Research Statement
Alan D. Christian
My research interests lie in the three goals of my research program: 1) conduct research in the field of aquatic ecology that addresses fundamental ecological questions, 2) conduct research in the field of aquatic ecology that addresses applied issues, and 3) train undergraduate, masters, and doctoral students to be scientists and to ask fundamental questions and solve pressing environmental and conservation problems. I am a broadly trained aquatic ecologist interested in understanding the role of freshwater mussels in the structure and function of freshwater systems ranging from molecular to ecosystem ecology. Mussels are an interesting and important group to study because they are relatively large (biomass), long lived (2 - 30+ years), sessile organisms that have unique life histories (e.g. obligate parasites on fish during part of their larval development). In addition, freshwater mussels are one of the most imperiled groups in North America, with nearly 70% of the nearly 300 species being extinct, endangered, or of some special concern. Indeed, freshwater mussels represent an ecologically interdependent group that lends to asking interesting and exciting basic and fundamental ecological questions as well as representing an imperiled group that can be used as a focal species for applied environmental and conservation issues.
As stated above, studying freshwater mussel ecology provides the opportunity to ask exciting and interesting basic and fundamental questions, and I use this opportunity build a research program asking both pure and applied questions from the molecular to ecosystem scale. At the molecular scale, I am interested in the mating system(s) and the population genetic structure of freshwater mussels. The mating system(s) of freshwater mussels is not well understood other than male mussels release their sperm in the water column for females to filter in and become fertilized internally. Recently myself and my students developed methods to extract DNA from individual larvae, about the size of a pin head, for micro-satellite analysis to determine how many fathers contribute to a female’s brood. Through that effort, we recently published a paper on how to successfully extract and amplify both microsatellite and mitochondrial DNA from single larvae. Our work is the first to show evidence of multiple paternity in freshwater mussels. Current and future research on the mating system(s) of freshwater mussels includes investigating the relationship of spatial patterns of males and females and paternity assignments, investigation multiple paternity across taxa and sizes of populations, and the potential for sexual selection of sperm by females.
Because freshwater mussels are relatively sessile and their larvae are obligate parasites on fish, investigating the relationship between freshwater mussel population genetic structure and host fish population genetic structure could lead to better understanding migration and distribution of freshwater mussels which can be instrumental in their conservation. I am a co-author on several papers that have established population genetics models for conservation. For small stream mussels with host fish that have limited dispersal, show a great deal of variation between populations but little variation within a population. Meanwhile, large river species with host fish that have extensive dispersal show very little variation between populations, but a lot of variation within a population. The logical next step is to see if the population genetic structure of host fish follows the same population genetic structure as the mussels. One of my undergraduate research students is using micro-satellite analysis of the host fish of a small stream mussel who will compare the population genetic structure of these host fish to the population genetic structure of the small stream mussel. This research is expected to lead to better conservation and management not only of freshwater mussels, but show direct evidence that mussel conservation must also include fish and their habitat.
Management of freshwater mussels has not always used science to establish or test best management practices in freshwater mussel conservation. Over the last four years, my lab has been involved in a research project funded by the Federal Highways Administration and the Arkansas Highway and Transportation Department to investigate two management questions: 1) What is the success of relocating mussels during bridge construction? and 2) What are the effects of bridge construction sediments on local mussel assemblages? To see if relocation has an effect on mussels, we followed the movement, fitness, and mortality of freshwater mussels during a relocation event. We are comparing movement, fitness (glycogen, lipids, and C:N ratios) and growth and survival of relocated and native animals. To see if bridge construction sediments have an effect on in situ mussels, we are studying mussel assemblage composition and fitness, sediments, and sediment movement via hydraulic modeling using a Before, After, Control and Impact experimental design. The goal of this project is to establish best management protocols for bridge construction projects and to streamline the U.S. Fish and Wildlife and NEPA Biological Assessment and Biological Opinion process for future bridge construction projects.
Associated with investigating the effects of sediment on mussels, I investigate the abiotic and biotic factors that influence freshwater mussel distributions across multiple scales (i.e. watershed, stream system, stream segment, stream reach, pool/riffle and microhabitat). At the microhabitat to stream reach scale, I investigate abiotic and biotic factors that may be important to the distribution of freshwater mussels (and other aquatic organisms) and while incorporating the size and biomass distribution of freshwater mussels across these scales. At the reach, pool/riffle and microhabitat scale, I use spatial statistics to determine the relationship between the distribution of mussels and abiotic factors such as water depth, water velocity, and substrate composition and biotic factors such as other macro-invertebrates and biofilm communities. At the stream system and stream segment scale, I collaborate with hydrologists and hydraulic engineers to look at stream segment and stream reach hydraulic and geomorphological features that may influence mussel distribution and abundance in streams. At the watershed scale, I collaborate with hydrologists to relate the distribution and abundance of mussels and fish to cumulative watershed sediment effects. Cumulative watershed effects integrate land use and habitat to assess the effects on aquatic organisms including fish, mussels, and other aquatic invertebrates. We have found that modeled sediments delivery (using GIS and WEPP) and habitat are correlated with the distribution and abundance of fish and mussels. This landscape level approach is an invaluable conservation and management tool used for identifying potential sediment impacts (e.g. roads, loss of riparian buffers) to aquatic systems and to set thresholds at which these impacts are observed.
Abiotic factors that influence freshwater mussel abundance and distribution are not the only poorly understood aspect of freshwater mussel biology and ecology. Our understanding of what mussels feed on, both qualitatively and quantitatively, also is not well understood. In order to identify food resources of mussels, I use multiple methods to determine the food quality and quantity such as nutrient analysis (CNP), organics analysis (NVSS/AFDM), C and N stable isotopes, chlorophyll concentrations, microbial biomass and community structure, and mussel gut fluid enzyme analysis. During previous work, I found that stable isotopes do not provide the appropriate resolution to pinpoint the source of freshwater mussel food. I plan to continue my research on identifying the food source(s) of freshwater mussels by radiolabeling autotrophic and heterotrophic microbes and feeding labeled microbes to freshwater mussels in controlled laboratory or mesocosm settings to determine if mussels incorporate microbial cells, microbial exopolymers, both, or neither. I also plan to continue my collaboration with microbial ecologists for the seston microbial biomass and community structure. This work has food web implications determining which trophic level freshwater mussels feed and also can be used to determine if different species of mussels are using the same food source (functional redundancy) or partitioning the resources. My previous research using stable isotopes and gut fluid enzyme activities showed that two co-occurring species of mussels had similar gut fluid enzymes and stable isotope signatures indicating that they were using a similar low protein diet as indicated by high protease activity. In addition to radiolabeling, I plan to move down the river continuum from small to large streams to determine if and how mussel food use changes down the continuum.
As a logical progression from my research interests listed above, I conduct research on the role mussels play in nutrient cycling investigating the ecological stoichiometry of mussel’s food, body tissue, and excretion and egestion. Since the quality and quantity of seston (food source) changes from season to season, event to event, and from site to site, it is important to understand the spatial and temporal patterns of mussel nutrient recycling rates and ratios. Further, species have different size structure and nutrient composition of tissues, the concentrations, ratios, and demand for nutrients are different. Therefore, it is possible two species of mussels feeding on the same general resource with the same nutrient composition may assimilate different amounts of nutrients due to their body content and demand, i.e. they are stoichiometrically different. The implications of differences in demand will result in different rates and ratios of soluble and fecal nutrients being recycled into the water column. Therefore, even though mussels are feeding on similar food items, they may recycle the nutrients in different rates and ratios, providing different ecosystem functions, providing for a unique ecosystem function. Conversely, two or more species may be redundant in recycling nutrients, providing for more stability in the system if one or more species should become extinct. I have had two papers published on the nutrient excretion and ecological stoichiometry of mussels and one of my form masters students is working the manuscript for a third paper.
After establishing the rates and ratios mussels excrete nutrients, I plan to model, using empirical data, the contribution mussels provide to nutrient fluxes in streams and to meeting the demand for nutrients by other organisms such as primary producers or heterotrophic bacteria. During my dissertation work, I found that the mussel assemblage in the Ouachita River headwaters (Arkansas) provided ~10% of the N and P flux of the stream during low flow conditions in the fall and that nutrients were limiting to primary producer growth during this time. The final step in determining the role of mussels in streams will be to conduct nutrient spiraling experiments in reaches with and without mussels. I predict that reaches without mussels will have shorter nutrient spirals than those with mussels providing nutrients to the reach. In conjunction with the nutrient spiraling experiment, I will determine the demand for these nutrients by incorporating primary production and possibly heterotrophic production measurements and calculations.
In closing my research program focuses on the role of freshwater mussels in freshwater ecosystems across spatial and temporal scales. Indeed, this information will lead to a better understanding of basic mussel biology and ecology and for better conservation and management of this imperiled fauna. This research in not only limited to my experience with freshwater mussels, but also my broad interest in the community composition of other freshwater organisms including fish and other macro-invertebrates. My research involves a variety of laboratory and field techniques, experiments, and surveys and involves a variety of training opportunities.