Does algae and shell hash from riprap make it into neighboring sediments?


You can find subtidal riprap all over the place in urban marine ecosystems.  It’s often like a patchy network of little islands in a sea of soft sediment.  On each of the islands is vibrant hub of reef-associated species – fish, algae, invertebrates of all shapes, colors, and sizes.  Collectively, these riprap-dwelling species generate a lot of biomass, particularly in the form of algae and shell hash.  Eventually, I’ll be testing whether these materials alter the soft sediment communities nearby.  But for now, I simply want to test whether the materials produced by riprap-dwelling organisms make it into adjacent habitats.

Divers entering water to collect soft sediment

This was my reason for embarking on a series of dives recently at Alki Pipeline and Elliott Bay Marina’s breakwater.  With the invaluable help of two dive buddies, Rhoda Green and Dave Thoreson, I was able to get my hands on lots and lots of dirt!  At each site, we lay out three transects extending perpendicularly from riprap.  We collected core samples at 1m, 7m, and 15m along each transect and put them into plastic bags.  After a lot of heavy lifting, we got the samples back to the lab, where I sit now (procrastinating by writing this) with my work cut out for me.  Over the coming weeks, I’ll be sorting through the sediment, quantifying algae, shell hash, sediment grain size, and macrofauna.

I’ll keep you posted on findings as they arise. For now, it’s time for me to get busy.