Predator-mediated nutrient transfers and recycling in ecosystems: Fine-scale variation in the contribution of Antarctic fur seals (Arctocephalus gazella) on and around the Kerguelen Islands

Abstract

Waters on the Kerguelen Plateau are characterised by a high productivity and unique nutrient dynamics compared to the surrounding Southern Ocean. They support large populations of amphibious marine predators such as seabirds and marine mammals, which participate in nutrient recycling at sea and nutrient transfers from sea to land on their colonies. This study investigates the contribution of Antarctic fur seals (Arctocephalus gazella) to the biological recycling of 15 essential and 5 non-essential nutrients on and around the Kerguelen Islands during their four-month breeding season. Nutrient concentrations of 59 fur seal scats from two colonies were examined and used to determine faecal nutrient output from lactating females using a bioenergetic model. We evidenced individual variability in the multi-nutrient composition of fur seal scats, with different types of potential fertilisers. Colony differences in scat compositions resulted in multi-nutrient release levels that did not simply reflect colony abundances. This suggests that estimating predators' nutrient release based on averages (diets, prey or faecal nutrient contents) likely overlooks important fine-scale spatial variability in their contributions. Collectively, fur seals deposit up to 628 kg (95 % CI [548–710]) of phosphorus (limiting on land), 46.4 [39.8–53.2] kg of iron (limiting at sea), and 5.9 [5.2–6.8] g of lead (toxic to most organisms). At sea, fur seal defecation could trigger locally significant enrichment events. On land, fur seal faecal nutrient deposition is spatially concentrated in coastal colonies where phosphorus deposition is in the range of global crop fertiliser use. We argue that the role of marine predators in the local nutrient dynamics of the Kerguelen area should not be overlooked, and that fine-scale variability in the nutrient input by such marine predators is important to consider in any local nutrient dynamic study.