Paleolimnological proxies reveal continued eutrophication issues in the St. Lawrence River Area of Concern

Abstract

Recent surface-water surveys suggest that high nutrient concentrations and nuisance algae remain issues in the St. Lawrence River Area of Concern (AOC) at Cornwall, Ontario, specifically in the tributaries and nearshore zones of Lake St. Francis (LSF). In particular, it is unclear whether management actions designed to reduce nutrient inputs, first implemented in the 1990s as part of the Remedial Action Plan for the AOC, have reduced algal production or influenced assemblage composition. To address this issue, a paleolimnological approach was used to provide a historical context for the present-day nutrient concentrations and to quantify the extent of change in water quality in LSF since the early 1990s. A sediment core was collected near the north shore of LSF and was examined for changes in the concentrations and compositions of fossil diatoms and pigments, as well as stable isotope (δ 15 N and δ 13 C) values.Analyses of diatom and pigment concentrations indicated that overall algal abundance has risen in the last few decades, including trends of increasing occurrences of potentially toxic cyanobacteria, despite ongoing remediation efforts. Temporal patterns of stable isotope signatures in the core suggest a steady increase in nutrient influx since the mid-20 th century, with the post-1990 increase in algal production likely attributable to recent inputs associated with land-use changes in local contributing watersheds. These patterns suggest that the AOC delisting goals for the LSF tributaries will not be reached without a drastic change in land management practices.

Subfossil Chironomid Assemblages as Indicators of Remedial Efficacy in the Historically Contaminated St. Lawrence River at Cornwall, Ontario

“…The decrease of M. pedellus-type and M. radialis-type from ~ 1980 onward suggests a secondary ecological shift. The decline of M. pedellus-type could be caused by a decrease in nutrients, as widespread remedial ecological actions in the 1990s decreased nutrient loading to LSF (Moir et al 2018).…”

“…The increase in D. nervosus-type and Parantanytarsus, which prefer organic, macrophyte-dominated environments(Brooks et al 2007), is consistent with an increase in plant biomass. While an increase in metal concentrations co-occurred with these changes, these concentrations are likely insu cient to cause such a dramatic shift (based on sediment quality guidelines;Persaud et al 1993) in contrast to documented changes in habitat and food source, which are often the most important factors governing chironomid assemblage structure(Moore 1978).While Grenadier Basin was selected as a reference site due to its distance from major tributaries and point-source agricultural inputs, LSF experienced elevated nutrient concentrations, terrestrial inputs, eutrophication, and undesirable algae through the mid-to-late 20th century(Reavie et al 1998;Moir et al 2018). Our sediment core shows a carbonate peak at 10.25 cm (~ 1940-50s) which suggests that Grenadier Basin experienced elevated terrestrial inputs potentially due to local ooding from water level rise.…”