Many of the most species-rich ecosystems on Earth, such as coral reefs and tropical rain forests, are characterized by high plant and animal productivity despite an apparent lack of the nutrients required to sustain plant growth. The east African rift valley lakes may be the only high-productivity, low-nutrient freshwater ecosystems. The profusion of life in these lakes is concentrated at their edges, where sufficient light reaches the lake bottom to allow for high rates of photosynthesis of the algae growing on the rocks. But how is high algal productivity maintained in the face of 1) the extreme scarcity of critical nutrients such as nitrogen and phosphorus in the environment and 2) the intense grazing pressure imposed by high densities and rich diversity of herbivorous fish? The answer may lie in the grazing fish themselves. The hypothesis that the negative impacts of fish on their algal food resource are offset by the positive effects of fish on nutrient availability will be tested. Lab and field experiments will be used to test whether fish promote ecosystem productivity by 1) slowing the loss of nutrients to deeper waters by storing nutrients in their bodies; 2) rapidly recycling dietary nutrients between algae, animals, and the environment; and 3) promoting the growth of types of algae that are able to use and retain forms of nutrients unavailable to most algal species. Thus, algae-eating fish may encourage the growth of their own food resources by speeding up the nutrient cycle, stockpiling nutrients, and increasing the influx of new nutrients into the ecosystem. Testing the importance of fish in sustaining lake productivity requires measuring nutrient storage and recycling by fish, experimentally testing whether algae grow faster when fish are present, measuring inputs of nutrients from the depths of the lake, and using theoretical models to compare the importance of fish and other nutrient inputs. This combination of activities will be pursued both under field conditions in Africa and in laboratories in the United States.
This project will help to guide efforts to protect the hundreds of unique species and the globally-important fishery of Lake Tanganyika by clarifying two critical issues. First, fishermen are catching too many fish in many lakes worldwide, including Lake Tanganyika. This overharvest may remove too many nutrients from the lake, or reduce the rate of nutrient recycling so that algae grow more slowly. By that mechanism, fishing could actually undercut the future productivity of the lake. Second, climate change is warming the surface waters of the lake and reducing the seasonal winds that cause cold, nutrient rich waters to periodically well up from the depths of the lake. Reduction in the frequency of influx of these deep-water nutrients is cutting off the algal growth that sustains the fish. This research will offer the first thorough evaluation of how these human-imposed factors will affect the productivity of Lake Tanganyika, which supports a regional human economy. The project will increase public awareness of Lake Tanganyika?s aquatic life through a website, an article for an aquarium hobbyist magazine, and a popular science article. Finally, many students from Wright State University and the University of Michigan will gain professional experience through involvement in the project. This will include students from Africa as well as Americans. Partnerships with African and global non-profit organizations will further broaden the impact of the research.
Context: When algae become abundant enough to be noticed by citizens, something is amiss. Bright green water or long, hairy strands of algae on rocks indicate that an aquatic ecosystem has been polluted by nitrogen and phosphorus coming from farm fertilizers or sewage. Although unsightly or toxic blooms give them a bad name, algae are an immensely important food for aquatic animals, including fish. Through photosynthesis, algae convert sunlight and inorganic nutrients into living biomass that feeds the food web. Understanding the basic ecological relationships between availability of inorganic nitrogen (N), phosphorus (P), algae and aquatic animals is essential to maintaining healthy and clear lakes, rivers and oceans. Our research demonstrated that interactions between grazing fish and algae promote the growth of nutritious algae that become rapidly entrained in the food web rather than forming nuisance blooms. Intellectual Merit: Lake Tanganyika in East Africa is the world’s second largest freshwater lake in depth and volume. It is a regionally important source of fish protein. An amazing, diverse endemic fauna has evolved in this ancient lake. The biodiversity is concentrated in a narrow band of shallow water at the lake edge, the littoral zone, where there is sufficient light for microscopic algae attached to the bottom to photosynthesize. Our research greatly furthered our understanding of how the abundant life in the lake thrives even though the nitrogen and phosphorus necessary for algal growth are undetectable in the water. The attached algae in the lake are highly productive and support a diversity of fishes. We showed that fish themselves were critical to the high productivity of the algae and the whole ecosystem. About half of Lake Tanganyika’s fish species graze on attached microscopic algae. We demonstrated that these fish selectively feed on rapidly growing, nutritious algae called diatoms. Crucially, the algae that the fish leave behind, the cyanobacteria, are responsible for high rates of nitrogen fixation in the littoral zone, which is essential for the production of algae and fish. The fish have only a slight negative impact on their algal food. Fish reduce algal biomass by 50-90% but reduce algal growth only by < 15%. Fish also affect the algae in a positive way. By constantly grazing on the algae, fish create optimal light conditions for the remaining algae, and recycle nutrients by releasing N and P in their urine and feces. The algae rapidly sequester these nutrients, promoting algal growth and leaving little accumulation in the water. Where fish are abundant, grazing leaves only a thin, nearly invisible, layer of algae coating the rocks. Nevertheless, the algae grow rapidly, producing continuous, nutritious food for the fish. We showed that fish cannot overgraze their algal food. Rather, areas with rapid algal growth support more fish than deeper areas where algae grow slowly. Unfortunately, deforestation is causing increased sediment loading, and our research shows that sediment lowers the food quality of algae. We also demonstrated that the direct and indirect effects of climate change will reduce fish growth. Our data show that the densities of algivorous fish are already declining, likely due to compromised food quality. Ongoing environmental degradation is likely reducing the capacity for Lake Tanganyika to provide fish protein for people. Broader Impacts: In addition to better understanding the basic ecological interactions in lakes, our research was used to support conservation in Lake Tanganyika, to enhance the public understanding of ecosystem science and to develop methods to detect pollution from human sewage. A Nature Conservancy report based on our research was used to promote conservation research in Lake Tanganyika. The report has been presented to governments, donors, and conservation experts to educate audiences about environmental concerns in Lake Tanganyika. The Africa Agriculture Director at the Nature Conservancy is using our data to craft new proposals to reduce sediment loading to the lake. We used naturally occurring forms (isotopes) of nitrogen in snails to demonstrate that nitrogen from sewage was entering the food web even though water quality analysis failed to detect it. For remote areas, this method may prove far superior to conventional water quality monitoring because it is sensitive, sample collection is simple and samples are stable at ambient temperatures. The Jane Goodall Institute is using our data to prioritize locations for beach latrines in villages. We are creating a website with Caravan Lab, a media company that helps make science accessible to the public. The website will use our work in Lake Tanganyika to present basic principles of ecosystem ecology, emphasizing the importance of maintaining healthy links among all aspects of the ecosystem in order to support human well-being. We are developing teacher tools and a 10th grade science curriculum. Most importantly, the website presents science as an interactive, collaborative and exciting human endeavor that includes people of any race or gender.
