From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes

Abstract

Increased concentrations of dissolved organic carbon (DOC), often labelled “browning”, is a current trend in northern, particularly boreal, freshwaters. The browning has been attributed to the recent reduction in sulphate (S) deposition during the last 2 to 3 decades. Over the last century, climate and land use change have also caused an increasing trend in vegetation cover (“greening”), and this terrestrially fixed carbon represents another potential source for export of organic carbon to lakes and rivers. The impact of this greening on the observed browning of lakes and rivers on decadal time scales remains poorly investigated, however. Here, we explore time-series both on water chemistry and catchment vegetation cover (using NDVI as proxy) from 70 Norwegian lakes and catchments over a 30-year period. We show that the increase in terrestrial vegetation as well as temperature and runoff significantly adds to the reduced SO4-deposition as a driver of freshwater DOC concentration. Over extended periods (centuries), climate mediated changes in vegetation cover may cause major browning of northern surface waters, with severe impact on ecosystem productivity and functioning.

Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon

“…Considering that northern boreal lakes are facing both brownification (Monteith et al 2007, Finstad et al 2016 and increased N deposition (Vitousek et al 1997, Galloway et al 2008 our study provides important whole lake insights concerning the consequences of brownification and increased N availability on pelagic food webs, and phytoplankton specifically. Our study clearly illustrates that with increasing DOC phytoplankton production and biomass decrease, and that phytoplankton communities differ, shifting towards a dominance of high DOC adapted phytoplankton species such as autotrophic flagellates.…”

Decoupled trophic responses to long‐term recovery from acidification and associated browning in lakes

“…While recovery from acidification has been identified as a primary driver of browning (Clark et al, ; Monteith et al, ), other drivers such as landscape change (Kritzberg, ), climatically induced increases in precipitation (Brothers et al, ; Couture, Houle, & Gagnon, ; Hongve, Riise, & Kristiansen, ), or warming‐induced increases in terrestrial productivity (Finstad et al, ; Larsen, Andersen, & Hessen, ) and tree line advance (Hofgaard, Tømmervik, Rees, & Hanssen, ) may also cause browning in some regions. In these other cases, browning may be a more important driver of ecological change than observed here, because other alternate drivers may have different or muted concomitant water chemistry changes.…”

Coupling dissolved organic carbon, CO2 and productivity in boreal lakes

“…For most boreal freshwater systems as well as some coastal regions influenced by forested catchments, there is a current trend with increased export of terrestrially fixed carbon (C), causing elevated concentrations of coloured dissolved organic matter (DOM) (Finstad et al., ; Garmo et al., ; Monteith et al., ). This “browning” will play a multitude of partly contrasting roles for primary and secondary productivity in lakes (Hessen, ; Solomon et al., ; Williamson et al., ).…”