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

Abstract

River discharge considerably influences marine ecosystems through hydrological and biogeochemical drivers. Measuring baseline variability of riverine inputs, and their relation to anthropogenic land‐use impacts and global climate stressors affecting watersheds is therefore crucial for managing coastal marine ecosystems. This is particularly relevant in the relatively undisturbed Northwest Patagonia region (NWP; 41°S–46.5°S), where an extensive estuarine system influenced by high river runoff supports much of Chile's aquaculture economy (est. $6+ billion USD). Combining an unprecedented weekly citizen‐monitoring effort and daily streamflow data with a weighted‐regression modeling approach, we estimated inputs of macronutrients, dissolved iron, dissolved silica, and total suspended solids as well as characterized dissolved carbon concentrations from the five largest watersheds to the NWP marine environment. We used these estimates to evaluate export magnitude, seasonality, synchrony, and potential influence on marine ecosystems. Our results suggest that large rivers have a net dilution effect on total nitrogen and phosphorus, with freshwater concentrations substantially lower than those reported for NWP fjords and comparable coastal temperate rainforest rivers globally. Conversely, freshwater concentrations of dissolved silica and iron were markedly higher than those in marine waters and other coastal temperate rainforest rivers. Collectively, these results suggest a potential regulating function of large watersheds to marine ecosystems in Northwest Patagonia, with implications for harmful algal blooms and for managing external inputs from aquaculture and changing watershed land‐use. Our findings provide one of the first comprehensive regional baselines for major watersheds in reference conditions linked to temperate fjord ecosystems.