Dominance of particle-attached bacteria in the Columbia River estuary, USA

Particle-attached bacteria are a central component of the detrital food web of many turbid coastal and estuarine ecosystems. The Columbia River estuary, at the terminus of a 660 000 km(2) watershed in northwestern North America, is a turbid, partially mixed system that has a flushing time of 1 to 3 d. Several large, well-defined estuarine turbidity maxima (ETM) extend the residence time of both mineral and organic particles transported through the estuary. Water samples collected in the North Channel of the estuary every 2 h for 148 h (6 tidal cycles) in May 1995 were analyzed to determine the concentration and production of particle-attached and free-living bacteria, ext-acellular enzyme activity, turbidity, salinity, and particulate organic carbon (POC) concentration. The concentration of particle-attached bacterial defined as those caught by a 3 mu m filter, averaged 1.02 x 10(6) (SD = 1.00 x 10(6)) cells ml(-1) and correlated with turbidity and POC, and thus to some extent with the tidal cycle that maintains the ETM. The concentration of free-living bacteria was more constant, averaging 1.25 x 10(6) (SD = 0.4 x 10(6)) cells ml(-1). Particle-attached bacterial carbon production, calculated from the rate of incorporation of H-3-thymidine, averaged 1.61 (SD = 1.10) mu g l(-1) h(-1), accounted for 90 % (SD = 9 %) of total bacterial carbon production, and correlated with turbidity and POC. Extracellular enzyme activity, measured as the rate of hydrolysis of fluorescently labeled compounds; increased with turbidity and was predominantly associated with particles. Particle-attached bacteria probably account for most of the bacterial degradation of particulate organic material in the estuary, and the transfer of that material into the detrital food web. The hydrodynamics of the estuary contribute to the dominance of particle-attached bacteria by extending the residence time of particles in the ETM, and by quickly flushing free-living cells through the estuary, perhaps preventing the development of an estuarine population of free-living bacteria.