Low ratios of silica to dissolved nitrogen supplied to rivers arise from agriculture not reservoirs

Abstract

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.

Stoichiometry of nitrogen, phosphorus, and silica loads in the Mississippi‐Atchafalaya River basin reveals spatial and temporal patterns in risk for cyanobacterial blooms

“…But the high N and P yields, relative to DSi yields, make this subbasin the key determinant not only of the mass of annual N and P loads but also the stoichiometry of the annual loads. The N‐ICEP results thus support the argument by Downing et al () that N runoff from agriculture, as opposed to Si retention in reservoirs, is the primary driver of N:Si stoichiometry in surface waters of the Upper Mississippi subbasin.…”

“…An inverse relationship between dissolved Si concentrations in the Mississippi River and the mass of P fertilizer used in the U.S. provides support for this hypothesis (Turner and Rabalais ). Others argue that increased N:Si ratios are the result of N runoff from agriculture and that reservoirs play a minor role (Downing et al ). In the latter case, regions of the Mississippi basin dominated by agriculture should disproportionately affect N:Si stoichiometry of nutrient loads received by the Gulf of Mexico.…”

Nutrient export and elemental stoichiometry in an urban tropical river

“…, Downing et al. ). Because land use changes such as urbanization and agriculture can dramatically alter the availability of Si as well as N and P that are delivered from land to surface waters (Carey and Fulweiler ), it is a potentially important consideration in management of water quality.…”

“…Recent work highlights a growing realization that Si should receive more consideration in contemporary assessments of nutrient limitation (Downing et al 2016). Increasingly, in both lentic freshwater and marine systems, the role of Si has been considered in the context of nutrient limitation (Garnier et al 2010, Downing et al 2016.…”

“…Recent work highlights a growing realization that Si should receive more consideration in contemporary assessments of nutrient limitation (Downing et al 2016). Increasingly, in both lentic freshwater and marine systems, the role of Si has been considered in the context of nutrient limitation (Garnier et al 2010, Downing et al 2016. Because land use changes such as urbanization and agriculture can dramatically alter the availability of Si as well as N and P that are delivered from land to surface waters (Carey and Fulweiler 2012), it is a potentially important consideration in management of water quality.…”