Riverine Biogeochemical Exports from Major Watersheds to the Northwest Patagonian Estuarine Network

Abstract

River runoff linking terrestrial and marine ecosystems is a significant driver of marine processes (Lewis & Cook, 2023). The effect of river runoff on marine ecosystems is determined by variability in its timing, quality, and quantity. Human watershed perturbations – both direct, e.g. land-use change, and indirect, e.g. climate change – alter all three of these properties. Measuring baseline variability of these properties is crucial to quantifying the effects of human perturbations, which is necessary for tailoring watershed management to limit disruption of marine ecosystem health. This is particularly important in Northwest Chilean Patagonia (NCP), where a semi-enclosed estuarine system highly influenced by river runoff supports much of Chile's $7+ billion fisheries industry. Thanks to a citizen-science effort of unprecedented scale, we were able to characterize baseline inputs of key nutrients to the NCP marine environment for the 2022-2023 water year from the region's five largest watersheds: the Puelo, Yelcho, Palena, Cisnes, and Aysen, from North to South, respectively. Given the pristine nature of NCP's watersheds, these estimates represent one of the world's first regional characterizations of freshwater nutrient inputs to a temperate fjord system in the absence of extensive human intervention. Across all five rivers, we found freshwater concentrations of total nitrogen (TN), total phosphorus (TP), and dissolved organic carbon (DOC) to be lower than those of the receiving subantarctic marine waters, and well below global averages for comparable coastal temperate rainforest (CTR) ecosystems. (For the purposes of this paper, the metrics TN, TP, and DOC will be considered proxies for nitrogen (N), phosphorus (P), and carbon (C).) Conversely, large watersheds were found to be significant contributors of iron (Fe) and dissolved silica (dSi), collectively discharging ~ 79.9 and 291 metric tons during the 2022-23 water year. Taken together, these results indicate a net regulating function of NCP's five largest rivers on marine ecosystems, possibly inhibiting harmful algal blooms by favoring diatoms over dinoflagellates. The results suggest the importance of large watersheds in regulating marine ecosystems, highlight the potential value of watershed conservation for retaining ecosystem services of TN and TP dilution, and provide a baseline characterization of land-sea linkages in a temperate coastal system.