Turbid estuaries can obscure the seagrass signal. How do we go about mapping these areas? Do you have any tips?
Hi David - acoustic techniques don’t care about turbidity. I’d recommend side scan sonar, I had a lot of success with it in turbid Massachusetts. Even better if you can combine it with imagery, and obviously you must have a groundtruthing plan. Where are you mapping?
Hi Jill, we’re working in turbid, shallow Chesapeake Bay. I’ve seen some great results from side-scan sonar in other areas. How shallow have you been able to work? Since most of our grass grows at less than a meter, I worry that we can’t get high enugh above it in the water column with sonar to use it effectively. I understand that the swath width also depends on depth. I would love to hear more about how you’re using it.
- Dave
I’ve been able to use side scan in 1m water, but you’re right that the swath is reduced, especially if mapping taller seagrasses like Z.marina that can take up the whole water column and block the acoustic signal. In that case, I’ve run the side scan just shoreward (or seaward) of the shallow (or deep) edge to get a detailed edge delineation, and used underwater camera survey work to confirm bed characteristics within the meadow. I used a ~19ft maritime skiff with a consumer grade fish-finder side scan (Humminbird Helix 9 CHIRP), which sits just below the hull, and this has worked well in shallow areas, but optimal in > 3-5m for sure.
-Jill
I mapped eelgrass with a kayak-mounted sidescan (Helix 7) in a turbid Eastern Pacific estuary, but I found it impractical to cover large meadows with the reduced swath width in such shallow water. I had some success working at spring tides using mix of sensor platforms: Surveying intertidal/shallow parts of seagrass meadows with UAV at low tide, followed by sonar surveys of deeper areas at high tide.
-Johannes
We have used Biosonics MX Habitat Echosounder in turbid and deep waters in the Pacific Northwest with good results. I agree with Johannes above that a combination of techniques or sensors is ideal especially since the echosounder data should be groundtruthed. It can map at 3-4 knots and can cover large areas quickly. Coupling with UAV for shallower survey areas during low tide is a great approach to cover large beds, especially if there is an intertidal component.
Another technique for mapping in turbid systems could be based on ensemble satellite classifications. Using satellites that have a short revisit period (e.g., Planet revisits almost daily), one could select multiple scenes with favorable attributes (tide, turbidity, clouds, etc.) for classification. These classifications can be combined into a seagrass probability map, assigning high probability to areas where different classifications show consensus and low probability where they differ. This would be an example of that approach. Google Earth Engine would probably be suitable for this application, too.
-Johannes