DIRECT AND INTERACTIVE EFFECTS OF ALLOCHTHONOUS DISSOLVED ORGANIC-MATTER, INORGANIC NUTRIENTS, AND ULTRAVIOLET-RADIATION ON AN ALPINE LITTORAL FOOD-WEB

Allochthonous dissolved organic matter (DOM) may regulate littoral foo d webs by both modifying nutrient availability and attenuating potenti ally damaging ultraviolet radiation (UVR). These hypotheses were teste d in a three-factor (DOM, inorganic nutrients, and WR) experiment usin g 24 littoral enclosures in an alpine lake located in Banff National P ark, Canada. DOM was extracted from treeline soils and added (3 mg DOM liter(-1)) to +DOM enclosures over 1 month. These amendments were int ended to simulate increases in allochthonous inputs that might occur a s climate warming promotes the development of treeline soils and veget ation. DOM amendments significantly increased epilithon biomass (as to tal chlorophyll, carotenoids) but did not affect the abundance of eith er epipelon or phytoplankton. in addition, natural UV irradiance signi ficantly enhanced the positive effect of DOM on epilithon and directly increased the abundance of epipelon. Threefold amendments of ambient nitrogen (N) and phosphorus (P) (20 N:1 P, by weight) significantly in creased epilithon abundance. The positive effects of DOM, NP, and UVR on phytobenthos were primarily attributable to the increased abundance of diatoms, whereas NP affected phytoplankton by increasing chrysophy tes and dinoflagellates. DOM amendments did not significantly affect t he final abundances of heterotrophic bacteria and protists but did res ult in significantly higher densities of omnivorous copepod nauplii. T hese results show that allochthonous DOM is an important resource for littoral food webs in oligotrophic lakes; however, its effects are med iated by UVR and differ between benthic and planktonic habitats. Our f indings suggest that littoral food-web structure in clear, shallow lak es and ponds is altered by fluctuations in allochthonous inputs and UV R exposure arising from droughts and long-term climatic change.