Long-term seasonal effects of dreissenid mussels on phytoplankton in Lake Simcoe, Ontario, Canada

Abstract

The invasion of dreissenid mussels to Lake Simcoe coincided with notable changes in the ecosystem, including a period of lower phytoplankton biovolume, particularly at shallower sites. Dreissenid grazing effects are typically most prominent during the summer season in the nearshore areas of lakes. Grazing effects in the winter are small because dreissenid filtration rates, especially those of zebra mussels (Dreissena polymorpha), decrease in cold temperatures. Phytoplankton dynamics in the winter season are poorly characterized, particularly in lakes that experience ice cover. The purpose of this study was to examine the effects of dreissenids on the seasonality of phytoplankton dynamics in the nearshore waters of the lake using long-term monitoring data from unchlorinated water treatment plant (WTP) intake pipes. The long-term dataset from the WTPs showed significant and sustained declines in phytoplankton biovolumes and chlorophyll a (Chl-a) concentrations in the 12-year period following the invasion of dreissenid mussels to Lake Simcoe. The decline was smallest during the ice-covered winter months (Jan-Mar). The fall phytoplankton peak observed in the pre-dreissenid years shifted to a smaller peak during the winter in post-dreissenid years. We hypothesized that reduced dreissenid grazing pressure during the winter, and possible improved under-ice light conditions that we attribute to climate change, may be contributing to this shift. Phytoplankton biovolume data collection continues to be important in Lake Simcoe as a measure of phytoplankton biomass. Our results also indicate that winter phytoplankton biovolumes should be considered when managing lakes experiencing both climate change and dreissenid mussel effects.

Internal phosphorus loading in Canadian fresh waters: a critical review and data analysis

“…There have, however, been significant decreases in the hypoxic factor (i.e., the number of days that an area equivalent to the bay's surface area is overlain by water with ≤2 mg·L −1 of dissolved oxygen; Nürnberg et al 2013a). Internal P loading is an important regulator of pelagic productivity in this large lake, as its timing during the autumn months corresponds to the year-round peak in primary production , total algal biomass (Baranowska et al 2013), and cyanobacteria biomass (Nürnberg et al 2013b). …”

Under-ice and open-water ecosystem metabolism in temperate water bodies

“…The under-ice light environment (estimated by Ē24) is dictated by convective mixing dynamics (Yang et al 2020), where convective cells maintain the phytoplankton at the top of the water column (Bertilsson et al 2013), improving the Ē24 Bouffard et al 2019), resulting in increased phytoplankton biomass (Suarez et al 2019). This is now a well-documented phenomenon on Lake Simcoe , related to winter phytoplankton peaks (Baranowska et al 2013;Yang et al 2017). Our companion studies on Lake Simcoe documented these under-ice phytoplankton blooms that were 10 m thick ) and composed of small centric diatoms (e.g., Stephanodiscus), representing 3x more biomass in a single event than measured during the summer sampling (Kim et al 2015).…”

Morphological, physiological, and transcriptional responses of the freshwater diatom Fragilaria crotonensis to elevated pH conditions

“…For example, here we demonstrate high pH results in smaller, browner, rougher diatoms. Based on prior research, we postulate that these morphological alterations will modify grazing patterns of secondary consumers and selective filtration of higher trophic consumers (Vanderploeg et al, 2001(Vanderploeg et al, , 2013Baranowska et al, 2013). This decrease in size will also likely increase diatom predation by Dreissenids, as smaller diatoms seem to be selectively filtered (Reavie and Barbiero, 2013).…”