Mesocosm Sampling Activity Report

Within the framework of the mesocosm experiment—with a total of 27 sampling sessions—the first session provided us with a multidimensional dataset along with several anomalous phenomena that warrant attention. The report below details the sampling procedures, the specialized measurement instruments employed, and key observations, while also discussing limitations such as the “wall effect” observed in mesocosm systems.

Mesocosm experiments are a crucial research tool designed to recreate and control ecological and chemical processes in semi-natural environments. This approach allows researchers to analyze the interactions between biological components and environmental factors. The initial sampling session for this experiment spanned three days and incorporated various sampling and analytical steps to ensure comprehensive and accurate data collection.

Sampling Procedures and Measurement Methods

1. Sampling Schedule

Day 1:

• Field Data Collection: Data were collected on-site using multiparameter instruments—namely, the EXO2-sonde, Algaetorch, and nephelometer.
• Periphyton Sampling: Periphyton samples were collected, and system maintenance and cleaning were performed.

Day 2:

• Water Sampling: Water samples were collected from the mesocosm tanks.
• Laboratory Analysis: Samples were filtered in the laboratory to analyze toxins and chlorophyll a content. Additionally, SRP (Soluble Reactive Phosphorus) analyses were conducted, and samples were fixed for subsequent biomass quantification.

Day 3:

• Zooplankton Sampling: Zooplankton were sampled during nighttime to assess the zooplankton community.

2. Field Data Collection Procedures

Upon arriving at the mesocosm area, the measurement process commenced by inserting the EXO2-sonde into the first tank. For each tank, an average value was computed from at least six consecutive measurements, with each tank requiring approximately 4 minutes for data acquisition. Given that 32 tanks were scheduled for measurement, immediate initiation of measurements was critical to optimize the overall data collection process.

EXO2-sonde:

This multiparameter instrument is capable of measuring various environmental parameters, including:

• Temperature

• Conductivity

• Salinity

• pH

• Dissolved oxygen

• Chlorophyll content

• Cyanobacteria concentration (blue-green algae)

The device records and internally stores the measured values, allowing for seamless transition between subsequent measurements without interrupting the process.

Algaetorch:

This device utilizes the fluorescence phenomenon of algal cells to measure chlorophyll a content and to estimate cyanobacteria biomass directly in the field. It provides rapid assessment data regarding the biological status of the ecosystem.

Nephelometer:

This instrument measures water turbidity by recording the intensity of light scattered by suspended particles, thereby estimating the concentration of suspended solids.

In parallel with these primary measurements, periodic inspections of the entire mesocosm system were conducted to identify any maintenance issues. Temperature, dissolved oxygen, and photosynthetically active radiation (PAR) sensors were cleaned and verified. During these inspections, an anomaly was noted in the form of several instances of fish mortality. This occurrence may be associated with a sudden drop in dissolved oxygen levels (only reaching approximately 27% compared to over 65% in other tanks), although the specific cause requires further investigation.

Finally, the day’s task also included the collection of periphyton strips from the tanks for subsequent quantitative and comparative analyses.

Observations and Discussion

The “Wall Effect” in Mesocosms

One notable limitation of mesocosm experiments is the “wall effect” – an artifact resulting from an increased tank wall surface area-to-volume ratio, which can lead to the unnatural growth of periphyton. Previous studies (Chen et al., 1997) have indicated that:

• Periphyton biomass increases quadratically as the ratio of tank wall area to volume increases.

• There is a significant negative correlation between periphyton biomass and phytoplankton abundance.

To mitigate this effect, we monitored periphyton development on a monthly basis by determining the dry weight of pre-measured periphyton strips collected in the field. The accompanying illustrative image shows marked differences in biomass and the composition of periphyton groups across the tanks, underscoring the importance of data calibration to exclude the impact of the “wall effect.”

Influencing Factors and Anomalous Observations

The occurrence of dead fish in several tanks prompted a review of the water quality indices. Specifically, the abrupt reduction in dissolved oxygen (recording only about 27% compared to over 65% in other tanks) is believed to be one of the contributing factors to this anomaly. These observations suggest that, in addition to controlling physical parameters, maintaining stable ecological conditions within the system is essential.

The first sampling session of the mesocosm experiment provided us with valuable data on the physical and biological parameters of the system. The use of advanced measurement devices such as the EXO2-sonde, Algaetorch, and nephelometer enabled multidimensional and precise data collection. Moreover, observations regarding the “wall effect” and anomalies like the reduction in dissolved oxygen highlight certain limitations that must be addressed in both the operation and design of the experiment.

The collected data will be meticulously processed and analyzed to implement necessary adjustments, thereby ensuring the efficacy of subsequent sampling sessions and enhancing the reliability of the research outcomes. Consequently, the mesocosm experiment will continue to serve as a foundational platform for a deeper understanding of the interactions between biological processes and environmental factors in semi-natural ecosystems.