Kelp-forest dynamics across broad spatiotemporal scales: Stability and drivers of kelp distribution in the Mingan Archipelago (Gulf of St. Lawrence, Canada)Macroecology

Ladd Johnson^1,2, Patrick Gagnon³, Anne St-Pierre^3,4

¹Université Laval, Canada - ²Québec-Océan, Canada - ³Memorial University, Canada - ⁴Fisheries and Oceans Canada, Canada

Drivers of stability and change in benthic systems operate at multiple spatiotemporal scales. In submerged kelp forest systems, most studies to date have been limited to fine spatiotemporal scales due to the scuba-based monitoring techniques applied, which limits our understanding of the occurrence and drivers of broader patterns. We applied remote sensing and landscape ecology approaches to investigate spatial configuration and persistence of kelp forests over broad spatial (km2) and temporal (decades) extents in the Mingan Archipelago (Gulf of St. Lawrence, Canada). Aerial imagery covering 2.85 km2 of shallow (<7 m) seabed surrounding five islands was acquired every five to 11 years between 1983 and 2016 (six sampling years in total) and divided into two benthic classes based on cover type: kelp and non-kelp (barrens). Spatial pattern metrics were used to quantify the spatial characteristics of kelp forests (including kelp coverage, number of kelp patches, mean patch area, and largest patch index) and to examine the correlations between these metrics and atmospheric or oceanographic conditions. Overall, kelp cover increased from 1999 to 2016, but among the sampled years, harsh oceanographic conditions in late winter to spring (e.g., long ice-covered season, high North Atlantic Oscillation index) led to decreases in kelp cover, smaller mean patch sizes, and an increased number of patches. The distribution of kelp-covered areas was, however, relatively stable over time, with 75% of the total area exhibiting two changes in cover type or less over the entire study period. Analysis of the effects of depth, bottom slope, and exposure to waves on the persistence of kelp forests and variability in cover type showed a significant effect of depth only, with the cover on the deeper seabed more dynamic compared to shallower areas, likely due to variable grazing by urchins. The present study offers a unique, broad-scale perspective of the variability in distribution patterns of completely submerged kelp forests in the northern Gulf of St. Lawrence and the drivers of long-term stability in kelp-urchin systems.