OPUS: CRS Synthesis to add dissolved organic matter to the trophic paradigm: the importance of water transparency in structuring pelagic ecosystems

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Water of higher clarity adds economic, recreational, and other value to lake ecosystems and the services they provide. Yet, lakes worldwide are threatened with decreasing clarity due not only to increased nutrient inputs and subsequent algal growth, but also due to increased dissolved organic matter inputs. Some of the compounds produced during the decomposition of organic matter, which is primarily dead plant material, dissolve in water and turn lake water brown. This process is similar to how a steeping tea bag discolors a clear cup of hot water. Increases in rainfall, which are pronounced in some areas, are increasing the flow of dissolved organic matter into lakes from surrounding land, significantly decreasing water clarity and quality. Dissolved organic matter strongly and selectively absorbs ultraviolet radiation, which would otherwise disinfect surface waters by killing harmful human and wildlife pathogens. Dissolved organic matter also absorbs wavelengths of light that are used by photosynthesis, which is the only source of oxygen in the deep waters of lakes. Decreasing water clarity thus results in the degradation of water quality, a greater risk of pathogen survival in surface waters, and depletion of the oxygen in deeper waters, leading to “dead zones” like those in Lake Erie and the Gulf of Mexico. Through a synthesis of data from over 400 lakes that have been monitored over multiple decades, this project will provide new insights into how dissolved organic matter influences long-term changes in water clarity, and the resulting consequences for lake ecosystems. The effects of dissolved organic matter will be integrated into the conventional paradigm for water clarity, so that models can more accurately predict future consequences of dissolved organic matter inputs for lake ecosystems and the services they provide. The products of this research will be: (1) a unique, comprehensive, fully annotated, and publicly available lake database, (2) a suite of synthesis papers that integrate dissolved organic matter (DOM) and UV into a lake ecosystem model, (3) a broader conceptual model for research and educational purposes, and (4) educational and public outreach for diverse audiences, including undergraduates, lake association members, lake managers, and policymakers.The strong relationship between nutrients and chlorophyll and the classification of lakes along a single axis of lake productivity has been the most central paradigm in lake ecology for decades. However, one of the major changes in lakes in recent decades is up to a doubling or more of DOM, leading to browning of many inland waters in North America, Europe, and beyond. Recent increases in precipitation extremes (drought to flood), and associated browning of lakes, require extension of the traditional paradigm to include DOM and the optical properties of lakes. This project will synthesize the most geographically extensive and site intensive database in existence on ultraviolet radiation (UV), optical properties, and associated limnological survey and experimental data on hundreds of lakes worldwide, and a set of three core study lakes. The synthesis will add a new dimension to the current trophic paradigm for lakes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.