Seagrasses at World Heritage Site still suffer from heat wave

Massive seagrass beds in Western Australia’s Shark Bay have showed little recovery from a devastating 2011 heat wave, according to a new study.

The team’s findings demonstrate how certain vital ecosystems may change drastically in a warming climate.

“We were studying a relatively pristine ecosystem, but in summer 2011 we had the hottest water temperatures on record at the time, and we saw 70-90 percent losses of seagrasses at our study sites. No one expected it to be that bad,” said Rob Nowicki, a researcher at Mote Marine Laboratory who conducted the fieldwork while a doctorate student in Marine Sciences at FIU. “After our colleagues documented the losses, we wanted to know how much the ecosystem might recover over a few years. If you take a punch and get up quickly, you’re ready for the next punch. But our study has suggested this system took a punch, and in the short term, it has not gotten back up.”

Several years after a harsh 2011 heat wave, a once-lush seagrass bed still struggles to recover.

Shark Bay, a UNESCO World Heritage site, features 1,853 square miles of seagrass beds, which UNESCO’s website calls the “richest in the world.” This vast, subtropical ecosystem hosts thousands of large sharks, other fish, sea turtles, bottlenose dolphins and a critical population of dugongs, plant-eating mammals related to manatees.

The researchers surveyed 63 sites in Shark Bay four times between 2012 and 2014 to assess seagrass recovery and changes. Before the heat wave, many sites were dominated by the temperate seagrass, known as “wireweed,” whose dense and tall thickets provide ample food and shelter for numerous species. The heat wave drastically thinned many wireweed beds, and in many places their underground stems blackened and died, leaving bare sand.

The new study showed that surviving wireweed beds appeared stable but didn’t reclaim much turf. Instead, a tropical seagrass that is a close relative of a seagrass native to Florida, began filling the gaps.

“The seagrass hit hard was the most common species — and was dense like a mini forest,” said Mike Heithaus, dean of FIU’s College of Arts, Sciences & Education. Heithaus was the doctoral advisor to Nowicki and co-author of the study. “Losing that cover is really huge; it’s like going from a bushland in Africa to a well-mowed lawn.

Losing that much structure has consequences, including loss of water clarity. Fewer seagrasses were available to trap sediments, and decaying seagrass may have nourished a bloom of microscopic algae observed in 2014. Study authors say these consequences aren’t surprising, given the valuable ecosystem services healthy seagrass beds provide.

Seagrass beds stabilize sediments, preventing erosion and clarifying water. More seagrass biomass can store more carbon dioxide, decreasing its availability to harm ecosystems through climate change and ocean acidification. Dense seagrass beds are also critical for economically important fisheries. Seagrass meadows are valued at $1.9 trillion worldwide just for their role in cycling nutrients, according to a 2009 study by others in Proceedings of the National Academy of Sciences. However, major seagrass ecosystems around the world have declined by about 7 percent per year since 1990, reminiscent of declines in coral reefs and other critical ecosystems.

Some take-home messages are clear — It’s critical to monitor ecosystems well after a disturbance because they are not guaranteed to bounce back.

“It shows the importance of these long-term, comprehensive, ecosystem-level studies,” said Heithaus, referring to team efforts to study Shark Bay. “If we hadn’t been doing this since 1997, we wouldn’t have had the baseline data to know that the declines were a big deal.”

Also, if relatively pristine seagrass beds of Shark Bay are vulnerable to extreme weather, then it’s unclear how seagrass beds damaged by human activity will fare in the coming decades. Nowicki says that minimizing local stressors, such as nutrient pollution from fertilizer runoff into bays and estuaries, may give seagrasses better odds amid climate change and other global stressors.

“If Shark Bay had poorer water quality, we might have lost a lot more,” Nowicki said.

The latest study, published recently in Marine Ecology Progress Series, included partners from FIU, Deakin University in Australia and Nova Southeastern University in Fort Lauderdale.