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

Abstract

Freshwater phytoplankton communities are currently experiencing multiple global change stressors, including increasing frequency and intensity of storms. An important mechanism by which storms affect lake and reservoir phytoplankton is by altering the water column's thermal structure (e.g., changes to thermocline depth). However, little is known about the effects of intermittent thermocline deepening on phytoplankton community vertical distribution and composition or the consistency of phytoplankton responses to varying frequency of these disturbances over multiple years. We conducted whole‐ecosystem thermocline deepening manipulations in a small reservoir. We used an epilimnetic mixing system to experimentally deepen the thermocline via five short (24–72 hr) mixing events across two summers, inducing potential responses to storms. For comparison, we did not manipulate thermocline depth in two succeeding summers. We collected weekly depth profiles of water temperature, light, nutrients, and phytoplankton biomass as well as bottle samples to assess phytoplankton community composition. We then used time‐series analysis and multivariate ordination to assess the effects of intermittent thermocline deepening due to both our experimental manipulations and naturally occurring storms on phytoplankton community structure. We observed inter‐annual and intra‐annual variability in phytoplankton community response to thermocline deepening. We found that peak phytoplankton biomass was significantly deeper in years with a higher frequency of thermocline deepening events (i.e., years with both manipulations and natural storms) due to altered thermal stratification and more variable depth distributions of soluble reactive phosphorus. Furthermore, we found that the depth of peak phytoplankton biomass was linked to phytoplankton community composition, with certain taxa being associated with deep or shallow biomass peaks, often according to functional traits such as optimal growth temperature, mixotrophy, and low‐light tolerance. For example, Cryptomonas taxa, which are low‐light tolerant and mixotrophic, were associated with deep peaks, while the cyanobacterial taxon Dolichospermum was associated with shallow peaks. Our results demonstrate that abrupt thermocline deepening due to water column mixing affects both phytoplankton depth distribution and community structure via alteration of physical and chemical gradients. In addition, our work supports previous research that phytoplankton depth distributions are related to phytoplankton community composition at inter‐annual and intra‐annual timescales. Variability in the inter‐annual and intra‐annual responses of phytoplankton to abrupt thermocline deepening indicates that antecedent conditions and the seasonal timing of surface water mixing may mediate these responses. Our findings emphasise that phytoplankton depth distributions are sensitive to global change stressors and effects on depth distributions should be taken into account when predicting phytoplankton responses t...

Reservoir Drawdown Highlights the Emergent Effects of Water Level Change on Reservoir Physics, Chemistry, and Biology

“…Interestingly, due to the altered light environment (i.e., shallower euphotic zone depth) in the reservoir during drawdown, C max depth was associated with approximately the same light availability in 2022 as in previous years (∼1% of surface light), though at much shallower depths (Figure 6, Figure S4 in Supporting Information ). Our results support previous work in a nearby reservoir that observed the reciprocal effect, in which thermocline deepening led to a deepening of C max depth (Lofton et al., 2022). Ultimately, these results highlight the substantial plasticity of phytoplankton to adapt to changing physical conditions, optimizing their location at the depth that best matches their nutrient and light requirements.…”

“…Interestingly, our work highlighted the potential for drawdown to increase the local strength of thermal stratification at the thermocline (i.e., maximum buoyancy frequency; Figure 4). Maximum buoyancy frequency characterizes the likelihood of mixing between surface and bottom layers in a stratified waterbody (e.g., Foley et al., 2012; Mackay et al., 2014), which will affect depth profiles of water chemistry (Bush et al., 2017; MacIntyre et al., 1999; Osborn, 1980) and phytoplankton (Cullen, 2015; Leach et al., 2018; Lofton et al., 2020, 2022). Consequently, changes in thermocline strength have the potential to play an important role in modulating the effect of drawdowns on reservoir chemistry and biology, particularly during smaller drawdowns (i.e., those that do not result in destratification), like in Beaverdam Reservoir.…”

“…We calculated two metrics from the FluoroProbe data to characterize the depth distributions of fluorescencebased phytoplankton biomass in Beaverdam Reservoir, following Lofton et al (2020Lofton et al ( , 2022. First, we calculated the chlorophyll maximum (C max ) depth as the depth at which the maximum concentration of fluorescencebased biomass of each spectral group (green algae, brown algae, cyanobacteria, mixed algae, and total phytoplankton) was observed.…”

“…Taken together, these divergent changes in nutrient and light availability could result in positive or negative net effects on total phytoplankton biomass (Ma et al., 2023, Figure 1b). Moreover, alteration of thermal stratification by drawdown could also alter the distribution of phytoplankton biomass throughout the water column (Leach et al., 2018; Lofton et al., 2020, 2022). Drawdown could alter biomass distributions either by inducing mixing, thereby homogenizing biomass across depths (Kasprzak et al., 2017; Planas & Paquet, 2016; Rinke et al., 2009; Wu et al., 2015), or by facilitating the formation of deep maxima of biomass if thermal stratification increases post‐drawdown (Alldredge et al., 2002; Cullen, 2015; Lewis et al., 2017).…”

Analysis of morphological traits as a tool to identify the realized niche of phytoplankton populations: what do the shape of planktic microalgae, Anna Karenina and Vincent van Gogh have in common?

“…According to Huisman et al (2002), phytoplankton can positively grow in the euphotic zone within a "turbulent window" characterized by intermediate turbulence levels allowing phytoplankton organisms to avoid both sedimentation losses and dilution beyond their growth capacity, while being passively transported within the mixed layer. The depth of the mixed layer is therefore another crucial point for phytoplankton (e.g., Lindenschmidt & Chorus, 1998) and can influence its taxonomic and morphological structure (Lofton et al, 2022). It is in the very low-turbulence environments of boundary layers and tight metalimnetic gradients where velocities may fall to < 10 −3 m s −1 , that entrainment is critically lost and phytoplankton sinks to a greater depth (Wheeler et al, 2019).…”