Linking ecosystem dynamics and biogeochemistry: Sinking fractionation of organic carbon in a Swedish fjord

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We studied the growth and sedimentation of a phytoplankton bloom in the Gullmar Fjord in spring 2001. Sinking fractionation, as measured by differential sinking of components having varied carbon isotope signals, was an important process in determining carbon fluxes within this complex coastal ecosystem. Large, rapidly sinking diatoms aggregated with their own carbon exudates. This exudate material was the single largest contributor to vertical carbon flux in the fjord over the study period, and amounted to an order of magnitude greater carbon flux than the live biomass reaching the sediment traps. We estimate that diatoms' primary production contributed 75% of the total integrated production, ~100% of new production and ~100% of the total sedimentary flux during the primary sedimentation event, despite the fact that the surface bloom biomass was dominated by high concentrations of the flagellate Chattonellasp. Diatoms carried a heavy carbon isotope signature (δ13C = -19‰) to depth; this moved downward as a layer through the water column, distinct from other particular organic carbon (POC) at the surface having a very light signature (-23 to -26‰). This light δ13C signature at the surface coincided with a surface peak in particulate and dissolved organic carbon (POC/DOC) and transparent exopolymer particles (TEP), and material carrying the low δ13C signature had a negligible sinking rate. This distinctive surface signature probably contained small POC typical of the microbial loop and its products, a component of which was the very low -32‰ of the organic colloids. Measurable aggregation of these colloids occurred, and they may have contributed up to ~20% of the vertical flux very early in the study. Sinking fractionation of POC caused isotope composition shifts on the order of 3–7‰.