Browning-Related Oxygen Depletion in an Oligotrophic Lake Lesley B
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Fisherc aItasca Biological Station and Laboratories, University of Minnesota Twin Cities, Lake Itasca, MN, USA; bDepartment of Biology, Miami University, Oxford, OH, USA; cDepartment of Statistics, Miami University, Oxford, OH, USA ABSTRACT ARTICLE HISTORY In recent decades, terrestrial dissolved organic matter (DOM) has increased in many northeastern Received 13 September 2017 North American and European lakes and is contributing to long-term browning. We used a long- Accepted 9 March 2018 term dataset (1988–2014) to study the consequences of browning-related decreased water KEYWORDS transparency on dissolved oxygen dynamics in 2 small temperate lakes in Pennsylvania, USA, anoxia; browning; dissolved that differ in their dissolved organic carbon concentrations. The oligotrophic (“clearer”) lake has – organic carbon; dissolved low productivity and historically oxygenated deep waters. The mesotrophic slightly dystrophic oxygen; lakes (“browner”) lake also has relatively low productivity but historically anoxic deep waters. We examined whether browning coincided with changes in summer dissolved oxygen dynamics, with a focus on deep-water oxygen depletion. In the clearer lake, we found that minimum oxygen concentrations decreased by ∼4.4 mg L−1 over the 27-year period, and these changes were strongly associated with both decreased water transparency and increased water column stability. We also found a shallowing of the maximum dissolved oxygen depth by ∼4.5 m and anoxic conditions established in more recent years. In the browner lake, the metrics we used did not detect any significant changes in dissolved oxygen, supporting the prediction that vertical temperature and oxygen patterns in clearer lakes may be more sensitive to increasing DOM than darker lakes. Anoxia is traditionally considered to be a consequence of anthropogenic nutrient loading and, more recently, a warming climate. We show that browning is another type of environmental change that may similarly result in anoxia in oligotrophic lakes. Introduction (Williamson et al. 1999). Research has also highlighted Water color and concentrations of terrestrial dissolved the importance of lake DOM pools, transformations, organic matter (DOM) are increasing in lakes in parts and fluxes to regional and global carbon cycling (Cole of northeastern North America and northern Europe et al. 2007, Tranvik et al. 2009). Thus, long-term browning (Monteith et al. 2007). The terms browning and brow- has the potential to greatly alter aquatic systems, and nification have been used to describe this phenomenon increasing evidence shows profound effects on lake (Roulet and Moore 2006, Kritzberg and Ekström 2012). ecosystem structure and function (Solomon et al. 2015, Increases in DOM that lead to lake browning are likely Williamson et al. 2015). For example, a unimodal the result of multiple drivers. Recovery from acidification response to changes in DOM and related changes in was initially suspected to play the primary role (Evans light and nutrient availability has been observed for et al. 2006, Monteith et al. 2007), and more recent both phytoplankton production (Seekell et al. 2015)and work suggests that climate variables such as increased fish production (Finstad et al. 2014) in arctic and boreal precipitation and temperature may also be important, lakes. The shading effect of increased colored DOM can even in regions where acid deposition has decreased also lead to stronger thermal stratification and shallower (Zhang et al. 2010, Couture et al. 2012). Increased terres- mixing depths in small lakes via changes in vertical heat trial vegetation cover from climate and land use change absorption and distribution (Fee et al. 1996, Snucins and (Finstad et al. 2016) as well as increased iron concen- Gunn 2000). Shallower mixing depths can in turn lead trations (Kritzberg and Ekström 2012, Weyhenmeyer to increases in organic carbon burial in lake sediments et al. 2014) can also contribute to browning. (Fortino et al. 2014). DOM acts as an ecosystem regulator in lakes by attenu- Naturally occurring brown-water lakes are often net ating light, altering nutrient availability, and influencing heterotrophic and can experience low oxygen conditions the balance between heterotrophy and autotrophy at depth during the summer (Nürnberg and Shaw 1998, CONTACT Lesley B. Knoll [email protected] Supplemental data for this article can be accessed at https://doi.org/10.1080/20442041.2018.1452355 © 2018 International Society of Limnology (SIL) 256 L. B. KNOLL ET AL. Wetzel 2001). This relationship has also been documen- as in coastal marine systems such as those in the Gulf ted in prior studies in high DOM lakes that show the of Mexico (Diaz and Rosenberg 2008) or the Chesapeake effects of short-term increases in DOM on oxygen Bay (Kemp et al. 2005). Warming air temperatures can dynamics. In a 4-year whole-lake manipulation, a near also lead to oxygen depletion in lakes because oxygen doubling of dissolved organic carbon (DOC) concen- solubility is reduced in warmer waters, and warmer sur- − trations (7.5–14.1 mg L 1) resulted in a shift to shallower face waters lead to increased strength and duration of chlorophyll a distributions and a simultaneous shift to a stratification with reductions in vertical mixing (Jan- shallower depth of maximum oxygen concentrations kowski et al. 2006, Foley et al. 2012, North et al. 2014). (Christensen et al. 1996). A second study illustrated a Warming-induced oxygen declines are not just a concern short-term response to browning in a shallow, highly for lakes. Recent evidence suggests that since 1960, the humic lake (Brothers et al. 2014). In this shallow lake sys- Earth’s oceans have lost 2% of their oxygen content, tem dominated by benthic primary production, 2 years and the volume of anoxic water has increased 4-fold of heavy precipitation increased already-high DOC con- (Schmidtko et al. 2017). centrations in the lake, which led to decreases in light Here we examined the potential for an alternative availability, reduced benthic primary production, and pathway to oxygen depletion via long-term increases in increased anoxia. terrestrial DOM in 2 undisturbed lakes in a region Few studies have examined the link between long- experiencing browning. We also explored mechanisms term browning and deep-water oxygen depletion. A by which browning may influence dissolved oxygen recent modeling study for an oligotrophic lake examined trends. Specifically, we examined lake stability (Schmidt the combined and independent effects of warming water stability) and water transparency (1% photosynthetically temperature and increasing DOM on dissolved oxygen active radiation [PAR] depth). We analyzed a long-term (Couture et al. 2015). The authors found that the com- dataset (1988–2014) for 2 small lakes—an oligotrophic bined effect resulted in the largest impact on oxygen con- (“clearer”) lake and a mesotrophic–slightly dystrophic sumption, but the long-term increases in DOM played a (“browner”) lake—located in protected watersheds. larger role in controlling dissolved oxygen consumption Both lakes have experienced significantly decreasing than warming lake temperatures. Another study pro- water transparency (i.e., 1% PAR depth) over the 27- vided a brief summary of observed changes in oxygen, year dataset and have shown trends of increasing DOC along with many other limnological changes, that concentration, but because of the high interannual varia- accompanied lake browning in 2 temperate lakes, Lake bility of DOC concentration in the browner lake, the Giles and Lake Lacawac (Williamson et al. 2015). This trend is statistically significant only in the clearer lake study described observed shifts in the depth of peak (Williamson et al. 2015). The observed reductions in and minimum dissolved oxygen saturation in a subset water clarity do not seem to be associated with an of years in a long-term dataset, but only in summary increase in phytoplankton biomass because summer form. We extend the findings in Lakes Giles and Lacawac chlorophyll concentrations have not changed signifi- by using an expanded long-term dataset to more fully cantly over the past 27 years in either lake (Williamson document and statistically analyze the relationship et al. 2015). Rather, DOC is the primary factor control- between long-term browning and changes in oxygen ling light attenuation in these lakes (Morris et al. 1995, over depth and time. We also relate dissolved oxygen Williamson et al. 1996). The mechanism driving brown- trends with potential drivers to better understand the ing is likely related to both increases in precipitation and mechanisms behind oxygen changes. recovery from acidification because during the study Exploring the connection between long-term brown- period, total precipitation significantly increased and sul- ing and anoxia, defined here as dissolved oxygen fate deposition decreased, with