Lakes as sensors in the landscape: Optical metrics as scalable sentinel responses to climate change

As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longerterm interdecadal trends with regression r2 values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a*320) and spectral slope (S275–295), where the ratio a*320 to S275–295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a*320 and S275–295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC-related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton. Climate change is driving us toward an overall warmer and wetter world, with more frequent and severe droughts as well as more intense precipitation and floods. Future climate scenarios point to dry regions and seasons getting drier and wet regions and seasons getting wetter (IPCC 2012, 2013), yet deciphering the effects of the temperature vs. precipitation components of a changing climate is notoriously difficult. Long-term climate trends are clouded by variability that creates noise in the system (Clark et al. 2010). To add to the challenge, climate change is manifested not only in long-term trends but also in extreme events and geographic variation across the landscape (IPCC 2012). For example, while precipitation has increased by 7% on average across the United States in the past 100 yr, the amount of rainfall in extreme precipitation events (the top 1% of all daily events) has increased by 20% (Karl et al. 2009). Some regions, such as the northeastern United States, have experienced up to a 67% increase in extreme precipitation events, while in other regions, such as the southwestern United States, this increase has been only 9% (Karl et al. 2009). These heavy rainfall events can lead to more intense runoff into lakes, which can in turn increase terrestrially derived dissolved organic carbon (DOC) concentrations that substantially alter thermal structure and ecosystem metabolism of lake ecosystems (Klug et al. 2012; Sadro and Melack 2012). In recent decades, trends of increasing DOC concentration have provided one of the strongest signals of change in inland aquatic ecosystems (Williamson et al. 2009a), with as much as a doubling of DOC concentrations in lakes in some regions of North America and Europe (Monteith et al. 2007). The causes of increasing DOC are a subject of active debate. While earlier work (Monteith et al. 2007) seemed to be converging on a central role for recovery from anthropogenic acidification, more recent work (Zhang et al. 2010; Couture et al. 2012) suggests that climate plays an equally or perhaps more important role, even in systems in which recovery from acidification has been observed. Furthermore, climate change effects on terrestrial ecosystems are predicted to increase future DOC concentrations in boreal lakes by as much as 65% (Larsen et al. 2011). A clearer understanding of these increasing DOC concentrations is critical because DOC is one of the most fundamental regulators of aquatic ecosystem structure and function (Williamson et al. 1999; Couture et al. 2012). In most inland waters the primary source of DOC is from the surrounding terrestrial landscape, though phytoplankton and macrophytes also produce DOC. Due largely to its ability to absorb light (evidenced by color), terrestrially derived DOC plays an important role in determining water transparency, mixing depth, and oxygen depletion in deeper waters, as well as the processing and bioavailability of nutrients and toxic compounds (Williamson et al. 1999). Increases in DOC can also alter the * Corresponding author: [email protected] Limnol. Oceanogr., 59(3), 2014, 840–850 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.3.0840