MARINE SNOW DERIVED FROM ABANDONED LARVACEAN HOUSES - SINKING RATES, PARTICLE CONTENT AND MECHANISMS OF AGGREGATE FORMATION

The dynamics and formation mechanisms of marine snow aggregates from a bandoned larvacean houses were examined by laboratory experiments and field sampling during a spring diatom bloom in a shallow fjord on the west coast of the USA. Intact aggregates were sampled both from sedime nt traps and directly from the water column by divers. ALI aggregates were composed of 1 abandoned house of the larvacean Oikopleura dioica to which numerous diatoms, fecal pellets, ciliales, and amorphous detr itus were attached. High vertical flux rates (20000 to 120000 houses m (-2) d(-1)) and settling velocities (average 120 m d(-1)) imply a rapi d turnover of suspended larvacean houses, and concentrations of diatom s and fecal pellets in the aggregates exceeding ambient concentrations by 3 to 5 orders of magnitude suggest their potential importance in d riving the vertical flux of particles. Identical particle assemblages were observed in aggregates collected in the water column and in sedim ent traps. Most of the fecal pellets found in the houses were most Lik ely produced by the larvaceans themselves. Numbers of diatoms per hous e corresponded with the diatom concentrations in the ambient water and , on average, each aggregate contained diatoms in abundances correspon ding to those found in 4.5 ml of ambient water. Laboratory measurement s showed that larvacean houses scavenge diatoms and fecal pellets whil e sinking, and observed scavenging rates were similar to those predict ed from theory. However, both predicted and observed scavenging rates in experiments were orders of magnitude too low to account for the par ticle content observed on aggregates from the water column. Based on m odels, shear coagulation was also assessed to be insignificant in form ing aggregates. It is concluded that most of the particles become atta ched to the incurrent filters of the larvacean house while it is still inhabited, and that shear and sinking insignificantly contribute to p article collisions and adhesions on the abandoned house.