Date

Spring 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Subtidal surveys are critical for ecosystem monitoring, traditionally involving diver underwater visual census (UVC), though such methods can impose time or logistical restrictions on the study. Imagery-based techniques, such as diver-operated stereo-video (stereo- -dimensional measurements from 2-dimensional images, facilitating accurate size and distance estimates as well as the ability to survey large areas quickly. This study compared two stereo-DOV angles (45° and 90°) with UVC to estimate the abundance of benthic macroinvertebrates and macroalgae, and habitat characteristics (relief, substrate, benthic cover) within kelp forests and urchin barrens. We also explored associations between habitat complexity, measured as relief and substrate transitions, and kelp and urchin densities. Stereo-DOV and UVC surveys yielded similar estimates for species richness, diversity, and purple sea urchin (Strongylocentrotus purpuratus) densities in urchin barrens, and for stipitate algae species densities in kelp forests. However, the presence of macroalgae significantly reduced the effectiveness of both stereo-DOV angles compared to UVC in characterizing community structure and quantified densities of key species, such as urchins. The 90° stereo-DOV surveys performed better in kelp forests relative to 45° surveys, but still measured lower species richness and diversity compared to UVC surveys. Cryptic species found in UVC surveys were often missing from both stereo-DOV survey types, particularly in kelp forests. Habitat characterization accuracy was similar between methods, though these similarities occurred mainly for broad substrate categories (e.g., sand, bedrock) and intermediate relief (>10 cm - < 1 m). Habitats with relief transitions (>1 per 10m-2) were associated with higher densities of giant kelp (Macrocystis pyrifera) and reduced densities of purple urchins. Substrate transitions were positively associated with stipitate algae density. Our findings suggest stereo-DOV may be suitable for monitoring community scale changes in urchin barrens, with reduced performance in dense kelp forests. The 90° Stereo-DOV survey estimates were more similar to UVC estimates than 45° stereo-DOV surveys, but habitat was a more significant influence on stereo-DOV performance. This study provides insight into using stereo-DOV as a benthic surveying tool in kelp forests and urchin barrens and the potential for habitat transitions to support kelp populations while limiting urchin movement.

Download

Share

COinS