Research project PlantFish
Coastal manager’s struggle globally both with declining stocks of large predatory fish and decreased submerged vegetation; the "foundation species" that forms the basis for many coastal ecosystems. The research project Plant-Fish has thoroughly examined the functional role of vegetation and predatory fish in a large number of shallow Baltic Sea bays.
The researchers in the project have combined studies at different spatial scales and methods to address the hypothesis that vegetation and predatory fish facilitate each other via a positive feedback loop, which enhances ecosystem health.
Plant-Fish is a co-operation between researchers at the Department of Ecology, Environment and Plant Sciences at Stockholms University, Stockholms University Baltic Sea Centre, Swedish University of Agricultural Sciences, University of Groningen and AquaBiota. The project was financed by the Swedish research council FORMAS during 2014-2018, and continues with the new project Spatially cascading regime shifts, the "stickleback wave".
A map showing all examined bays (blue) and the detaily studied bays (orange).
Modelling of vegetation and juvenile fish data across the entire Swedish Baltic Sea Coast have been combined with detailed examinations of the food web in 32 bays, and small-scale manipulative field experiments at the marine research station Askö Laboratory.
Results from the studies show that:
- High cover of benthic vegetation results in less turbid water partly through stabilizing the soft sediment bottoms.
- The benthic vegetation is important for coastal fish; the abundance of juveniles of pike, perch and cyprinids increase with increasing cover of vegetation.
- Boat marinas have a negative effect on the cover and height of benthic vegetation, and alter the species composition. This can in turn influence the recruitment of fish.
- Predatory fish, such as pike and perch, have a positive effect on benthic vegetation though a trophic cascade, where they control meso-predatory fish, such as sticklebacks, which in turn regulate small invertebrates such as insect larvae, snails and amphipods. These invertebrates can in turn, through grazing, control nuisance algae which smother the benthic vegetation.
- The density of benthic vegetation is important for maintaining healthy plants when large predatory fish disappear and the water is enriched with excessive nutrients.
- The small fish three-spined stickleback have gradually taken over larger parts of the Baltic Sea’s coastal ecosystem. The stickleback contributes to local ecosystem ‘regime shifts’, where young-of-the-year pike and perch decline in individual bays, and these shifts gradually spread like a wave, from the outer archipelago into the mainland coast. More stickleback and fewer predatory fish also intensifies the eutrophication symptoms, in accordance with the results above.
Strong links between vegetation and fish
Sampling in a shallow bay. Photo: Joakim Hansen.
The results from the project strengthen previous suggestions that the seabed vegetation perform important ecosystem functions such as improving water quality and recruitment success of coastal fish, whereas predatory fish indirectly control nuisance algae that otherwise smother the vegetation.
The research project have also documented the large-scale change that have occurred along the Swedish Baltic coast, where stickleback gradually have increased and caused local regime shifts in one bay after another; to a condition with few predatory fish and a lot of nuisance algae.
The jungle-like vegetation is perfect nursery habitats for fish like this young pike. Photo: Ulf Bergström.
In shallow bays with many jetties and a lot of boat traffic the vegetation was both short and sparse, and the most sensitive species were missing. Photo: Joakim Hansen.
A spatial regime shift to stickleback dominance. Large numbers of three-spined stickleback have gradually taken over larger parts of the Baltic Sea’s coastal ecosystem, shows a new scientific study. Stickleback is a small prey fish common in aquatic food webs across temperate Europe. The stickleback contributes to local ecosystem ‘regime shifts’, where young-of-the-year pike and perch decline in individual bays, and these shifts gradually spread like a wave from the outer archipelago into the mainland coast.
“Together we stand”: aquatic plants help each other endure stress. In coastal ecosystems, eutrophication and declines of large predatory fish have negative effects on large aquatic plants by increasing the amount of filamentous algae. Now, new results from an experimental study show that plant density is important for maintaining healthy vegetation when large predatory fish disappear and the water is enriched with excessive nutrient content. The study highlights the possibility of counteracting negative effects of eutrophication and loss of large predatory fish by maintaining high plant density in coastal ecosystems.
Rich and tall vegetation in a shallow undisturbed bay. Photo: Joakim Hansen.
Fish habitats degraded by recreational boating. People around the Baltic Sea love boats and Swedes and Finns in particular love their archipelago. But the boating activities can put pressure on the coastal environment. In a new scientific article, researchers from Stockholm University and the Swedish University of Agricultural Sciences show that small boat marinas have a clear impact on both the amount and the species composition of vegetation on the seabed. The study also shows that this vegetation is very important for recruitment of several coastal fish species, including perch and pike.
The Project in English media
Science In depth, Sep 4, 2020, An ecosystem goes topsy-turvy as a tiny fish takes over
Science, Aug 27, 2020, A tiny fish takes on its predators—and wins, transforming the Baltic coast
For an overview over more coverage, in Swedish, see the Swedish project page.