What’s changed? Post dam removal benthic surveys start at the mouth of the Elwha River
By Sean Sheldrake, Steve Rubin, and Alan Humphrey
In this second part of our story (see our earlier blog post), we return to the Elwha to talk more about the techniques involved with the survey.
This USGS-led survey involves counting over 65 species of invertebrates and 23 species of algae—all of which we had to memorize before the survey began. As if that wasn’t enough homework for the dive crews, you have to “sneak” up on your critters to actually count them!
Species like Mya truncata clams can “see” you coming and will retract if they can feel the pressure wave of the diver approaching. Likewise, tubeworms are also underwater detectives with their own early warning sensors for approaching divers. Once Schizobranchia insignis or Eudistylia polymorpha tubeworms retract they look remarkably similar!
In buddy teams, divers go down and count algae (kelp, for example) on one side of the transect, and invertebrates (such as clams) on the other. Our divers must adjust for this “shy” behavior when they reach the bottom and “change things up.” Since each diver must count critters and algae on one side of the transect only, the invertebrate scientist tries to count on the downcurrent side of the transect line. After all, the algae-counting scientist has the benefit of their “prey” not running away from them!
In addition to counting all the species within one meter of the transect tape for 30 meters for algae and invertebrates respectively, a separate survey is done called a “uniform point count.” Every ½ meter, the diver puts their finger down along the transect tape and counts only what is beneath it. (Even if the most amazing anemone is an inch away, it doesn’t count!) Statistically, the point count and overall tally of species will give a representative assessment of life in the ocean ecosystem near the Elwha River mouth.
Early survey results included a decrease in algae abundance compared to levels seen before the start of dam removal. The decrease may have been due to light deprivation rather than loss of suitable substrate as there was little obvious accumulation of sand or mud on the seafloor. The divers deployed light sensors at many stations to help to document what sort of change in light penetration was occurring at each site. In addition, it seems that tubeworms are on the increase.
What other changes are there? The study will show the changes for the nearly 100 species of algae and invertebrates, in addition to fish, for the largest dam removal effort in North America to date.
Find out more about the wild survey conditions next week in part three of our story.
For more information on the study, see: http://www.usgs.gov/elwha.
For more information about the EPA dive program, check out their Facebook page at: http://www.facebook.com/EPADivers.
About the authors: Sean Sheldrake is part of the Seattle EPA Dive unit and is also a project manager working on the Portland Harbor cleanup in Oregon. He and Alan Humphrey both serve on the EPA diving safety board, responsible for setting EPA diving policy requirements. In addition, they both work to share contaminated water diving expertise with first responders and others. Steve Rubin is an aquatic biologist specializing in algal species with the USGS and a lead scientist on the survey.
Editor's Note: The opinions expressed in Greenversations are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.