Time series analysis of nutrient inputs to the Baltic Sea and changing DSi:DIN ratios

Abstract

Increasing nutrient loads have characterized the Baltic Sea during the last century. However, the detection of long-term trends in the water column has been difficult due to both paucity of data and high variability. Analysis of water quality data with robust non-parametric methods has shown statistically significant increases in total nitrogen, total phosphorus, nitrate (NO3), and dissolved inorganic phosphate, although with considerable spatial and temporal differences. Significant decreases in dissolved silicate (DSi) and ammonium (NH4) concentration have also been reported. We report here significant decreases in the DSi:DIN ratio (where DIN, dissolved inorganic nitrogen, is the sum of NO3, NO2, and NH4 concentrations) in the Baltic Sea from 1970 to 1990. The molar ratios prior to the formation of the spring bloom are now approaching unity, with further decreases expected with continued eutrophication of the Baltic Sea. This can be explained by an increased net sedimentation of biogenic silica due to increased primary production attributable to increased nutrient loading. While the Baltic proper is generally assumed to be N limited, declining DSi:DIN ratios indicate that spring diatom growth may become DSi limited in the near future, as the optimal DSi:DIN ratio for diatom growth is approximately 1:1. This decrease in the DSi:DIN ratio cannot be statistically detected in the river input to the Baltic proper. Only a few significant tests were found in the sea, with both upward and downward trends detected. Ecological implications of this observed reduction in the DSi:DIN ratio may include DSi- limited diatom growth and changes in species composition and, subsequently, food web dynamics.