Friday, December 4, 2015

Sandwich eelgrass results

The area we studied in Sandwich does not have, as far as I know, previous maps or estimates of eelgrass coverage.  I don't know how old this meadow might be.  The closest previously mapped eelgrass is from aerial photographic analysis Mass DEP does.  They mapped a bed to the northwest.  There was also work done by Woods Hole Group, a consultant for a town beach nourishment project.  They mapped a small bed near the spit in the middle of the picture and some patches off a groin in the southeast corner of the picture below.  The US EPA identified this bed as a good location for a blue carbon study.  Eric Nelson of EPA snorkeled in the area to check it out and gave us two points indicating the approximate shoreward edge of the bed.  Those are the two bigger dots just south and east of the spit in the middle of the picture below.  The area is about 0.5 miles away from the meadow DEP mapped.
Our goal was to map the extent of the bed near the EPA points.

We mapped a total of 21.1 acres and interpolated 8.3 acres based on the aerial photo we had.  Total acreage = 29.4 acres.  This is less accurate than what we did in Cohasset since there is no previous information and there is a significant eelgrass-algae mix with many boulders.  The seaward side has a lot of Fucus algae which has large air bladders and looks the same as eelgrass in the sidescan.  We weren't sure if we made it to the southeastern extent of the bed with the sidescan, so we examined the 2013 USGS aerial photos.  Vegetation is really obvious in the aerials, but differentiating eelgrass and any type of algae in the aerials is impossible without groundtruthing.  Therefore, interpreting the aerial photo without groundtruthing is, well, a crapshoot.  We did due diligence and studied bathymetry to define a shoreward and seaward edge, but this is a tough area.  Take areas without groundtruthing with a grain of salt.

Here are some photos to give an idea of what it looks like on the bottom:

Patchy eelgrass
Dense eelgrass
Red algae found in deeper part of study area
Cobble bottom
The last image is from the spit that defined the northwestern edge of our study area.  It was a pretty cool spot.  As it deepened, more and more algae encrusted on the cobbles.


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