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Climate change effects on phytoplankton and periphyton in streams and shallow lakes

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Periphyton and phytoplankton are key communities in freshwater ecosystems. They contribute importantly to primary production, regulate biogeochemical cycles and influence food webs interactions. Predicted scenarios of global warming are expected to drive profound changes in the structure and functioning of freshwater ecosystems, which will impact, among others, the primary producers. Although some potential effects of global warming on pelagic production are relatively well known, the effects on periphyton and particularly, on phytoplankton-periphyton interactions have been much less studied. This limits the understanding of how the effects of multiple stressors associated with climate change can affect algal production in freshwater ecosystems.
The general objective of this thesis was to elucidate how a set of climate change sensitive environmental variables and their interactions affect periphyton and phytoplankton biomass and composition in shallow lakes and streams, with special focus on:
1) how a decline of dissolved N, simulating increased denitrification and reduced N loadings, affects the biomass and composition of periphyton under different temperature scenarios in shallow lakes.
2) the effects of warming on the growth of periphyton and two macrophyte species (Potamogeton crispus and Elodea canadensis) in mesocosms with high P loadings and contrasting N levels.
3) the combined effects of nutrient enrichment and light availability on the periphyton biomass and composition along a discharge gradient in lowland streams, simulating eutrophication, loss of riparian coverage and climate-driven long-term discharge alterations.
4) the potential environmental consequences of climate change and eutrophication for the invasion of the phytoplankton species Ceratium furcoides in subtropical lakes.
5) the effects of omnivory by small-sized fish, warming, eutrophication and their potential interactions on phytoplankton and periphyton.
This PhD combined field data and experiments in mesocosms resembling shallow lakes and lowland streams and analysed the responses of phytoplankton and periphyton as the main communities along with other potential interactions with complementary communities.
We found contrasting effects on periphyton and phytoplankton. The decline in N loadings induced changes from pelagic to periphyton biomass dominance. At high N loadings periphyton biomass was negatively affected by warming, possibly due to increased shading by phytoplankton, while this effect disappeared at low N loadings because of the lower phytoplankton biomass.
Periphyton biomass and composition in lowland streams are likely to be differently affected by changes in discharge depending on the light and nutrient conditions. The combined effect of high light availability and nutrient enrichment resulted in higher periphyton biomass with increased discharge, contrary to what happened under shaded and low nutrient conditions. This indicates synergies between eutrophication and loss in riparian coverage, which increase periphyton biomass and drive compositional changes with increased discharge.
Eutrophication along with climate change may also favour biological invasions as we identified with the example of C. furcoides invasion in Uruguay. Eutrophic conditions before the bloom of C. furcoides, and extreme wind events causing cyst resuspension may lead to its rapid growth. This biological invasion and the ensuing blooms may in some cases be associated with fish kills.
The presence of small-sized omnivorous fish caused drastic cascading effects, promoting planktonic over periphytic production through important compositional changes in zooplankton and macroinvertebrates, and this effect may potentially be more important than warming and eutrophication combined.
The thesis demonstrates that phytoplankton and periphyton are significantly affected by climate-driven changes in stressors through changes in both biomass and composition, altering the structure and function of stream and shallow lake ecosystems. However, these effects are largely regulated by nutrient and light availability and influenced by trophic structure. Such interactions are not only of scientific interest, they also have implications for environmental management strategies and warrant further studies.
ForlagAarhus Universitet
Antal sider166
ISBN (Trykt)978-87-93129-64-1
StatusUdgivet - aug. 2021

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