The experiment will simulate different run-off scenarios that transport dissolved organic matter (DOM) and inorganic nutrients into lakes at different pulse frequencies and intensities. Experiments with the same setup were conducted in summer 2022 at SITES stations Erken and Bolmen. In 2023, we plan to do the experiment in spring, as close to ice-off as feasible, in order to compare the response to the different run-off treatments, (1) between seasons and different starting communities in Erken and Bolmen (replication in time), and (2) at an additional lake at SITES Skogaryd station (replication in space). The start time is dependent on ice melt, but tenatively for Bolmen and Skogaryd a start around mid-April and for Erken late April/early May is anticipated. Each experiment will last for 45 days, including time for preparations and deployment. 

Environmental change encompasses stochastic to periodic fluctuations in environmental conditions, gradual environmental changes as well as pulse events, i.e. abrupt unpredictable changes in the environmental factors affecting ecological parameters. The latter become more important in many ecosystems as a result of increasing intensities and frequencies of extreme weather events due to climate change. Lakes, for example, will experience to a greater extent heavier and flashier rainfalls, storm-induced mixing and prolonged drought periods in summer, which will result in more variable resource (nutrient and carbon) regimes in lakes. This, in turn, will affect the temporal heterogeneity of available resources that plankton communities encounter, which can potentially alter ecosystem process rates as well as biomass and community composition, stoichiometry, diversity, temporal turnover and community assembly mechanisms across multiple trophic levels. In general global change research has to a large extent focused on altered mean environmental conditions whereas the importance of stochastic event driven dynamics and their characteristics, such as their intensities, frequencies, timing, chronology are less studied, even though they may be more important for ecological communities and ecosystem processes than long-term changes in mean conditions (e.g. Jentch et al 2007, Forsman et al 2016). 
 
To manage effects of increasing extreme weather events we further need to know if their effects are persistent over different types of lakes or if local lake characteristics and local climate strongly affect the response. To address such questions, spatially and temporally coordinated experiments that study responses to standardized manipulations with standardized equipment and methods are crucial tools. SITES AquaNet is well suited here because it is designed to implement standardized experiments across lakes. Moreover, the mesocosms are equipped with a high frequency sensor system for high-resolution measurements of phytoplankton and cyanobacterial biomass and ecosystem metabolism, such as primary production and respiration, at various time scales.

We will manipulate 3 run-off scenarios that vary in the intensity of frequency of organic matter and nutrient pulses during a 3-week ‘simulated rainfall’ period. The total amount of added DOM/nutrients will be kept constant and thus fewer pulses of higher intensity on the one hand and/or more frequent but smaller pulses on the other hand, thereby creating a gradient from ‘extreme’ to more continuous DOM/nutrient loadings:

As we aim to start the experiment as close after ice break off in spring as feasible the starting times will differ between the lakes depending on the geographic location. The stations that are prioritized for this experiment are Bolmen and Erken (because they did the experiment already in 2022) and Skogaryd.
 
For Bolmen and Skogaryd a start around mid-April is anticipated, and for Erken late April/early May.
 
The duration of the experiment will be 45 days in total, including time for set-up and deployment of mesocosms, sensors etc. 

Parameters/Measurements:Information here

How different disturbance characteristics (i.e. intensity, frequency, disturbance type (e.g. press vs. pulse)) affect community and ecosystem responses is an important question in ecology. 

One environmental stressor that has recently received increasing attention is salinization of inland waters related to increased droughts, saltwater intrusion, road salt applications etc. (Cunillera-Montcusi et al. 2022). Findings from the global GLEON salt mesocosm experiment recently highlighted that negative effects of salinisation can already be seen at low levels of salinity changes (NaCl in this case) and that responses are widely variable between different zooplankton groups and systems (Hintz et al. 2022). For example, responses of large-bodied zooplankton (cladoceran and copepod populations) were highly variable across sites, but they were, overall, considerably more sensitive than rotifer populations (Hébert et al. 2022). Moreover, thresholds where negative effects of salinity are found differ largely between systems and might not even exist at all in some cases (Hintz et al 2022). In this and other experiments, the salt was added to the mesocosms at the start of the experiment at the final target concentration, thereby simulating a strong pulse of sodium chloride as deicing agents used for roads and urban areas at high latitudes. However, it is still unclear how responses would differ between various salinity disturbance regimes driven by different run off scenarios transporting salts from land to water, e.g.  different incremental increases towards the target concentration or variation in the intensity and frequency of salinity pulses over a given period of time. Moreover, more work is needed to look particularly into effects at low salt concentrations to depict threshold concentrations where negative effects can be seen in a variety of different response variables, including, for example, those of phytoplankton and bacterial communities. Further, high-frequency sensor data can be used to possibly detect early warning signals in response to salinization (Urrutia Cordero et al in preparation).

Manipulation of different run-off scenarios similar to Experiment 1. The rationale here is that also transport of ions from the catchment occurs according to different flow regimes, and that different trajectories of incremental increases towards the same final salt levels could lead to strongly divergent compositional and functional differences in plankton communities if different regimes select organism that differ in their adaptive potential and plasticity. 
 

The same total concentration of salt (NaCl) will be added to the mesocosms over a fixed period of time (approx 3 weeks) in different additions, e.g. (1) small daily additions, (2) adding all at once, and a scenario with several intermediate-sized additions at stochastic frequencies. This will be followed by a 2 week ‘recovery period’ without any additions to test if effects of differences seen between treatments are transient or if they lead to more persistent effects. Each treatment will be replicated 4 times.

Time plan
The experiment will primarily involve stations that were part of the GLEON Salt experiment in 2018, in particular Asa and Svartberget. The anticipated start is early-mid June. The experiment will run for approximately 45 days, including a control without any salt additions.