Tuesday, September 3, 2013

Ampelisca...easily my favorite amphipod

A close up of Ampelisca sp. collected from the sea floor off of Green Point

What is your favorite amphipod? Everyone should have one. In 2011 I co-authored a short piece on the potential role of benthic amphipods (and Ampelisca in particular) in nearshore ecosystems of the Strait of Juan de Fuca, which appeared as a sidebar (p 170-171) in Chapter 6 of "Coastal Habitats of the Elwha River, Washington - Biological and Physical Patterns and Processes Prior to Dam Removal". That piece was prompted by observations of the amphipod Ampelisca at two sites we use as reference for our Elwha sub-tidal surveys: Green Point at the mouth of Siebert Creek and Low Point at the mouth of the Lyre River.

Site 7SS1 off of Green Point in the Strait of Juan de Fuca

As part of this year's effort we re-surveyed a variety of sites that we hadn't visited since 2009, including a site called 7SS1 just to the east of Green Point. This site was one of the locations that first led me to sit up and take notice of these benthic amphipods because they absolutely cover the seafloor

A shot of the seafloor at site 7SS1...each of those things is an Ampelisca tube

This time around I decided to do a quick and dirty estimate of the density of these amphipods at this site by collecting a single sample (in this case using a 6 inch diameter tube pressed into the seafloor). I then sieved and hand sampled the amphipods out of that bulk sample, counted the individuals then dried the entire sample (in my home oven...much to my family's chagrin) and weighed the dried sample.

the mass of amphipods pulled out of the sediment sample from 7SS1, a total of 613 individuals

When all was said and done the numbers were astonishing...613 amphipods (and I think I did pretty well here, plus or minus maybe 10% on the count) in the sample, which equates to ~36,000 amphipods per square meter. The dry mass of the sample came out to 5.8 g, which equates to ~340 grams per square meter - or the equivalent of about two or three quarter-pounders with cheese. In 2009 we observed bite scars at this site that we attributed to gray whale feeding:

A divot in the seafloor, observed in 2009, that we attributed to Grey Whale feeding

in part because we observed whales feeding at this site in 2008 (see below) - no wonder when there is so many available calories, and a literature on gray whale feeding on Ampelisca, both off Vancouver Island and at summer feeding grounds in the Arctic.

A Grey Whale observed on the surface near site 7SS1 in 2008

Ampelisca seem to require particular grain sizes to build their tubes, and the hypothesis we presented in the report call-out wondered if the dam removal may make particular grain sizes more available to these amphipods and promote their recruitment in the Elwha coastal zone (where they are relatively infrequent)...still watching on that one.

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