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

The Columbia River estuary is dominated by a detrital food web that feeds p rimarily on river-borne particulate organic material in the estuarine turbi dity 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 mum filter. Earlier studies deter mined that the relationship between particle-attached bacterial activity an d suspended particulate mass (SPM) was variable, suggesting that some parti cles supported a larger fraction of bacterial activity. In order to charact erize these bacterially-active particles, suspended particulate material fr om the estuary was fractionated by size and in situ settling velocity, and analyzed for H-3-thymidine incorporation rate, total particle mass and part iculate organic carbon (POC) concentration. The location and movement of ba cterially-active particles in the estuary was traced by measuring H-3-thymi dine incorporation rate and SPM in near-bottom depth profiles collected at 4 or 5 time points during ETM resuspension events. The smallest particle si ze fraction (3 to 10 mum) supported 87 % (SE = 13) of bacterial activity, b ut contained only 38 % (SE = 5) of total POC and 38 % (SE = 6) of total par ticle mass. However, when particles were separated by in situ settling velo city using a method that preserved the integrity of aggregated particles, t he settling velocity of the majority of bacterially-active particles varied from < 0.07 mm s(-1) in some samples to >0.75 mm s(-1). Microscopic analys is of bacterially-active particles revealed that they were aggregates of sm aller (<10 <mu>m) particles held together by a transparent matrix. This stu dy shows that small, slow-settling particles host most of the bacterial act ivity in the Columbia River estuary, and suggests that they contain the mos t rapidly consumed pool of organic matter in the system. Furthermore, it de monstrates that the configuration of these particles may be highly dynamic as they are regularly aggregated 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 som etimes remained in the water column after most ETM particles had settled ba ck 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-feed ing detritivores. Formation of large, fast-settling macroaggregates during the late stages of flood and ebb tides and during slack tides is probably t he mechanism by which bacterially-active particles are trapped in the ETM, and may be essential to maintaining the estuarine community of particle-att ached bacteria.