Spatial and temporal trends of Minnesota River phytoplankton and zooplankton

Plankton communities have important roles in aquatic ecosystems, but studies of plankton in lotic systems are infrequent. We collected over 100 water, phytoplankton, and zooplankton samples during 2016-2018 to explore spatiotemporal trends in Minnesota River plankton communities and evaluate relationships with physico-chemical factors. Phytoplankton and zooplankton community structure exhibited temporal patterns but only the zooplankton community differed spatially. Cyanobacteria (M +/- SE; 11.27 +/- 1.43 mm(3)/L) and diatoms (8.12 +/- 1.08 mm(3)/L) dominated phytoplankton biovolume with seasonal peaks in Cyanobacteria occurring during July-September and peaks in diatoms occurring during May, August, and September. All phytoplankton taxa except Cryptophyta exhibited a negative relationship with relative discharge. Crustacean zooplankton biomass was greatest at two upstream sites (146.7 +/- 32.6 mu g/L) where cladocerans and copepods were likely exported from upstream of dams where water residence time is greater. Within the lower free-flowing reach rotifers dominated the zooplankton community (207.9 +/- 40.9 individuals/L and 6.5 +/- 1.0 mu g/L). Thus, spatial differences in zooplankton community structure were primarily attributed to the influence of dams. Seasonal patterns in zooplankton community structure included peaks in Chydoridae, cyclopoid, immature copepod, and rotifer biomass during May and Bosminidae biomass during October. Excluding the influence of dams on zooplankton, the cumulative effects of month and relative discharge were the most important for explaining variability in plankton community structure. Baseline understanding of plankton community dynamics provides the ability to quantify responses to future perturbations such as climate change and establishment of invasive planktivores.

Automated summary

Plankton communities play important roles in aquatic ecosystems, but studies on plankton in lotic systems are scarce. The study on Minnesota River plankton communities from 2016 to 2018 showed temporal patterns in phytoplankton and zooplankton community structure, while spatial differences in zooplankton community structure were primarily attributed to the influence of dams.