The influence of a severe reservoir drawdown on springtime zooplankton and larval fish assemblages in Red Willow Reservoir, Nebraska

Abstract

Reservoirs can be dynamic systems, often prone to unpredictable and extreme waterlevel fluctuations, and can be environments where survival is difficult for zooplankton and larval fish. Although numerous studies have examined the effects of extreme reservoir drawdown on water quality, few have examined extreme drawdown on both abiotic and biotic characteristics. A fissure in the dam at Red Willow Reservoir in southwest Nebraska necessitated an extreme drawdown; the water level was lowered more than 6 m during a two-month period, reducing reservoir volume by 76%. During the subsequent low-water period (i.e., post-drawdown), spring sampling (AprilÀJune) showed dissolved oxygen concentration was lower, while turbidity and chlorophyll-a concentration were greater, relative to pre-drawdown conditions. Additionally, there was an overall increase in zooplankton density, although there were differences among taxa, and changes in mean size among taxa, relative to pre-drawdown conditions. Zooplankton assemblage composition had an average dissimilarity of 19.3% from pre-drawdown to post-drawdown. The ratio of zero to non-zero catches was greater post-drawdown for larval common carp and for all larval fishes combined, whereas we observed no difference for larval gizzard shad. Larval fish assemblage composition had an average dissimilarity of 39.7% from pre-drawdown to postdrawdown. Given the likelihood that other dams will need repair or replacement in the near future, it is imperative for effective reservoir management that we anticipate the likely abiotic and biotic responses of reservoir ecosystems as these management actions will continue to alter environmental conditions in reservoirs.

Does artificial drawdown affect zooplankton structure in shallow lakes? A short-term study in a tropical reservoir

“…Such water level changes may also affect zooplankton by influencing their water quality conditions (Duggan et al, 2002;Watkins et al, 2013;Perbiche-Neves et al, 2013a), competition for resources, and vulnerability to predation by fish and invertebrates during the low water phase (Havens et al, 2007). Indeed, these processes can interactively affect the taxonomic structure of zooplankton communities, with subsequent changes to species dominance and community composition (Danielsdottir et al, 2007;Deboer et al, 2016).…”

“…Artificial drawdowns are now commonly used for the management of aquatic macrophytes in many tropical reservoirs (Thomaz, 2002;Thomaz et al, 2006), and studies evaluating the success of applying these practices have warranted greater attention in recent years (Havens et al, 2007;Deboer et al, 2016). This study assessed the broader ecological implications of using an artificial drawdown over a short timeframe (13 days) to reduce the abundance and distribution of Egeria in a reservoir.…”

Zooplankton: A Valuable Environmental Indicator Tool in Reservoir Ecological Management?

“…Most zooplankters including Ceriodaphnia and T. prasinus, can only take particles smaller than 20 µm [23]. Those assemblage patterns are typical in reservoirs subjected to periodic water level variations [24,25,12,13]. According to these authors, the above mentioned species evidence adaptive advantages over larger Cladocera and Copepoda, not only on account of their feeding behaviour but also because they have shorter generation times and greater tolerance to large amounts of organic matter in the water.…”

“…This value stems from their position in the food web, sandwiched between the top-down regulators (fish) and bottom-up factors (phytoplankton), thus providing information about the relative importance of top-down and bottomup control and their impact on water clarity [7]. Consequently, few studies have been undertaken to assess and understand the responses of zooplankton communities to reservoir water level fluctuations [8,9,10,11,12,13]. Serra Serrada reservoir is located in the Iberian Peninsula, within the Portuguese part of the River Douro catchment.…”

Factors influencing recruitment of walleye and white bass to three distinct early ontogenetic stages

“…; DeBoer et al. in press). We indexed age‐0 larval walleye, white bass and gizzard shad abundances (from the larval trawls) as the maximum mean daily densities for each reservoir and year.…”