The effects of seascape pattern on algal patch structure, sea urchin barrens, and ecological processes

Abstract

Seascapes are the marine analog of terrestrial landscapes, and their importance for ecological processes in marine ecosystems is generally poorly known. This is especially true for giant kelp (Macrocystis pyrifera) forests for which the acquisition of high resolution acoustic terrain data is problematic due to canopy cover. The goal of this study was to determine the effects of geologically forced seascape structure on algal patch structure, and to address the potential for seascape pattern to affect higher order ecological processes within giant kelp forests. The difficulty of terrain data acquisition within giant kelp forests was overcome using a narrow single beam sonar system and the development of processing algorithms that resulted in high resolution (10m) models of bathymetry and algal patch structure. Algal patch structure was determined for the canopy kelps Macrocystis pyrifera and Pelagophycus porra, and pooled understory algal species. The analysis also enabled estimation of the spatial distribution of a large (>2ha) persistent sea urchin barrens. The results highlight the importance of bottom topography and bottom depth distributions on algal distributions, algal patch structure, and the dynamics of the sea urchin barren. Large scale topographic features (ca. 500m), depth, and their interaction are significantly related to canopy and understory distributions among the forests and known patterns of algal resiliency and persistence. Canopy is preferentially distributed toward large scale topographic highs, whereas understory is preferentially distributed towards topographic lows. Pelagophycus porra, however, does not exhibit a distributional preference among hard bottom topographic features. Large scale topography clearly controls algal patch structure. Greater algal patch density and complexity are associated with topographic highs and lows, whereas patches in planar areas exhibit large core areas with less edge habitat and less interspersion with different canopy guilds. Comparisons of the joint probability distributions of bottom and algal depth distributions among forests indicates that density dependent positive feedback processes potentially reinforce the predominance of their differential canopy guild composition. Seascape structure (1) controls and interacts with algal patch structure to define edge habitat structure and potentially foraging and migration pathways, and (2) defines the margins of the large persistent sea urchin barrens and likely the dynamics of sea urchin feeding and movement. The results reinforce the need for adequate sampling design and replication for probability based benthic assessments when high resolution habitat structure is not available, or the utilization of model based purposive sampling strategies when habitats are known.