High-resolution sampling of particulate organic carbon in a coastal upwelling system

Graduation date: 2010Summertime, wind-driven upwelling off the Oregon coast delivers nutrient rich water to the surface that fuels the autotrophic production of particulate organic carbon (POC). This POC can be transported horizontally by fluid motions and vertically by sinking to the bottom where it can be entrained in the benthic boundary layer (BBL). POC can be transformed during transport by heterotrophic metabolism, thus changing its concentration and composition. To better understand the dynamics of POC within the water column of this highly variable system, we developed a semi-automated filtration system that, when coupled to a towed profiling/sampling vehicle, allowed us to collect POC samples at higher spatial and temporal resolution than previously possible. During late May of 2009 we used this system to collect around 400 POC samples from two cross-shelf transects off the central Oregon coast spanning the ranges between BBL and surface mixed layer, and shelfbreak to shoreline. These samples were collected in conjunction with in-situ measurements of temperature, salinity, chlorophyll fluorescence, optical backscatter, and beam attenuation coefficients. Analyses of both the optical and bulk measurements indicate the presence of three distinct particle pools. The first pool is rich in POC and shows elevated fluorescence and beam-c relative to optical backscatter. The second pool is elevated in both fluorescence and optical backscatter, and is rich in POC relative to beam-c. The third pool is depleted in POC and shows proportionately elevated backscatter. Using variations in the optical properties of these three particle pools, we created multiple POC – beam-c calibrations, which allowed us to derive high-resolution POC distributions within the water column. This derived distribution indicates a decoupling between sediment and carbon in the BBL, and an unanticipated elevation of POC in the mid water column