Epiphytic algae, growing on submerged macrophyte surfaces were studied in the middle reaches of the River Great Ouse during 1989 and 1990. Submerged macrophytes are largely restricted to the shallower margins, partly because of the turbid waters but also by weed cutting and dredging, and those macrophytes growing near the surface of the water, where light is not severely restricted, provide a suitable substratum for colonization by epiphyton. Nuphar lutea was chosen as an example of a predominantly submerged macrophyte, and Scirpus lacustris and Phragmites communis as examples of fringing, emergent macrophytes. There are two major components to the algal ยฏora of the river; a phytoplankton component, largely conยฎned to the spring and early summer (Marker and Collett, 1997a) and the periphyton that reaches its greatest development later in the summer as the underwater light climate improves.
The most productive component of the epiphyton is believed to occur on submerged Nuphar leaves where biomass reached a maximum of 6ยฑ8mg cm 72 during the period of this study. Although periphyton associated with submerged Scirpus and Phragmites may attain higher levels of biomass (ca. 25mg cm 72 ) locally, it is much more patchily distributed, being largely conยฎned to the outer fringes of the reed beds, just below the water surface and it develops mainly after midsummer. Diatoms dominate the algal ยฏora for much of the season and Cocconeis placentula is particularly important on Nuphar leaves. Towards the end of the summer several species of Nitzschia become progressively more important. Nitzschia amphibia and N. dissipata are particularly notable. Other diatoms were occasionally abundant, including Navicula meniscula, N. viridula, N. gracile, Gomphonema parvulum, Rhoicosphenia curvata, Nitzschia palea and Achnanthes spp. Species of Synedra (S. afยฎnis, S. pulchella and S. ulna) and Gomphonema augur were conspicuous, owing to their large size, but were never numerically abundant.