During the last two summers the Edwards lab from SDSU and the Konar lab from UAF headed up to sample kelp forests and urchin barrens across the Aleutian island archipelago. With the loss of sea otters, sea urchins were able to eat down algae composing kelp forests, What tends to dominate these areas where kelp once was is hard (calcium carbonate) coralline algae which covers the bottom and deals with urchin grazing by having tough morphological defenses. Other more soft/fleshy algae can also survive, for example a green alga called Codium ritteri, and I was interested in determining why that may be. I flew up early to Alaska the summer of 2017 to start some experiments off Adak Island joined by Ben Weitzman (UAF) and Doug Rasher (Bigelow). We wanted to determine if this green alga that seems to thrive in some of the urchin barrens was actually being consumed by urchins. The most straightforward way to do this would be to remove some material, weigh it, then either place it out again where it could be consumed by urchins, partially cage it allowing grazing but blocking light, and then cage it eliminating urchin grazing but shading the algae as the partial cage will, let this run for a few weeks and reweigh the algae. We found evidence that urchins did consume the algae in the open and partial cages while the caged Codium grew. The hypothesis that urchins cant consume the algae was rejected. We had other questions like does the increased light in these urchin barrens change how much of the Codium that gets consumed and will urchins consume the same amount of Codium regardless of the islands the Codium comes from?
We conducted a few experiments using the Kasitsna Bay NOAA lab across Kachemak Bay from Homer to try to answer these questions. We collected algae and urchins from 7 islands across the chain and brought them back to the lab. To reduce the chance of changing local genetic pools, canister filters were used to remove particulates and UV filters to eliminate the foreign reproductive material from algae and urchins collected across the Aleutian Islands. I made 5 different light treatments to simulate light levels from urchins barrens which are relatively bright due to the grazing down of algae that block light and the kelp forests which are dark due to the canopy, subcanopy, and understory layers of algae that block light for Codium. This experiment would test if urchins ate different amounts of Codium depending on the light level over 20 days. I also set up a grazing experiment with Codium from urchin barrens off 7 islands and estimated the amount consumed over 20 days. These experiments helped us understand how this Codium alagae is able to survive when many other algae become urchin food. There are a few other algae that we observe in urchin barrens and they may all possess traits like morphological or chemical defenses allowing them to exist in the urchin dominated worlds that are these urchin barrens.