Characterization of the bacterially-active particle fraction in the Columbia River estuary

Abstract

The Columbia River estuary is dominated by a detrital food web that feeds primarily on river-borne particulate organic material in the estuarine turbidity maxima (ETM). Approximately 90% of the bacterial activity and most of the extracellular enzyme activity in the water column of the estuary were associated with particles captured by a 3 µm filter. Earlier studies determined that the relation- ship between particle-attached bacterial activity and suspended particulate mass (SPM) was variable, suggesting that some particles supported a larger fraction of bacterial activity. In order to characterize these bacterially-active particles, suspended particulate material from the estuary was fractionated by size and in situ settling velocity, and analyzed for 3 H-thymidine incorporation rate, total particle mass and particulate organic carbon (POC) concentration. The location and movement of bacterially-active particles in the estuary was traced by measuring 3 H-thymidine incorporation rate and SPM in near- bottom depth profiles collected at 4 or 5 time points during ETM resuspension events. The smallest particle size fraction (3 to 10 µm) supported 87% (SE = 13) of bacterial activity, but contained only 38% (SE = 5) of total POC and 38% (SE = 6) of total particle mass. However, when particles were separated by in situ settling velocity using a method that preserved the integrity of aggregated particles, the set- tling velocity of the majority of bacterially-active particles varied from 0.75 mm s -1 . Microscopic analysis of bacterially-active particles revealed that they were aggregates of smaller (<10 µm) particles held together by a transparent matrix. This study shows that small, slow- settling particles host most of the bacterial activity in the Columbia River estuary, and suggests that they contain the most rapidly consumed pool of organic matter in the system. Furthermore, it demon- strates that the configuration of these particles may be highly dynamic as they are regularly aggre- gated and disaggregated in ETM. In the estuary, these particles often appeared in the water column early during developing flood tides prior to the appearance of the most turbid part of ETM, and some- times remained in the water column after most ETM particles had settled back to the bed. This cycle places bacterially-active particles in the water column longer than most ETM particles, making them more likely to be washed out of the estuary, but also making them more available to suspension-feeding detritivores. Formation of large, fast-settling macroaggregates during the late stages of flood and ebb tides and during slack tides is probably the mechanism by which bacterially-active particles are trapped in the ETM, and may be essential to maintaining the estuarine community of particle-attached bacteria.