What are the impacts to seagrass from dredging?

A project is planned to dredge near seagrass beds. What studies or sources of expertise are available to assess the impact of this type of work?

Hi Mark,

We’ve spent a fair bit of time studying recovery of SAV areas scarred by boats. Here’s a 2017 paper where we report on some of what we have learned.

Orth, R., Lefcheck, J., & Wilcox, D. (2017). Boat Propeller Scarring of Seagrass Beds in Lower Chesapeake Bay, USA: Patterns, Causes, Recovery, and Management. Estuaries and Coasts, 40. https://doi.org/10.1007/s12237-017-0239-9

This figure summarizes detailed field monitoring results.

  • Dave

Thanks Dave, Very cool!

Hi all, I know this topic is a year old now, but I am interested in finding any information about impacts to seagrass from dredge material being placed on an adjacent marsh habitat. Has anyone looked at this not necessarily in terms of direct dredging impacts to beds, but indirect impacts due to runoff of the newly placed dredge spoil from the adjacent marsh habitat? Thanks!

Hi Erin (@eshields) We haven’t done any specific studies, but the few living shorelines & marsh creation projects I’ve been involved has used turbidity curtains during construction. I’m not sure how long they leave them out but I can ask our contacts at The Nature Conservancy.
Also Kris Kaufman (@kris.kaufman) may have some info or know some folks she can put you in contact with.

Hi Erin (@eshields) and Dottie (@dotbyron) - I reached out to a few folks, who then reached out to a few more folks and collectively we compiled some info to share. I reached out to scientists working on Louisiana’s Coastal Planning Protection and Restoration Act who design and build wetlands using thin layer placement, while they don’t encounter many adjacent seagrass habitats near their work they noted they successfully contain material with dikes/berms but can loose material from dewatering structures (a possible weak point in any project to be aware of) but they also add turbidity curtain to wier boxes to protect nearby oyster habitat. They pointed me to a USGS publication (https://pubs.er.usgs.gov/publication/ofr20211062) that provided information on the burial of seagrass at the Chandeleur Islands due to overwash from constructed sand berms, this document also has references contained within that might be of interest. Additional reach out by this team to Gulf of Mexico contacts found additional literature that might be of interest: 1) recovery by benthos and seagrass after dredge material placement, see P.Sheridan’s work https://www.sciencedirect.com/science/article/abs/pii/S0272771403003111?via%3Dihub ,and; 2) Carruthers et al. 2002 is a synthesis listing many data sources related to habitat function and sediments in seagrass https://www.ingentaconnect.com/openurl?genre=article&issn=0007-4977&volume=71&issue=3&spage=1153&epage=1169&aulast=Carruthers ,and finally; 3) Ben Longstaff studied pulsed sediment effects on seagrass in northeast Australia (Harvey Bay, Moreton Bay) and Eva Koch has some Chesapeake Bay studies. While not all of this work is directly related to marsh thin layer placement work and its possible indirect impacts on seagrass it might help connect some dots? This is a very interesting point to bring up as we are going to start seeing these large-scale wetland restoration/recovery/resilience approaches being applied not only in La but in many parts of the US trying to combat subsidence and sea level rise. Maybe conversations need to be had between experts in the restoration application and the seagrass community in order to better understand what needs to be studied and where in the project implementation process the vulnerability to seagrass occurs (and how to best abate that).

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Thank you both so much for all of this great info. I agree, this is likely to be coming up in the future quite a bit, and we don’t want to see seagrass concerns left out of the process.