EFFECTS OF BURROWING BY A STREAM CADDISFLY ON CASE-ASSOCIATED ALGAE

Diel burrowing behavior of Gumaga nigricula (McL.) (Trichoptera:Serico stomatidae) was investigated in Big Sulphur Creek (Sonoma Co., Califor nia). Most of the population burrows during the day and surfaces at ni ght, a behavior that facilitates feeding on periphyton while retaining many of the advantages of burrowing (e.g., protection from predators) . Because of daytime burrowing, case algae (primarily diatoms) are pot entially light-limited by overlying substrate and, indeed, chlorophyll a concentrations on stream-collected cases and diatom colonization on cleaned cases increased dramatically when daytime burrowing was preve nted. Cases and case-associated algae are normally abraded during burr owing; therefore experimental abrasion had little effect. However, aft er cases were cultured to accrue algae, experimental abrasion drastica lly reduced the accrued algae. Blue-green algal filaments and diatoms were removed from exposed surfaces but were protected in crevices betw een sand grains. Comparison of cases of different caddisfly genera sho wed a trend between abrasion level and case-associated algae. Under lo w abrasion, grazer-resistant diatoms occurred on exposed surfaces and upright diatoms occurred in crevices (e.g., Glossosoma and Discosmoecu s cases). As abrasion increased, diatoms were lost from exposed surfac es, although they remained in crevices (e.g., Gumaga cases); with cont inuous burrowing, cases were nearly devoid of algae (e.g., Agarodes ca ses).