Abstract
Allochthonous dissolved organic matter (DOM) may regulate littoral food webs by both modifying nutrient availability and attenuating potentially damaging ultraviolet radiation (UVR). These hypotheses were tested in a three‐factor (DOM, inorganic nutrients, and UVR) experiment using 24 littoral enclosures in an alpine lake located in Banff National Park, Canada. DOM was extracted from treeline soils and added (3 mg DOM liter−1) to +DOM enclosures over 1 month. These amendments were intended to simulate increases in allochthonous inputs that might occur as climate warming promotes the development of treeline soils and vegetation.DOM amendments significantly increased epilithon biomass (as total chlorophyll, carotenoids) but did not affect the abundance of either epipelon or phytoplankton. In addition, natural UV irradiance significantly 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 increased 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 chrysophytes and dinoflagellates. DOM amendments did not significantly affect the final abundances of heterotrophic bacteria and protists but did result in significantly higher densities of omnivorous copepod nauplii. These results show that allochthonous DOM is an important resource for littoral food webs in oligotrophic lakes; however, its effects are mediated by UVR and differ between benthic and planktonic habitats. Our findings suggest that littoral food‐web structure in clear, shallow lakes and ponds is altered by fluctuations in allochthonous inputs and UVR exposure arising from droughts and long‐term climatic change
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