The role of water transparency in regulating carbon dynamics in lakes: Experimental, comparative and high-frequency approache

Chapter 1: Comparing the relative importance of photodegradation and biodegradation for transforming dissolved organic matter quality in three temperate lakes of varying trophic status. In this chapter, I tested the relative importance of photodegradation vs. biodegradation for altering dissolved organic matter (DOM) quality and quantity from three temperate lakes ranging in color and productivity. For all three lakes, photodegradation led to larger decreases in DOM color and molecular weight than biodegradation. In addition, DOM with low prior sunlight exposure responded more to photodegradation than previously sunlight exposed DOM. This chapter is currently in preparation for submission to the Journal of Geophysical Research: Biogeosciences. .;Chapter 2: Sunlight-driven degradation of terrestrial organic matter exceeds microbial respiration. For Chapter 2, I extended Chapter 1 by more completely exploring how sunlight and microbes control the production of CO2 for terrestrial DOM from the watershed surrounding the same temperate lakes and a sub-tropical lake. Here we show that sunlight led to higher amounts of CO2 production than microbial respiration for terrestrial DOM from the watersheds of the brown-water lakes. However, sunlight-driven reductions in DOM quality were greater than CO2 production for DOM from the watersheds of the oligotrophic and eutrophic lakes. This chapter is in preparation for submission to Biogeochemistry. .;Chapter 3: The potential of high-frequency profiling to assess vertical and seasonal patterns of phytoplankton dynamics in lakes: an extension of the Plankton Ecology Group (PEG) model Here, I compared nightly profiles of chlorophyll fluorescence (proxy for phytoplankton biomass) from 11 global lakes to test the drivers of seasonal changes in sub-surface phytoplankton layers. High-frequency profiles captured the short-term phytoplankton dynamics better than traditional sampling, and physical drivers including light availability and thermal stratification were important determinants of when sub-surface phytoplankton layers form. This research was published as part of the GLEON special issue in Inland Waters. (Brentrup et al. 2016).;Chapter 4: Drivers of metalimnetic oxygen maxima in lakes: the importance of physical processes. In this chapter, I tested the relative contributions of physical and biological processes for metalimnetic oxygen maxima (MOM) formation. We found that physical processes played a larger role in MOM formation than previously thought, especially in transparent, oligotrophic lakes. Net ecosystem production at the MOM was also typically net heterotrophic, suggesting a stronger physical rather than biological signal and high rates of respiration. This chapter is in preparation for submission to Limnology and Oceanography..