Light and nutrient control phytoplankton biomass responses to global change in northern lakes
Global environmental change affects the transportation of nutrients and organic matter to lakes. Researchers of the Climate Impacts Research Centre (CIRC) at Umeå University studied how the transporation of these substances into lakes influences phytoplankton, an organism found in aquatic ecosystems. The study suggests that colour changes, which are induced by high levels of organic matter as coloured dissolved organic carbon (DOC), and nutrient levels influence the abundance of phytoplankton. This leads to lakes being increasingly covered with biomass.
The figure shows how much phytoplankton is available in three different situations: (1) Browning alone (2) Browning with single Nitrogen enrichment (3) Browning with Nitrogen and Phosphorus enrichment.
Northern lakes which are characterised by low nutrient content dominate global lake populations. As these lakes become more acidic and are further influenced by the consequences of climate change as well as changed land use, the colour of these lakes changes to brown (brownification). Additionally, the usage of fertilisers and fossil fuels due to forestry and agriculture leads to increased nutrient content in lakes. Lakes with high nutrient contents have a high amount of biological activity which means that the growth of plants within the lake is increased.
Phytoplankton, photosynthesising organisms, are key actors in food webs in lakes. Phytoplankton as well as other plants develop and grow differently depending on the water properties, e.g. the nutrient content. In this study, the effect of nutrient content as well as browning on phytoplankton abundance was studied on a large scale to draw conclusions on how whole lake systems respond to changes in the environment. Therefore, the relationship between phytoplankton biomass and environmental changes, which was found to be non-linear in previous studies, is modeled for whole lakes.
The results, as visualised in the above figure, show that a combination of different factors, i.e. browning and high nutrient content, lead to more phytoplankton being present in the lake than for browning alone. For a threshold value of DOC, the phytoplankton abundance will be maximal. In northern Sweden, the treshold value for DOC is not reached. Thus, phytoplankton is likely to grow even further, leading to higher biomass coverage in northern lakes.
Based on the model developed in this study, predictions about the lake responses to future environmental changes may be made.
A.-K. Bergström, J. Karlsson, Light and nutrient control phytoplankton biomass responses to global change in northern lakes, Global Change Biology, 2019, doi.org/10.1111/gcb.14623