Chronicling the complex ecological changeswrought by increasingly volatile El Niño – LaNiña cycles22 December 2016

Brown biology graduate student Robbie Lamb cages offan area of algae as part of an experiment to investigatepatterns of grazing by herbivorous fish and urchins.Credit: Jon Witman

When Brown University Professor Jon Witmanreturned this year to a small patch of coral off theGalápagos Islands that he had first marked off forstudy in 2000, he saw virtually nothing—and thattold him a lot. An underwater ecosystem that hadbeen teeming with diverse life 16 years ago wasnow a barren patch, an apparent victim ofincreasingly strong El Niño weather systems amidglobal climate change.

"The 2015-16 El Niño was exceptionally strong,"Witman said. "Climatologists predict that thefrequency of strong El Niños will increase with climate change."

This year, with funding from the National ScienceFoundation, Witman has made several expeditionswith graduate and undergraduate students towitness the effects of the latest El Niño. Hejourneyed to the Galápagos in January, March and

again last summer, and he's there again nowthrough February on sabbatical. He celebratedThanksgiving with graduate student Robert Lamband colleagues at the Charles Darwin ResearchStation with a holiday meal of ahi tuna, cornbread,pumpkin soup and baked sweet potato.

It's been a busy year.

"During this past year of fieldwork in Galápagos,there were so many surprises that we could hardlykeep up with all the effects of the strong El Niño,"Witman said.

Amid the warmest water at the beginning of theyear, for example, Lamb discovered the emergenceof a skin-wasting disease in the reef fishes that hestudies.

"We were surprised to find this novel wildlifedisease running rampant in at least 20 differentspecies," Lamb said. "Such outbreaks of diseaseare one of the least understood but potentiallydevastating manifestations of climate change."

The same patch of coral in 2000 (left) and 2016 (right)shows a severe decline in its biodiversity. Credit: JonWitman

The disease abated later in the year as the coolerLa Niña took over from El Niño. But other changesassociated with the warm water were apparent, too:

1 / 4

rampant grazing of algae by sea urchins, forexample, and the ominous emergence of mats ofcyanobacteria.

"The cyanobacteria is a species that we haven'tseen at these sites in all the time we've beenworking there," said Fiona Beltram, a Brown juniorfrom Glocester, R.I., who has been a member ofthe Witman lab for two years. "I'm interested indetermining if the cyanobacteria cover leads tobleaching in coralline algae, which are importantfoundation species on the Galápagos reefs."

Among the responsibilities assigned to Beltram,who joined the team in the Galápagos this summer,was to set up experiments to study thecyanobacteria. She also helped to measure what'sbeen going on at about two dozen specific areas, or"transects," that Witman has been monitoring foryears.

"I've been really passionate about marine sciencesince I was a kid, so it was incredibly exciting totravel down there and actively contribute tofieldwork," Beltram said. "It's also just a beautifulplace, especially underwater."

A cycle out of balance

Graduate student Robbie Lamb encounters a sea lionwhile diving. Credit: Brown University

The question, though, is whether that beautifulplace will last. Over the years, Witman said, the El

Niño - La Niña cycle has yielded a pattern thatalternately seems to destroy, but at other timesredeem the subtidal ecosystem's stunningbiodiversity.

"Corals bleach and sometimes die during theextreme temperature variability from the hightemperatures during El Niño and the lowtemperatures of the ensuing La Niña," he said. "Butas La Niña brings more productive water, barnaclesreproduce and settle on the stressed corals and allover the bottom."

That deteriorates the corals further, but thebarnacles serve as food for many other species,promoting a rebound in the undersea life. But themore Witman and his team dives in—literally andfiguratively—the more they see that the long-termtrend is worrisome.

"With continued El Niños, the Galápagos subtidalecosystem may be shifting to a system with morebarnacles and their predators and fewer corals," hesaid. "I'm concerned, however, that this may be alower diversity system, which may make it lessresilient to future climate shocks."

Who eats whom

Warm El Niño waters harm the corals. Credit: BrownUniversity

At least for now, the food webs in the area remain

2 / 4

full of characters, all of whom have a role to bestudied. Lamb and Witman spent Novemberformulating experiments to test hypotheses aboutwhat species are grazing on algae and how much.They deployed cages that selectively excludeherbivorous fishes or sea urchins, the two majorgroups of herbivores in this ecosystem.

"We want to understand the individual andcombined ecological roles of these two importantgroups," Lamb said. "Because of their different lifehistories and feeding strategies, they may be vastlydifferent in how they respond to changes in theenvironment such as increased wave turbulencefrom storms generated during El Niño."

Because algae serve as the basis of much of anecosystem's productivity, the answers to thesequestions come with profound implications. Toomuch algae consumption can undermine the foodsupply for other species and reduce biodiversity.

The food webs play out in complicated ways,Witman has found, as species fight to survive. As itdid for Darwin, the Galápagos still deliver plenty ofnew lessons about how nature works—the criticalrole of triggerfish, for example.

"Our research has discovered that triggerfish arekeystone predators," Witman said. "They preyheavily on sea urchins and consequently increasethe productivity of the marine ecosystem byreleasing algae from consumption by sea urchins.Sea urchins are like underwater lawnmowersmowing down bottom-dwelling algae; anything thatchanges their consumption of algae has a dramaticeffect on productivity. We found that Spanishhogfish slow the rate of triggerfish predation on seaurchins by trying to take the urchin out of thetriggerfish's mouth."

Around the world

The colder La Niña encouranges the growth of barnacles,a major food source, on the corals. Credit: BrownUniversity

As Witman and his students—over the years he'strained nine undergraduates, six doctoral studentsand three postdoctoral scholars in the Galápagosalone—continue to piece together how subtidal life ischanging with climate in the islands, he's alsoworking globally. He took a side trip in earlyDecember, for example, to check on another of hisregular spots: coral reefs near Easter Island, far offthe coast of Chile.

When he's home in New England, Witman canoften be found in the Gulf of Maine at CashesLedge, the largest and deepest offshore kelp forestin the North Atlantic. There, too, climate changemay threaten biodiversity.

"We are studying the resilience of the kelp forest towarming, as it is such an ecologically importantspecies, and also trying to achieve permanentprotection for the spectacular marine communitieson Cashes Ledge as a U.S. Marine NationalMonument," Witman said.

Lamb is working concurrently to show how theunique kelp forests on Cashes Ledge are home tosome of the healthiest populations of cod andpollock in the southern Gulf of Maine.

3 / 4

A triggerfish (top) swims along with its rival the hogfish,which will take urchins out of its mouth. Credit: BrownUniversity

Whether in the warm waters of the South Pacific orthe much chillier North Atlantic, the goal is tounderstand how ocean ecosystems work so thatpeople can help them thrive rather than threateningtheir survival.

"Sound management of the oceans requiresunderstanding the roles that key species andbiodiversity play in ecosystem functioning," Witmansaid. "And, of course, how this is being altered byhuman disturbance."

Provided by Brown UniversityAPA citation: Chronicling the complex ecological changes wrought by increasingly volatile El Niño – LaNiña cycles (2016, December 22) retrieved 16 November 2021 from https://phys.org/news/2016-12-chronicling-complex-ecological-wrought-increasingly.html

This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, nopart may be reproduced without the written permission. The content is provided for information purposes only.

Powered by TCPDF (www.tcpdf.org)

4 / 4