TP ratios for both atmospheric deposition and river water indicate that changes in atmospheric nutrient deposition affect loading rates and N : P ratios. These changes, especially the latter, could have significant ecological effects in eutrophic systems by lowering the N : P ratios, which could induce cyanobacteria blooms.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

“…The cascades of effects due to anthropogenic shifts in N:P ratios are similar in aquatic systems (lakes, estuaries, streams) and terrestrial ecosystems, where water and planktonic N:P ratios tend to increase in response to atmospheric deposition, leading to lower “growth rates,” complexity of community structure, and trophic diversity (Figure ; Table S1). Exceptions to these trends, however, have been recorded for aquatic systems, such as a decrease in N:P ratios in Japan due to the increasing deposition of P from dust dispersed from countries in southeastern Asia (Miyazako et al, ), and for European and North American lakes in areas with recent reductions in N deposition (Gerson et al, ; Isles et al, ). Although most studies of urban and crop wastes and leachate loads to rivers and estuaries (83.3%) have found increasing N:P ratios associated with increasing N:P ratios from human inputs, other studies (13.7%) tended to find decreasing ratios in areas with high livestock densities (Arbuckle & Downing, ; Johnson, Heck, & Fourqurean, ; Figure ; Table S1).…”

“…Atmospheric P deposition is also increasing due to the rising levels of anthropogenic emissions of P to the atmosphere (3.5 Tg P/year), which have led to current net continental and oceanic rates of P deposition of 2.7 and 0.8 Tg P/year, respectively (Wang, Balkanski, et al, 2015). This deposition has been particularly intense in areas of the world with emerging economies, such as eastern Asia, which may account for the low N:P ratios reported in some freshwater systems in Japan (Miyazako et al, 2015).…”

Nutrient Pollution and Its Dynamic Source-Sink Pattern in the Pearl River Estuary (South China)

“…The atmospheric deposition is one of the potential nutrient sources and is important in regulating the phytoplankton biomass in an estuary (Yadav et al, 2016). Atmospheric deposition of TN and TP display annual cycles (Miyazako et al, 2015). Compared with other source paths of nutrients into the PRE, the contribution of atmospheric dry and wet deposition was relatively low (Supplementary Table 2; Li et al, 2011), accounting for 12.64% of TN and 1.14% of TP.…”

Impacts of Hydrological Changes on Nutrient Transport From Diffuse Sources in a Rural River Basin, Western Japan

“…Since TN and TP concentrations increased with increasing precipitation and discharge (Figures S3 and S4), our results in agricultural and residential areas indicate that drainage water from a small area of anthropogenic land covers (Figure S5) enriches TN and TP concentrations in river water during storms. Miyazako et al () reported an increasing trend of the atmospheric TP deposition in the basin over the past decade, which in turn affected the atmospheric ratio of TN to TP, and explained that this trend may have increased the phosphorus leakage from the soil. These may in turn persist at the present level or increase the concentrations of TN and TP and the eutrophic level in downstream rivers and waters.…”