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Spatiotemporal phytoplankton patterns in the Upper Mississippi River in response to seasonal variation in discharge and other environmental factors

Decker, J.K., J.D. Wehr, J.N. Houser, and W.B. Richardson.  2015 (online). Spatiotemporal phytoplankton patterns in the Upper Mississippi River in response to seasonal variation in discharge and other environmental factors.  River Systems.  DOI:10.1127/rs/2015/0103

Abstract

The Upper Mississippi River (UMR), as a relatively unconstrained floodplain river, has a lateral expanse of contrasting aquatic areas. Differences in environmental sensitivities and tolerances among phytoplankton are thought to result in spatial heterogeneity in species composition across aquatic areas. Identifying phytoplankton abundance patterns is critical for understanding water quality properties, nutrient cycling, and ecosystem processes. This study aims to identify (1) spatial (main channel vs. contiguous backwater) and temporal (seasonal and annual) patterns in phytoplankton species abundance in the UMR near LaCrosse, Wisconsin and (2) the environmental factors that explain these spatiotemporal patterns. Our study contained two components: (1) a spatiotemporal study that spanned 5 years (1999–2004), two seasons (spring and summer), and two contrasting aquatic areas (main channel and backwater) and (2) a time series of monthly mean environmental conditions and phytoplankton community structure in a single backwater that included the growing season of two consecutive years (2000–2001). Non-metric multi-dimensional scaling (MDS) ordinations indicated that the largest differences in phytoplankton species composition were seasonal, and associated with seasonal differences in river discharge. Spatial differences (main channel vs. backwater) also accounted for compositional differences but to a smaller degree. These differences were more pronounced in years when discharge was higher. Comparisons with historical phytoplankton data identified a long-term shift towards more eutraphentic species, including bloom-forming cyanobacteria such as Aphanizomenon flos-aquae, indicating that nutrients are important to phytoplankton dynamics in the UMR. Variation in TP and Si best explained spatiotemporal species abundance patterns, while variation in TP alone best explained the periodicity of cyanobacterial abundance. Short-term changes (4-week intervals) in species composition in the backwater were best explained by variation in NH4+, Si, and Mg concentrations, light, and temperature. Our study highlights the necessity for long-term monitoring for identifying factors that drive species composition shifts.

Keywords

Upper Mississippi River; cyanobacterial blooms; hydrological connectivity; phytoplankton; river-floodplain systems