Filling in the Flyover Zone: High Phosphorus in Midwestern (USA) Reservoirs Results in High Phytoplankton Biomass but Not High Primary Productivity

Abstract

In lakes and reservoirs, climate change increases surface water temperatures, promotes thermal stability, and decreases hypolimnetic oxygen. Increased anthropogenic land-use and precipitation enhance nutrient and sediment supply. Together, these effects alter the light and nutrient dynamics constraining phytoplankton biomass and productivity. Given that lake and reservoir processes differ, and that globally, reservoir numbers are increasing to meet water demands, reservoir-centric studies remain underrepresented. In the agricultural Midwest (United States), ubiquitous reservoirs experience eutrophy and hypolimnetic anoxia. Here, we explore influences of eutrophication and landuse on the proximate light and nutrient status of phytoplankton communities in 32 Missouri reservoirs. Light and nutrient status indicators include mixed layer irradiance, phosphorus (P) and nitrogen (N) stoichiometry/debts, photosynthetic efficiency, and photosynthetic-irradiance parameters. Contributing to the ongoing P vs. N and P management debate, we evaluate if phytoplankton biomass and productivity are constrained by light, P, N, or a combination thereof, across gradients of trophic status and land-use during two contrasting wet and dry summers. Despite agricultural prevalence, P-deficiency is more prominent than either Nor light-deficiency. In 2018, ∼46% of samples were P-deficient with ∼36% indicating neither light nor nutrient deficiency. Gross primary productivity per unit chlorophyll-a (GPP B) demonstrates negative relationships with nutrients, biomass, and turbidity, and positive relationships with light availability. GPP B is highest in oligotrophic reservoirs where light utilization efficiency is also highest. Overall, phytoplankton biomass and productivity appear constrained by P and light, respectively. If Midwestern reservoirs are precursors of future inland waters affected by climate change and eutrophication, our crystal ball indicates that both P and light will be important regulators of phytoplankton dynamics and subsequent water quality.

Vertical Distribution of Bacterial Community in Water Columns of Reservoirs With Different Trophic Conditions During Thermal Stratification

“…Compared to natural rivers, artificial reservoirs generally exhibit hydrodynamic variations and longer exchange periods (Jardim et al, 2020). Additionally, numerous nutrients, such as nitrogen (N) and phosphorus (P) are intercepted, which alter the nutrient cycle, drive the rapid propagation of algae and other plankton, and decrease the dissolved oxygen (DO) content in the water body (Van Cappellen and Maavara, 2016;Shi et al, 2017;Petty et al, 2020), thereby worsening the eutrophication of the water body. According to 2018 data from the United Nations Water Resources Organization, over 75% of closed water bodies (such as lakes and reservoirs) are eutrophic to a certain extent, with deterioration in water quality.…”

Nonlinear multidecadal trends in organic matter dynamics in Midwest reservoirs are a function of variable hydroclimate

“…Land cover is predominantly agricultural in the north and forested in the south. This statewide land cover pattern also leads to a gradient in reservoir trophic status (Jones et al 2004, 2008, 2020; Petty et al 2020).…”

Under-ice and open-water ecosystem metabolism in temperate water bodies

“…The phytoplankton pigment absorption coefficient was estimated using the quantitative filter technique (Tassan and Ferrari 1995;Petty et al 2020). AGPF was calculated with the R package 'phytotools' (Silsbe and Malkin 2015) and integrated through depth and time (Petty et al 2020).…”

“…An inherent assumption in lake management is that primary production is positively related to phytoplankton biomass. A disconnect, however, between production and biomass has been observed in relatively turbid water bodies (Dubourg et al 2015;Petty et al 2020). In our similarly winter light deficient conditions, 50 % of the time under-ice PAR was deficient for biomass accrual in our Canadian water bodies, although in Lake Erie, winter phytoplankton growth rates were comparable to summer (Twiss et al 2014).…”