Carbon from land is eaten by benthic fauna in lakes and seas. In this way, carbon from land enters the aquatic food web. Through isotope studies, researchers have been able to follow carbon from its arrival in coastal waters to its path up the food chain.
Text: Kristina Viklund
We know that climate change leads to an increased influx of terrestrial carbon, that is, carbon originating from land and often brown in color. Exactly how this influx will affect our coastal food webs is still unclear, but thanks to this study, some pieces of the puzzle have fallen into place. Rivers transport terrestrial carbon to the sea, and earlier studies have shown that brown water reduces light penetration in the water and that bacteria production is favoured over algae production. But where does the terrestrial carbon go once it has entered the coastal waters?
Benthic fauna eat the carbon
By labelling the terrestrial carbon with deuterium, heavy hydrogen, the researchers were able to trace its path through different parts of the food web. They showed that some of this carbon was transported up the food web to higher trophic levels, such as fish. The path into the food web seems to go through benthic fauna which probably eat the terrestrial carbon directly when particles fall to the bottom. Some animal groups eat more of the terrestrial carbon, while others mainly feed on bottom-living algae, i.e. marine carbon.
Zooplankton feed mainly on marine carbon, and not on the carbon transported from land to sea.
Zooplankton feed on marine carbon
Bacteria in the water column, which are often favoured by terrestrial carbon, could conceivably contribute to carbon entering the food web by being eaten by small zooplankton. However, this study indicates that this is not the case to any great extent. Zooplankton does not contain much terrestrial carbon, but instead appears to feed on carbon produced in the marine environment.
Perch feed on terrestrial carbon
The researchers also studied sticklebacks and perch, and saw great differences between the two species. The sticklebacks were not very dependent on the terrestrial carbon, while perch seemed to heavily depend on it. The results therefore show that perch to a large extent eat benthic fauna.
Variation during summer
The degree to which different organisms depend on terrestrial carbon varies during the summer. Benthic fauna use this carbon throughout the summer, with a slight dip in early June. Values were highest near the estuary, and lower further out in the sea. The values were also higher in deep areas, which indicates that the dependence on terrestrial carbon from these organisms is higher where there is a shortage of marine produced carbon. Zooplankton, on the other hand, showed a very low dependence on the terrestrial carbon during most of the summer, but with a slightly higher value in May. The study emphasizes that climate change will change the basic energy sources in coastal ecosystems.
The article has earlier been published in the EcoChange annual report 2018. The text is based on: