Browsing by Author "Heald, Emily"

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    Effects of Whole-Lake Mixing on the Diet of Rainbow Smelt (Osmerus mordax)
    (2016) Dobosenski, Jamie; Heald, Emily; Hrabik, Thomas
    Rainbow smelt, an invasive coldwater fish species native to the eastern coastal US, were intentionally introduced into the Laurentian Great Lakes Region in the early 1900s to serve as forage for predatory fishes. Rainbow smelt reduce populations of native fish in the systems they invade. To eradicate invasive cold-water rainbow smelt in Crystal Lake, the lake underwent whole-lake mixing. The manipulation successfully created isothermic conditions and removed all coldwater habitat required by rainbow smelt. Although the induced mortality rate was significant (90%), the mixing did not eradicate rainbow smelt from Crystal Lake. For my research I hypothesized that a shift in diet may have helped the rainbow smelt population persist during the thermal manipulation. The mixing event removed the coldwater habitat, thereby inducing a spatial shift of rainbow smelt in an attempt to find new habitat. This movement could have changed the prey availability and prey selection, and those that were able to adapt to that change survived. There was evidence that rainbow smelt diet was effected by whole-lake mixing. Overall, the proportion of general zooplankton decreased during mixing and returned to a high proportion after mixing. The proportion of larger diet items increased during mixing. These finding may have future implications for management strategies.
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    Experimental mixing in a north temperate lake: examination of variability in spatial autocorrelation of fish and zooplankton populations
    (2015-07) Heald, Emily
    In lakes, biotic and abiotic variables interact at multiple spatial and temporal scales, resulting in heterogeneous horizontal distributions of organisms. Although habitat heterogeneity is a vital aspect of ecosystem function and performance, few studies recognize spatial autocorrelation and scale dependence of biotic communities within their abiotic environment. Zooplankton and zooplanktivorous fish represent two trophic levels of pelagic lake food webs whose heterogeneous horizontal distributions may be affected by water movements, prey availability, predatory avoidance, swimming capacity, and thermal tolerance. The most active driver of these distributions depends heavily on scale of analysis. We used hydroacoustic surveys and variogram analysis in a small, north-temperate lake in Vilas County, WI to compare whole-lake horizontal patterns of fish and zooplankton separately, then examined patterns in their interactions. We tested the durability of these patterns in response to a whole-lake manipulation project in which we experimentally destratified the lake, and examined how horizontal patterns change in response to alteration in vertical structure. Lake manipulation decreased the scale of spatial autocorrelation for fish populations. Fish subsequently returned to pre-manipulation spatial patterns after mixing ceased, suggesting the manipulation induced a flight response by fish in response to removal of preferred coldwater habitat. We did not detect changes in zooplankton spatial patterning with mixing, but found support for the hypothesis that external factors, such as wind and wind-induced water movements, may drive large scale horizontal spatial patterns in zooplankton distributions. We also found that fish and zooplankton aggregations exhibit spatial structure at different scales, which may have implications for sampling designs measuring both fish and zooplankton. We detected bottom-up effects where fish aggregate in regions of high zooplankton biomass for at least a portion of the year, and did not detect top-down effects.
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