A document by Jacob Cram. Click on the document to view its contents.

Particulate matter modulates the transport of carbon and nutrients through estuarine environments. In the Chesapeake Bay, sinking of particles and their consumption by microbes likely modulates the emergence of a seasonal oxygen deficient zone. The relationship between particle size and abundance affects the transport dynamics of the particles and the biology of associated organisms. The variability of particle characteristics has not previously been characterized across the length of the Chesapeake Bay, nor has it been compared to the oxygen deficient zone. Therefore, we measured the size to mass and size to abundance relationship of suspended particles along the Chesapeake Bay during a major deoxygenation event. A laser scattering instrument measured particle size and abundance at six stations. Five particle size classes were sampled at surface and bottom depths. Particles in the less saline northern end of the Bay were less massive relative to size than particles farther south. Estimates of total particle mass, calculated by combining particle size to mass and particle size to abundance data, suggested that the anoxic region has lower particulate mass than overlying oxic water, perhaps because stratified water above the oxygen minimum zone keeps particles from the productive top layer from mixing into this region. Total particle mass was higher just above the sediment, suggesting resuspension of benthic particles. Our data provide the first systematic survey of size resolved particle abundances across the Chesapeake Bay oxygen minimum zone and provide context to future work in evaluating the biogeochemical role of particles in this environment.

Models and observations suggest that particle flux attenuation is lower across the mesopelagic zone of anoxic environments compared to oxic environments. Flux attenuation is controlled by microbial metabolism as well as aggregation and disaggregation by zooplankton, all of which also shape the relative abundance of differently sized particles. Observing and modeling particle spectra can provide information about the contributions of these processes. We measured particle size spectrum profiles at one station in the oligotrophic Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ) using an underwater vision profiler (UVP), a high-resolution camera that counts and sizes particles. Measurements were taken at different times of day, over the course of a week. Comparing these data to particle flux measurements from sediment traps collected over the same time-period allowed us to constrain the particle size to flux relationship, and to generate highly resolved depth and time estimates of particle flux rates. We found that particle flux attenuated very little throughout the anoxic water column, and at some time-points appeared to increase. Comparing our observations to model predictions suggested that particles of all sizes remineralize more slowly in the ODZ than in oxic waters, and that large particles disaggregate into smaller particles, primarily between the base of the photic zone and 500 m. Acoustic measurements of multiple size classes of organisms suggested that many organisms migrated, during the day, to the region with high particle disaggregation. Our data suggest that diel-migrating organisms both actively transport biomass and disaggregate particles in the ODZ core.