The elusive nature of fluorescent organic matter in Lake Kinneret, Israel

The dynamics of fluorescent organic matter (FOM) in lakes have generally been studied over extended periods of weeks to months rather than short time scales. We employed spectrofluorometry to examine whether short-term in situ variations in the three components of FOM (protein-like, terrestrial, and marine fluorescence) were related to changes in production in Lake Kinneret, Israel. Protein-like fluorescence correlated strongly with phytoplankton biomass, chlorophyll a, and Secchi depth in situ, suggesting it might serve as an indicator of aquatic production. In vitro dark incubations of filtered and unfiltered surface lake water (without added nutrients or organic matter) revealed considerable protein-like and marine fluorescence production in the unfiltered samples, suggesting that FOM is linked to particulate matter dynamics and to microbial processing of this suspended material. Terrestrial fluorescence did not significantly change in incubation, indicating that this component is probably produced deeper in the lake. Moreover, the lack of marine FOM accumulation in surface waters, despite producing in vitro, suggests that FOM photobleaching may regulate this component.