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The shrimp-like herbivores, called mesograzers, are smaller than a thumbtack, but gobble up substantial amounts of algae. Keeping the plants in check makes for clearer waters, which gives seagrass beds access to light and oxygen, researchers report in Ecology. The wee arthropods, in turn, serve as a meal for small fish, which are eaten by larger fish and birds, and on up the food chain. 

Seagrass beds act as nurseries for commercially important fish and shellfish. The rich, long grasses offer protection from predators, as well as filter out toxins in the water column and protect coastlines from storms and erosion. Now, seagrass beds in estuaries and seas across the world are declining, often because harmful algae blooms choke them out. Rising water temperatures due to climate change are also stressing the aquatic grasses. 

Seagrass ecosystems are difficult to research in the field, and are therefore not fully understood. So, researchers from the Virginia Institute of Marine Science and the U.S. Geological Survey set out to crack the relationship between algae-chomping crustaceans and eelgrass, a species of seagrass, in Virginia’s York River.

Throughout the fall of 2008 and the summer of 2009, they ran experiments in plots within the river. They added carbaryl (known to kill off household insects, arthropod cousins to the aquatic mesograzers) to some plots to examine algal behavior in absence of mesograzers, and added fertilizer to others to determine what effect nutrients had on the system. 

The confirmed that restricting crustaceans and adding extra nutrients led to algal blooms. An influx of fertilizer spurred rapid algae growth in the fall; in the summer experiment, carbaryl input eliminated the mesogazers, and algae again flourished, causing eelgrass dieback. 

“Coastal managers have been concerned for years about excess fertilizer and sediment loads that hurt seagrasses,” said coauthor J. Emmett Duffy of the Virginia Institute of Marine Science in a . “Our results provide convincing field evidence that grazing by small animals can be just as important as good water quality in preventing nuisance algae blooms and keeping seagrass beds healthy.” 

The findings indicate that scientists should take mesograzers into account when creating conservation management plans for these aquatic ecosystems, say the authors. They compare the relationship between mesograzers and seagrass to that between earthworms and soils, or honeybees and tree crops. 

The study is the first in a series of seagrass investigations that the (ZEN), a collaborative group of scientists, are undertaking. Their name comes from the scientific name of eelgrass, Zosterna marina, the world’s most widespread marine plant. They have engaged in international research since the 2008 and 2009 experiments, trekking to waters of Japan, Portugal and other nations.

ZEN is exploring everything about eelgrass, from how they influence biodiversity, climate change, to natural variability worldwide, and shedding light on these vital but poorly understood ecosystems.