Nitrogen enrichment leads to changing fatty acid composition of phytoplankton and negatively affects zooplankton in a natural lake community

www.nature.com/scientificreports OPEN Nitrogen enrichment leads to changing fatty acid composition of phytoplankton and negatively affects zooplankton in a natural lake community Gabriele Trommer1,2*, Patrick Lorenz1, Ameli Lentz1, Patrick Fink 3,4,5 & Herwig Stibor1 Secondary production in freshwater zooplankton is frequently limited by the food quality of phytoplankton. One important parameter of phytoplankton food quality are essential polyunsaturated fatty acids (PUFAs). Since the fatty acid composition of phytoplankton is variable and depends on the algae’s nutrient supply status, inorganic nutrient supply may affect the algal PUFA composition. Therefore, an indirect transfer of the effects of nutrient availability on zooplankton by changes in algal PUFA composition is conceivable. While the phosphorus (P) supply in lakes is largely decreasing, nitrogen (N) inputs continue to increase. This paper presents data from a mesocosm field experiment in which we exposed phytoplankton communities to increasing N enrichment. As a consequence, the PUFA composition of the phytoplankton community changed. With increasing nitrogen fertilisation, we observed lower quantities of essential PUFAs, together with a decrease in the abundances of the dominant herbivorous zooplankton Daphnia sp. Their biomass was significantly correlated with phytoplankton PUFA content (C18:3 ω3, C20:5 ω3, C18:2 ω6). Our data therefore indicate that changes in nitrogen supply, together with the resultant changes in phytoplankton food quality, can negatively affect the secondary production of herbivorous zooplankton by reducing the availability of essential polyunsaturated fatty acids. Global anthropogenic activities drastically alter nutrient cycles by increasing energy consumption and biomass production1 thereby strongly affecting global ecosystem services2,3. Since the key nutrients nitrogen (N) and phosphorus (P) are essential components of the biomass of organisms’ and often limit primary production, changes in their biogeochemical flows can have drastic consequences on ecosystem dynamics. The biogeochemical pathways of both elements are to a large degree influenced by anthropogenic activities resulting in increasing amounts of N and P entering the ecosystems by means of waste water, excessive fertiliser application and soil erosion soils1,4,5. There has been an increasing effort for several decades to reduce nutrient loads, especially in freshwater systems where P often limits the primary production6. Replacing the P compounds in detergents and/or providing purification plants with highly efficient P elimination techniques has resulted in successful P reduction and the reoligotrophication of water bodies7,8. However, N loads have continued to increase, since the diffuse N inputs are difficult to control by means of targeted measures. N is much more mobile than P, and its high dispersal potential means that it can easily enter groundwater and atmospheric pools9. In lakes, P enrichment is often clearly visible by an apparent increase in primary production6. Furthermore, P enrichment can potentially result in eutrophication characterised by blooms of toxic or undesired algal species, the oxygen reduction of deep waters, and other undesirable consequences10. By contrast, N enrichment of fresh 1 Ludwig-Maximilians-University Munich, Department II Biology, Aquatic Ecology, Großhaderner Str. 2, 82152, Planegg-Martinsried, Germany. 2Present address: Water management office Ansbach, Dürrnerstr. 2, 91522, Ansbach, Germany. 3University of Cologne, Institute for Zoology, Zülpicher Street 47b, 50674, Cologne, Germany. 4 Helmholtz Centre for Environmental Research, Department River Ecology, Brückstraße 3a, 39114, Magdeburg, Germany. 5Helmholtz Centre for Environmental Research, Department Aquatic Ecosystem Analysis, Brückstraße 3a, 39114, Magdeburg, Germany. *email: Scientific Reports | (2019) 9:16805 | https://doi.org/10.1038/s41598-019-53250-x 1 www.nature.com/scientificreports/ www.nature.com/scientificreports waters is often much more inconspicuous. Only in lakes where N availability limits primary production, eutrophication signatures are visible and observed11–13. However, in addition to the quantitative effects of increasing primary production, more subtle qualitative effects can occur in terms of community and biochemical composition. For example, N enrichment can favor certain algal groups, such as mixotrophic algae14 via an alteration of the bacterioplankton composition15. Such changes in algal community composition can result in changes in food quality for herbivorous zooplankton, as not all algal groups are equally well-suited as zooplankton food. Consequently, recent mesocosm experiments with natural plankton communities suggest that N enrichment is accompanied by a lower trophic transfer efficiency16. While zooplankton growth decreases with an increasing N supply, the decline is most pronounced in the case of cladoceran zooplankton16, which are a particularly important food source for fish. Cladoceran zooplankton, especially Daphnia sp., have been thoroughly investigated and several food quality related factors that influence their fitness have been identified17,18. These factors are algal size, gelatinous sheaths, and toxicity, but also nutrient stoichiometry and biochemical composition17–19. The fatty acid composition of phytoplankton, especially the contribution of polyunsaturated fatty acids (PUFAs), is a biochemical factor that is already well known from laboratory studies for its consequences for Daphnia growth20–22. There is evidence that the PUFA composition of phytoplankton appears to exert a higher influence on the somatic growth rates and reproduction of Daphnia sp. than do stoichiometric effects such as the C:P or N:P ratio of the phytoplankton biomass23,24. There is a general mismatch in nutrient management strategies that recent reoligotrophication continues to reduce the amount of P in a large number of water bodies, whereas the N supply continues to increase. This unbalanced reoligotrophication can have various undesirable effects. Although large quantitative effects from N supply to P limited systems are not expected, qualitative changes on the biomass stoichiometry14 or biochemical composition cannot be excluded. We therefore investigated the qualitative effects of increasing N enrichment on biochemical phytoplankton PUFA composition in an already P deficient system. We exposed a natural spring plankton community to a gradient of N enrichment in an enclosure experiment. The experimental N enrichment was conducted over an ecologically meaningful time scale of ten weeks25,26 in order to include not only the short-term direct enrichment effects on phytoplankton, but also the subsequent bottom up effects on higher trophic levels. Results Water chemistry. Prior to fertilisation, the nutrient concentrations in Lake Brunnensee were 8.4 mg L−1 for NO3, 121 µg L−1 for NH4 a (...truncated)