Comparing invertebrate and seaweed diversity in kelp forests and urchin barrens in Owen Bay, British Columbia, using Autonomous Reef Monitoring Structures (ARMS)

Artificial Reef Monitoring Structures (ARMS) are standardized units consisting of a stack of PVC plates that have been globally deployed in coral reef environments to monitor biodiversity. We tested the applicability of ARMS as a biodiversity monitoring system in NE Pacific temperate ecosystems, including kelp forests and urchin barrens. 10 ARMS were deployed for one year, five in a kelp forest and five in an adjacent urchin barren. Each ARMS unit was augmented with the addition of a 14x14 cm PVC settlement plate and a 20x7cm scrub brush; these two devices were replaced every 2 months. Seawater samples were collected in parallel every 2 months for the purposes of analyzing eDNA. We recorded a total of 420 animal species and 167 seaweed species across all devices and 305 animal species and 135 seaweed species from just the ARMS. ARMS accumulated species rapidly and did not reach an asymptote, suggesting that more than 10 replicates may be required to capture the full diversity able to settle on the ARMS. ARMS in the kelp and barren habitats were distinct, indicating a high potential for habitat resolution. We observed a very low degree of similarity between the ARMS and all other datasets included in this investigation – similarity was below 40% regardless of comparison (ARMS vs plates, ARMS vs eDNA). We observed a trend of decreasing similarity between the ARMS and the plates from April to January, indicating some influence of priority effects on structuring ARMS communities. However, the ARMS did support an additional richness component that is likely the result of successional processes, including facilitation.