Outsized nutrient contributions from small tributaries to a Great Lake

Abstract

Excessive nitrogen (N) and phosphorus (P) loading is one of the greatest threats to aquatic ecosystems in the Anthropocene, causing eutrophication of rivers, lakes, and marine coastlines worldwide. For lakes across the United States, eutrophication is driven largely by nonpoint nutrient sources from tributaries that drain surrounding watersheds. Decades of monitoring and regulatory efforts have paid little attention to small tributaries of large water bodies, despite their ubiquity and potential local importance. We used a snapshot of nutrient inputs from nearly all tributaries of Lake Michigan—the world’s fifth largest freshwater lake by volume—to determine how land cover and dams alter nutrient inputs across watershed sizes. Loads, concentrations, stoichiometry (N:P), and bioavailability (percentage dissolved inorganic nutrients) varied by orders of magnitude among tributaries, creating a mosaic of coastal nutrient inputs. The 6 largest of 235 tributaries accounted for ∼70% of the daily N and P delivered to Lake Michigan. However, small tributaries exhibited nutrient loads that were high for their size and biased toward dissolved inorganic forms. Higher bioavailability of nutrients from small watersheds suggests greater potential to fuel algal blooms in coastal areas, especially given the likelihood that their plumes become trapped and then overlap in the nearshore zone. Our findings reveal an underappreciated role that small streams may play in driving coastal eutrophication in large water bodies. Although they represent only a modest proportion of lake-wide loads, expanding nutrient management efforts to address smaller watersheds could reduce the ecological impacts of nutrient loading on valuable nearshore ecosystems.

Water Quality Characteristics of a Water Hyacinth Infested Tropical Highland Lake: Lake Tana, Ethiopia

“…Lakes receive nutrients from external and internal sources (Ramm and Scheps, 1997;Pettersson, 1998;Jeppesen et al, 2007;Mooney et al, 2020). External sources consist of point and non-point sources.…”

Tributary Loading and Sediment Desorption as Sources of PFAS to Receiving Waters

“…In all sites that show elevated aqueous PFAAs, the tributary discharge is relatively low (<1 m 3 /s). 64 Areas with low PFAA concentrations include the eastern and northern shores of the bay (i.e., the Door County peninsula and the upper peninsula of Michigan, respectively), corresponding to a regional background level of 22 ng/L for summed PFAAs (Table S8). Areas with elevated aqueous PFAAs tend to be individual tributaries rather than entire regions around the bay, suggesting that PFAAs are closely associated with nearby sources.…”

“…PFAA concentrations in Green Bay tributaries including (a) concentrations in the water column, (b) loading rates into the bay, and (c) concentrations in sediment. Loading is calculated using aqueous PFAA concentrations and estimated discharge rates . Loading is not calculated for sites denoted with an X because these sites do not drain directly into the bay.…”

“…Portage Creek receives runoff from the Delta County Airport in Escanaba, Michigan, where significant PFAS contamination has been found, and clusters with Green Bay sites due to its high PFHxS content. In all sites that show elevated aqueous PFAAs, the tributary discharge is relatively low (<1 m 3 /s) . Areas with low PFAA concentrations include the eastern and northern shores of the bay (i.e., the Door County peninsula and the upper peninsula of Michigan, respectively), corresponding to a regional background level of 22 ng/L for summed PFAAs (Table S8).…”

“…Loading is calculated using aqueous PFAA concentrations and estimated discharge rates. 64 Loading is not calculated for sites denoted with an X because these sites do not drain directly into the bay. Sediment data was not collected for sites denoted by an * due to inaccessibility or rocky riverbeds.…”