Seasonal dynamics of chromophoric dissolved organic matter in Lake Kinneret: An investigation of fluorescent component changes in relation to microbial and phytoplankton productivity.
Jonathan Liberson, Dror Angel and Yohay Carmel
Energy and biomass flow through aquatic food webs as a result of photosynthesis, grazing and predation, and also through microbial consumption of detritus and dissolved organic matter (DOM). In some systems, such as Lake Kinneret, Israel, the microbial loop can represent the majority input into the heterotrophic food web, and yet the specific pathways of organic matter cycling within the water column remain poorly understood. Specifically, the interactions of chromophoric DOM and the microbial community have been difficult to characterize. Chromophoric (colored) dissolved organic matter (CDOM) is a complex mixture of amino acids, proteins, humic acids and sugars which absorb light. CDOM affects a number of ecosystem properties, including light availability, biological oxygen demand, micronutrient and reactive oxygen concentrations. Its chemical composition, and subsequently its specific optical absorbance and reactivity, is dependent on its source, which may be terrestrial (soil leachates) or biological. Biological production in the water column (algal, animal and microbial metabolic byproducts) takes the form of proteins and “marine” humic acids. Photoreactive processes break down CDOM into compounds of lesser molecular weights, either increasing or decreasing its lability as a microbial substrate. Due to its compositional variability, the controlling factors for CDOM lability remain poorly characterized. Still, researchers have developed new tools for quantifying and characterizing CDOM in natural systems, the most recent of which is spectrofluorometric analysis of the fluorescent fraction of CDOM. This method is able to separate and quantify the fluorescence of individual fractions of the total CDOM pool according to their optical excitation/emission peaks. The present study uses spectrofluorometry to investigate seasonal trends in CDOM fluorescence in Lake Kinneret, and attempts to relate the dynamics of CDOM fluorescence to microbial processing of algal-produced biomass within the lake. The study tracked three principal fluorescent components of the total fluorescent organic matter (FOM) pool in the upper mixed layer. In-situ timecourses showed that surface FOM was characterized by highly dynamic (max. ∆ = 14%/week) protein-like fluorescence (p-comp: Excitation/Emission 280/322-382) and weaker, relatively stable ‘terrestrial’ and ‘marine’ humic-like components (t-comp: 360/458 and m-comp: 310/394) during the winter production period. T-comp and m-comp were linearly related from turnover until late summer. All three components showed gradual reductions after lake stratification. P-comp was linearly related to phytoplankton biomass, chlorophyll a and secchi depth in situ. Subsequent dark incubations of water sampled at naturalistic DOM concentrations in situ revealed p-comp and m-comp production, with up to 91% (m-comp) and 73% (p-comp) of integrated FOM production and up to 52% of integrated microbial carbon production originating from the particulate fraction. T-comp concentrations did not significantly change in incubation. Seasonal increases in phytoplankton productivity and microbial production were reflected by increased FOM production in incubation. This study supports the relationship between microbial processing of algal biomass and the production of protein-like and marine humic-like FOM. The lack of m-comp accumulation in surface waters, despite m-comp production in incubation, supports the role of rapid photobleaching in the regulation of this component. The ability of p-comp to trace seasonal algal dynamics is demonstrated, though this relationship was highly dependent on the temporal resolution of sampling.
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