Joelle D. Young

The state of Lake Simcoe (Ontario, Canada): the effects of multiple stressors on phosphorus and oxygen dynamics

Abstract Lake Simcoe, the largest lake in southern Ontario outside of the Laurentian Great Lakes, is affected by numerous stressors including eutrophication resulting from total phosphorus (TP) loading, climate change, and invasions of exotic species. We synthesized the long-term responses of Lake Simcoe to these stressors by assessing trends in water quality and biological composition over multiple trophic levels. Evidence for climate change included increasing thermal stability of the lake and changes in subfossil diatom communities over time. Although the deep water dissolved oxygen (O2) minimum has increased significantly since TP load reductions, it is still below estimated historical values and the Lake Simcoe Protection Plan end-of-summer target level of 7 mg O2 L-1. Low deep water O2 concentrations corresponded with a decline in coldwater fish abundance. Since 1980, some nutrient concentrations have decreased (spring TP) while others have increased (silica), but many show no obvious changes (ice-free TP, nitrate, ammonium). Increases in water clarity, combined with declines in chlorophyll a and phytoplankton biovolumes in Cook’s Bay, were temporally consistent with declines in TP loading and the lake-wide establishment of dreissenid mussels as a major component of the Lake Simcoe ecosystem. Using an investigative tool, we identified 2 periods when abrupt shifts potentially occurred in multiple parameters: 1986 and 1995-1997. Additional ecosystem level changes such as declines in zooplankton, declines in offshore benthic invertebrate abundance, and increased nearshore invertebrate abundance likely reflect the effects of invasive species. The interaction of these multiple stressors have significantly altered the Lake Simcoe ecosystem.

Temperature-dependent Allee effects in a stage-structured model for Bythotrephes establishment

Whether the invasive freshwater cladoceran Bythotrephes longimanus can establish after introduction into a water body depends on several biotic and abiotic factors. Among these, water temperature is important because both development rates and mode of reproduction (parthenogenetic or sexual) in Bythotrephes are influenced by temperature. We built a stage-structured model for the population dynamics of Bythotrephes based on the temperature-dependency of events in its life cycle and used the density of resting eggs at the end of each year to track changes in population density. The model was parameterized using data from published laboratory experiments and data on the Bythotrephes population in Harp Lake, Canada, from 1994 to 2005. The parameterized model was then used to simulate the outcome of invasions with different initial resting egg densities under different temperature regimes. A strong Allee effect emerged from the model, i.e. there is a critical threshold density above which the population can establish and below which it goes extinct. We showed analytically that the existence of an Allee effect arises from the model structure and is therefore robust to the parameter values. An increase in temperature reduces the establishment threshold for introductions in the same year as well as for introductions in the previous years. We therefore hypothesize that climate warming might facilitate Bythotrephes invasions. Finally, we study how the establishment threshold is influenced by the timing of the introduction event and thus identify time periods during the year when lakes may be particularly susceptible to Bythotrephes invasions.

Increased dependence on nearshore benthic resources in the Lake Simcoe ecosystem after dreissenid invasion

Abstract Changes in the lower trophic level of Lake Simcoe, Canada, coincident with the invasion of dreissenids have been well documented, but little is known regarding the effects of these changes on the pathways of energy flow to higher trophic levels. To evaluate pathways of energy flow, we analyzed stable isotopes of zooplankton, benthic invertebrates, and fish over a 26-year period. Using stable isotopes of carbon (C) and nitrogen (N), we found evidence for a dramatic increase in the importance of benthic-derived nearshore primary production in Lake Simcoe after the invasion of dreissenids; δ13C of benthos collected at 5 and 10 m depth increased by 4–5‰ on average, and benthic warmwater fishes demonstrated a similar increase. In contrast, δ13C of profundal (≥20 m) benthos and pelagic zooplankton were not significantly different during this time period, while offshore pelagic and profundal fishes changed more subtly in magnitude but in the same direction as nearshore benthos and warmwater fishes. The range of δ13C values observed across the fish community increased from 3 to 10‰, primarily due to a positive temporal shift among warmwater fishes. Our study highlights the increase in functional heterogeneity in Lake Simcoe after dreissenid invasion, likely due to an increase in nearshore benthic production. Accounting for increased contributions of benthic-derived C with time is critical in accurately modelling C and energy transfer in the lake, and for better understanding the degree of nearshore–offshore coupling in the lake.

Dynamics of the invasive spiny water flea, Bythotrephes longimanus, in Lake Simcoe, Ontario, Canada

Abstract The invasion of the nonindigenous zooplankter Bythotrephes longimanus in Lake Simcoe has the potential to impact overall ecosystem functioning, yet there is currently no information regarding the status and dynamics of its population. We used a combination of uni- and multivariate analyses to examine variation in Bythotrephes interannual and seasonal abundance, and life-history characteristics in Lake Simcoe from 1999 to 2007, and we explored the biotic (predators, prey) and/or abiotic (water quality, temperature) factors that may have regulated its population over this period. The mean annual abundance of Bythotrephes varied by an order of magnitude over the period of study and was significantly associated with epilimnetic temperature and prey and predator abundance. Bythotrephes typically appeared in early summer and had 2 seasonal abundance peaks, which varied in magnitude and timing within and among years. August and September Secchi depth and total phosphorus concentration predicted groups of years with similar Bythotrephes phenologies, lending further support to the role of water clarity as a significant component of this visual predator’s habitat in its invaded range. Bythotrephes body size and reproductive patterns also varied seasonally, possibly reflecting the seasonal availability of cladoceran prey, predation pressure from planktivorous fish, or adaptations to avoid unfavourable conditions. Overall, a complex interaction of fish predation, food availability, temperature, and water clarity controlled the dynamics of Bythotrephes in Lake Simcoe. Changes to the Lake Simcoe ecosystem due to Bythotrephes invasion may have important consequences for the ecosystem services provided by the lake and merit further investigation.