Illuminating riverine dissolved organic carbon dynamics using carbon age

Understanding the global carbon cycle is essential to predicting future climate change. Dissolved organic carbon (DOC) is one of the largest carbon reservoirs globally; the amount of carbon stored in oceans alone as DOC exceeds the amount of CO2 in the atmosphere. Despite being one of the largest carbon reservoirs globally, riverine DOC sources are poorly quantified. Radiocarbon (14C) can provide important information on riverine DOC age. An increase in DOC age in rivers over time suggests the mobilization of once-sequestered terrestrial carbon stocks in rivers, potentially from direct or indirect anthropogenic activity. DOC that was once “removed” from the global carbon cycle (e.g., stored in terrestrial soils) that is laterally exported to rivers could potentially be respired or photochemically oxidized and evaded to the atmosphere, thus contributing to global carbon dioxide emissions. I measure 14C-DOC on the Connecticut River seasonally and long-term to address if and when aged DOC is mobilized in rivers. This is necessary to predict anthropogenic perturbations to terrestrial carbon storage. This work is funded by the NASA Connecticut Space Grant.

The coupling of dissolved iron and dissolved organic matter

In rivers, iron speciation and concentration contribute to water color, affect the cycling of nutrients such as phosphorous, and support downstream (i.e., coastal and ocean) aquatic productivity and CO2 uptake. This is because iron is often a limiting nutrient in the coastal and open ocean due to the loss of iron in estuaries. Significant riverine export of iron to coastal zones is largely dependent on interactions with dissolved organic matter (DOM), often sourced from terrestrial environments; thus, the cycling of DOM and iron vary temporally, as terrestrial sources of iron and DOM depend on changing hydrologic and seasonal drivers. I evaluate the concentrations and downstream fluxes of two size fractions of dissolved iron in relation to water temperature, river discharge, and DOM composition using long-term frequent monitoring on the Connecticut River mainstem. This approach allows us to establish how the timing of the downstream export of iron might impact coastal and ocean productivity and CO2 uptake and determine the mechanistic drivers behind changing iron concentrations.

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Ferric iron impacts dissolved organic matter UV-vis absorbance in natural waters

Dissolved organic matter (DOM) composition and amount impacts the functioning of aquatic ecosystems by limiting the amount of UV and visible light (UV-vis), controlling nutrient cycling and trace metal speciation and fate, and by supporting heterotrophy. Because DOM absorbs light, UV-vis absorbance measurements have been widely used as a cost- and time- efficient method of estimating DOM composition and amount. However, the presence of ferric iron in filtered water results in overestimations of DOM UV-vis absorbance. I quantify the contributions of dissolved ferric iron to UV-vis absorbance on commonly measured DOM UV-vis parameters for natural water samples collected from the Connecticut River watershed over 1.5 years. Further, I evaluate how ferric iron interference in UV-vis absorbance measurements contributes to systematic biases in the interpretation of DOM composition and amount by land-use, season, and hydrology.

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