Using UV-Fluorescence Fingerprints to Assess Lake Browning Effects - A Mesocosm Experiment in Lake Stechlin, Germany
Résumé
During the summer of 2021, a browning experiment was conducted at the LakeLab mesocosm facility at Lake Stechlin, Germany, as part of the AQUACOSM Transnational Access program. From mid-July to mid-August, a total of 10 sampling campaigns in 16 enclosures was run to assess the effects of artificial browning (HuminFeed® addition) on water clarity and light penetration in the epilimnion. Four different treatments: control (CC), +nutrients (CN), control +browning (BC) and +nutrients +browning (BN) were applied to a subset of 4 enclosures; additional control measurements were taken at different locations in Lake Stechlin. Each sample aliquot was analyzed with a fluorescence spectrophotometer (Hitachi F-7000) to obtain its spectral fingerprint. Common
fluorescence peaks were measured and their respective behaviors analyzed throughout the study period. The results indicate that all fluorescence peaks and fluorescence-based indicators were attenuated and remained low after addition of artificial humic substances. Only peak C fluorescence, reflecting humic-like compounds in water, increased in enclosures BC and BN that received the browning treatment; thus highlighting the specific HuminFeed® signature. Interestingly, after a 2-week lag phase, fluorescence of peaks B and T (tyrosine- and tryptophan-like compounds) and BIX, the Biological Index characterizing the biological production of DOM, increased significantly in enclosures CN and BC. These observations may be linked to phytoplankton growth which did not occur in photic zones of BN enclosures. Spectral signatures of CC enclosures mimicked those from lake controls and none demonstrated significant fluorescence variations.