Mesocosms; environmental laboratories

Studying the biology of the marine environment can prove tricky, there are many factors you must consider when looking at it. The influence of water chemistry, physics, weather can all impact the biology. We can work in the laboratory, and control a number of these. We can determine how much light or nutrients organisms get, which allows us to test specific variables. But there is an “in between” of the lab and the environment, and this is what is known as a mesocosm.

Mesocosms are an experimental system, usually within the natural environment, but under controlled conditions. For instance, they can be large tanks, or floating bags within an aquatic environment, for instance at the Espegrend marine field station in Bergen, Norway.

The mesocosms at Esepgrend on a floating platform in the fjord (

They can range in size and also experimental treatments. They have been used to study the impact of CO2 on marine algae, the influence of increasing temperature and acidity on biological organisms, and also to gain a better understanding of the marine food web.

This understanding of the marine food web, is where my experiment comes in.

As part of my research mobility exchange funded through the World Universities Network, I am in Bergen trying to gain a greater understanding of the marine food web and the flow of nutrients and carbon through the system.

Diagram of the marine food web, showing the different trophic levels from algae to fish (

The experiment we are running is to look at the impact of gelatinous zooplankton (feed on zooplankton and young fish) on the marine food web. For this, we are using small mesocosms (approx. 30L buckets) which have water from a local fjord within them, and a bloom of algae stimulated through the addition of nutrients and additional light.

The tanks set up in the temperature controlled room, light panels provide light on an appropriate day:night cycle length 

Our room is set up with a controlled temperature and light intensity, by controlling these variables we can limit the number of factors which may differ and impact our experiment. Our main objective is to observe the impact  of gelatinous zooplankton. To study this we will have 3 tanks with gelatinous zooplankton and 3 with none added, to act as our “control”. For this experiment, we want to observe, over 2 weeks, what the impact of the ctenophore, Mnemiopsis leidyi, is on marine systems. For 2 weeks we will observe the changes in the abundance of biological organisms (from bacteria to larger grazers), the oxygen consumption and release (through photosynthesis and respiration) and the changing nutrient conditions.

The ctenophore Mnemiopsis leidyii

One reason why we are interested in these results are because gelatinous zooplankton appear to be becoming more abundant in marine systems, we want to know what the impact of them is on the base of the food chain. Do they, as some studies suggest, stimulate algal production through the release of nutrients which are limiting growth? Or, do they reduce phytoplankton biomass by feeding on zooplankton which enhances the population of algal grazers (the smaller micro-zooplankton)?

Stay tuned to find out more!


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