To measure chlorophyll or phytoplankton biovolume: an aquatic conundrum with implications for the management of lakes

Abstract

Canfield DE, Jr., Bachmann RW, Hoyer MV, Johansson LS, Søndergaard M, and Jeppesen E. 2018. To measure chlorophyl or phytoplankton biovolume: an aquatic conundrum with implications for the management of lakes. Lake Reserv Manage. 35:181–192.

The log₁₀-transformed relationship between measured phytoplankton biovolumes and chlorophyll concentrations, surrogates for algal biomass, was examined using 13,000-plus paired samples collected from lakes in Denmark (250), the continental United States (1835), and Florida (159). A positive (R² = 0.57) relationship was found but predicted biovolumes had a 95% confidence interval of 11–912%. Regressing chlorophyll as opposed to phytoplankton biovolume against total phosphorus (TP) (R² = 0.43 vs. R² = 0.21), total nitrogen (TN) (R² = 0.24 vs. R² = 0.08), and Secchi disk (SD) (R² = 0.60 vs. R² = 0.39) yielded stronger relationships. Three algal groups (Cyanophyta, Chlorophyta, and Bacillariophyceae) contributed approximately 100% of the biovolume in some samples. For these groups and samples, biovolume increased significantly with chlorophyll (R² = 0.35, R² = 0.27, and R² = 0.31, respectively) and TP (R² = 0.18, R² = 0.13, and R² = 0.13) and decreased significantly with increases in SD (R² = 0.29, R² = 0.31, and R² = 0.17). All empirical relationships had substantial confidence intervals. Contingency tables for variance within the independent (horizontal variance) and dependent (vertical variance) variables are presented, providing managers information on how much change is required to insure noticeable effects. If resources are limited, chlorophyll is recommended for monitoring long-term trends because it provides an estimate of biomass magnitude and has better relationships with nutrients and SD. Managers may integrate occasional biovolume measurements if concerned with Cyanophyta abundance, taste and odor production, or changing algal population dynamics.

Keywords:

• Algal biomass
• chlorophyll
• nutrients
• Secchi
• contingency tables

Acknowledgments

We thank all the individuals who worked to collect the samples that helped establish the large database used here. Your names will never be known to posterity, but your efforts shall have contributed to the advancement of the science of lake management.