High-Frequency Data Reveal Patterns of Dissolved Organic Matter Production and Nitrate Removal Within a Low-Head Coastal Reservoir

Dissolved organic matter (DOM) is often the primary form of organic nitrogen and carbon in aquatic ecosystems. As such, it is an important source of nutrients and energy for both freshwater and coastal ecosystems. DOM can be often found in high concentrations in fluvial wetlands and impounded water bodies such as reservoirs due to the accumulation of woody debris and particulate organic matter. This makes these aquatic ecosystems potential hotspots for DOM production. Similarly, due to the hydrological and chemical characteristics of impounded water bodies, they are often sites of considerable nitrate (NO3) retention. The accumulation of organic matter may lead to reduced conditions where denitrification can be the dominant nitrogen transformation process. As such, ponded water bodies can be sites with substantial implications for alterations of both DOM and NO3 fluxes. With the removal of dams becoming a high priority for many advocacy groups in recent years, this can have serious implications for DOM processing as well as NO3 removal in river networks. We monitored fluorescent dissolved organic matter (fDOM), turbidity, and nitrate (NO3) concentrations at the inflow and the outflow of a low-head, run-of-the-river reservoir in coastal Massachusetts using paired suites of in situ sensors. These sensors were deployed to determine the effect of the reservoir primarily on nitrogen and organic matter fluxes. Preliminary data suggest a shift from moderate retention of fDOM during the mid-spring to considerable fDOM production in the late spring and early summer that coincided with an increase in the abundance of primary producers. During a similar time period in a previous year, there was substantial increase in NO3 retention that coincided with the beginning of an extreme low flow period. Increased residence times and potentially higher denitrification rates within the reservoir likely led to high nitrate removal. These results suggest that reservoirs have the potential to rapidly transform both organic matter as well as nitrogen as a result of changing biological and hydrological conditions. With dam removals becoming a more prevalent river restoration solution, consideration should be given to the biogeochemical processes that may be lost due to such a technique.