LB 37-311 - Green gravel as a technique to restore kelp forest ecosystems threatened by climate change

Kelp forests are threatened by climate change, which has motivated the development of methods for restoring these critical coastal ecosystems. Giant kelp (Macrocystis pyrifera), a foundational kelp species along the temperate Pacific coast of North America, has been severely reduced in abundance and range due to ocean warming and overgrazing by sea urchins. Green gravel is a potentially cost effective, scalable technique to restore lost kelp forests that involves seeding kelp spores onto small gravels, growing them to a viable size in aquaculture systems, then outplanting them in the field. Despite showing promise as a large-scale solution, this technique is very new, and many fundamental questions remain to optimize green gravel for different species and unique local conditions.We conducted a restoration experiment using giant kelp grown on green gravel to understand how genetic diversity and source population impact outplanting success (i.e., survivorship, growth) along a temperature range. We sourced kelp from two populations along Barkley Sound (Vancouver Island, British Columbia): a “cool” site towards the mouth of the sound, and a “warm” site toward the edge of giant kelp’s local range where considerable dieback occurred during a recent marine heatwave, and where thermally tolerant strains may have persisted. Green gravel was seeded by kelp spores in four experimental treatments defined by combinations of their genetic diversity (two parents vs. ten parents) and source population (warm vs. cool) and grown under identical conditions. After four weeks in culture, green gravel was outplanted into experimental plots at the two source sites. After four weeks in the field, the kelp showed 70% survivorship and ~40mm of growth on average across all plots. The number of parents and the source population had little impact on survivorship and growth, but the outplant site had a significant effect, with kelp having 55% greater survivorship and 26% more growth at the cool site. Monitoring will continue through the summer and results from later timepoints will also be presented. This experiment provides an important early trial of the green gravel technique for giant kelp in British Columbia. Our results show that, while genetic diversity may be important at later stages of restoration, within-site conditions have a large impact on early growth and survivorship.