Do algal subsidies from riprap alter soft sediment community structure?


In my last post, I provided some background information about spatial subsidies and how I think they may be playing a role in urban marine ecosystems. Specifically, I considered whether the red algae that grows on riprap may have an effect when it gets incorporated into adjacent soft sediments.

To test this, I set up a field experiment at the beginning of the summer.

Photo of enrichment plot
Enrichment plot treated with shredded read algae

The experiment consisted of a series of half-meter, circular plots, each marked at the center with a construction flag. The plots were set up in a large square grid.  In the days leading up to the experiment, I collected large amounts of shell hash and red algae from nearby dive sites.  All of the shell hash was sieved through a 0.5mm sieve. The algae were all ground up using a cheese grater so that they resembled the tiny pieces of red algae that I often find in sediment samples. Each plot then received one of the following treatments: 100mL of algae, 100mL of shell hash, 100mL of algae and 100mL of shell hash combined, 500mL of algae, 500mL of shell hash, or no treatment (as a control).  There are obviously many specific details to this that I’m leaving out, but please don’t hesitate to contact me if you have questions.

Photo of enrichment plot
Enrichment plot treated with shell hash

I’ll be collecting core samples from the plots at 8 weeks and at 16 weeks. Each sample will then be sorted and all macrofauna (worms, clams, snails, etc) will be removed.  By comparing the inhabitants of each sample, I should be able to detect whether the community of species that lives in soft sediment is affected by the introduction of shell hash and red algae, which appear to originate from riprap.