Short-Term Photochemical and Biological Unreactivity of Macrophyte-Derived Dissolved Organic Matter in a Subtropical Shallow Lake

Macrophytes have been associated with low bacterial metabolism in the littoral zones of lake Mangueira, but an explanation for this pattern is largely unknown. In this study, macrophyte-derived DOM was incubated in situ for the measurement of the effect of grazers, bacteria, and light on its degradation in three experiments. The water was separated in bulk, bacterial, and control (+？HgCl2) fractions and exposed to or hidden from sunlight for 120？h. Unchange in bacterial variables in the bulk fraction suggested a combined control of radiation and grazing on bacteria. Light treatment increased bacterial density but not biomass and biovolume, while bacterial density decreased in the dark. Significant fading of water color in the bacterial fraction only occurred after light exposure, indicating a complementary pathway of light and bacteria. DOC and the Abs250？:？365 ratio did not change with incubation, indicating no net change of DOC pool and reactivity. Due to continuous carbon loading from macrophytes and low UV irradiance, the very low rates of DOM degradation provide the mechanistic explanation for the observed impacts of macrophytes in lake’s carbon metabolism in littoral zones. 1. Introduction Macrophytes are important sources of carbon to the littoral zones of lakes, and these plants directly and indirectly (via support of epiphytes) contribute higher amounts of dissolved organic carbon (DOC) than do algal sources [1]. They are the main sustainers of bacterial production in some systems (e.g., [2]), but knowledge of their impact on the entire lake metabolism is still relatively sparse [1, 2]. Loading of macrophyte-derived carbon in littoral zones can create within-lake patchiness in the quantity and quality of organic carbon, thus causing differences in the composition of bacterial assemblages [3]. Patchy utilization of heterogeneous DOC by compositionally or adaptively different bacterial communities could affect the entire lake metabolism, because bacterial secondary production and respiration can be affected by the consumption of either high or low-molecular-weight compounds [4]. DOC derived from macrophytes is composed mostly of aromatic and aliphatic polymer-like compounds of high molecular weight [5] and hence presumably refractory and hence highly unreactive (i.e., with low capacity to undergo chemical reaction, specially oxidation) to bacterial consumption. This requires bacterial assemblages that inhabit humic lakes to use a different, more energy-expensive mechanism to exploit these compounds [1, 6]. Moreover, this type of DOC is