Susan E. Knight

Benthic-Pelagic Links: Responses of Benthos to Water-Column Nutrient Enrichment

Although the responses of pelagic algae and invertebrates to gradients of nutrient enrichment are well known, less is known about the responses of benthos to such gradients or how benthic and pelagic responses may interact. We performed a 9-wk experiment in 2000-L mesocosms in the field to test for the effect of water-column nutrient enrichment on phytoplankton, algae on sediments (epipelon) and hard surfaces (plastic strips), as well as pelagic and benthic primary consumers. The experimental design consisted of 4 nutrient enrichment rates (0, 0.5, 1.0 and 2.0 μg P L-1 d-1, together with N to yield an N:P ratio of 20:1 by weight). Nutrient enrichment induced significant increases in chlorophyll a in phytoplankton and attached algae, but not epipelon. Zooplankton biomass was significantly higher in enriched mesocosms than in controls over the initial 4 wk of enrichment, but the effect was not sustained over the course of the experiment. Densities of sediment-dwelling, and hard-substrata-associated invertebrates were higher in enriched treatments relative to controls. Emergence of benthic insects also increased with enrichment. Size and species composition of benthic macroinvertebrates differed between enriched treatments and controls. Our results suggest that nutrients added to the water column were quickly converted into benthic biomass, likely reducing pelagic responses to enrichment.

Genetic consequences of outcrossing in the cleistogamous annual, Impatiens capensis. II: Outcrossing rates and genotypic correlations

The genetic consequences of a plants mating system depend on both the degree of outcrossing and the genetic relationship between mates. We examined the electrophoretic genotypes of seeds derived from cleistogamous (CL) and chasmogamous (CH) flowers in six populations of the facultatively cleistogamous annual, Impatiens capensis. Multilocus estimates of the outcrossing rates for the strongly protandrous CH flowers ranged from 0.29 to 0.71 and were higher than estimates based on single‐locus data. Such results suggest that the CH flowers experience variable levels of both geitonogamous self‐fertilization and biparental inbreeding. A new and generally applicable technique based on the relative level of inbreeding within progeny groups provided direct estimates of the correlation between the genotypic values of outcrossed mates. These correlations varied widely among populations and contributed up to half of the inbreeding observed among the CH progeny. Such biparental inbreeding biases estimates of the outcrossing rate based on the mixed‐mating model downward and influences mating‐system evolution by decreasing the “cost of meiosis.”

Costs of carnivory in the common bladderwort, Utricularia macrorhiza

SummaryCarnivorous plants are usually restricted to nutrient-poor environments, suggesting that there is a cost to caputuring animals that is offset by the benefits of carnivory only under unusual circumstances. One such cost could involve a reduced photosynthetic capacity associated with the growth and maintenance of prey-capture organs. This hypothesis is tested using the common bladderwort, Utricularia macrohiza, which bears numerous distinct prey-capture bladders. Measurements of the photosynthetic and respiration rates of leaves and bladders were incorporated into growth models to estimate the growth rates of plants with and without bladders. Comparisons were made in three lakes which differed in nutrient status and in which plants exhibited marked differences in their densities of prey-capture bladders. Overall, photosynthetic rates for leaves were approximately twice those for bladders while respiration rates did not differ significantly between tissues. Calculations incorporating these values indicate that plants producing both bladders and leaves would grow to as little as 21% or as much as 83% of plants that produced leaves alone. Comparisons among lakes led to the rejection of the hypothesis that plants from some lakes are able to produce more bladders per leaf because bladders differ in their photosynthetic productivity.

Genetic consequences of outcrossing in the cleistogamous annual, Impatiens capensis. I: Population-genetic structure

We examined the genetic structure of 11 populations of Impatiens capensis, a cleistogamous annual herb, using starch gel electrophoresis. We sampled both cleistogamous (CL) and chasmogamous (CH) progeny (if present) from maternal parents in each population to infer maternal genotypes and to estimate the extent and pattern of inbreeding within and among populations. Only eight of 31 loci were polymorphic, with one to six (mean = 3.1) loci varying within each population. Mean heterozygosity per individual is quite low (mean = 3.9%) and comparable to highly self‐fertilized species. Gene flow is low, and genetic distances do not parallel geographical distances, suggesting a population structure similar to Wrights Island model with drift among the populations. Fixation indexes within populations (f̂ or FIS) span the largest range yet reported for a plant species (0.26 to 0.94, mean = 0.57). Further inbreeding results from population substructuring (θ^ or FST=0.46) , resulting in a total average inbreeding coefficient (F̂ or FIT) of 0.77. Despite these high overall levels of inbreeding, chasmogamy significantly reduces fixation, which may account for the observed greater fitness of CH progeny.

Testing a methodology for assessing plant communities in temperate inland lakes

Abstract We outline and test an aquatic plant sampling methodology designed to track changes in and make comparisons among lake plant communities over time. The method employs a systematic grid-based point-intercept sampling design with sampling resolution adjusted based on littoral area and lake shape. We applied this method in 72 Wisconsin lakes ranging from 6.5–245 ha in size, recording species presence–absence and depth at approximately 20,000 unique sample points. To assess how reductions in sampling effort might affect data quality, we used Monte Carlo simulations (100 iterations at each of 9 levels of sampling intensity) to reduce total lake sample points by 10% through 90% using a stratified random selection approach. Species accumulation curves were fit using the Michaelis-Menten 2-parameter formula for a hyperbola, and the predicted asymptote was similar to observed species richness. In a subset of lakes, oversampling (200% effort) did not yield significant increases in species richness. However, even a modest reduction (10–20%) in sampling effort affected species richness, while frequencies of occurrence of dominant species and estimations of percent littoral area and maximum depth of plant growth were less sensitive to sampling effort. In addition, we provide results of a power analysis for detecting changes in plant communities over time. Future applications of this protocol will provide information suitable for in-lake management and for assessing patterns in aquatic plant communities state-wide related to geographic region, hydrological characteristics, land use, invasive species and climate.

Genetic consequences of outcrossing in the cleistogamous annual, Impatiens capensis . III. Interlocus associations

Electrophoresis was used to examine levels of association between alleles at different variable loci within eight natural populations of the annual plant, Impatiens capensis. This species produces both obligately self-fertilizing cleistogamous (CL) and strongly protandrous chasmogamous (CH) flowers. Values of gametic (Dij) and composite (Δij) disequilibrium estimated for all pairs of covarying loci revealed significant levels of disequilibrium in most of the populations. A bootstrap technique was used to obtain weighted means and standard errors of normalized disequilibrium estimates and these were compared among populations and groups. Mean composite disequilibrium (Δ′) spanned a remarkable range among populations (0·9−1·0, mean = 0·43), as did fixation indexes (f), but these were only weakly correlated with each other (r = 0·50). Levels of disequilibrium in the CL and CH progeny generally resembled those found in their maternal parents, but values for the CH group were more variable. A reduction in disequilibrium was sometimes associated with outcrossing, but it is unlikely that selection to reduce disequilibrium favours outcrossing in this species.

A field test on the effectiveness of milfoil weevil for controlling Eurasian watermilfoil in Wisconsin lakes

We tested the effectiveness of milfoil weevils (Euhrychiopsis lecontei) for reducing biomass of Eurasian watermilfoil (Myriophyllum spicatum, EWM) under natural lake conditions in a 3-year field experiment. In each of four lakes, we randomly chose two EWM beds for stocking and two beds as controls. A total of ca. 40,000 weevils were added to the eight stocked beds. During June and August 2013–2015, we measured plant diversity, biomass of EWM and native plants, and weevil abundance in the 16 study beds. Background weevil densities varied widely among beds and were often greater than the densities stocked. Weevil stocking had no significant effect on EWM or native plant biomass. Nevertheless, weevil damage to EWM was common and its extent appeared strongly related to observed densities of the weevil. ANCOVA results indicated that weevil density was a significant predictor of EWM biomass in both June and August, but not on growth during summer. Overall, our study found that weevil density is an important factor for predicting EWM biomass, while weevil density is likely affected by a large number of environmental factors. This work highlights the importance of carefully considering lake conditions that may influence the efficacy of stocking for biological control.

Abundance of milfoil weevil in Wisconsin lakes: Potential effects from herbicide control of Eurasian watermilfoil

ABSTRACT Havel JE, Knight SE, Miazga JR. 2017. Abundance of milfoil weevil in Wisconsin lakes: potential effects from herbicide control of Eurasian watermilfoil. Lake Reserve Manage. 00:1–10. Although extensive research has revealed the milfoil weevil (Euhrychiopsis lecontei) to have potential for biological control of Eurasian watermilfoil (EWM, Myriophyllum spicatum), herbicides are widely used for controlling this invasive aquatic plant. We hypothesized that density of the milfoil weevil would be lower in lakes with a history of treating EWM with herbicides than in untreated lakes. We surveyed 3 groups of lakes in the Northern Highland Lake District of Wisconsin to see if densities of the milfoil weevil and extent of weevil damage to milfoil depended on history of herbicide control in the lakes or on species of milfoil (EWM or northern watermilfoil, NWM, Myriophyllum sibiricum). We detected milfoil weevils in 28 of 36 lakes surveyed. Despite healthy EWM beds, mean weevil density in lakes that had herbicide treatment of EWM within the past 10 yr was only one-fifth of that in comparable lakes without a recent history of herbicide control. Densities were also significantly higher in untreated EWM lakes than in lakes with only NWM and no EWM. The frequency of milfoil showing evidence of weevil damage depended strongly on the density of the milfoil weevil. The pattern of low weevil density in treated lakes may be explained either by slow recolonization of weevils after destruction of host EWM or by historically low densities of weevils releasing EWM to grow in abundance, triggering aggressive treatment with herbicides.