Coastal marine systems are greatly altered by toxic marine algae, eutrophication and hypoxia. These problems have been linked to decreased ratios of dissolved silica to inorganic nitrogen (Si : DIN) delivered from land. Two mechanisms for this decline under consideration are enhanced nitrogen (N) fertiliser losses from agricultural lands or Si sequestration in reservoirs. Here we examine these mechanisms via nutrient concentrations in impoundments receiving water from 130 watersheds in a landscape representative of the agriculture that often dominates coastal nutrient inputs. Decreased Si : DIN was correlated with agriculture, not impoundment. Watersheds with > 60% agricultural land yielded highest DIN, whereas Si was uncorrelated with agricultural intensity. Furthermore, eutrophic lakes were dominated by Cyanobacteria that use little Si, so reservoirs did not diminish Si : DIN. Instead, Si : DIN increased slightly as reservoir residence time increased. These data suggest that impoundments in agricultural watersheds may enhance the water quality of coastal ecosystems, whereas fertiliser losses are detrimental.
Bibliographical noteFunding Information:
J.A.D. thanks J. Ramstack, K. Lee and K. Haapa-aho for assistance in collecting nutrient data, two anonymous reviewers for helpful comments, and B. Danielson for discussions of percolation theory. We also thank the Iowa Department of Natural Resources and the Iowa Geological Services Bureau for funding. R.W.F. thanks Boston University for sabbatical support and Harvard University for the Bullard Fellowship, which provided support and time for her to help work on this manuscript.
© 2016 John Wiley & Sons Ltd/CNRS
- harmful algae blooms
- marine ecology