How will large storms alter particulate organic matter exports and composition and impact water quality of receiving aquatic ecosystems?

Abstract:

Climate analysis indicates that there has been a 71% increase, over the past 50 years, in the most intense storms (top 1%) for the northeast US. Climate projections reveal that this trend will continue. This has major implications for fluvial organic matter exports from watersheds, which typically, increase with increasing stream discharge. Large storms, however, result in a disproportional increase in the particulate versus dissolved forms of organic matter (POM and DOM, respectively). We investigate: How will the relative proportions of POM and DOM and their sources change with increasing discharge? How does the composition (C and N content and lability) and particle size distribution of POM change with flow? What role does storm magnitude, intensity, and seasonal timing play in shaping the nature of POM exports? How do POM exports vary with watershed landuse and legacy? How does the change in the amounts and composition of POM inputs impact nutrient processing and cycling in receiving aquatic ecosystems?

We address these questions through a synthesis of our own work and existing studies in literature. Our previous research in headwater, forested, Piedmont watersheds in the US has shown that POM exports increase exponentially with stormflow while DOM follows a more muted linear increase. There appears to be a hydrologic threshold beyond which POM exports outpace the DOM values. Upland erosion associated with the largest storms mobilize greater amounts of carbon (C) and nitrogen (N) rich coarse fractions of POM, especially in headwater forested catchments. In contrast, in higher order streams, stream bank erosion could be an important contributor to POM. The relative amounts of POM could vary with landuse, with agricultural watersheds yielding lower amounts of POM compared to forested systems. Large POM inputs to receiving water bodies could serve as a long-term source of nutrients, thereby altering the nutrient cycling and processing regimes in these systems. Understanding these changes and the driving mechanisms behind them is critical to developing effective management strategies to mitigate impacts of climate change.