Epilimnetic Mixing Deepens Maximum Phytoplankton Biomass and Reduces Phytoplankton Biodiversity in a Eutrophic Reservoir

Mary E. Lofton, Ph.D. Candidate, Virginia Tech Dept. of Biological Sciences

Dexter W. Howard, Ph.D. Student, Virginia Tech Dept. of Biological Sciences

Ryan P. McClure, Ph.D., Postdoctoral Associate, Virginia Tech Dept. of Biological Sciences

Heather L. Wander, Ph.D. Student, Virginia Tech Dept. of Biological Sciences

Phytoplankton are primary producers which are critical for reservoir ecosystem function. However, certain phytoplankton, particularly cyanobacteria, can form noxious blooms in freshwater ecosystems. Epilimnetic mixing is a management strategy intended to disrupt surface scums of harmful cyanobacteria while promoting less-harmful phytoplankton taxa, such as diatoms. Many studies have examined the degree to which epilimnetic mixing decreases cyanobacterial biomass. However, fewer studies have addressed how mixing affects other aspects of the phytoplankton community, especially the vertical distribution of phytoplankton biomass and biodiversity across a wide range of taxa. Quantifying the effects of mixing on phytoplankton diversity and vertical distribution is important to avoid unintended consequences, such as causing problematic blooms of non-cyanobacterial taxa, shifting phytoplankton bloom biomass closer to drinking water outtakes, or disrupting reservoir food webs. We conducted periodic epilimnetic mixing in Falling Creek Reservoir in Vinton, VA during the summers of 2016-2017, and then did not operate the mixing system in 2018-2019. From approximately May to October in all four years, we collected weekly depth profiles of phytoplankton biomass using a fluorescence-based sensor. We also collected water samples at the depth of maximum phytoplankton biomass to assess phytoplankton biodiversity via microscopy. Epilimnetic mixing resulted in a deeper thermocline and shifted the depth of maximum phytoplankton biomass downwards (on average, by 1.6 m) during 2016-2017 relative to 2018-2019. Using time series models to assess how environmental variables affect phytoplankton, we found that epilimnetic mixing was associated with decreased phytoplankton genus richness and Shannon diversity as well as increased abundance of cryptophytes and total phytoplankton biovolume. Notably, cyanobacterial abundance did not respond to mixing. Our findings have important implications for reservoir food webs, as many organisms depend on phytoplankton as a food source, and cryptophytes generally have more nutritional value than cyanobacteria. In addition, our work indicates that mixing can change the depth of maximum phytoplankton biomass in the water column, which could affect the optimal depth for drinking water outtake. In sum, epilimnetic mixing can alter both phytoplankton distribution and community composition, with implications for ecosystem function and water management.


Author Bio

Authors continued:
Whitney M. Woelmer
Ph.D. candidate, Department of Biological Sciences, Virginia Tech, (540) 231-0788, wwoelmer@vt.edu
Alexandria G. Hounshell
Postdoctoral Associate, Department of Biological Sciences, Virginia Tech, (540) 231-0788, alexgh@vt.edu
Abigail S.L. Lewis
Ph.D. student, Department of Biological Sciences, Virginia Tech, (540) 231-0788, aslewis@vt.edu
Cayelan C. Carey
Associate Professor, Department of Biological Sciences, Virginia Tech, (540) 231-8938, cayelan@vt.edu

Bio for Mary Lofton:
Mary Lofton is a doctoral student in Dr. Cayelan Carey’s freshwater ecology lab at Virginia Tech. She joined Dr. Carey’s lab in fall 2016, and is most interested in examining how changing climate, land use, and water management interact to affect the ecosystem functioning of drinking water reservoirs, and phytoplankton community dynamics in particular. With colleagues in the Carey Lab, she conducts both basic and applied research in drinking water reservoirs.