Can small fish control food webs in aquatic systems?

Experimental results in Lake Mývatn (Iceland) and its relationship to climate change

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Climate change is one of the most important threats affecting loss of biodiversity and degradation in natural systems. Particularly, lakes in the polar region are in the front line of research due to their vulnerability to the rapid increase of global temperature. These lakes have the characteristic to show simple food webs with very low trophic levels due to the presence of few species. This implies that they are good model systems suitable for experimental manipulations and allow to test general predictions in relation to climate change.

Recently, members of the Aquatic Ecology group from the University of Vic-Central University of Catalonia (UVic-UCC), leaded by Sandra Brucet, have published a paper about food webs in polar region. A three-month experiment was conducted in Lake Mývatn (Iceland) during the summer of 2014, together with scientists from Aarhus University (Denmark) belonging to professor Erik Jeppesen’s group.

The main purpose of the experiment was to assess the predation role of a small fish (three-spined stickleback, Gasterosteus aculeatus L.) in the food web of the lake (Fig. 1). This fish species feeds on very small aquatic organisms called plankton (microscopic algae and animals, phytoplankton and zooplankton, respectively), whose abundance and size can change as a result of the predation process. However, it remains unclear whether fish predation influences the food web of the Arctic lakes, which are nutrient-poor compared to temperate and tropical ones. Clarifying this question is important because Artic lakes are expected to experience an increase in temperature and nutrient concentration, which benefit the abundance of small fish such as sticklebacks.

Figure 1: Different individuals of sticklebacks caught in the Lake Mývatn. Can these small fish dominate the lake?

The experimental set-up consisted of eight polyethylene cylinders (a.k.a. enclosures) located in the Western part of the lake (Fig. 2). We added sticklebacks to half of the enclosures (4 fish per enclosure) and left the other half with no fish addition (controls). After setting similar chemical and biological conditions in all cylinders, the experiment was run from the 20th of June to the 20th of August (a total of 61 days). Physicochemical (phosphorus, nitrogen, temperature, oxygen etc.) and biological (macroinvertebrates, zooplankton, phytoplankton etc.) parameters were recorded from each enclosure every two weeks (i.e. in total five samplings).

Figure 2. Experimental set-up in the Western part of the Lake Mývatn.

Our results showed that the small fish plays an important role in controlling the abundance of other group of organisms by decreasing zooplankton abundance and size, leading to an increase of the phytoplankton biomass. However, unexpectedly, the blooming of filamentous cyanobacteria (Anabaena) in all enclosures after the second sampling period masked the effects of fish predation on the food web. Because cyanobacteria are toxic and not edible by zooplankton, they reduced the abundance and size of zooplankton in both enclosures with and without fish. This suggested that cyanobacteria blooms may have the potential to interfere with the predation role of sticklebacks in lakes. Despite the cyanobacterial bloom, an increase in the density of sticklebacks in Lake Mývatn could have some consequences on the food web leading to an increased abundance of phytoplankton. Our results can be applied to other shallow lakes of polar region, where there may be stronger predation from smaller fish and increased occurrence of cyanobacterial blooms with the global warming.   

Reference: Cañedo-Argüelles, M., Sgarzi, S., Arranz, I., Quintana, X.D., Ersoy, Z., Landkildehus, F., Lauridsen, T., Jeppesen, E., Brucet, Z. Role of predation in biological communities in naturally eutrophic subarctic Lake Mývatn, Iceland, Hydrobiologia (2016) 790: 213-223