Salt marsh habitat is threatened by massive algal blooms, according to a new study conducted by scientists at the Elkhorn Slough National Estuarine Research Reserve (ESNERR), published in the scientific journal, Biological Conservation.
The study reveals that piles of green algae form mats along bank edges that leads to dieback of the salt marsh below. When the marsh plants die, the roots no longer help to hold the bank together causing large pieces of the bank to fall into the channel in response to strong tidal currents or wind-driven waves in the slough. The result: marshes convert to mudflats at the edges, and, in the face of sea level rise and low sediment volumes, the conversion is permanent.
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Citizen scientist, Elkhorn Slough Reserve volunteer, and avid kayaker Ron Eby, sparked the study with an observation of the algal mats and eroding bank chunks. “It just seemed there was a connection between the algal mats on the marsh edges and mud chucks calving off the banks,” Eby states. He brought his wonderings on whether the mats caused the erosion to ESNERR Research Coordinator Kerstin Wasson, Ph.D, who oversees the diverse research projects occurring at Elkhorn Slough.
“It’s heartening to see the curiosity and observational skills of our volunteer Ron Eby lead to significant discoveries about the challenges we face in the Elkhorn Slough,” said Reserve Manager Dave Feliz. With the question raised, ESNERR researchers led a collaborative investigation including Reserve staff, academics, summer research interns, and Eby himself. The team monitored 15 sites along the main channel of Elkhorn Slough where the edges had varying amounts of algal mats. The year-long surveying found that plant health decreased and bank erosion increased the more extensive the algal mats were.
To complement this observational study, an experiment involving 30 test plots was conducted. Interns lugged buckets of algae onto a marsh to cover marsh plants for either 0, 8 or 12 weeks. The plots showed the same results as the observational monitor — impaired marsh growth and retreat of the marsh edge —indicating the more often or the more algae the worse the impact on the marsh.
Green algae are natural parts of the estuary community, but their abundance can be tremendously increased by nutrients in the water. To determine whether algal blooms at Elkhorn Slough were natural or the result of human activities, the research team did some historical detective work. They examined aerial photographs dating from the 1930s to the present and found that algal mats on the marsh have increased dramatically during the 85 year period. Data on nitrate concentrations in the water and on fertilizer sales in the county were also compiled. The findings showed exactly the same pattern of exponential increase of nitrate concentration, fertilizer sales, and algal blooms over the same period. The study thus concludes that agricultural fertilizer is not only fertilizing crops in the region, but also, through run-off, algal blooms in the estuary. These fertilizer inputs are responsible for marsh dieback along the channel edges of Elkhorn Slough, as well as for increased erosion rates of the banks.
A related study by Reserve scientists published late last year similarly concluded that nutrient-enrichment of Elkhorn Slough waters is harming biodiversity there. Sites with poor water quality had low oxygen conditions that killed fish and stopped oyster growth. Both of these investigations illustrate the importance of decreasing polluted run-off to the estuary and improving water quality. “Salt marshes, fish and oysters will be healthier at Elkhorn Slough if fertilizer inputs are decreased,” says Dr. Kerstin Wasson, Research Coordinator of the Elkhorn Slough Reserve and author on both papers. “The loss of marsh will reverberate into the future,” Wasson explains “we are learning that marsh is excellent at taking up carbon dioxide from the air and storing it deep in marsh soils, which can help to slow climate change.”