Casual observations, random musings and wild extrapolations based on some actual data on the impact of invasive tunicates to eelgrass

Abstract

Tunicates, more commonly known as sea squirts, can grow as a solitary organism or as part of extensive colonies. Tunicates are more frequently associated with hard substrate, but a number of invasive tunicate species have now been found to colonize eelgrass. We attempted to quantify what impact these tunicates may have on eelgrass. Using HOBO light sensors, we measured the ability of 5 different tunicate species to block light. In several ponds on Martha\u27s Vineyard, we measured a number of eelgrass parameters (canopy height, leaves per shoot, growth rate, tissue sugar concentration) in both plants without tunicates and plants heavily colonized by tunicates. We found that all species of tunicates tested blocked between 70-85% of ambient light. We found that plants colonized by tunicates had fewer leaves per shoot, smaller canopy height, lower growth rates, but high tissue sugar concentrations than plants not colonized by tunicates. In 2013, we attempted to assess the extent of the problem, by coordinating a study of 19 different sampling locations from New Jersey to Canada. Particiapnts sampled eelgrass meadows and documented the presence/absence of tunicates, identified tunicate species, estimated the extent of the colonization, measured eelgrass shoot density and canopy height and in some locations collected water temperature. Invasive tunicates were observed on eelgrass in all meadows sampled from Newfoundland to New Jersey, though the number of species present and the extent of the colonization varied greatly