Abstract
Lake Scugog is an important headwater to the Trent Severn-Waterway in Ontario, Canada. In recent years, notable ecosystem-level changes have occurred coinciding with the emergence of the non-native invasive charophyte Nitellopsis obtusa. Despite N. obtusa arriving in North America in the early 1970s, studies documenting the impact of N. obtusa on invaded ecosystems are scarce. Given the increasing dominance of N. obtusa in inland waters of the Great Lakes basin, we investigated the ecosystem-level impacts of N. obtusa in Lake Scugog over a 3-year period. We show for the first time a strong association between N. obtusa occurrence and biomass with benthic anoxia in this shallow, polymictic lake. Benthic dissolved oxygen concentrations were significantly lower (p-value < 0.001) at sites with N. obtusa compared to sites without N. obtusa. Additionally, N. obtusa biomass was a negative predictor of near-bed oxygen concentration (R2 = 0.59, p-value < 0.001). Knowing that anoxia can promote the internal loading of phosphorus, we measured soluble reactive phosphorus (SRP) in the pore-water of sediments at each site, and found N. obtusa biomass explained 90% of sediment pore-water SRP (R2 = 0.90, p-value < 0.001). These notable associations between N. obtusa and key lake elements indicates that N. obtusa may be acting as an ecosystem engineer in invaded lakes by altering the biogeochemical fate of oxygen and phosphorus.
Highlights
Despite increasing range of distribution in the Great Lakes basin, information pertaining to the negative effects on ecosystem processes, habitat structure, and biota associated with N. obtusa invasion are in short supply and remain largely anecdotal
Based on the physical effect that dense beds of N. obtusa could have on water column mixing and near-bed biogeochemistry, we propose that N. obtusa may be an ecosystem engineer in invaded lakes and wetlands
Was low benthic dissolved oxygen (DO) associated with N. obtusa presence, but N. obtusa biomass was a negative explanatory variable for benthic DO (R2 0.59) (Figure 4A) and positive explanatory variable for sediment pore-water soluble reactive phosphorus (SRP) (R2 0.90) (Figure 4B)
Summary
We surveyed N. obtusa across both basins in Lake Scugog from 2016–2019, and saw a clear increase in the dominance of this invasive species in the macrophyte community (Harrow-Lyle and Kirkwood, 2020a). To improve our understanding of the ecosystem-level impacts of N. obtusa, and its role in Microcystis blooms in Lake Scugog, we analyzed 3-years (2017–2019) of benthic dissolved oxygen (DO) and sediment soluble reactive phosphorus (SRP) from twelve sites spanning the entire lake. We examined sediment pore-water SRP in response to N. obtusa biomass, based on the premise that depleted benthic DO concentrations would enhance internal phosphorus loading (Lake et al, 2007). We detected strong statistical relationships between DO and SRP with N. obtusa biomass, respectively These results infer a mechanistic role for this invasive macrophyte that impacts near-bed habitat condition. Based on the physical effect that dense beds of N. obtusa could have on water column mixing and near-bed biogeochemistry, we propose that N. obtusa may be an ecosystem engineer in invaded lakes and wetlands
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