Collaborative Research: Consumer control of high-productivity, low-nutrient ecosystems: Enhancement of primary productivity by grazing fish in Lake Tanganyika

地球上许多物种最丰富的生态系统，如珊瑚礁和热带雨林，其特点是动植物生产力很高，尽管它们明显缺乏维持植物生长所需的营养物质。东非裂谷湖泊可能是唯一高产、低营养的淡水生态系统。这些湖泊中丰富的生命集中在湖泊的边缘，那里有足够的光线到达湖底，使生长在岩石上的藻类能够进行高速率的光合作用。但是，面对1)环境中氮和磷等关键营养物质的极度匮乏，以及2)高密度和丰富多样性的草食性鱼类所带来的强烈放牧压力，高藻类生产力是如何保持的？答案可能就在食草鱼本身。关于鱼类对藻类食物资源的负面影响被鱼类对营养物质供应的积极影响所抵消的假设将得到检验。实验室和实地试验将用于测试鱼类是否通过以下方式促进生态系统生产力：1)通过在体内储存营养物质来减缓营养物质向较深水域的流失；2)膳食营养素在藻类、动物和环境之间快速循环；3)促进藻类的生长，这些藻类能够利用和保留大多数藻类无法获得的营养物质。因此，以藻类为食的鱼类可能会通过加速营养循环、储存营养物质和增加新营养物质流入生态系统来促进自身食物资源的增长。测试鱼类在维持湖泊生产力方面的重要性需要测量鱼类的营养储存和循环，实验测试当鱼类存在时藻类是否生长得更快，测量来自湖泊深处的营养输入，并使用理论模型来比较鱼类和其他营养输入的重要性。这种活动组合将在非洲的实地条件下和在美国的实验室中进行。通过澄清两个关键问题，该项目将有助于指导保护坦噶尼喀湖数百种独特物种和具有全球重要意义的渔业的努力。首先，渔民在包括坦噶尼喀湖在内的世界上许多湖泊捕捞了太多的鱼。这种过度捕捞可能会从湖中带走太多的营养物质，或者降低营养物质循环的速度，从而使藻类生长得更慢。通过这种机制，捕鱼实际上会削弱该湖未来的生产力。其次，气候变化正在使湖的表层水变暖，并减少了季节性的风，这种风会导致寒冷、营养丰富的水周期性地从湖的深处涌出。这些深水营养物质流入频率的减少，切断了维持鱼类生存的藻类生长。这项研究将首次全面评估这些人为因素将如何影响坦噶尼喀湖的生产力，而坦噶尼喀湖支撑着区域人类经济。该项目将提高公众对坦噶尼喀湖的认识。通过一个网站，一篇水族馆爱好者杂志的文章和一篇科普文章来了解水生生物。最后，许多来自赖特州立大学和密歇根大学的学生将通过参与该项目获得专业经验。这将包括来自非洲和美国的学生。与非洲和全球非营利组织的伙伴关系将进一步扩大这项研究的影响。

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.