Large storms and particulate organic matter (POM) export: Changes in particle size, composition and source

Abstract:

Large storm events not only have potential to increase the total amounts of organic matter (OM) exported from catchments but can also result in a dramatic shift towards particulate (POM) from dissolved forms (DOM) of organic matter. This change in fluvial “OM regime” can have significant implications for nutrient transport, lateral carbon flux, and metabolism and nutrient cycling in aquatic ecosystems. The environmental impacts may vary with the amounts of POM, sources, composition and particle size. We studied exports of coarse (CPOM: >1000 μm), medium (MPOM: 250-1000 μm), and fine (FPOM: < 250 μm) in a nested first (12 ha catchment) and second order (79 ha) forested stream system in the Piedmont physiographic region in Maryland, USA. Sampling was performed for multiple storms over two years. The composition of POM sources and sediments were investigated using stable isotopes of C and N and a suite of molecular biomarkers analyzed by thermochemolysis-gas chromatography-mass spectrometry. Significant differences observed between catchment end members using this combination of molecular and isotopic analyses allowed for the differentiation of the main sources of POM and a Bayesian mixing model using stable isotopes was paired with a multivariate analysis of biomarkers to distil the strongest trends in particle source. The source of POM varied among flow regimes of varying runoff and rainfall intensity, among different particle sizes, and with position along the stream. Higher rainfall intensities mobilized CPOM from the forest floor litter in greater proportions, whereas finer POM was largely mobilized from wetlands and forest floor humus. A different response was observed for events of lower rainfall intensity but higher discharge, in which CPOM was still predominantly sourced from forest litter, but finer POM had a larger fraction of stream beds and banks. Generally, low antecedent moisture conditions promoted humus in POM while high antecedent moisture and/or smaller rainfall events promoted more POM from in-channel sources. Understanding these shifts in POM source and quality under different hydrologic conditions is especially critical considering climate-change projections for the northeast US indicate increasing intensity and magnitude of the largest storms.