Shading impacts of small docks and piers on salt marsh vegetation in Massachusetts

Funding for this project was provided by MassBays to Division of Marine Fisheries (2013 and 2014)

Part 1: An Experimental Evaluation of Dock Shading Impacts on Salt Marsh Vegetation in a New England Estuary

Proliferation of small docks and piers in salt marsh habitats poses potential cumulative impacts through shading and displacement of marsh vegetation. Environmental permitting guidelines provide construction recommendations regarding various dock characteristics (e.g., height, orientation, deck spacing, deck materials), but few quantitative data exist to substantiate these guidelines. 

In a project funded by the 2013 MassBays Research and Planning Grant, the Massachusetts Division of Marine Fisheries constructed an experimental dock matrix in Marshfield, MA  to specifically address the effect of dock height on underlying marsh communities. These results provided useful information on the ideal dock height and data on short-term shading effects. Findings after one year indicated the need to maintain at a minimum the 1:1 height to width ratio and that taking into consideration that other dock features were designed to maximize light penetration, results suggest that standard four foot wide docks with traditional decking set over low marsh vegetation at less than a 1:1 ratio would significantly reduce impact marsh vegetation under all dock orientations. Results were observed over a brief, single season time scale and preliminary experimental results provide cautious support for the 1:1 height to width ration. Further monitoring was recommended to apply existing guidelines to a more rigorous test. 

Details of the methodology applied, results obtained, and recommendations are described in the final report: Shading Impacts of Small Docks and Piers on Salt Marsh Vegetation in Massachusetts. MA Division of Marine Fisheries. 2013 

Part 2: Environmental Impacts of Docks and Piers on Salt March Vegetation Across Massachusetts Estuaries - A Quantitative Field Survey Approach

To further build on the results of the 2013 controlled short-term study, MassBays, through the 2014 MassBays Research and Planning Grant funded MarineFisheries to conduct a large scale survey of vegetation characteristics under existing private docks and piers along coastal Massachusetts using the same survey approach employed in the 2013 dock matrix study. Specifically, clip plot samples of marsh vegetation were collected to measure aboveground biomass under docks and piers, and from adjacent unshaded control locations in order to quantify changes in marsh stem density, height, biomass, and elemental composition a) in relation to unshaded areas, and b) in relation to different dock designs. 

Results of the 2014 study provided support for height-based construction design guidelines as a means of reducing shading impacts. Docks that met or exceeded the 1:1 height to width ratio guideline had greater stem density and biomass than docks that failed to meet this threshold. Dock height was consistently a significant variable and data on stem densities and weights strongly suggest that height is the main driver in impacts on salt marsh communities. Results indicate that dock heights of ≥ five feet provide the least impact on density and biomass overall. However it was pointed out that such docks still impact salt marsh vegetation growth, with a lower stem density compared to unshaded marsh. The study results also provided some support for the benefits of grated decking as an alternative to traditional decking (higher stem biomass), but grated docks also showed a sensitivity to height. For this reason, it was concluded that grated decking alone would not provide a reliable means of minimizing shading but would need to be coupled with adequate height to minimize shading impacts. Consequently, even docks designed to promote light penetration will result in salt marsh loss. Given that over 1,000 docks are currently constructed over salt marsh in Massachusetts and new docks are continuing to be installed, cumulative impacts to marsh habitat and production at the system and state levels are worth exploring.

A comprehensive description of the methods, findings, and recommendations for future work including examining potential effects of cumulative impacts is provided in the report: Environmental Impacts of Docks and Piers on Salt March Vegetation Across Massachusetts Estuaries - A Quantitative Field Survey Approach. MA Division of Marine Fisheries.2014 

Data generated from this project, combined with data being generated from the 2013 study, will provide managers with information on dock shading impacts for use in future planning and regulation of coastal development. These data may be used to refine existing regulations to effectively minimize impacts to marsh vegetation and will help coastal towns with future management decisions pertaining to dock and pier proliferation across existing salt marsh habitats.   

Additional Resources for Part 2: Environmental Impacts of Docks and Piers on Salt March Vegetation Across Massachusetts Estuaries - A Quantitative Field Survey Approach

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