Documenting the effect of substrate angle on invertebrate assembly patterns: An approach for identifying sessile benthic invertebrate assemblages in the San Juan Channel rocky subtidal zone

Abstract

Understanding benthic community distributions, and using such information in marine restoration ecology, requires an understanding of the effect of substrate angle on assembly patterns. To date, there is no information regarding in situ identification and remote identification of sessile benthic invertebrate assemblages on substrate angles from 0° to 90°. This study aimed to document the effect of substrate angle on sessile invertebrate assemblages and to both develop and validate an approach for remotely identifying assemblages based on substrate angle. The in situ photoquadrat method was used to document invertebrate assemblages at five 15 m deep sites in the San Juan Channel. Substrate angle was recorded for each photo based on six 15° categories. Univariate and multivariate statistical analyses suggested that there are three distinct assemblages of invertebrates: horizontal (0-15°), sloping (15-75°), vertical (75-90°). Crustose coralline algae, non-calcified (maroon) encrusting red algae, encrusting bryozoans, tube complex (amphipods, polychaetes), and porifera contributed more than 5% to assemblage differences due to angle at all sites. These results necessitate a review of factors thought to cause differences in assemblages due to substrate angle. The relationship between species and substrate angle was applied to multibeam echosounder (MBES) bathymetry data, gridded at 2 m, in ArcGISTM. Transect locations were plotted and ArcGIS generated slope values. ArcGIS scale was smaller than the photoquadrat scale; therefore, the scales were not comparable. This technique for remotely identifying assemblages based on substrate angle may be utilized in the future by following a revised protocol that includes the collection of bathymetric and photoquadrat data at the similar Paxton, 2 scales. This approach may be used both in the San Juan Channel and internationally for restoration and long-term monitoring efforts.