Sierra Nevada Aquatic Research Laboratory

01 Jul, 11 AM - 01 Oct, 05 AM

Consequences of tadpole declines on the diversity of algae in Sierra Nevada alpine lakes. Thomas C. Smith Department of Ecology, Evolution, and Marine Biology University of California, Santa Barbara Objective(s) of study: The objective of this study is to evaluate how tadpoles of an endangered frog species influence the number, identity, and abundance of algae species in alpine lakes in the Sierra Nevada. Background/content of investigation Alpine lakes in California?s Sierra Nevada are an important economic and ecological resource. Water from snowmelt is collected and stored in these lakes before it flows down into the state?s reservoirs and aqueducts. That water is habitat and a resource for plants and animals, many of which are unique to California. The diversity and health of those plants and animals buffer ecosystems from natural and anthropogenic environmental change. Apart from the potential for abiotic stressors like climate change and air pollution to influence ecological processes, interactions among organisms influence ecosystem properties. Mountain yellow-legged frogs in Sierra alpine lakes are being driven to extinction by chytridiomycosis, a disease caused by a parasitic fungus. Chytridiomycosis is responsible for worldwide declines of amphibian populations, even in protected areas such as Yosemite and Sequoia-King?s Canyon National Parks in the Sierra Nevada. Biologists worldwide are exploring the impacts of this disease on amphibian individuals, populations, and species, but few have addressed the effect that amphibian declines and extinctions have on freshwater ecosystems. For the Sierra Nevada ecosystem, disease-caused frog declines and extinctions are the most dramatic ecological event since the introduction of trout. Given the importance of these lakes to California?s economics and ecology, it is critical to understand the consequences of Mountain yellow-legged frog extinctions on Sierra Nevada alpine lake ecosystems. Algae convert sunlight into energy which flows up through the alpine food web. Tadpoles and insects eat the algae; their energy exits the lake via adult frogs and insects, where it becomes available to terrestrial consumers like birds and bears. Algal growth and diversity is influenced by the availability of nutrients, such as nitrogen; ammonia is the preferred source of nitrogen for most algae. One source of ammonia in Sierra lakes is tadpoles urine. Mountain yellow-legged frog tadpoles frequently form schools, or aggregations, in warm shallow water. These aggregations form in the same location every day of the summer and can contain thousands of individuals; tadpoles disperse after sunset. Over the course of each day, the areas where these aggregations form develop ammonia concentrations that are orders of magnitude higher than in the rest of the lake. Tadpoles cannot increase the amount of nitrogen in a lake, but they do distribute ammonia heterogeneously throughout a lake, and create localized daily fluctuations in ammonia. By aggregating, tadpoles affect the heterogeneity of nitrogen available to algae in both space and time. These observations result from fieldwork I conducted in 2010. Ecological principles suggest that more species may coexist using a single resource when that resource fluctuates in time and/or space than when that resource is held constant. One species will be most successful when a resource is at low concentration, and another will be most successful when the resource is abundant. If the resource were always supplied in low concentration, only the former species can thrive. When the resource fluctuates, conditions favor each species in turn, and both species are able to coexist. A community is a group of coexisting species; the number of species in a community is referred to as richness; the particular species in a community are described by community composition. I hypothesize that algal richness will be higher where tadpoles aggregate. Certain algae species will thrive in low-nitrogen parts of lakes where tadpoles are scarce, but more species will thrive on the pulses of nitrogen created when tadpoles aggregate. I also hypothesize that algal richness will be higher in lakes that have tadpoles than in those that do not. Research approach and procedures Field Experiment: I have identified six alpine lakes in the southern Sierra Nevada in which tadpoles are abundant. Two sampling grids will be established in each lake. One will be centered on a tadpole aggregation so that the outer sampling points lie just outside the tadpole aggregation; the second will be placed in an area where tadpoles do not aggregate. At each sampling point, I will measure nutrient concentrations as tadpoles gather and disperse throughout the day. I will collect algae at each point, and calculate richness, composition, and the abundance of each species. Analysis will compare these metrics of the algae community to nutrient concentrations and fluctuations at each point. Field Surveys: Surveys to compare algae richness in lakes with and without tadpoles will occur in 10 lake pairs. Each lake pair will contain one lake with tadpoles and one nearby lake without tadpoles. For samples of algae collected from each lake, the richness and composition of the community and the abundance of each species will be compared to presence or absence of tadpoles. Laboratory Experiments: Laboratory experiments will corroborate how communities of algae, collected in the field, respond to nutrient fluctuations. Experimental treatments will supply nitrogen in constant-low concentrations, constant-high concentrations, and fluctuating-low to high concentrations. Analysis will compare richness, composition, and abundance of each species to nutrient supply regime. Schedule of research The field experiment will be set up in late June and early July. Samples of algae will be taken following setup and again in September. Nutrient surveys will be conducted in mid-summer. Field surveys will occur in July and August. Lab experiments will occur during fall quarter 2011. Analysis will continue until June 2012. Anticipated results and significance of results I anticipate that algal richness will be higher in the midst of tadpole aggregations compared to where tadpoles are scarce, and that algal community composition will differ. I anticipate that lakes with tadpoles will have higher richness and different composition than lakes where tadpoles were driven to extinction by chytridimycosis. One of the fundamental questions in ecology addresses the mechanisms which allow species to coexist. My field experiment and surveys examine the relationship between species coexistence (richness) and resource dynamics in a non-model system under completely natural conditions. The results will enhance our understanding of the generality of these mechanisms of coexistence and at what ecological scales they apply. Mountain yellow-legged frogs in the Sierra Nevada are faced with extinction due to chytridimycosis, as are many amphibian species worldwide. This study complements the few existing studies which found changes in freshwater communities following similar amphibian extinctions. I believe natural resource managers can use my findings to plan responses to amphibian declines. Decreases in algal richness and shifts in algal community composition following frog extinctions may prompt managers to seek alternative ways to maintain algal richness. Maintaining integrity of the algae community should help buffer Sierra Nevada alpine lake ecosystems to the stresses of climate change or pollution.

Visit #24669 @Sierra Nevada Aquatic Research Laboratory

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Under Project # 23458 | Research

Response of periphyton diversity to consumer-driven nutrient heterogeneity in alpine lakes.

research_scientist - University of California, Santa Barbara

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Thomas C. Smith Jul 1 - Oct 1, 2011 (93 days)
Thomas C. Smith Jul 1 - Oct 1, 2011 (93 days)

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