Ocean food web “disrupted” by marine heat wave

A study carried out by Oregon State University has shed light on a marine heat wave that has been “[disrupting]” the ocean food web.

The researchers have explained that marine heat waves in the northeast Pacific Ocean are creating “ongoing and complex disruptions”, that one the one hand may “benefit some species”, but could also “threaten the future of many others”.

The study was published in the journal Nature Communications and examined the impacts of marine heat waves on the entire ocean ecosystem in the northern California Current, the span of waters along the West Coast from Washington to Northern California.

Researchers found that the biggest beneficiary of marine heat waves is gelatinous zooplankton – predominantly cylindrical-shaped pyrosomes that explode in numbers following a marine heat wave and shift how energy moves throughout the food web.

Working on the study was lead author Dylan Gomes, a postdoctoral scholar with Oregon State University’s Marine Mammal Institute.

“If you look at single species interactions, you’re likely to miss a lot. The natural effects of a disturbance are not necessarily going to be straightforward and linear. What this showed us is that these heat waves impact every predator and prey in the ecosystem through direct and indirect pathways,” explained Gomes.

Completed in collaboration with the National Oceanic and Atmospheric Administration, the project was supported by Joshua Stewart, an assistant professor with the Marine Mammal Institute.

“What I found both alarming and fascinating is the extent to which these pyrosomes absorb all of the energy in the system,” Stewart said.

“Because nothing else really eats the pyrosomes, they just become this dead end, and that energy is not available for anyone else in the ecosystem.”

Defined as period of prolonged, unusually warm ocean temperatures, marine heatwaves have been found to be increasing in prevalence and intensity around the globe.

“While the impacts of these heat waves on marine species have been well-documented on individual and population levels, the effects on the entire ecosystem have not been well understood,” continued Gomes.

In order to obtain a more holistic view of the impact of marine heat waves, Gomes updated an end-to-end ecosystem model with new data on marine life throughout the ocean food web that was collected during local biological surveys. Following this Gomes compared how the food web worked before and after a recent spate of marine heat waves, including a large, well-documented event in 2013-2014 known as “the blob.” Much of the new data used in the model was then collected following that event as researchers tried to better understand its impacts.

The research unveiled various impacts including some that were predictive including pyrosomes, which are known to thrive in warmer waters – but the analysis also showed that the ecosystem functions in ways that are not intuitive.

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“For example, the modeling showed how the dominance of pyrosomes drew energy out of the food web. That loss of energy is most likely to affect fish and marine mammals that are higher up the food chain, potentially impacting economically important fisheries and recovery efforts for threatened or endangered species,” said Stewart.

According to the researchers, their updated model could help commercial fisheries adapt harvest strategies that are impacted when fish commonly found in one area move to escape the encroaching warm water or their populations drop due to lack of available food following a marine heatwave.

In addition the team has suggested that their methods could also provide a template for future research to understand the impact of these events elsewhere.