Phenology of stranded kelp degradation by the kelp fly Paractora dreuxi mirabilis (Helcomyzidae) at Marion island

Abstract

During heavy seas, bull-kelp Durvillaea antarctica (Cham.) Har. plants are stranded either singly or in wrack beds on sub-Antarctic Marion Island's (46°54′S, 37°45′E) few pebbled beaches. In the supralittoral zone, Durvillaea wrack is consumed chiefly by kelp fly larvae (the endemic Paractora dreuxi mirabilis Seguy), whilst two marine amphipods attack wrack in the eulittoral and sometimes in the supralittoral zone. During the summer of 1984 on Marion Island, artificial Durvillaea deposits were established in the supralittoral zone of a beach close to the meteorological station and were closely monitored for six weeks thereafter. Eight days after deposition, temperatures inside the protected wrack bed reached 10°C above ambient, and P. dreuxi larvae attained a maximum biomass of 27 g/kg (dry mass) of kelp in the bed. After 30 days, wrack strings and a protected wrack bed had lost 80 and 65%, respectively, of their original dry mass. A bed left exposed to trampling by seals had disappeared. Over 30 days, P. dreuxi larvae accounted for 35% of the dry mass loss in the wrack bed. P. dreuxi has an opportunistic and flexible ecological strategy, facilitated by a stable, interstitial biotope of particulate kelp detritus from which it colonizes fresh wrack.