Biogeochemistry of wetlands in watersheds affected by aerial deposition of metals, and linkages to aquatic ecosystem recovery.

Wetlands are prevalent in the boreal shield, and are efficient at retaining metals and thus modifying the chemistry of downstream receiving waters. The Sudbury region of Ontario, Canada has suffered over a century of elevated metal and sulphur deposition from mining and smelting activity, and wetlands in the area have provided an important ecosystem service by adsorbing large concentrations of metals over decades of emissions. However, metals can be released from peat in some circumstances such as during dry periods that result in a drop in water table height. With emissions declines in the region, there is the potential that wetlands have remained contaminated and are now acting as a source of metals at annual scales. Stream metal concentrations and dissolved organic matter (DOM) exports were evaluated in six Sudbury wetland-draining streams to address the question: Are wetlands in mining-impacted watersheds releasing potentially toxic metals to receiving waters? Water table declines occured in response to summer droughts, and some metals (Al, Co, Cu, Fe, Mn, Ni, and Zn) were released during rewetting events. Other metals that have a high affinity for DOM (Cu, Al, Fe) were also released from some wetlands in the summer in response to mineralization and DOM release. Concentrations in streams exceeded provincial guidelines and fluxes of some metals (e.g. Ni, Co) exceeded atmospheric deposition inputs to lakes by as much as 12 times. Changes in DOM concentration and composition occurred concurrently with the mobilization of metals after drought, with large decreases in aromaticity (SAC340) coupled with decreased DOC concentration. Biotic ligand modelling indicated that these drought-related DOM changes had a large effect on Cu toxicity, and while predicted Ni toxicity increased because of drought-related increases in Ni concentrations, predicted Cu toxicity increased primarily because of drought-related DOM changes. It is likely that these metal stores will take a very long time to deplete, iv and this offers important insight into the recovery trajectory of aquatic communities in watersheds affected by smelting. The results can be used to advance restoration efforts in the Sudbury region and other similar smelter-impacted areas affected by aerial depositon of metals.Doctoral These

Dry conditions disrupt terrestrial-aquatic linkages in northern catchments.

Aquatic ecosystems depend on terrestrial organic matter (tOM) to regulate many functions, such as food web production and water quality, but an increasing frequency and intensity of drought across northern ecosystems is threatening to disrupt this important connection. Dry conditions reduce tOM export and can also oxidize wetland soils and release stored contaminants into stream flow after rainfall. Here, we test whether these disruptions to terrestrial-aquatic linkages occur during mild summer drought and whether this affects biota across 43 littoral zone sites in 11 lakes. We use copper (Cu) and nickel (Ni) as representative contaminants, and measure abundances of Hyalella azteca, a widespread indicator of ecosystem condition and food web production. We found that tOM concentrations were reduced but correlations with organic soils (wetlands and riparian forests) persisted during mild drought and were sufficient to suppress labile Cu concentrations. Wetlands, however, also became a source of labile Ni to littoral zones, which was linked to reduced abundances of the amphipod H. azteca, on average by up to 70 times across the range of observed Ni concentrations. This reveals a duality in the functional linkage of organic soils to aquatic ecosystems whereby they can help buffer the effects of hydrologic disconnection between catchments and lakes but at the cost of biogeochemical changes that release stored contaminants. As evidence of the toxicity of trace contaminant concentrations and their global dispersion grows, sustaining links among forests, organic soils and aquatic ecosystems in a changing climate will become increasingly important.Natural Environment Research Council (Grant ID: NE/L006561/1)This is the final version of the article. It first appeared from Wiley via https://doi.org/10.1111/gcb.1336

Bridging between litterbags and whole-ecosystem experiments: a new approach for studying lake sediments

Nearshore sediments have a major influence over the functioning of aquatic ecosystems, but predicting their response to future environmental change has proven difficult. Previous manipulative experiments have faced challenges controlling environmental conditions, replicating sediment mixing dynamics, and extrapolating across spatial scales. Here we describe a new approach to manipulate lake sediments that overcomes previous concerns about reproducibility and environment controls, whilst also bridging the gap between smaller microcosm or litterbag experiments and whole-ecosystem manipulations. Our approach involves submerging moderate-sized (~15 L) artificial substrates that have been standardised to mimic natural sediments within the littoral zones of lakes. We show that this approach can accurately mirror the absolute dissolved organic carbon concentrations and pH of pore water, and to a lesser degree inorganic carbon concentrations, from natural lake sediments with similar organic matter profiles. On a relative basis, all measured variables had similar temporal dynamics between artificial and adjacent natural sediments. Late-summer zooplankton biomass also did not differ between natural and artificial sediments. By offering a more realistic way to manipulate freshwater sediments than previously possible, our approach can improve predictions of lake ecosystems in a changing world

Global change-driven effects on dissolved organic matter composition : Implications for food webs of northern lakes

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans