Marine snow aggregates: life history sequence and microbial community of abandoned larvacean houses from Monterey Bay, California

Abandoned larvacean houses and unfiltered sea water were sampled using SCUBA techniques from near-surface waters of Monterey Bay, California during different phases of the seasonal upwelling period. Concentrations of 5 trophic and size categories of microorganisms found on these aggregates, i.e. bacteria, autotrophs with equivalent spherical diameters both < 5 pm (biovolume < 65 pmJ) and > 5 pm (biovolume > 65 pm3), heterotrophic flagellates and ciliates, were greater than surrounding water values by factors of 102 to 103. Measured differences among house aggregates led us to propose a successional history divided into 3 phases: (1) an initiation phase while the larvacean is present; (2) after abandonment, a microcosm phase with an associated microbial community; and (3) after utdizatlon of the outer house, a remnant phase. Absolute abundances of bacteria and protozoans on Phase 2 aggregates were sirmlar to those of detrital microbial co~nmunities associated with decaying macroalgae and estuarine macrophytes. The ratio of the biomass of bacterial predatordbactena was significantly greater on Phase 2 aggregates than for the microbial community in the surrounding water during active upwelling, but values for the ratio from both systems were similar during the postupwelling strabfied penod. Thus, aggregates somet~mes represent small-scale patches of detrital or late-successional microbial comrnunites suspended in a water column undergoing early successional , events.