Growth Rate Consequences of Coloniality in a Harmful Phytoplankter

Allometric studies have shown that individual growth rate is inversely related to body size across a broad spectrum of organisms that vary greatly in size. Fewer studies have documented such patterns within species. No data exist directly documenting the influence of colony size on growth rate for microscopic, colonial organisms.To determine if similar negative relationships between growth rate and size hold for colonial organisms, we developed a technique for measuring the growth of individual colonies of a bloom-forming, toxic cyanobacterium, Microcystis aeruginosa using microscopy and digital image analysis. For five out of six genotypes of M. aeruginosa isolated from lakes in Michigan and Alabama, we found significant negative relationships between colony size and growth rate. We found large intraspecific variation in both the slope of these relationships and in the growth rate of colonies at a standard size. In addition, growth rate estimates for individual colonies were generally consistent with population growth rates measured using standard batch culture.Given that colony size varies widely within populations, our results imply that natural populations of colonial phytoplankton exist as a mosaic of individuals with widely varying ecological attributes (since size strongly affects growth rate, grazing mortality, and migration speed). Quantifying the influence of colony size on growth rate will permit development of more accurate, predictive models of ecological interactions (e.g., competition, herbivory) and their role in the proliferation of harmful algal blooms, in addition to increasing our understanding about why these interactions vary in strength within and across environments

Genome evolution and host‐microbiome shifts correspond with intraspecific niche divergence within harmful algal bloom‐forming Microcystis aeruginosa

Intraspecific niche divergence is an important driver of species range, population abundance and impacts on ecosystem functions. Genetic changes are the primary focus when studying intraspecific divergence; however, the role of ecological interactions, particularly host‐microbiome symbioses, is receiving increased attention. The relative importance of these evolutionary and ecological mechanisms has seen only limited evaluation. To address this question, we used Microcystis aeruginosa, the globally distributed cyanobacterium that dominates freshwater harmful algal blooms. These blooms have been increasing in occurrence and intensity worldwide, causing major economic and ecological damages. We evaluated 46 isolates of M. aeruginosa and their microbiomes, collected from 14 lakes in Michigan, USA, that vary over 20‐fold in phosphorus levels, the primary limiting nutrient in freshwater systems. Genomes of M. aeruginosa diverged along this phosphorus gradient in genomic architecture and protein functions. Fitness in low‐phosphorus lakes corresponded with additional shifts within M. aeruginosa including genome‐wide reductions in nitrogen use, an expansion of phosphorus assimilation genes and an alternative life history strategy of nonclonal colony formation. In addition to host shifts, despite culturing in common‐garden conditions, host‐microbiomes diverged along the gradient in taxonomy, but converged in function with evidence of metabolic interdependence between the host and its microbiome. Divergence corresponded with a physiological trade‐off between fitness in low‐phosphorus environments and growth rate in phosphorus‐rich conditions. Co‐occurrence of genotypes adapted to different nutrient environments in phosphorus‐rich lakes may have critical implications for understanding how M. aeruginosa blooms persist after initial nutrient depletion. Ultimately, we demonstrate that the intertwined effects of genome evolution, host life history strategy and ecological interactions between a host and its microbiome correspond with an intraspecific niche shift with important implications for whole ecosystem function.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151861/1/mec15198_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151861/2/mec15198.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151861/3/mec15198-sup-0001-Supinfo.pd

Seston quality drives feeding, stoichiometry and excretion of zebra mussels

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136507/1/fwb12892.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136507/2/fwb12892_am.pd

Limits to Genetic Bottlenecks and Founder Events Imposed by the Allee Effect

The Allee effect can result in a negative population growth rate at low population density. Consequently, populations below a minimum (critical) density are unlikely to persist. A lower limit on population size should constrain the loss of genetic variability due to genetic drift during population bottlenecks or founder events. We explored this phenomenon by modeling changes in genetic variability and differentiation during simulated bottlenecks of the alpine copepod, Hesperodiaptomus shoshone. Lake surveys, whole-lake re-introduction experiments and model calculations all indicate that H. shoshone should be unlikely to establish or persist at densities less than 0.5–5 individuals m−3. We estimated the corresponding range in minimum effective population size using the distribution of habitat (lake) sizes in nature and used these values to model the expected heterozygosity, allelic richness and genetic differentiation resulting from population bottlenecks. We found that during realistic bottlenecks or founder events, \u3e90% of H. shoshone populations in the Sierra Nevada may be resistant to significant changes in heterozygosity or genetic distance, and 70–75% of populations may lose \u3c10% of allelic richness. We suggest that ecological constraints on minimum population size be considered when using genetic markers to estimate historical population dynamics

Effects of cyanobacteria on fitness components of the herbivore Daphnia

Cyanobacteria are known to negatively affect their herbivores either by being of low nutritional value, by clogging the feeding apparatus or by producing toxins, and specifically, the role of toxins has been debated. Hence, in order to assess to what extent cyanobacterial toxins affect a major herbivore (Daphnia magna) that has had previous experience of cyanobacterial toxins, we conducted a life-table study using two otherwise-similar strains of Microcystis aeruginosa, one producing and one not producing the toxin microcystin. In contrast to previous studies, we found that Daphnia population growth was positive (r > 0.1 day(-1)) on a diet containing toxic Microcystis. However, we also found that the presence of the toxin negatively affected early survival and population growth of a microcystin-tolerant D. magna clone. Although there was no effect of toxin presence on per-capita fecundity of surviving adults, Daphnia produced smaller neonates when fed toxin-containing M. aeruginosa than when fed the non-toxic mutant. Hence, although Daphnia survival, population growth and neonate size were negatively affected by microcystin presence, Daphnia populations that have prior experience with toxic cyanobacteria may show positive population growth even at high concentrations of cyanobacterial toxins. This conclusion may have considerable implications for interactions between toxic cyanobacteria and herbivores in natural systems

The effect of Mating Behavior and Temperature Variation on the Critical Population Density of a Freshwater Copepod

At low density, population growth rates of dioecious zooplankton depend on the encounter rate of potential mates, resulting in a demographic Allee effect and a critical density for population establishment and persistence. Empirical evidence confirms a critical density for the calanoid copepod Hesperodiatomus shoshone, but existing estimates of the critical density span an order of magnitude. Combining three-dimensional video analysis of mating behavior with life history data from natural populations we estimated H. shoshone critical density to be 0.44–1.44 m−3. The critical density was highly dependent on body size, primarily as a result of the latter\u27s influence on swimming speed. Swimming speed also depended on temperature, increasing \u3e 25% as temperature increased from 5°C to 16°C. Rapid swimming (1.25–2.4 cm s−1) and the ability to follow pheromone trails greatly improved the ability of H. shoshone to find mates. The large effect of temperature on mating behavior means that environmental variation can have a major effect on critical density and indicates that recovery or colonization events may be more likely to succeed in warmer lakes and/or warmer years. Considering the potential for critical densities to vary with environmental conditions is important for understanding how these thresholds determine population establishment and persistence in sexually reproducing aquatic organisms and other populations subject to Allee effects

Appendix E. Meta-analysis of the slope of Daphnia clearance rate vs. log food concentrations < 200 µgC/L from eight studies.

Meta-analysis of the slope of Daphnia clearance rate vs. log food concentrations < 200 µgC/L from eight studies

Allee Effect Limits Colonization Success of Sexually Reproducing Zooplankton

Understanding the dynamics of populations at low density and the role of Allee effects is a priority due to concern about the decline of rare species and interest in colonization/invasion dynamics. Despite well-developed theory and observational support, experimental examinations of the Allee effect in natural systems are rare, partly because of logistical difficulties associated with experiments at low population density. We took advantage of fish introduction and removal in alpine lakes to experimentally test for the Allee effect at the whole-ecosystem scale. The large copepod Hesperodiaptomus shoshone is often extirpated from the water column by fish and sometimes fails to recover following fish disappearance, despite the presence of a long-lived egg bank. Population growth rate of this dioecious species may be limited by mate encounter rate, such that below some critical density a colonizing population will fail to establish. We conducted a multi-lake experiment in which H. shoshone was stocked at densities that bracketed our hypothesized critical density of 0.5–5 copoepods/m3. Successful recovery by the copepod was observed only in the lake with the highest initial density (3 copepods/m3). Copepods stocked into small cages at 3000 copepods/m3 survived and reproduced at rates comparable to natural populations, confirming that the lakes were suitable habitat for this species. In support of mate limitation as the mechanism underlying recovery failure, we found a significant positive relationship between mating success and density across experimental and natural H. shoshone populations. Furthermore, a mesocosm experiment provided evidence of increased per capita population growth rate with increasing population density in another diaptomid species, Skistodiaptomus pallidus. Together, these lines of evidence support the importance of the Allee effect to population recovery of H. shoshone in the Sierra Nevada, and to diaptomid copepods in general