| In aquatic ecosystems, dissolved organic matter (DOM) is an important contributor to the carbon (C), nitrogen (N), and phosphorus (P) pools, representing the base of the microbial food-web and a key component of multiple elemental cycles. Human activities affecting water bodies and their catchments have the potential to modify the ecological and biogeochemical roles of DOM. However, there are important knowledge gaps regarding DOM dynamics in human-altered aquatic ecosystems, particularly with respect to the mechanisms behind the connections among DOM and inorganic nutrient cycles. My research has focused on filling these knowledge gaps by characterizing the DOM in streams draining agricultural landscapes, and investigating the interactions between DOM and inorganic nutrient availability. In agricultural streams, spatial and temporal trends in biochemical properties of the DOM pool have shown that (1) in-stream microbial transformations of OM can be an important contributor to the DOM pool, (2) the relative contribution of DOM to the N pool is small relative to inorganic N availability, and (3) microbial demand for organic N often appears to be fulfilled by OM in the sediments rather than water column DOM. In addition, results of manipulative and descriptive studies in streams and rivers across gradients of land use suggest that the coupling between DOM and inorganic P and N cycling depends on both the availability and the chemical form of inorganic nutrients. Furthermore, the mechanisms behind this coupling seem to be associated with changes in the microbial allocation of resources, which is in turn affected by human alterations of the inorganic nutrient pools. Overall, this research highlights the complexity of DOM dynamics and provides empirical evidence of coupling between DOM and inorganic nutrient cycling. This work shows that DOM and coupled nutrient cycles are an important, but currently underappreciated, aspect of effective nutrient management in flowing waters. |
