FERTILIZER RUNOFF spurring ALGAE GROWTH Research finds Elkhorn Slough reserve is suffering from effects of agricultural waste
As scientist Kerstin Wasson trudges along the banks of Elkhorn Slough, her rubber boots crunch through a white, brittle crust of dead algae that encircles nearly the entire shoreline.
The live version of the algae is green and seaweed-like and can be seen accumulating in clumps in the shallow water near shore. When the algae dies, it washes onto the banks and hardens into a white, desiccated layer blanketing the plants. The thicker the layer, the more likely it is to kill the plants underneath, said Wasson.
“If you had been here 100 years ago, you would not see the green or the white (live or dead algae),” said Wasson, a research coordinator at the Elkhorn Slough National Estuarine Research Reserve.
Algae growth is natural in salt marshes, but levels have increased drastically since the 1970s, largely due to agricultural runoff. Fertilizers used by farmers have increased the nitrate levels in Elkhorn Slough’s waters, which fueled the algae’s growth, according to a paper recently published in the scientific journal “Biological Conservation.”
“We really have a smoking gun here,” said Wasson, who said that fertilizers make the algae grow faster. She was a lead author of the paper, along with others from the Elkhorn Slough National Estuarine Research Reserve, UC Santa Cruz, the University of New Haven, Drexel University and Duke University.
Other factors — such as septic tanks, warmer weather, upwelling in
the Monterey Bay, bird feces — have played a role in increasing the nitrate levels in the water, but the blame for algal blooms lies primarily on agricultural runoff, Wasson said.
“None of these factors have increased exponentially as fertilizer nitrates have,” said Wasson. “It’s the only plausible explanation.”
Another important finding from the team’s three years of research: The dead algae kills the native pickleweed plant beneath it, which loosens the soil, speeding erosion.
Citizen scientist Ron Eby, an avid kayaker, sparked the team’s research with his observation of the algae and the eroding banks, which he brought to an Elkhorn Slough weekly science club meeting in 2014.
“Every time I would go along the slough I would look at the banks and see them all covered with the dead algae,” said Eby, who also noticed the dead pickleweed underneath, which left the mud banks vulnerable.
The currents then could cut the mud banks in chunks, he said.
“Just like an iceberg calving off, that kind of thing — you’d see these big chunks of mud in the slough and the algae was still on top of it,” Eby said.
Eby helped the research team with grunt work: dumping buckets of algae in plots across the slough to test its effect on marsh health, tracking algae buildup and staking the banks to measure erosion.
Several factors can cause erosion — such as large tidal swings, crab burrows and sea level rise — but the algal crust speeds the rate, said Wasson.
California salt marshes are a rare ecosystem, and Elkhorn Slough is the largest one south of San Francisco.
The marsh plants improve water quality, reduce carbon dioxide levels and stabilize the shoreline against waves, said Wasson.
Since the 1800s, Elkhorn Slough has changed drastically, and about half of the historical salt marsh has been lost due to diking and draining of the waters, she said.
The solution for the slough’s algae problem is two-fold, said Wasson, with the first step being continued restoration of the marsh.
The second part is decreasing the amount of fertilizer that gets to the slough’s waters, she said. One way to do that is by building sediment basins — ponds that trap runoff at the lowest elevation on farmland.
The Resource Conservation District of Monterey County offers technical assessments to farmers for sediment basin construction.
The district has an engineer on staff who can be contracted for design work. The district also assists farmers in applying for USDA funding, which can pay for up to 75 percent of the costs for sediment basin design and construction, said Paul Robins, the district’s executive director.
“The obstacles (for farmers) are permitting, and definitely cost,” said Robins, who added that another issue is the size of the pond, which takes some land out of production.
North Monterey County has plenty of sloped agricultural land, and the steeper the land, the greater the runoff. In the 1990s and early 2000s the USDA had a comprehensive program focused on decreasing agricultural runoff around Elkhorn Slough, but funding dried up in the mid-2000s, said Robins.
“A lot of folks who were used to having a lot of help available are (now) challenged, working on their own,” Robins said. “And the county is now playing catch up with how they’re handling it.”