Evol Ecol Res 11: 421-432 (2009)     Full PDF if your library subscribes.

Terrestrial limitation of Amazon River productivity: why the Amazon River is not green

Phillip Dustan

Department of Biology, College of Charleston, Charleston, South Carolina, USA

Correspondence: P. Dustan, Department of Biology, College of Charleston, Charleston, SC 29424, USA.
e-mail: dustanp@cofc.edu


Questions: What factors control the continental scale distribution of phytoplankton biomass in the Amazon River? How do its tributaries influence the overall Amazon River system?

Data: Continuous along-track measurements of chlorophyll, pH, temperature, and beam transmittance along the length of the Amazon River during the high-water season of 1982. Additional vertical stations downstream, upstream, and in the mouth of every major tributary measured the same parameters and collected discrete samples for nitrate/nitrite and phosphate analysis.

Results: Along-track chlorophyll values were low (0.6–2.5 µg · l−1), while light attenuation and nutrients were high along the entire mainstem. Erosion in the Andean headwaters generates heavy sediment loadings that block light. Strong mixing offers no chance for stratification or settling that might increase light levels. When the river leaves the highlands, suspended sediment decreases. However, light attenuation remains very high due to the input of polyphenolic plant products (humic acids) from black water tributaries. These biologically produced compounds absorb light very strongly across the visible spectrum. When the river meets the Atlantic Ocean, chlorophyll values soar to 10–20 times those of the mainstem.

Conclusions: Amazon River plankton are light- not nutrient-limited along the entire course of the mainstem due to physical and evolutionary forcing functions. The high concentrations of humic acids in the mainstem illustrate the continental scale influence that a terrestrial system can exert on aquatic systems as a consequence of the evolutionary responses to an entirely different set of challenges on land. Thus the very biogenic compounds that have evolved to ‘keep the terrestrial Earth green’ reverse ecological roles when they enter the aquatic realm.

Keywords: Amazon, black water, chlorophyll, continental scale ecology, HSS, humic acids, limiting factors, productivity, tannins.

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