The influence of periphyton on boundary layer conditions: a pH microelectrode investigation

The effect of the physical presence and activity of an increasing layer of periphyton on the pH, CO2 and thickness of the boundary layer surrounding a rtificial plants was investigated. Details of a microelectrode apparatus su itable for use in natural water, with probes 5 mu m tip diameter and with a sensitivity of 0.01 pH are given. Detailed pH gradients illustrate an incr ease in length of gradient and amplitude of pH shift with increased periphy ton abundance. A logarithmic increase in pH at the leaf surface was found w ith increasing periphyton abundance measured as areal chlorophyll density a nd periphyton thickness, which translated into an exponential decline in ca lculated free CO2 availability. Periphyton was capable of reducing the esti mated free CO2 concentration to less than 2 mu mol 1(-1). Boundary layer th ickness increased Linearly with periphyton thickness. The outer edge of the periphyton effectively became the surface of the object, with interstitial water static. Periphyton has the potential to detrimentally affect the gro wth of the host plant through its effects on the rate of carbon supply. (C) 2000 Elsevier Science B.V. All rights reserved.