Underwater Meadows and Resilient Seas
Smithsonian scientists work to solve how water quality, fishing pressure, and herbivore grazing determine the future of underwater grasslands.
If not for the flash of an octopus as it propels through the water or the swish of sediment as a manatee lumbers through an expanse of emerald green fronds, seagrass meadows might be mistaken for a lush and unruly lawn, maybe even the tall grasses of a mountain meadow. To scientists, however, they are more like tropical rainforests. Home to an amazing diversity of life, seagrass beds are important indicators of the overall health of coastal ecosystems—and they can be just as sensitive as their terrestrial counterparts to changes in the environment.
Seagrasses are angiosperms—plants that produce flowers and seeds—that have adapted to life in the sea. With long narrow blades, they grow in extensive meadows throughout the world’s shallow coastal regions, both temperate and tropical, providing important ecosystem services. Seagrasses help stabilize sediment, recycle nutrients, and protect our shorelines. They also serve as juvenile nurseries for fishes and shellfish, homes for endangered animals like manatees and sea turtles, and as habitats for commercial and recreational fisheries.
But these complex ecosystems are in danger, says Justin Campbell, a Smithsonian postdoctoral fellow who developed an interest in the effects of climate change on seagrasses during his graduate work at Florida International University in Miami. “In addition to climate change, there are a number of local and regional factors harming seagrasses, such as shifts in water quality, overfishing, and shoreline modification. We need rigorous, powerful experimentation to begin to understand how these stressors interact and ultimately influence seagrass beds.”
A network of experiments to understand seagrass ecology
As a Smithsonian postdoctoral fellow, Dr. Campbell has continued his work with seagrasses by helping launch the Thalassia Experimental Network (TEN). A MarineGEO initiative, TEN has established monitoring and parallel experiments in beds of turtlegrass (Thalassia testudinum), the dominant seagrass found throughout south Florida and the Caribbean. Based at the Smithsonian Marine Station (SMS) at Ft. Pierce, Florida, TEN is replicating these experiments at a Smithsonian field site in the Florida Keys as well as at the Smithsonian Tropical Research Institute (STRI) at Bocas del Toro, Panama, and the Smithsonian’s Caribbean Coral Reef Ecosystems Program (CCRE) at Carrie Bow Cay, Belize.
The goal of TEN is to understand vulnerability of seagrasses to nutrient pollution (eutrophication) and decline of grazing animals, and to investigate how those mechanisms vary among locations. This issue is important for coastal systems worldwide, where fertilizer and septic runoff are common contributors to altered ecosystems. Results from this project will provide scientists with a wealth of data on how human impacts affect vegetated near-shore ecosystems and, by extension, the species that depend on them for shelter and sustenance.
Just like any other plant, seagrasses need sunlight to thrive and reproduce. Excess nutrients can increase the growth of algae, which cloud the water and overgrow the seagrasses, reducing the amount of sunlight reaching the plants. Although such algal blooms represent a major cause of seagrass decline around the world, the complex interactions among nutrients, plants, and grazers that influence them remain poorly understood.
The TEN experiments mimic these stressors, says Campbell, thus examining how seagrasses at different locations respond to nutrient loading and fishing pressure. Each site consists of 60 small field plots of turtlegrass in shallow waters that receive similar amounts of light. Half the patches are treated with the same amount of slow-release nutrient fertilizer. The other 30 seagrass plots are left untreated. These beds are being monitored over time to describe and compare the structure and functioning of the seagrasses within the plots. “We monitor various aspects of seagrass health, such as growth rates and shoot density,” says Campbell. “It is vitally important that we do exactly the same thing in exactly the same way across all four sites. That way we ensure that differential responses between sites are not due to distinctions in our experimental design or methods.” This standardization of methodology among sites is a key theme of MarineGEO, and TEN represents an important first set of coordinated experiments across the sites.
Full analysis of the data gathered by the TEN experiment will start in fall 2014, but early findings are compelling. “What seems to be emerging initially is that nutrients influence vulnerability of seagrasses to grazing, but that there are differences in how seagrass beds respond,” says Campbell. Attention now is shifting to what parameters vary across geographic location that might explain these differences. The coordinated experiments thus open up a host of opportunities for further research.
Other factors affecting seagrass beds
Campbell and the other researchers are also interested in the role that larger herbivores such as turtles and urchins play in determining the health of tropical seagrasses. “These animals can consume substantial quantities of seagrass vegetation, and in some locations drastically change the structure of the seagrass bed,” Campbell explains. “While seagrasses are sensitive to ‘bottom-up’ influences of excessive nutrients in certain regions of the world, other areas may be increasingly influenced by changes in these herbivore populations.” The TEN experiment thus is also assessing changes in herbivory. Of the 60 plots at each site, half are grown in cages to exclude these herbivores, while the other half are left exposed.
Finally, increased acidification of seawater is another factor impacting seagrass health. Oceanic CO2 levels have been increasing over the past 200 years, and the Smithsonian has recently begun to develop a baseline for measuring CO2 concentrations in their marine monitoring sites at SMS in Ft. Pierce, at SERC in Maryland, and at STRI’s Bocas del Toro facility in the Caribbean. These measurements are tracking variations in pH and acidity but—importantly—also assisting scientists to identify reasons for those variations.
For instance, monitoring has revealed extreme variability in pH at local levels. Campbell explains: “Just like terrestrial plants, seagrasses use CO2 during the day in order to produce carbohydrates, which are essentially their fuel. During the day CO2 levels decline, due to photosynthesis. At night they increase, due to respiration.”
And while excess CO2 has proven generally to slow growth of coral reefs, Campbell’s research shows that certain seagrass beds actually thrive in waters with high acidity. Those studies could prove invaluable to understanding how certain marine ecosystems can resist, or even thrive under, increased levels of CO2.
The power of MarineGEO partnerships
Campbell is excited to be part of research that meshes so closely with his own long-term goals. “I’m broadly interested in how coastal environments respond to environmental change. Many of these changes stem from human activity, and my research examines the ultimate consequences of our actions. Seagrass decline can result from multiple factors, thus we have a complex interplay of local and regional stressors such as eutrophication, and broader global stressors such as elevated temperatures and CO2 from climate change. My work aims to address these factors, both singly and combined, across a variety of spatial and temporal scales.”
Tracking and understanding these causes and effects is a rigorous and long-term challenge. “These systems are amazingly complex and comprised of a diverse array of organisms, and they all show slightly different responses. Which means, unfortunately, there is no general blanket statement that we can make regarding how a particular organism will respond to some sort of stress, whether nutrient enrichment, CO2, or even elevated temperature.”
The Smithsonian’s MarineGEO initiatives make it possible for the kind of in-depth, broadly based, long-term research necessary to understand this complexity—and the role human beings play in changing the coastal environments of which seagrass beds are such a vital part. “The work that we can do across the network is increasingly powerful,” says Campbell, “because now we can paint a picture of how these systems respond not only in one location but across larger geographic scales, such as across the Caribbean.”