Variation in top-down control of red algae epibiosis in the White Sea

ABSTRACT: Epibiosis is shaped by a complex interplay of biotic interactions involving hosts, epibionts, and mobile consumers. In temperate waters, consumer control by mesograzers prevents complete overgrowth of seaweeds. In polar waters, the mechanisms determining the abundances of sessile organisms associated with seaweeds are unknown. We empirically assessed the strength of the consumer control effect on the colonization of the sub-arctic red seaweed Phycodrys rubens by caging individual plants in the field in the shallow subtidal of the White Sea (65°N). We compared epibiosis on plants in consumer exclosure cages, in cages with the mesopredatory shrimp Spirontocaris phippsii, in semi-enclosed cages, and on unmanipulated plants in a cold year (2014) and a warm year (2015). Despite the dramatic interannual variation in consumer control, the mean total cover of epibionts in the absence of consumers never exceeded 15%. While consumers had a substantial effect on the total epibiont cover in the warm year and a nearly negligible effect in the cold year, the total cover of unmanipulated algae was similar in 2014 and 2015. Bryozoans, which were selectively impacted by consumers—particularly shrimp—dominated in both years. However, bryozoan abundance was much lower in 2015, when the abundance of hydroids, sponges, and bivalves—less affected by consumers—increased. Consumer control is not a key factor preventing most Phycodrys plants from being heavily overgrown. Yet, smaller plants, which have a higher epibiont cover, may indirectly benefit from consumers. Future climate changes are likely to make the Phycodrys epibiosis increasingly top-down regulated.