PATTERNS IN THE EPILITHIC COMMUNITY OF A LAKE LITTORAL

1. The algae and sedentary macroinvertebrates on the upper surface of stones from the littoral of Crosemere were investigated over 13 months . This lake is one of a series of eutrophic meres of glacial origin in the English Midlands. 2. Stones were taken from shallow and deep area s in the littoral and from areas shaded by bankside trees and those aw ay from trees. This gave four habitat types: open/ shallow; open/deep; shade/shallow; shade/deep. Epilithic algae and sedentary macroinverte brates from the upper surfaces of the stones were quantified monthly f or each habitat type. 3. Chlorophyll a and ash-free dry mass both show ed a strong seasonal pattern common to all habitat types, with a sprin g peak declining to a summer minimum, followed by a small autumn recov ery, and a winter minimum. 4. In terms of percentage cover, Cladophora glomerata showed a markedly different pattern. There were strong diff erences between habitat types, with shaded stones from the shallows, i n particular, having very sparse Cladophora cover. In the open, Cladop hora cover was high in summer and low at other times. 5. The invertebr ate community was dominated by retreat-dwelling larvae of the psychomy iid caddis, Tinodes waeneri, and four species of chironomids with tube -building larvae, Cricotopus sylvestris, Microtendipes pedellus, Glypt otendipes pallens and Endochironomus albipennis. For Tinodes, Cricotop us and Microtendipes, peaks of density occurred chiefly beneath trees in spring and summer. 6. The seasonal pattern of algal abundance showe d little relationship with that of invertebrate biomass. The ratio of chlorophyll a to ash-free dry mass also declined in summer, despite th e higher invertebrate biomass. This indicated that grazing was not the dominant factor diminishing algal abundance seasonally. It seems like ly that algae were limited chiefly by physical factors, such as light and temperature, and by nutrients, particularly nitrates, which declin e in summer in the epilimnion of the lake. 7. Grazing may have contrib uted to spatial patchiness of algae in summer, however, particularly t hat of Cladophora. The scarcity of Cladophora on shallow, shaded stone s coincided with a high abundance of Tinodes on these stones in early summer. Riparian trees could thus have affected epilithic algae, not o nly by shading but also indirectly through the supply of grazers.