ION-EXCHANGE MECHANISMS AND THE ENTRAPMENT OF NUTRIENTS BY RIVER BIOFILMS

Biofilms envelop all surfaces in aquatic ecosystems. They possess an e xtremely efficient nutrient entrapment mechanism which is widely belie ved to be mediated through ion exchange processes. During a field expe riment, potassium and bromide were transported along a 105 m reach at different rates. The distance between the two solute pulses increased with increasing distance downstream. And, in a laboratory experiment, changing the ionic composition of waters overlying the biofilm influen ced the retention of (phenolic) material by that biofilm. An analogy w as drawn with ion chromatography (IC): In IC, different ions show diff erent rates of progress through the column (retention times), and also show increasing separation between peaks with increasing distance fro m the point of injection (column length). Likewise, the affinity of a given ion for the column can be modified by manipulation of the ionic composition of overlying waters (eluent). The observed similarities be tween IC columns and the biofilm-coated stream channel may therefore r epresent a degree of experimental support for the putative involvement of ion exchange in the biofilm nutrient entrapment mechanism.