Whole-Ecosystem Experiments Reveal Varying Responses of Phytoplankton Functional Groups to Epilimnetic Mixing in a Eutrophic Reservoir

Abstract

Water column mixing can influence community composition of pelagic phytoplankton in lakes and reservoirs. Previous studies suggest that low mixing favors cyanobacteria, while increased mixing favors green algae and diatoms. However, this shift in community dominance is not consistently achieved when epilimnetic mixers are activated at the whole-ecosystem scale, possibly because phytoplankton community responses are mediated by mixing effects on other ecosystem processes. We conducted two epilimnetic mixing experiments in a small drinking water reservoir using a bubble-plume diffuser system. We measured physical, chemical, and biological variables before, during, and after mixing and compared the results to an unmixed reference reservoir. We observed significant increases in the biomass of cyanobacteria (from 0.8 ± 0.2 to 2.4 ± 1.1 μg L−1, p = 0.008), cryptophytes (from 0.7 ± 0.1 to 1.9 ± 0.6 μg L−1, p = 0.003), and green algae (from 3.8 to 4.4 μg L−1, p = 0.15) after our first mixing event, likely due to increased total phosphorus from entrainment of upstream sediments. After the second mixing event, phytoplankton biomass did not change but phytoplankton community composition shifted from taxa with filamentous morphology to smaller, rounder taxa. Our results suggest that whole-ecosystem dynamics and phytoplankton morphological traits should be considered when predicting phytoplankton community responses to epilimnetic mixing.

Experimental thermocline deepening alters vertical distribution and community structure of phytoplankton in a 4‐year whole‐reservoir manipulation

“…In contrast, another study did not report homogenisation, but instead a deepened biomass maximum after a storm (Garneau et al, 2013;represented in Figure 1b). Finally, abrupt experimental thermocline deepening caused increases in small, silica-containing flagellates and decreases in colonial, filamentous phytoplankton taxa, but inconsistent total biomass responses during a single summer (Lofton et al, 2019).…”

“…The EM is installed at a depth of 5 m and extends throughout the lacustrine region of the reservoir (Figure 2). The system comprises an onshore air compressor coupled to a diffuser line of porous hose which can inject bubbles of atmospheric air at ambient air temperatures into the water column at rates up to 0.012 standard m 3 /s (see Lofton et al, 2019 for an in-depth description of the EM system). Although slight lateral mixing and entrainment of water just below the EM is possible due to the configuration of the porous hose, the majority of the mixing effect from EM operation is limited to the top 5 m of the water column, preventing the reservoir from becoming destratified (Chen et al, 2018).…”

Storm impacts on phytoplankton community dynamics in lakes

“…MFGs differentiate between, for example, large, colonial centric diatoms (MFG 6a1; e.g., Aulacoseira ) that are dependent on turbulent resuspension, and small unicellular centric diatoms (MFG 7a; e.g., Cyclotella ) that are better adapted for calm, nutrient‐rich conditions (Rühland, Paterson, & Smol, 2015), and can be conceptually mapped onto C‐S‐R strategies and light/nutrient gradients (Figure 5). Although they have not been widely applied to study storms, trait‐based functional classifications like MFG and C‐S‐R have been used to predict compositional shifts in phytoplankton communities in response to changes in nutrients, wind, rain, thermal stability, and thermocline structure over a variety of timescales (Abonyi et al, 2014; Deng, Salmaso, Jeppesen, Qin, & Zhang, 2019; Lofton, McClure, Chen, Little, & Carey, 2019; Tolotti, Thies, Nickus, & Psenner, 2012).…”

The “Bright Side” of Cyanobacteria: Revising the Nuisance Potential and Prospecting Innovative Biotechnology-Based Solutions to Integrate Water Management Programs

“…Despite the increasing interest and use of these techniques, the information is still scarce to assume their efficiency as a tool applicable to different types of water bodies and to different types of phytoplankton blooms . These methods are based mostly in response of different phytoplankton groups at the laboratory scale and not through experiments using model ecosystems . The costs associated with water destratification are variable according to the situation and size of the reservoir, the reference for large systems being within the hundred thousand dollars range .…”

“…43 These methods are based mostly in response of different phytoplankton groups at the laboratory scale and not through experiments using model ecosystems. 45 The costs associated with water destratification are variable according to the situation and size of the reservoir, the reference for large systems being within the hundred thousand dollars range. 46 For example, after a damaging bloom of Plankthothrix rubescens with high microcystin concentrations in Lake Boerhringen (0.735 km 2 ), in Germany, an artificial destratification device was installed with associated costs of EU 80 000 plus EU 3 000 annually for installation, coupled with EU 10 000 annually for monitoring.…”