Brian C. Peterson

Water quality parameters of a Nebraska reservoir differ between drought and normal conditions

Abstract Increased anthropogenic disturbance to watersheds and rivers is exacerbating the effects of oscillating wet-dry periods common in the Midwestern United States, consequently impacting the maintenance and health of lake and reservoir systems. A change in flow regime should affect water quality parameters in reservoirs, but few data exist to document such changes. During a four-year drought, the mean water volume of Harlan County Reservoir, in south-central Nebraska, dropped more than 50%, changing this irrigation reservoir from a lotic to a lentic system. We monitored changes in chlorophyll a, turbidity, temperature and dissolved oxygen over 7 months (April–October) during 2003–2006 (drought conditions) and 2007–2009 (normal conditions) at 15 locations. Chlorophyll a and turbidity were both significantly greater during drought conditions in most months. Dissolved oxygen predominately decreased during drought conditions, while water temperature did not change. Chlorophyll a and turbidity were also greater in the upper (river inflow) than the lower (near dam) reaches of the reservoir during the drought, but were similar during normal years. The observed substantial increases in chlorophyll a levels caused the reservoir to be classified as hypereutrophic, using the Trophic State Index, during drought years but eutrophic during normal years. These data emphasize the need to include precipitation and inflow patterns when examining water quality parameters and assigning trophic status to a particular water body.

Food habits and abundance of larval freshwater drum in a South Central Nebraska irrigation reservoir

During certain times of the year high densities of larval fish can impact the aquatic community. The specific role of freshwater drum (Aplodinotus grunniens) is not known but larval freshwater drum can be abundant in some systems and have been shown to feed on zooplankton. To assist with understanding the role larval freshwater drum play in reservoirs, we investigated changes in peak abundance from 2003–2010 in addition to food habits and prey electivity in 2008 in a South Central Nebraska reservoir. We found that larval freshwater drum abundance varied 100-fold during this study. Cladocerans were an important food item for larval freshwater drum. Freshwater drum >10 mm total length selected for Daphnia spp. (46–80 % of organisms found in stomach), while freshwater drum ≤10 mm total length selected for Bosmina spp. (37% of organisms found in stomach), which is a less common group of zooplankton in this reservoir. All larval freshwater drum showed a negative selectivity for copepod nauplii (the Strauss index value of ≤−0.35 for all length groups). Based on the food habits described in this study, larval freshwater drum have the potential to compete with larval sport fish species for desirable zooplankton (ie, Daphnia spp.).

Changes in relative abundance of adult walleye and egg density following the addition of walleye spawning habitat in a midwest irrigation reservoir

Cobble substrate for walleye (Sander vitreus) spawning was added at Sherman Reservoir, Nebraska, in January 2008. We evaluated changes in relative adult walleye abundance and egg density in response to the cobble substrate and estimated site fidelity of adult walleye during the spawning seasons of 2007, 2008, and 2009. A 15% increase in electrofishing catch-per-unit-effort (CPUE) of adult walleye was observed on the cobble substrate the first season it was available (2008), but a 5% decrease in electrofishing CPUE was observed the second season (2009) compared to the 2007 season. While gill nets were not run during the first season the cobble was available, a 113% increase in gill net CPUE was observed the second season. During the first season the cobble was available, a 215% increase in egg density was observed, but no increase in egg density was found in the second season. Site fidelity of adult walleye within a spawning season was 94% and was 81% between seasons. These results indicate that the addition of the cobble substrate increased the relative abundance of adult walleye and density of eggs on the cobble substrate.

Long-term changes in biotic and abiotic factors influence larval gizzard shad (Dorosoma cepedianum) annual peak density

ABSTRACT Gizzard shad (Dorosoma cepedianum) are an influential forage fish and driver of zooplankton resources in many reservoirs. The ability to identify the biotic and abiotic factors that influence the timing of elevated gizzard shad densities can be important to better utilize this forage fish and manage sport fish. The objective of this study was to investigate which biotic and abiotic factors influence larval gizzard shad annual peak density. We used combinations of six variables (CPUE of adult gizzard shad from the prior fall, mean zooplankton density, turbidity, chlorophyll a, relative reservoir elevation, and water temperature) from a long-term (2003–2014) monitoring program to create 28 a-priori candidate models to assess the relative support of explanatory variables using Akaikes information criterion (AIC). During the course of our study, larval gizzard shad annual peak densities were best explained by zooplankton density and relative reservoir elevation. Zooplankton density provides greater food availability which has been shown to increase larval gizzard shad growth. Relative reservoir elevation was negatively associated with higher densities of larval gizzard shad. This study describes two important factors that may result in greater larval gizzard shad densities, which can assist managers in anticipating available forage for sport fish and identify strategies to improve system management.

Assessment of Seasonal Changes in Abiotic and Zooplankton Communities in Highly and Moderately Alkaline Sandhills Lakes

Regionally proximate waters in the Sandhills of Nebraska are known to have distinctive alkaline conditions. However, little is known about the seasonal variability that exists for abiotic conditions and zooplankton communities within these same waters. This study assessed two highly alkaline and four moderately alkaline lakes for seasonal trends in conductivity, turbidity, pH, temperature, dissolved oxygen, and nitrate, as well as zooplankton biodiversity and density. These findings indicate that each water has seasonally dynamic conditions and communities. Future reporting should identify the season of collection and expand the temporal collection to more accurately characterize these waters. Additionally, managers should not assume that proximate waters demonstrate similar seasonal variation.