Production of dissolved organic matter from lily pads (Nymphaea odorata) in a mesotrophic freshwater lake

Abstract

Macrophytes are potentially an important dissolved organic matter (DOM) source in lakes. To assess the effect of lily pads (Nymphaea odorata) on DOM, optical properties and dissolved organic carbon concentrations (DOC) were measured over a year in Lake Louise, Pacific Northwest, USA. Lily pads were widespread around the shoreline during summer, dying back in the fall. Some optical indices (slope ratio, index of recent autochthonous contribution, fluorescence index) indicated the lake was dominated by DOM from terrestrial sources for most of the year. Indicators of autochthonous production in a few winter samples were attributed to phytoplankton. DOC concentrations and precipitation were not correlated, but DOC was positively correlated with absorption coefficient at 350 nm (α₃₅₀). DOC and α₃₅₀ were highest in summer and decreased in the winter wet season, which would be consistent with an in situ lake source being diluted by rainfall as a possible explanation for this. Excitation-emission matrix fluorescence spectra of lake waters and lily pad leachates indicated humic and protein material components. The plant leachates had slope ratio, fluorescence index, and index of recent autochthonous contribution values consistent with terrestrially derived DOM and lake waters. Higher DOC and α₃₅₀ in the summer were associated with some lower optical indices (specific ultra-violet absorbance, spectral slope, humification index, fluorescence/absorbance ratio ratios), consistent with material derived from lily pads. Estimates indicated lily pads contributed as much to the lake DOM pool in the summer dry season as watershed terrestrial inputs in the winter wet season, suggesting that DOM production by lily pads may account for a significant portion of the carbon pool in smaller lakes.