Experimental thermocline deepening alters vertical distribution and
community structure of phytoplankton in a four-year whole-reservoir
manipulation
Abstract
1. 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. 2. We conducted whole-ecosystem thermocline deepening
manipulations in a small reservoir. We used an epilimnetic mixing system
to experimentally deepen the thermocline in two summers, simulating
potential responses to storms, and 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
discrete 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. 3. 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 weaker thermal stratification and deeper 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. 4. 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 distribution and community composition interact at inter-annual
and intra-annual timescales. 5. 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 emphasize that
phytoplankton depth distributions are sensitive to global change
stressors and effects on depth distributions should be taken into
account when predicting phytoplankton responses to increased storms
under global change.