Coastal resources exploitation can mask bottom-up mesoscale regulation of intertidal populations

Abstract

We describe the spatial distribution patterns of rocky intertidal Patella spp. limpets (heavily collected by shellfishers) and top-shell snails belonging to the genus Osilinus (comparatively slightly harvested) through a multiscaled sampling design spanning five orders of magnitude of spatial variability (from 10s of m to 100s of km) throughout the Canarian Archipelago (eastern Atlantic); where rocky intertidal assemblages on opposite sides of the Archipelago (western vs. eastern islands) are subjected to different regimes of bottom-up effects, as large spatial variation in oceanographic conditions is recorded across an east–west gradient. We tested the hypothesis that the response of rocky intertidal populations to mesoscale oceanographic bottom-up variability (quantified using differences in Chlorophyll-a concentration among islands as an approximation to bottom-up effects) depends on the exploitation status of coastal resources, by means of a correlative approach. Our study represent another case in which mesoscale shore-associated physical processes seem to be correlated to large-scale differences (variability among islands, 10s to 100s of km apart) in the abundance of slightly harvested intertidal grazers (topshell snails). In contrast, we did not observe large-scale spatial differences for heavily collected grazers (limpets). In conclusion, our study suggests that the signal of bottom-up processes in coastal populations may be difficult to demonstrate under intense human exploitation.