Convergence of Terrestrial Dissolved Organic Matter Composition and the Role of Microbial Buffering in Aquatic Ecosystems

Substantial changes in vegetation are expected as global climatic patterns shift, altering terrestrial sources of dissolved organic matter (DOM) entering rivers and streams. Since differences in the chemical composition of plant litters are reflected in the DOM that is leached, changes in riparian vegetation can directly influence the bioavailability of DOM to local aquatic microbial communities. We assessed the degradation dynamics and optical compositional changes of DOM from a variety of vascular plant leachates through microbial and coupled photochemical‐microbial degradation pathways. Initial decay rates ranged from 0.029 ± 0.011 day−1 (microbial, mixed wetland) to 0.73 ± 0.62 day−1 (photochemical‐microbial, mixed grasses), and all decay rates decreased to below 0.05 day−1 after 1 week, converging below 0.029 day−1 after 2 weeks. Overall, we found a decrease in leachate optical diversity under microbial and photochemical‐microbial degradations, corresponding to a decrease in degraded leachate sample dispersion using principal component analysis. We show that despite initial variability across DOM leachates, successive degradation promotes kinetic and optical convergence such that, in aquatic environments with long residence times, terrestrial DOM source and composition are much less important on exported DOM composition than historically thought. In these systems, DOM compositional convergence may act as a natural buffer to provide stability of aquatic DOM cycling in the face of future landscape changes. Plain Language Summary Future changes in climate are expected to affect where and when plants will grow. By the year 2100, for example, native California oak trees may occupy only half their current land area and likely will grow only at higher latitudes, causing concern for organisms that depend on native oak trees for food. When leaves fall into water, they break down into small pieces that become part of the dissolved organic matter in aquatic systems. Dissolved organic matter provides food for aquatic bacteria, and these bacteria become food for zooplankton and fish. Since each plant species has its own chemical fingerprint, some dissolved organic matter is more difficult to degrade. Thus, if a plant like the California oak tree disappeared from its current geographic area, bacteria might not have another easily degradable food source. However, we found that after being processed (through partial degradation by bacteria or exposure to sunlight), d