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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. Commonfluorescence 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.

Plastic debris is widespread in freshwater ecosystems, but a rigorous assessment of its global distribution has been hindered by a lack of comprehensive and comparable data. We performed the first standardized global survey of lakes to assess the quantity and type of plastics (>250μm). We included 38 lakes located in 23 different countries distributed across five continents, spanning different environmental gradients and varying levels of anthropogenic stress. All samples were collected by horizontal trawling of a plankton net and subsequently treated with hydrogen peroxide. We identified 9425 plastic particles, which were classified based on shape, color, and size. Polymer identification was carried out using Raman micro-spectroscopy. Our results showed that the concentration of plastics spanned four orders of magnitude (10-3-101 particles/m3). Fibers (49%) and fragments (41%) were the most frequently detected particles, suggesting a secondary origin of plastic contamination. The most commonly identified polymers were polyester (30%), polypropylene (20%), and polyethylene (16%), which are widely used in short life-cycle products and account for the majority of global plastic production. Further, we found that urban-related attributes of lakes/watersheds influenced the occurrence and type of plastics in lentic systems and larger and deeper lakes with higher retention times are accumulating plastic debris at higher concentrations. info:eu-repo/semantics/publishedVersion

The majority of microplastic research has focused on seawater, with fewer than 4% of microplasticsrelated studies occurring on freshwaters. The limited available information suggests that the abundance of microplastics in freshwaters is often as high or even higher than marine environments. However, comprehensive investigations on occurrence and fate of microplastics in freshwaters are scarce and highly fragmented, partly because detection and identification of microplastic particles is rather complex. In addition, up to now, harmonized and standardized protocols for the sampling and analysis of microplastics in freshwaters do not exist, and studies with different research aims and hypotheses often report unstandardized results, making comparison among studies difficult. In the present study, we performed the first global standardized sampling and analysis effort to investigate the occurrence and distribution of microplastics in surface water of lakes and reservoirs with different anthropogenic impacts. Participants aim to collect water samples of freshwater systems with different features (e.g., area, depth, thermal behavior, watershed), following a common protocol. This 18 establishes the collection of samples by horizontal trawling of a plankton net and, after treatment with hydrogen peroxide, the polymer identification through micro-Raman spectroscopy. This GLEON project will allow obtaining comparable data about microplastic contamination in different freshwater systems around the globe. With this global dataset, our goals are to determine whether a relationship exists between the abundance of microplastics and the waterbody/watershed attributes and understand which factors are likely to influence the occurrence of microplastics in surface water of lentic systems. info:eu-repo/semantics/publishedVersion