Abstract
In a changing world, phytoplankton communities face a large variety of challenges including altered light regimes. These alterations are caused by more pronounced stratification due to rising temperatures, enhanced eutrophication, and browning of lakes. Community responses toward these effects can emerge as alterations in physiology, biomass, biochemical composition, or diversity. In this study, we addressed the combined effects of changes in light and nutrient conditions on community responses. In particular, we investigated how light intensity and variability under two nutrient conditions influence (1) fast responses such as adjustments in photosynthesis, (2) intermediate responses such as pigment adaptation and (3) slow responses such as changes in community biomass and species composition. Therefore, we exposed communities consisting of five phytoplankton species belonging to different taxonomic groups to two constant and two variable light intensity treatments combined with two levels of phosphorus supply. The tested phytoplankton communities exhibited increased fast reactions of photosynthetic processes to light variability and light intensity. The adjustment of their light harvesting mechanisms via community pigment composition was not affected by light intensity, variability, or nutrient supply. However, pigment specific effects of light intensity, light variability, and nutrient supply on the proportion of the respective pigments were detected. Biomass was positively affected by higher light intensity and nutrient concentrations while the direction of the effect of variability was modulated by light intensity. Light variability had a negative impact on biomass at low, but a positive impact at high light intensity. The effects on community composition were species specific. Generally, the proportion of green algae was higher under high light intensity, whereas the cyanobacterium performed better under low light conditions. In addition to that, the diatom and the cryptophyte performed better with high nutrient supply while the green algae as well as the cyanobacterium performed better at low nutrient conditions. This shows that light intensity, light variability, and nutrient supply interactively affect communities. Furthermore, the responses are highly species and pigment specific, thus to clarify the effects of climate change a deeper understanding of the effects of light variability and species interactions within communities is important.
Highlights
In a world of climate change, phytoplankton communities face a large variety of challenges due to manifold changes
The quantum yield and the electron transport rate (ETR) were higher at variable light conditions at both light intensities (105 μmol photons · m−2 · s−1 and 195 μmol photons · m−2 · s−1) compared to constant light conditions (Figure 1, Table 1)
Photosynthesis as a fast response to light was highly influenced by light intensity and light condition
Summary
In a world of climate change, phytoplankton communities face a large variety of challenges due to manifold changes. Competition within communities is driven by all these factors and low light adapted species might not be outcompeted by the high light specialist, if they are able to use nutrients in a more efficient way within a community different mechanisms can become relevant e.g., physiological plasticity for various light acclimation mechanisms. This could (if possible) be distinguished from the effects caused by phenotypic plasticity of individual species within a community and from the effects caused by species sorting. We hypothesize that (4) light intensity and fluctuations have a larger impact on a community than nutrient (Phosphorus, P) supply
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