Jellyfish could soon dominate food chain
Researchers look at potential effects as Chesapeake Bay loses dissolved oxygen
Chesapeake Bay anchovy breathe through their gills; jellyfish breathe through their entire body.
As the world’s oceans and estuaries lose dissolved oxygen — about 2% internationally in the last 50 years — that’s something that could elevate the gelatinous zooplankton to a position of dominance in the Chesapeake Bay’s food chain, researchers from the University of Maryland Center of Environmental Science wrote in a report on ocean deoxygenation released in December.
Mike Roman and James Pierson of Horn Point Lab on the Eastern Shore authored a chapter in the 588-page International Union for Conservation of Nature and Natural Resources report, “Ocean Deoxygenation: Everyone’s Problem,” which involved 51 institutions and 17 countries.
In the report, Deputy Prime Minister of Sweden Isabella Lövin writes that in the last 50 years, global dissolved oxygen has declined about 2% and says while scientists have known about deoxygenation for decades, rising water temperatures could make the problem worse.
Roman said he works with other researchers around the world to raise awareness about deoxygenation and to advocate for government action. The ocean is losing its breath, he said.
The Chesapeake Bay has a summer dead zone with a volume of 3,500 square kilometers. This oxygen-less water is called hypoxic, and in the bay it is fueled by nutrient pollution from land, which causes algal blooms, which then sink to the bottom and decompose, eating up the oxygen in the lower water column.
Roman said the actions Maryland has taken to reduce nutrients make it a leader developing countries can look to when facing their own deoxygenation challenges. More work is ahead — the bay is expected to warm, and warm water holds less oxygen but also creates more demand from organisms, making the problem of deoxygenation all the worse.
“It’s really hurting marine food chains in many areas of the world,” Roman said.
Rockfish might not hang out in the same cool spots they prefer if those cooler bottom waters come with less oxygen, Roman said. The wind could blow water from the dead zone into a waterman’s crab pots, so when he pulls them up, all the crustaceans are dead — not a fresh catch.
In their contribution to the international report, Roman and Pierson wrote low oxygen levels in the bay can do a few things to disturb the food chain. One factor is the way a marine organism breathes. The zooplankton community is dominated by copepods, according to the report, crustaceans which are denser than salt water, causing them to sink to the bottom.
Those copepods and other plankton absorb oxygen through their entire body. Fish get their oxygen through gills. Therefore, the copepods and jellyfish can tolerate water with less oxygen, while fish swim away, setting the jellies up to take the place of plankton-eating fish like anchovy.
There could also be a shift to smaller zooplankton when oxygen is low because of the same respiratory function. Smaller zooplankton with a greater ratio of body surface area to body can breathe easier. Roman said it is similar to how in the Jurassic period, a dragonfly could be the size of a crow — it is because there was more oxygen in the air then.
Predators will need to eat more of the less-nutritious prey to get the same amount of energy.
“It’s like eating popcorn instead of steak,” Roman said.
Deoxygenation is a problem everywhere. Roman said there are low oxygen zones in the open ocean beyond territorial boundaries, which are rapidly expanding.
“That will take international action,” Roman said.