Aquatic-Terrestrial Linkages

Lakes are closely coupled to their surrounding watersheds through shared climatic fluctuations and fluxes of water, nutrients, and organic carbon. This aquatic-terrestrial coupling may give rise to synchronization, i.e., the persistent relatedness of fluctuations in state variables between lakes and the terrestrial landscapes in which they are embedded. Resolving when and why ecosystems fluctuate synchronously is necessary for understanding long-term dynamics under future environmental conditions (Wilkinson et al. 2020). We investigated the degree to which primary production in lakes is synchronized with primary production in the adjacent terrestrial landscape. Our findings suggest that hydrologic connectivity mediates the synchrony between terrestrial and aquatic primary production at inter-annual timescales (Walter et al. 2021).

A long-standing focus of our research has been quantifying the pools of terrestrial C in surface waters and understanding the dynamic flux of terrestrial material into lakes. Using stable isotopes in a comparative survey of north temperate lakes, we demonstrated that particulate and dissolved organic matter pools are dominated by terrestrial material, particularly in smaller ecosystems (Wilkinson et al. 2013). This terrestrial material is also assimilated into the food web, more so when aquatic resources such as phytoplankton are scarce (Wilkinson et al. 2013, 2014). Terrestrial C can also enter lakes as carbon dioxide where it is subsequently emitted to the atmosphere. Using high frequency measurements combined with whole-lake nutrient addition experiments, we demonstrated that even under eutrophic conditions lakes can act as vents of CO2 to the atmosphere due to influx from shallow groundwater (Wilkinson et al. 2016). Finally, accumulated organic C in the sediments of inland and coastal waters is another major pool of C in aquatic ecosystems. In a literature synthesis of organic C burial rates across aquatic ecosystems (freshwater to marine), we found that burial rates spanned four orders of magnitude and were poorly constrained for most ecosystems (Wilkinson et al. 2018). Given this paucity of information, we have investigated the contribution of terrestrial (salt marsh) carbon to sediment stocks in seagrass beds (Greiner et al. 2016, Oreska et al. 2018).