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Subsurface tile drainage speeds water removal from agricultural fields that are historically prone to flooding. While managed drainage systems improve crop yields, they can also contribute tothe eutrophication of downstream ecosystems, as tile-drained systems are conduits for nutrients to adjacent waterways. The changing climate of

A significant fraction of Earth's land surface is used for agriculture, which has led to extensive modification and degradation of streams and rivers. Although highly modified, agricultural streams offer important opportunities for advancing our understanding of agroecosystems and applying the principles of translational ecology. Using examples from the upper midwestern U.S., we discuss how highly modified agricultural streams can be used to gain insights into ecological and biogeochemical processes, though often this involves adjusting the paradigms and approaches used to study streams in less disturbed landscapes. We argue that highly modified agricultural streams could serve as a model for how less disturbed systems might respond to ongoing and future environmental change. Last, these streams often are targets for restoration, which opens the door to engaging with diverse stakeholders, including policymakers and nontraditional funding sources such as philanthropic foundations and industry groups.

Agriculture alters the biogeochemical cycling of nutrients such as nitrogen (N), phosphorus (P), and silicon (Si) which contributes to the stoichiometric imbalance among these nutrients in aquatic systems. Limitation of Si relative to N and P can facilitate the growth of non-siliceous, potentially harmful, algal taxa which has severe environmental and economic impacts. Planting winter cover crops can retain N and P on the landscape, yet their effect on Si concentrations and stoichiometry is unknown. We analyzed three years of biweekly concentrations and loads of dissolved N, P, and Si from subsurface tile drains and stream water in two agricultural watersheds in northern Indiana. Intra-annual patterns in Si concentrations and stoichiometry showed that cover crop vegetation growth did not reduce in-stream Si concentrations as expected, although, compared to fallow conditions, winter cover crops increased Si:N ratios to conditions more favorable for diatom growth. To assess the risk of non-siliceous algal growth, we calculated a stoichiometric index to quantify biomass growth facilitated by excess N and P relative to Si. Index values showed a divergence between predicted algal growth and what we observed in the streams, indicating other factors in uence algal community composition. The stoichiometric imbalance was more pronounced at high ows, suggesting increased risk of harmful blooms as environmental change increases the frequency and intensity of precipitation in the midwestern U.S. Our data include some of the rst measurements of Si within small agricultural watersheds and provide the groundwork for understanding the role of agriculture on Si export and stoichiometry.