Oxygen-deprived coastal waters, known as dead zones, are not as devoid of life as scientists once thought. Coastal dead zones form where nutrient pollution is severe. When water runs off of agricultural lands, it transports dissolved nutrients from animal waste and fertilizer. The polluted water makes its way into streams, rivers, and eventually to the ocean. There the nutrient-laden water nourishes a blossoming of plankton and algae which in time die and decompose, processes that consume oxygen from the water. As the levels of oxygen in the water plummet, marine life perishes in the oxygen-poor regions.
Recent ecological research by Brown University ecologist Andrew Altieri has revealed that some species can cope with low-oxygen levels and even thrive in the dead zones, where they live in a predator-free environment. One such species is the quahog clam (Mercenaria mercenaria), a species that is important to the Rhode Island shellfish industry.
To evaluate the ecology of coastal dead zones, Altieri studied four locations in Narranganset Bay, an estuary that spills into the northern end of Rhode Island Sound. Three of the four study locations become hypoxic (oxygen deprived) seasonally. Altieri placed an assortment of species at each study site: quahog clams, soft-shell clams (Mya arenaria) and blue mussels (Mytilus edulis). He then monitored how the populations of these shellfish coped during the summer and fall when oxygen levels in the water are most likely to plummet.
Altieri found that all three shellfish were able to tolerate mildly hypoxic conditions and benefited from it, as their predators were driven away by the low oxygen levels. But of the three study species, only the quahog was able to survive in a severely hypoxic environment, the soft-shell clams and the blue mussels both perished.
Altieri also compared quahog populations in polluted, oxygen-deprived waters to those in cleaner, oxygen-rich waters. What he found was intriguing: there were fewer quahogs in the cleaner water, where natural predators such as sea stars, fish and crabs, were abundant and able to hold the quahog populations in check. So cleaning up Narronganset Bay could result in fewer quahog clams for the shellfish industry to harvest.
Find out more:
- Brown Scientist Finds Coastal Dead Zones May Benefit Some Species (Eurekalert)
- Dead Zones as Safe Havens (Ecological Society of America Podcast)
Photo courtesy Brown University.