How do natural and human-discharged nutrient pulses impact tropical marine ecosystems?
STRI/DICYT The biggest nutrient injections into the Gulf of Panama come in two different forms. One happens every dry season when trade winds push surface water away from the Pacific coast, creating a vacuum that sucks cold, nitrogen-rich water from the depths to the surface. The other happens year-round as agricultural runoff and urban sewage gush into streams and rivers.
Andrew Sellers, a Panamanian pre-doctoral researcher based at Canada’s McGill University, studies both the natural and human-caused nutrient-input scenarios to fill gaps in our understanding of their impact on coastal tropical marine ecosystems.
His upwelling research compares how seasonal pulses of nitrogen — including heavy nitrogen from ocean depths — detected in the tissue of organisms — influence algae and their consumers on the rocky intertidal zones of the Gulf of Panama. Sellers compares organisms here with those growing during seasons when there is no upwelling in the Gulf and with other sites in Panama, like the Gulf of Chiriquí, which do not have upwelling.
“For a long time, we thought about rocky intertidal communities in the tropics as stable through time with very little variation,” says Sellers, who adds that the nutrient levels in algae-rich intertidal zones in temperate climes are known to vary with the seasons. “The idea is that the herbivores here are going to consume everything and what’s left behind is going to get fried under the sun.”
But this assumption does not account for variation in nutrient availability based on season or location in the tropics. “We’ve ignored the environmental context, the productivity of the system. What we’re doing here is a big-picture kind of thing to establish a baseline: What do these communities look like, how does the environment vary across time and space, and what kind of interactions you find in between herbivores and algae,” says Sellers, whose advisor at the Smithsonian Tropical Research Institute is Mark Torchin.
The rivers run with it
Sellers and colleagues are also learning how agriculture and urbanization impact the intertidal nutrient cycle. This “pollution effect” has not always been accounted for in upwelling studies and teasing pollution apart from natural inputs is necessary to understand human impacts on the ecosystem.
To get at that question, Sellers and his McGill advisor, Brian Leung, recently started sampling 20 rivers that flow into the Pacific. They plan to take monthly samples between the town of Pacora and the city of Davíd and to use the data as part of Leung’s Panama Research and Integrated Sustainability Model, or PRISM. This data on nutrient loading is not being captured systematically by anyone else.
“We know nutrient-dumping and fishing have an impact but we don’t really know how the impact varies based on where you are on the planet,” says Sellers, pointing to the algal takeovers of coral reefs that have been overfished and polluted. “It is important to understand how the environmental context of the system influences those processes if we’re going to develop proper management strategies.”