The dynamics of bacterial production and abundance in tropical lowland rivers of the Orinoco basin.

Factors controlling bacterial production were examined in relatively undisturbed lowland rivers of the middle Orinoco basin, Venezuela. Two clearwater and two blackwater rivers were sampled over two years for water chemistry, DOC, chlorophyll a, and bacterial production and abundance. The rivers have low pH (4.7--6.0) and conductivity (6.3--9.1 muS cm-1 and abundant nitrogen (total N, 273--314 mug L-1) compared to phosphorus (total P, 3.6--5.5 mug L-1). Dissolved organic carbon (DOC) concentrations were higher in blackwater (543--664 muM) than in clearwater rivers (184--240 muM), and were higher during the periods of rising and high water compared to low water. Bacterial production was low compared to most tropical and temperate freshwater systems. Average bacterial production was higher in clearwater (0.20--0.26 mug C L-1h-1) than in blackwater rivers (0.14--0.17 mug C L-1h-1), as was bacterial production per unit of DOC, suggesting that carbon availability also was higher in clearwater rivers, despite lower DOC concentrations. Mean bacterial abundance was similar among the study rivers (0.60.8 x 10 6 cells mL-1). Seasonal patterns in bacterial production and abundance were related to substrate availability and inputs of allochthonous bacteria. Periods of higher chlorophyll a concentrations (during low water) or flushing of terrestrial organic material (during rising water) were accompanied by higher bacterial production. Interannual variation in bacterial production was affected by the ENSO phenomenon and its influence on chlorophyll a and DOC concentrations; higher rainfall years have higher average bacterial production. Bioassay experiments with added glucose (C), ammonium (N), phosphate (P), leaf leachate, and algal-rich water demonstrated that inorganic P may be the primary limiting nutrient at certain times while carbon is an important secondary constraint to bacterial production in the study rivers. Compared to temperate zone lakes and rivers, bacterial production in these tropical systems had lower annual variance. This low variance is due in part to the low range of temperature in the tropics, and in part to the lower variability in the input rates of dissolved organic matter and nutrients. Because the study rivers are relatively undisturbed, these findings provide new baseline information to predict and evaluate future human activities in the region.PhDBiogeochemistryBiological SciencesEarth SciencesEcologyLimnologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132750/2/9990859.pd