Exploratory observations of marine aggregates at sub-euphotic depths

Aggregates >0.25 mm were collected over a 4-year period below the euphotic zone at depths between 40–251 m and at a 400–1000 m deep station in the Monterey Submarine Canyon system using a remotely operated vehicle (ROV). The aggregates were sized and characterized to determine their origins and possible association with local production cycles at a station regularly monitored by the Monterey Bay Aquarium Research Institute’s (MBARI’s) ocean monitoring program. We present a first-time inventory of particle types with their size distribution for Monterey Bay. Phytoplankton-containing aggregates number about half of the total aggregates in the upper, 40–251 m layer, but decline to 5% at 400 m, possibly due to offshore transport of sinking material, input of resuspended aggregates at depth, and/or intense biological recycling of aggregates in the midwater. In the upper layer, only the abundance of the phytoplankton containing aggregates was weakly correlated with local phytoplankton abundance as measured by in situ fluorescence on that day at a near-by mooring: correlations did not exist for total numbers of aggregates. Instead, aggregates were mostly of detrital origin, with organic matter of unrecognizable origin and sometimes with conspicuous lithogenic material. Particles of fecal origin were third in abundance, and their numbers are relatively low (25 l-1) compared with aggregates, likely because of their higher sinking rates. We suggest that many of the sub-euphotic aggregates in this system may be derived from additional sources or events not directly tied to production cycles. Because particles such as these are likely responsible for the observed mass flux at this site (see Pilskaln et al., 1998), the concept of a simple, surface-driven “biological pump” may not be applicable on short-time scales in predicting material flux in dynamic continental margin systems.