Healthy eelgrass on left vs. severe Lyngbya bloom in 2012 on right.
Maria Mitchell Research Associate Dr. Peter Boyce has managed the scallop and marine ecology research program for the last four years. Since 2006, the eelgrass covering the bottom of our harbor has decreased. With 2014 came a small recovery, a hopeful event after the devastating Lyngbya infestation in 2012. Lyngbya is an invasive macroalgae that smothers eelgrass. This coupled with single celled algae, both of rich thrive on nitrogen-rich water, filter out sunlight and inhibit eelgrass growth. Read below for more details.
Learn what part you can play in helping control the algae in our harbor!
Dr. Peter Boyce will present his findings to the Harbor and Shellfish Advisory Committee on Tuesday, December 16 at 5:00PM in the Community Room at 4 Fairgrounds Road.
Details on the study:
The amount of eelgrass in Nantucket Harbor has shrunk by as much as 40% over the last nine years, according to a long-running Maria Mitchell program of dive surveys. MMA Research Associate, Dr. Peter Boyce, who has managed the program for the last four years, discovered that the percent of the bottom covered by eelgrass has decreased dramatically since 2006 in all areas of the Harbor. The graph below shows the percent of the bottom covered by eelgrass year by year in two critical areas of the Harbor.
Third Bend is a productive bay scallop area, and the Head of the Harbor functions as a nursery. The scallops there tend to seed the rest of the Harbor with their larvae when they spawn. The insidious decline over the last nine years shown above is small enough not to be noticed year to year. It averages out to be only about 5% per year, and is often swamped by the larger year-to-year variations, but after nine years of scientific sampling we can say with certainty that the eelgrass is in danger of being totally lost in the next decade.
One of the threats is Lyngbya, an invasive macroalgae which first appeared in Nantucket Harbor in 2008 and can grow in sufficient quantities to smother eelgrass, as shown in the left picture at the top of the post, taken in 2012. The picture on the left of a healthy eelgrass bed dramatizes the importance of eelgrass as a safe nursery for young fish as well as tiny seed scallops. On a hopeful note, after dramatic declines in coverage of 37% in the harbor between 2012 and 2013, possibly due to Lyngbya, eelgrass did slightly rebound in 2014.
However, it is not just the black masses of Lyngbya that threaten Nantucket’s eelgrass; single celled algae flourish during the summer as well, abundantly nourished by high levels of nitrogen in the water. They too, filter out the sunlight and inhibit eelgrass growth. We are forcing our sun-loving eelgrass to grow in the shade, and it just isn’t working.
What can we do? Science says that the best method to keep algal levels down to reasonable numbers is to starve them by reducing the available nitrogen in the water. The rebound in eelgrass coverage in 2014 shows there is hope for our eelgrass. This little temporary success should encourage everyone to reduce fertilizer usage to no more than your lawn and plants can absorb and use. The 2014 rebound should, likewise, encourage everyone to maintain their septic system to ensure it is working at maximum efficiency. And it should encourage us to cheer for the raising of the jetties, which will, according to models, improve the water flow into and out of the Harbor.