Patterns and processes in the epilithic communities of a stony lake shore
This study on the littoral of Crosemere has shown that complex direct and indirect interactions exist between larvae of the gallery-building caddis Tinodes waeneri, the filamentous macro-alga Cladophora glomerata, and the larvae of several species of retreat-building chironomids. Co-existence between these species is facilitated by spatial and temporal patchiness in dispersal ability and behaviour by the dominant grazer/competitor Tinodes waeneri. Tinodes eats Cladophora and thus indirectly controls the abundance of chironomids which associate with the Cladophora mat. Adult oviposition under trees determines the initial distribution of Tinodes and several chironomids. While these latter can probably rapidly disperse by swimming, Tinodes dispersal is more limited. This creates spatial refugia for Cladophora and chironomids in areas of the littoral away from trees. Local-scale, temporal refugia are created by Cladophora being able to 'grow away' from Tinodes during periods in spring when Cladophora colonizes more vigorously than Tinodes. Local-scale spatial refugia are also created by modes' apparent reluctance to colonize taller stones. This spatial and temporal patchiness in species interactions in the littoral is set against a background of lake-wide, temporal variation which appears to constrain the entire littoral habitat. The summer stratification of Crosemere into upper and lower layers causes the surface waters, probably including also the littoral, to be seasonally nutrient-limited, particularly in nitrogen. This in turn will limit primary production in the littoral and thus also secondary production. The epilithic species studied all appeared to be food-limited in summer. The dominance of the Crosemere littoral by retreat-dwelling species may partly be due to their ability to recycle limited nutrients within or on their retreats and so to consume more algae than is otherwise available to mobile species.