Clearance of picoplankton-sized particles and formation of rapidly sinking aggregates by the pteropod, Limacina retroversa

Abstract

Findings from experiments showed that the web-feeding euthecosomatous pteropod, Limacina retroversa, can produce rapidly sinking, mucous aggregates. It is suggested that, by adhesion, these aggregates scavenged picoplankton-sized particles, which were thus effectively cleared from the medium. In contrast, Calanus finmarchicus was not able to clear these particles in our experiments. Sedimentation velocities of 10 aggregates measured in vivo were up to 1000 m day-1, with an average of ~ 300 m day-1 (not including two aggegates with neutral buoyancy). Mean velocities measured for feces of C. finmarchicus, Calanus hyperboreus and Thysanoessa sp. were considerably lower. We suggest that the sedimentation of L. retroversa aggregates was the source of mucous floes collected in sediment traps (Bathmann et al., Deep-Sea Res., 38, 1341-1360, 1991) and at the sea floor at 1200 m depth in the southern Norwegian Sea. This process may be an important mediator of sedimentation to the deep sea, when these pteropods are present in surface waters in large abundance.