The microclimate of the brown alga Feldmannia caespitula (J. Agardh) Knoepffler-Peguy interstitium was studied using microelectrode techniques. Zero water flow and irradiances of 170 and 1500 mu mol photons m(-2) s(-1) cause steep O-2 gradients peaking 3 to 4 mm below the outer surface of the tufts at 310 and 506% atmospheric saturation, respectively. The mean fluxes of O-2 from the interstitium to the surrounding bulk water were 87 +/- 21 and 262 +/- 68 nmol cm(-2) h(-1) at low and high quantum flux density. Except for the outer 2 to 4 mm thick margin, the alga interstitium became anoxic within 52 min after abrupt darkening. The rate of dark oxygen uptake was 52 +/- 5 nmol cm(-2) h(-1). The tufts were populated by 9 metazoan taxa: nematodes, harpacticoid copepods, ostracods, gastropods, bivalves, polychaetes, amphipods, isopods and halacarids. Our results suggest that the interstitium of fine-textured algal thalli is a microhabitat of variable water chemistry with temporary anoxia and hyperoxia in an otherwise relatively stable water column. Although the tufts are attractive for meiofauna by providing food and protection from currents and predators, rapid fluctuations in oxygen concentration probably cue temporal emigration of the algal infauna.