Energy availability and trophic relationships together are strong determinants of the structure and function of whole ecosystems. However, the relative strength of top-down and bottom-up limits to ecosystem productivity remains an area of active debate. While much of the research on the interactions of bottom-up resource limitation and top-down consumption has been conducted in aquatic ecosystems, few such studies have been conducted in the tropics. Factors affecting top-down and bottom-up forces on tropical aquatic ecosystem productivity are changing rapidly as fisheries are overexploited and agricultural development increases nutrient availability. To test the relative strength of top-down and bottom-up limits on algal biomass within a tropical river in northern Thailand, we experimentally manipulated both nutrient availability and fish presence using nutrient diffusing substrates (NDS) and fish exclosures across a gradient of human harvest pressure. Following three-week experimental deployments, we collected the algal biomass accrued on NDS enriched with nitrogen and phosphorus placed within paired fish exclosures and cage controls. Additionally, we collected algal biomass and macroinvertebrates from natural substrates within both fish exclosure treatments. Finally, we used visual survey techniques to estimate ambient fish density at each deployment site.
Algal biomass from standardized experimental substrates was significantly lower in fish exclosure treatments. These same treatments had significantly higher grazing macroinvertebrate density than adjacent exclosure controls, suggesting cascading top-down effects of fish on algal accrual. On NDS, we found that nutrient addition significantly increased algal accrual relative to unamended treatments in both fish exclosures and exclosure controls, providing evidence for bottom-up constraints from nutrient limitation. However, algal accrual on NDS in fish exclosures was substantially reduced relative to exclosure control treatments. Specifically, nutrient-enriched treatments from fish exclosures only accrued biomass approaching that of unamended treatments in cage controls, suggesting top-down effects have primacy in this system. Interestingly, enormous harvest pressure on fishes appears to have no effect on the strength of the trophic cascade. Macroinvertebrate abundance and algal accrual did not differ between small reserve areas, where harvest is prohibited, and areas of highest harvest pressure, despite reserves harboring orders of magnitude higher fish biomass. We hypothesize this is due to a trophic release of small invertivorous fishes having limited human consumption value and experiencing reduced predation risk outside of reserve areas. Our results illustrate the complexity of top-down and bottom-up effects within tropical aquatic systems intensively used by humans.