Fluxes and reactivities of organic matter in a coastal marine bay

Abstract

Vertical fluxes of bulk particulate material, organic carbon, nitrogen, lignin‐derived phenols, and neutral sugars through the water column and into surface sediments of Dabob Bay, Washington, were determined monthly for 1 yr by sediment trap deployments at 30, 60, and 90 m at a site 110 m deep. Vertical fluxes of sinking bulk particulate material in this marine bay were elevated during winter and increased in consistent proportion to sediment trap deployment depth throughout the year. Although annual average particle fluxes at 30 and 60 m bracketed the mean accumulation rate of the underlying sediment, the flux at 90 m was higher by a factor of 2 due to resuspension, horizontal advection, or both. The monthly fluxes of lignin‐derived phenols paralleled those of total particulate material, indicating a common riverine origin. The annual average fluxes of vanillyl and cinnamyl phenols through the water column closely matched the corresponding accumulation rates in the underlying sediment, whereas about a third of the total syringyl phenol input was degraded at the water‐ sediment interface. Although p‐hydroxyacetophenone exhibited a stability typical of lignin‐derived phenols, the distinctly higher reactivities (> 60% degradation) of p‐hydroxybenzaldehyde and p‐hydroxybenzoic acid indicate a predominantly nonlignin source. On average, 60 and 70%, respectively, of the total particulate organic carbon and nitrogen and 65–75% of all neutral sugars settling through the midwater column were degraded at the water‐ sediment interface. The elemental and carbohydrate composition of the degraded material was similar to that of local net plankton except for higher percentages of glucose and total neutral sugars. Land‐derived organic material accounted for about one‐third of the total organic carbon passing through the midwater column and two‐thirds of the organic carbon accumulating in the underlying sediments. The amounts of plankton‐derived organic matter sinking through the midwater column and being preserved in the sediments below corresponded to 14 and 3% of the annual mean primary productivity. Plankton‐derived organic matter exhibited about 5 times the reactivity of local land‐derived organic matter at the water‐sediment interface of Dabob Bay and supported essentially all of the benthic respiration.