The browning and re-browning of lakes: Divergent lake-water organic carbon trends linked to acid deposition and climate change

Abstract

Dissolved organic carbon (DOC) concentrations and water colour are increasing in many inland waters across northern Europe and northeastern North America. This inland-water “browning” has profound physical, chemical and biological repercussions for aquatic ecosystems affecting water quality, biological community structures and aquatic productivity. Potential drivers of this “browning” trend are complex and include reductions in atmospheric acid deposition, changes in land use/cover, increased nitrogen deposition and climate change. However, because of the overlapping impacts of these stressors, their relative contributions to DOC dynamics remain unclear, and without appropriate long-term monitoring data, it has not been possible to determine whether the ongoing “browning” is unprecedented or simply a “re-browning” to pre-industrial DOC levels. Here, we demonstrate the long-term impacts of acid deposition and climate change on lake-water DOC concentrations in low and high acid-deposition areas using infrared spectroscopic techniques on ~200-year-long lake-sediment records from central Canada. We show that acid deposition suppressed naturally higher DOC concentrations during the 20th century, but that a “re-browning” of lakes is now occurring with emissions reductions in formerly high deposition areas. In contrast, in low deposition areas, climate change is forcing lakes towards new ecological states, as lake-water DOC concentrations now often exceed pre-industrial levels.

Environmental drivers of cladoceran assemblages at a continental scale: A synthesis of Alaskan and Canadian datasets

“…Dissolved organic carbon is a component of lake water that is largely responsible for controlling clarity and light transmission. Dissolved organic carbon trends in the dataset closely follow those of TP, and also pH and Ca, and probably reflect regional soil conditions and wetland cover, as well as high pH, which is favourable to the mobility of DOC and its movement from soils to lakes (Meyer‐Jacob et al., 2019). In recent years, lakes in boreal regions that historically experienced elevated levels of acidic deposition across North America and Europe have experienced DOC increases, attributed to recovery from acidification, as well as warming temperatures and increased precipitation due to climate change (Meyer‐Jacob et al., 2019; Monteith et al., 2007).…”

Negative effect of turbidity on prey capture for both visual and non‐visual aquatic predators

“…Despite these methodological differences, understanding the influence of turbidity range variation is relevant to known which species can cope well with reduced water transparency and which species are more prone to be locally extinct in a scenario of increased human‐induced changes in turbidity. Aquatic environments are experiencing a reduction in water clarity via multiple stressors, including eutrophication and climate change (Asknes et al, 2009; Meyer‐Jacob et al., 2020; Škerlep, Steiner, Axelsson, & Kritzberg, 2020). Thus, considering the body of evidence summarized here, an increase in turbidity may reduce the effects of predation on prey populations.…”

Global lake responses to climate change

“…The mixing regimes of marginal lakes have also been described as being very sensitive to changes in water clarity 122,129 . Specifically, a browning of lake surface waters (resulting in a decrease in water transparency), due mainly to terrestrial inputs of dissolved organic matter [130][131][132] , affects the depth at which shortwave radia tion is absor bed within a lake. The browning has a profound influence on the vertical thermal structure 49 and can, for example, determine whether a lake mixes regularly or stratifies continuously throughout the summer period 129 .…”

“…Precipitation and snowmelt are among the major factors that affect nutrient and dissolved organic matter avail ability in lakes. In conjunction with other environ mental changes, wetter climates will lead to 'browner' lakes from terrestrial inputs of dissolved organic matter 131,132 , which has implications for carbon cycling and anoxia 154 , spe cies invasions 155 , the persistence of pathogens 156 and other ecological attributes 157 . Climate change in combination with browning and eutrophication will alter the function and fuelling of aquatic food webs 158 .…”