A new Cal Poly study shows water quality conditions in some parts of the Morro Bay estuary could be the driving factor in preventing eelgrass recovery after the iconic beds experienced a massive die-off over the past decade.
Although researchers have not been able to pinpoint the cause of the initial eelgrass collapse — having declined more than 90 percent since 2007 — environmental conditions likely limit growth in many parts of the bay, according to ecologist Jennifer O’Leary.
“Eelgrass is important because it supports a range of marine life,” O’Leary said in a news release. “It’s like the trees in a forest — these underwater plants provide food, structure and shelter to many of the animals that live in the bay.”
In bays and estuaries like Morro Bay, the long, ribbon-like leaves of eelgrass nurture a variety of sea life, everything from juvenile rockfish to invertebrates. It also is a vital food source for black brant geese during their annual migrations.
Efforts to restore eelgrass have failed in many parts of the bay, the release said, and the plant is now only found close to the mouth of the bay and sporadically in other regions.
A team of Cal Poly scientists connected with local, state and federal partners, including the Morro Bay National Estuary Program (MBNEP), to address the problem. The estuary program has tracked eelgrass health since the early 2000s.
MBNEP executive director Lexie Bell said the flowering underwater plant “is a critical component of a healthy, functioning Morro Bay ecosystem.”
Bell said the organization uses monitoring, restoration and research to help determine the best way forward to protecting and restoring the crucial habitat.
Ryan Walter, a physical oceanographer and physics professor at Cal Poly, and Edwin Rainville, a mechanical engineering student from Colorado, teamed with O’Leary to better understand how the movement and existing water quality conditions in different parts of the bay affect eelgrass survival.
The researchers set up oceanographic underwater mooring stations to measure a variety of water quality indicators at different sites in the bay during the summer of 2016.
They also tracked and analyzed hydrology, weather, dredging activity and eelgrass data back to the 1980 for additional context of the eelgrass ecosystem.
The study shows that in areas where eelgrass disappeared and had not returned — particularly in the southern or back part of the bay — water conditions were dominated by a combination of higher water temperatures and salinity, lower oxygen levels, high turbidity that limits light availability and limited flushing, where water gets “trapped” for longer periods of time.
“Previous research has shown that all of these factors can be stressors for eelgrass,” the release said. “Together, they may prevent eelgrass from bouncing back.”
Near the mouth of the bay, where ocean tides regularly refresh the region with relatively cool, clear and more oxygenated water, the researchers found eelgrass persisted — and recent restoration efforts with the MBNEP were successful.
Walter and others are exploring a hypothesis that sediment changes in the bay — driven by some combination of dredging, heavy precipitation and sediment loading events, followed by drought years — may have contributed to the initial decline.
The study also calls for more research and monitoring of similar estuaries to establish baselines to better assess drivers of ecosystem collapse when they occur.