Structuring of epilithic biofilms by the caddisfly Tinodes rostocki: Photosynthetic activity and photopigment distribution in and beside larval retreats

Abstract

Tinodes rostocki larvae (Trichoptera: Psychomyiidae) cover large proportions of stream hard substrata with retreats constructed of mineral particles and larval silk. We consider these retreats as 3-dimensional extensions of the epilithic biofilm that may possess a distinct microenvironment, community metabolism and composition. Therefore, we compared the photosynthetic/respiratory activities (O2 and pH microsensors) and the photopigment composition (HPLC) of larval retreats and the surrounding epilithic biofilms. In retreats, pigment contents and photosynthetic/respiratory activities were highest in sections with a visible microphytobenthic biofilm that were mostly the older parts of the retreats. In contrast, newly constructed sections of the retreats and the surrounding epilithic biofilm had approximately 5-fold lower values. The fucoxanthin-to-chlorophyll ratio of the retreat biofilm was high (fuco/chl a = 1.27) and indicated diatom dominance, which was not evident in the surrounding epilithic biofilm (fuco/chl a = 0.15). Experimental transplantation of larval retreats to microscope slides allowed microsensor measurements through the 500 to 700 μm-thick wall and inside the lumen. In the light, O2 concentration and pH values increased significantly across the wall and remained high in the lumen of the retreat, whereas in darkness O2 and pH depressions in both wall and lumen were moderate or even absent. Our data suggest that T. rostocki larvae construct and maintain retreats with a particular physico-chemical microenvironment that favours a distinct microbial community. Thereby, abundant T. rostocki larvae might significantly influence benthic primary production and heterotrophic metabolism in small streams.