Long-term trends in bottom water chemistry of Swedish lakes

Abstract: Long-term monitoring of lake water is essential for tracking local and long-range effects of anthro-pogenic pressure and to inform policy aiming at mitigating human environmental impacts. Swedish reference lakes are recovering from acidification; are subject to climate change and increasing inputs of terrestrial dissolved organic matter (DOM). The integrated effects of different environmental pressures alters lake water chemistry and, consequently, affects lake ecosystems. Most time trend analyses have focused on changes in surface water chemistry, whereas less is known about trends in bottom waters. The overall aim of this study was therefore to assess long-term trends (1988-2015) in bottom water chemistry of Swedish reference lakes (n=13) in relation to trends in surface waters. The hypothesis was that the prevalent increasing DOM concentrations in northern lakes have exacerbated the depletion of hypolimnetic dissolved oxygen (DO), as a result of DOM induced prolongation of stratification. Consequently, bottom water NH4-N (nitrate reduction to ammonia) and TP (internal loading) were expected to have increased over time in affected lakes. Time trend analysis (Mann-Kendall, p<0.05) showed that the yearly median bottom water DO has significantly decreased in 7 lakes. In 6 of these lakes (and 2 additional), surface water total organic carbon (TOC) has increased over time, however, rising TOC concentrations were more prevalent in bottom waters (12 lakes). The results further showed that bottom water nitrogen from ammonia (NH4-N) has increased in the 7 lakes with declining DO, whereas bottom water total phosphorus (TP) has increased in 4 of these lakes. In contrast, surface water TP has declined in 7 lakes, which may have masked additional increased internal loading of P. Closer observation of bottom water chemistry revealed that as DO levels dropped, TOC, NH4-N and TP concentrations peaked and were particularly high during periods of sustained anoxia (≥one year). The results also showed that bottom water Si concentrations have increased in most lakes. In the bottom waters of Brunnsjön, a sustained anoxic period coincided with a 32% step change in Si concentration. In conclusion, the underlying mechanism of the observed rising trends in bottom water TOC, NH4-N and TP are likely a result of prolonged or increased incidence of DO depleted hypolimnia. Clearly, monitoring of bottom waters can reveal important aspects of long-term changes in lake water chemistry. Future research is needed to further assess the long-term effects of brownification and climate change on bottom water DO in northern lakes.