Billions of larval blue crabs, or zoeae, are spawned each year into the many estuaries and bays of the Atlantic and Gulf coasts of the United States. These tiny translucent animals, comparable in size to the tips of fine sewing needles, only faintly resemble adult blue crabs.
A small fraction of zoeae successfully navigate the tides and currents many miles to the seaward margin of the continental shelf, develop through a series of larval stages, and eventually return as megalopae before molting into juveniles and, finally, adults. But new research suggests that environmental stressors within coastal zones that will intensify with climate change, namely low dissolved oxygen (hypoxia) and low pH (acidification), can increase mortality among early-stage blue crabs prior to their transport to the shelf.
Co-occurring hypoxia and acidification in the coastal zone
Largely as a result of nutrient overloading, many coastal water bodies worldwide endure prolonged periods of hypoxia and acidification. Huge quantities of nitrogen from sewage, agricultural fertilizer, and urban runoff make way from land to water, stimulating the growth of algal organic matter, a process known as eutrophication. This excess algal organic matter is then respired and degraded by bacteria, depleting dissolved oxygen and increasing carbon dioxide (and thus lowering the pH) of the water column.
Adult blue crabs are capable of tolerating periods of low oxygen and pH, and reports of hypoxia from regions notable for their blue crab populations such as the Chesapeake Bay and the Gulf of Mexico are ample. Adults are mobile, moving along the sediment surfaces, capable of migrating away from more stressful conditions.
Early life stages (zoeae), however, are found in the water column and swim at rates of 0.5 cm s-1, relying largely on tides and currents to be transported out of estuaries and bays where they are often spawned. Sometimes spending up to their first two zoeal stages (6-19 days from hatch) in an estuarine or near-shore environment, some blue crab larvae are likely exposed to poor water-quality conditions for prolonged periods.
In the laboratory, zoeae were exposed to low oxygen and low pH conditions, both individually and in combination, for either four or fourteen days. Experimental results suggest that both the severity and duration of exposure to hypoxia and acidification are important factors in controlling the mortality rate of blue crab larvae.
Relatively long-term (14 day) exposure to either hypoxic or acidified conditions negatively impacted zoeal survival, with the combination of stressors having the most severe effect, reducing survival by over 80% compared to larvae exposed to normal oxygen and pH levels. Over this time period, even at moderate oxygen levels, above the conventional “hypoxic” designation of less than 3 mg O2 L-1, survival was reduced relative to larvae of fully oxygenated waters.
Over four-day periods hypoxia reduced the survival of larvae, while acidification had no effect on survival. At oxygen levels between 2 and 3 mg O2 L-1, larval survival was reduced by over 90%, suggesting that even shorter exposure to hypoxic conditions could influence the mortality of blue crab populations. At these oxygen levels, zoeae spawned into estuaries that are not quickly flushed out to sea may be unlikely to survive.
Considerations for the future
In addition to eutrophication-driven hypoxia and acidification, rising temperatures and atmospheric carbon dioxide associated with climate change are also contributing to the deoxygenation and acidification of the ocean. While mitigation of climate change-driven hypoxia and acidification will take a global effort, local and regional measures to reduce nutrient loads and limit eutrophication-driven hypoxia and acidification have had success in a number of places, including Long Island Sound and Narragansett Bay.
To the human eye, blue crab zoeae look like mere specks of dust scattered throughout the water column. During their earliest life stages, these animals are vulnerable to hypoxia and acidification, and small changes in larval mortality rates could influence future recruitment. Thus, efforts to improve the water quality of blue crab spawning locations will likely benefit this iconic American fishery.
These findings are described in the research article entitled Individual and combined effects of low dissolved oxygen and low pH on survival of early stage larval blue crabs, Callinectes sapidus, recently published in the journal PLOS ONE. The work was conducted by Stephen Tomasetti, Brooke Morrell, Lucas Merlo, and Christopher Gobler from Stony Brook University (corresponding author: [email protected]).