Bacterial activity and community structure in the Columbia River estuarine turbidity maxima
The food web of the Columbia River estuary is centered in estuarine turbidity maxima (ETM) where allochthanous particulate organic material (POM), particle-attached bacteria, and consumers such as copepods and rotifers are trapped and concentrated by the hydrodynamics of the system. Bacteria play a central role in this food web as the principle consumers of dead and degraded organic matter and as a trophic link to protozoa and metazoa. Particle-attached bacteria dominated bacterial activity in all seasons of the year, subsisting primarily on river-borne POM. Most of this activity was associated with 4 subset of the particles in the water column characterized by small (<10 mum) aggregates of mineral grains and broken diatom frustules bound together with an organic matrix. These 'bacterially-active' micro-aggregates had a slow settling rate (<0.07 mm s-1), but often appeared to be associated with larger, faster-settling macro-aggregates in ETM. Phylogenetic analysis showed that ETM particles were substrata for many types of bacteria that were different than those in the river or the coastal ocean. These organisms are thought to compose the most active fraction of the bacterial community. Most archaea and free-living bacteria identified through phylogenetic analyses were similar to those in the river and the, coastal ocean, suggesting that they washed through the estuary quickly, and contributed little to estuarine bacterial and archaeal production. So bacterial production and consumption of POM in the Columbia River estuary were probably restricted to a specific estuarine community of particle-attached ached microorganisms. Analysis of seasonal and inter-annual data revealed that the production rate of these organisms was dependent on the quantity and quality of river-borne POM. Bacterial activity was greatest in the Spring and Summer when the principle source of fresh, high quality organic matter to the estuary, freshwater phytoplankton, was at its highest. The dominance of particle-attached bacteria, the concentration of bacterially-active particles, and the presence of a uniquely-adapted estuarine community of bacteria all indicate that Columbia River ETM have a major influence on the composition of the estuarine food web and the consumption and transformation of detrital organic matter.
- Oceanography