Mesocosms and other ecosystem-scale experimental approaches are among the very strongest tools to “take the pulse and assess the state of our aquatic systems, whether that is through reconstructing the past, observing the current state, or predicting the future of our lakes, rivers, estuaries, and oceans.” Mesocosms are not only powerful tools to obtain a mechanistic quantitative understanding of how various global change and other perturbations drive ecosystem responses, they also offer the unique ability to test possible measures to mitigate or counteract anthropogenic pressures through environmental engineering and nature-based solutions.

We welcome presentations on studies where scientific questions about various aspects of aquatic ecosystem functioning have been tested through using mesocosms or similar ecosystem level experimentations. We particularly invite scientists that study effects of increasing variability and extreme events on ecosystem responses including resistance, resilience, recovery of ecosystem structures, biodiversity or regime shifts that change ecosystem functions in response to global climate and environmental change. We strongly encourage early career participants to present results. We also encourage to present plans for future activities open to external participation, as this session also aims to serve as a meeting point for all persons interested in ecosystems scale experimentation on a global scale (see e.g. https://mesocosm.org). We indeed aim to host a session that will “reflect on how our decisions today will manifest in the environment tomorrow” by promoting a lively session fostering new collaborations across areas and disciplines, extending the success of the 3-day mesocosm session at ASLO-ASM 2023 Mallorca meeting.

Lead Organizer: Jens C Nejstgaard, IGB - Leibniz-Institute of Freshwater Ecology and Inland Fisheries (jens.nejstgaard@igb-berlin.de)

Co-organizers:

Stella A Berger, IGB - Leibniz-Institute of Freshwater Ecology and Inland Fisheries (stella.berger@igb-berlin.de)

Paraskevi Pitta, HCMR - Hellenic Centre for Marine Research (vpitta@hcmr.gr)

Meryem Beklioğlu, METU - Middle East Technical University (meryem@metu.edu.tr)

Behzad Mostajir, MARBEC - Marine Biodiversity, Exploitation and Conservation, University of Montpellier-CNRS-Ifremer-IRD (behzad.mostajir@umontpellier.fr)

Presentations

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04:30 PM

EFFECTS OF WATER RESIDENCE TIME ON PHYTOPLANKTON DYNAMICS IN CONNECTED LAKE ECOSYSTEMS - A LARGE-SCALE ENCLOSURE EXPERIMENT (9390)

Primary Presenter: Stella Berger, Leibniz Institute of freshwater Ecology and Inland Fisheries (IGB) (stella.berger@igb-berlin.de)

Lake ecosystems face severe anthropogenic forcing through land use alterations and climate change, which influence lake connectivity, compromise water quality and provision of ecosystem services. Effective lake management to sustain ecosystem functioning requires a mechanistic understanding of how ecosystems respond to environmental stressors. These responses are influenced by lake features, but also by connectivity to their surrounding catchment and neighbouring lakes and rivers. To test how lake connectivity drives synchrony of biological dynamics in lake chains, we conducted a mesocosm experiment in the IGB-LakeLab, installed in Lake Stechlin, Germany. Six experimental circular chains of four enclosures each were set up to mimic short and long water residence times observed in lake chains of the region. Water residence times were manipulated by pumping epilimnetic water between connected enclosures. Following an initial nutrient loading event in the head-enclosure, we monitored phytoplankton dynamics and ecosystem-scale processes related to ecosystem functioning by combining high-resolution in-situ multi-sensor profiles with high-throughput image-based flowcytometry (FlowCam). Our results indicate that short residence times synchronized the epilimnetic plankton community after four weeks, while long residence times led to pronounced differences in phytoplankton communities. This mesocosm experiment improves our understanding of biological dynamics in river-connected lakes and hold great promises for future lake monitoring and management.

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04:45 PM

Effects of a heatwave on the structure and composition of plankton food web components in a coastal zone of the Mediterranean Sea: a mesocosm study (8866)

Primary Presenter: Zoe Eglaine, Montpellier University, CNRS, MARBEC (zoe.eglaine@gmail.com)

Coastal lagoons are particularly affected by marine heat waves (HW) which impact the dynamics of plankton communities that are essential for the functioning and the productivity of ecosystems. To study the effect of a marine HW on plankton food web components, a mesocosm experiment was conducted in the Thau Lagoon (Mediterranean Sea, South of France) in May-June 2019. Two treatments were applied to six in situ mesocosms, each in triplicate. A HW of +3°C above the natural lagoon water temperature was applied to three mesocosms and maintained for the first 10 days of the experiment, after which heating was stopped and temperatures returned to natural levels for the remaining 10 days. The other three mesocosms followed the natural lagoon water temperature for the entire experiment. The simulated HW significantly reduced abundances of viruses, mixo- and heterotrophic flagellates, aloricate ciliates, picoeukaryotes, and diatoms (i. e. Cyclotella spp.). Conversely, cyanobacteria (i. e. Synechoccocus), autotrophic flagellates and pennate diatoms benefited from the HW. Stability parameters showed that diatoms, dinoflagellates, and ciliates exhibited low resistance and recovery but a high resilience. Conversely, inverse responses were observed for viruses, bacteria, and heterotrophic nanoflagellates. The contrasting resistance, resilience, and recovery of plankton functional groups to HW led to a restructuring of the plankton community. These changes may have repercussions within interactions of the food web and therefore could alter ecological functions of the pelagic ecosystem.

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05:00 PM

IMPACT OF RUN-OFF VARIABILITY ON PHYTOPLANKTON COMMUNITY STRUCTURE AND FUNCTION - RESULTS FROM A LAKE MESOCOSM EXPERIMENT (9311)

Primary Presenter: Katerina Symiakaki, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) (katerina.symiakaki@igb-berlin.de)

Extreme rainfall can cause high loadings of coloured dissolved organic matter (cDOM) and nutrients from the catchment into lakes. While phytoplankton and bacteria compete for nutrients, the outcome depends on whether the phytoplankton is limited by light reduction caused by cDOM, and how much the bacteria are able to utilise DOM-carbon. In addition, we propose that the temporal variability in loading may strongly impact the plankton community, specifically phytoplankton community composition, biomass and function. We tested the run-off variability in a mesocosm experiment at SITES AquaNet in Lake Erken, Sweden, in summer 2022. The same amount of cDOM and nutrients were added, but at different frequency and intensity: Daily, Intermittent, Extreme, no addition (Control) over 20 days, followed by a 16-day recovery phase of no additions. Phytoplankton community composition was analysed using microscopy and FlowCam with a semi-automated machine-learning approach for classification. We show that the Daily and Intermittent cDOM and nutrient additions promoted opportunistic taxa such as cyanobacteria and small green algae, while light limitation resulted in lower phytoplankton growth and biomass, but appeared to promote competition with bacteria in the Extreme additions. Phytoplankton functional groups showed pronounced differences between treatments in terms of size, coloniality and trophic strategy. We conclude that run-off variability impacts the taxonomic and functional composition of the phytoplankton community, thus affecting energy transfer and food web processes in lakes.

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05:15 PM

CONTROL OF BENTHIC AND HYPORHEIC STREAM BIOFILMS BY THE LIGHT:PHOSPHORUS SUPPLY RATIO – A MESOCOSM EXPERIMENT (8909)

Primary Presenter: Rizwan Ullah, Helmholtz Center for Environmental Research-UFZ (rizwan.ullah@ufz.de)

Streams around the world suffer heavily from various facets of anthropogenic land use activities. As a consequence, their ecological status is frequently deteriorated, e.g. through the inputs of excessive loads of dissolved nutrients and increased light availability at the streambed due to riparian deforestation. The light-nutrient hypothesis (LNH) is a stoichiometric framework that allows the prediction of thresholds for primary producer growth. We here tested its applicability in a small European river saturated in reactive nitrogen through human land use effects. We performed a flume mesocosm experiment in a small German river characterized by low dissolved phosphorus concentrations, but high nitrate loads. We applied a gradient of soluble reactive phosphorus concentrations (12 levels from < 10 to 220 µg P L^-1) at constant light conditions, thereby creating a wide range of light:phosphorus ratios. The biomass of benthic microbial communities (biofilms) decreased exponentially with the molar light:SRP ratio within one week, as predicted by the LNH. In contrast to the benthic (light exposed) biofilms, the hyporheic biofilm community did not respond according to the predictions of the LNH.  Our results demonstrate that the LNH approach is applicable for benthic biofilm growth in anthropogenically impacted, nitrogen-enriched stream ecosystems, and may help to derive management decisions for sustainable nutrient and light input levels.

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05:30 PM

THE RESILIENCE OF A MEDITERRANEAN LAGOON THROUGH PAST AND FUTURE THREATS (8936)

Primary Presenter: Laura Baldassarre, National Institute of Oceanography and Applied Geophysics - OGS (lbaldassarre@ogs.it)

Lagoonal systems represent some of the most important yet endangered transitional environments worldwide. The Grado-Marano lagoon (northern Adriatic Sea-eastern Mediterranean Sea) is a hotspot of biodiversity and a source of ecosystem services but it is historically polluted by mercury (Hg). This element exerts neurotoxic and genotoxic effects on animals once organized into the bioaccumulable monomethylmercury (MeHg). As the water temperature increases, the lagoon experiences occasional hypoxia/anoxia as consequences of high organic matter loads and water stratification. Under these conditions, the benthic MeHg production and release through the water column is stimulated. To test the resilience capacity of the lagoon system subjected to the synergistic effect of high temperature and contamination, we performed a field experiment, where we enclosed portions of the water column and surface sediments to simulate a plausible climate change scenario, i.e., high temperature in an organic-enriched shallow area. The short-term (few hours) and long-term (few days) effects on composition, abundance and structure of planktonic and benthic communities at different trophic levels (prokaryotes, microalgae, consumers) were investigated, together with physical-chemical changes in both the water column and in the sediments. To assess the effect of the combined stressors on the overall pelagic-benthic coupling, the main biological processes (respiration, primary production and prokaryotic heterotrophic production) rates were quantified in the enclosed water and sediments.

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