From the bottom of the ocean to the top of the food chain
A recent study of algae and the reefs at Papahanaumokuakea Marine National Monument in the Northwestern Hawaiian Islands highlighted something that might be considered common sense: disrupting algae on the ocean floor has a negative impact on the nearby reefs.
Randall Kosaki, the study’s co-author, said scientists look at a concept, such as the idea that a reef ecosystem is interrelated, and prove what might strike other scientists as a logical idea. In this study, Kosaki said they were really looking to see how much a reef depended on the oxygen produced, via photosynthesis, by the algae on the ocean floor, called benthic algae. They were also looking to see what impact that benthic algae had on the fish population around a reef.
The reef depends heavily on benthic algae to produce oxygen, Kosaki said the study showed. That’s important, he added, for several reasons.
“You can’t just protect fishes just by protecting fishes,” Kosaki said. “You have to protect the entire ecosystem.”
That’s not something people always want to hear, though, he added. Some people will argue that imposing a bag or size limit on a particular species can protect that species, but those limitations don’t address issues such as the fishes’ food supply or water quality.
Fertilizer and sediment running off the islands and damaging reefs and benthic algae will ultimately have an impact on fish populations, Kosaki said.
“It underscores that interrelatedness,” he said.
The scientists also studied the food chain in the reefs, from the benthic algae providing a base for the food web up to the large predatory sharks at the top of the chain. The predators eat smaller fish, which have eaten the algae on the seafloor, the scientists said.
Kosaki said scientists group the predators — from tiger sharks to ulua — in one category, apex predators. But the study showed something else Kosaki said might be considered a bit of common knowledge.
“The tiger sharks really are the king of the jungle,” he said, adding the sharks eat smaller fish, but also prey on the other apex fish.
“Benthic algae were found to support a majority of the fish production in this coral reef ecosystem,” lead author and National Oceanic Atmospheric Administration’s National Centers for Coastal Ocean Science oceanographer Anna Hilting said. “Even some coastal tunas, such as the kawakawa, were partially dependent on primary productivity occurring on the reef bottom.”
Researchers collected about 600 algae and fish samples, taking tissue biopsies smaller than the size of a pencil eraser.
Scientists tested the samples to see if it was the benthic algae or planktonic algae, closer to the surface, or both, supporting the reef’s productivity.