ABSTRACT
The impacts of structures over seagrasses are not clearly understood. Construction parameters such as height and orientation regulate the total amount of solar radiation received within the area shadowed by over-water structures. The effects of dock height and piling presence on
Halodule
wrightii
and
Syringodium
filiforme
were examined for 12 m transects at three sites at Indian River Lagoon, Florida. Seagrass cover, shoot density, and photosynthetically active radiation (PAR) were measured
in situ
over a 12-month period for treatments of pilings without decking, 0.91 m docks (above Mean High Water) and 1.52 m docks. Treatment and control transects showed post-construction decreases in cover and shoot density, partially because of an algal bloom produced by an unusually wet winter. Average percent decreases between 14 weeks pre-construction and 33 weeks post-construction showed predicted trends: cover at controls declined 12.3%; cover at pilings declined 21.3%; cover at 1.51 m docks declined 23.8%; and cover at 0.91 m docks declined 28.4%. Shoot counts nearest structures decreased 56% whereas shoot counts farthest from the structures decreased 40%. Analysis of seagrass change along treatment transects reveals a trend toward decreases nearest the docks and pilings, showing the greatest average treatment impacts closest to the structures. Monthly light data collected at the seagrass bed edge at the three sites predicted minimum seagrass light requirements of 33.3, 28.6, and 26.7% I
. Daylong PAR surveys showed irradiance values for 0.91 and 1.51 m docks as significantly lower than control and pilings values in peak growing season (July). Current trends and PAR measurements were used to predict the decline and perhaps demise of the seagrasses directly influenced by dock shading.
