Print PageAround the world, the hydrology of many large rivers has been significantly altered by dams and the diversion of water for human consumption. An understanding of the controls of algal primary production is essential for predicting how such changes will influence energy transfers to higher trophic levels. We are investigating how hydrology, light penetrance, and nutrient concentrations limit algal primary production and the production sources supporting consumers in five meandering floodplain rivers.
Our study system consists of two whitewater rivers with high loads of suspended sediment of fine grain size that limit algal primary production (the Brazos River in Texas and the Tambopata River in Peru); two blackwater rivers with sandy substrates, fairly low levels of suspended sediments, and lower pH (the Neches River in Texas and the Cinaruco River in Venezuela); and one clearwater river with low suspended sediment loads (the Guadalupe River in Texas). The rivers in Texas have higher nutrient concentrations and relatively unpredictable hydrologic regimes, and the rivers in South America have lower nutrient concentrations and strongly seasonal hydrologic regimes. In each of the rivers during the low- and high-water period, we are: 1) measuring discharge or stage height, light penetrance, and nutrient concentrations; 2) measuring respiration, net primary production (NPP), and gross primary production of the water-column and benthos in the littoral zone; and 3) using stable isotope analysis (carbon, nitrogen, and deuterium) to estimate the relative proportion of autochthonous-based versus allochthonous-based production sources supporting consumers.
Results/Conclusions
Ongoing research indicates that hydrology has a strong influence on light penetrance in the whitewater rivers, but not in the blackwater or clearwater rivers. As a result, littoral zones become heterotrophic in the whitewater rivers, but NPP is consistently positive in the blackwater and clearwater rivers. Hydrology also seems to cause seasonal variation in nutrient concentrations as a result of dilution, particularly in the tropical rivers. Thus, NPP values are frequently higher in the low-water period compared to the high-water period. Based on these results, we predict that the relative importance of autochthonous versus allochthonous production sources will vary seasonally in the whitewater rivers, but that algal-based production sources will support consumers during both the low-water and high-water period in the blackwater and clearwater rivers.
