Open Ocean Food Webs

As a graduate student based at Hopkins Marine Station, Elan Portner, PhD ’19 (biology), followed the daily commute that billions of small animals make between their deep-ocean daytime habitat and their nighttime feeding grounds near the ocean’s surface.

Called “micronekton” for their small size and swimming ability, these animals—fishes, crustaceans, and gelatinous organisms—provide a food source for most large oceanic predators. Their daily commute through ocean depths also plays a large role in the global carbon cycle. “By consuming carbon-rich food near the surface at night and releasing fecal pellets in deeper water during the day,” Portner explains, “micronekton ‘pump’ carbon into the ocean. Fecal pellets are metabolized by bacteria as they sink, releasing carbon dioxide at depth. This reduces the concentration of dissolved carbon dioxide at the surface and allows more atmospheric carbon dioxide to be absorbed.”

Because they are difficult and costly to observe, relatively little is known about micronekton. Supported at Hopkins by the Esther M. Lederberg Graduate Fellowship, Portner analyzed the diets of common predators to cost-effectively study how micronekton respond to environmental variation. “We use the predators like small nets that independently collect micronekton. Changes in predator diet composition under different environmental conditions can be related to changes in the abundance and distribution of micronekton,” Portner says. “My goal is that this work will improve the ecosystem models used to manage our marine resources.”