Spatial and temporal dynamics in marine aggregate abundance, sinking rate and flux: Monterey Bay, central California

Seasonal profiles of marine aggregate abundance and in situ sinking rate ex periments were carried out using a uniquely instrumented ROV platform in th e midwater column of Monterey Bay California between 1991 and 1994. Variati ons of an order of magnitude in the midwater 100-500 m abundance of aggrega tes within the 0.5 to > 5 mm size range were observed on an inter- and intr a-annual basis at the study site. Maximum midwater aggregate abundances of 15-40 aggregates l(-1) were common during the 1991 spring/summer upwelling season, with values of less than 5 aggregates l(-1) being more typical of t he non-upwelling winter months and the 1992 El Nino period. Midwater aggreg ate peaks represented the temporal signal of sinking aggregate material pro duced in the overlying waters, with no correlation observed between the ver tical distribution of aggregates and the density structure, or the relative suspended particle abundence measured as c(p). Additionally, significant i njection of aggregate material to the midwater profiles via lateral advecti on was not evidenced by the combined aggregate, hydrographic, and physical flow data sets obtained simultaneously at the study site. In situ aggregate sinking rates were measured using an ROV-mounted settling chamber. Mean ra tes ranged from 16.3 to 25.5 m day(-1) with a trend of increasing sinking r ate with aggregate diameter observed. An analysis of aggregate shape showed a decrease in spherical shape with increasing aggregate diameter and sinki ng rate. Seasonal aggregate POC fluxes at 450 m for 1991-1992 were calculat ed from the in situ aggregate property data sets to compare with coincident , 450 m trap-measured POC fluxes. The calculated aggregate POC fluxes range d from 26.3 to 481.2 mgC m(-2) d(-1) and were at least 1.5-2 times greater than the trap POC fluxes, with the exception of the non-upwelling winter mo nths where values are similar. Trapping efficiency and interannual variatio ns in aggregate sinking rates are believed to explain the differences betwe en the aggregate and trap-based POC fluxes. (C) 1998 Published by Elsevier Science Ltd. AU right reserved.