Peter, Kalista Higini (2014) Effect of climate warming on phytoplankton size structure and species composition: an experimental approach. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 93 pp.

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Abstract

Shrinking of body size has been proposed as one of the universal responses of organisms to global climate warming. Using phytoplankton as an experimental model system has supported the negative effect of warming on body size. However, there is no consensus about the underlying mechanisms. Explanation under the roof of Temperature Size Rule (TSR), clearly refer to size shift within species while community shift are often explained by intensified resource competition at higher temperatures and competitive advantages for smaller species. As an alternative explanation, intensified predation on larger prey items at higher temperatures has been suggested. This would apply only under specific food web configurations, e.g. if phytoplankton grazing is dominated by copepods which tend to feed on larger food particles while releasing small phytoplankton by predation from heterotrophic protists. The current study aimed to clarify the underlying mechanisms which induce size reduction of organisms in response to warming. In this study, marine phytoplankton is used as a model system. The first experiments were designed to test the mediating role of predation in the size response to temperature (TSR).Temperature was combined factorial with 3 types of grazing pressure, i.e. grazing by copepods, by microzooplankton and by nanozooplankton (Chapter 1). The predicted decrease in cell size with warming was confirmed for the majority of the phytoplankton species. Similarly, community mean cell size decreased with increasing temperatures. The results further showed that larger phytoplankton shrink more strongly than small ones, an effect which had not yet been reported in the literature before. Both, the interspecific and the community level size effects of warming were stronger under copepod gazing than under protists grazing. However, there was no reversal of sign under protist grazing, as would have been predicted from the feeding preference of protist grazers for smaller phytoplankton. This indicates that size selective predation has an influence on temperature-size relationship but that predation cannot be the dominant factor. Further factors are needed to explain the shrinking effect of temperature under protists grazing, which alone should be a selective advantage for bigger cell sizes. These results motivated the hypothesis that increasing nutrient stress at higher temperatures could be an important factor. Therefore, I performed a further experiment with a factorial combination of temperature and nutrient (nitrogen) stress (Chapter 2). Nutrient stress was manipulated by the rate of dilution according to the semi-continuous culture principle. However, a nutrient–independent role of temperature could not be assessed from a direct comparison of different treatments, because temperature itself influenced the strength of nutrient stress. Therefore C:N ratios of the biomass were taken as an indicator of the intensity of nutrient stress and the effects of temperature and nutrient stress assessed by multiple regression with temperature and C:N ratios as independent variables. The results indicate that the direct temperature effect is much weaker than nutrient effect. A further analysis of this experiment concentrated on the taxonomic response of phytoplankton to the impact of warming and of nutrient stress (Chapter 3). It confirmed the frequently reported replacement of large by small species under increasing temperature and nutrient stress. It was further asked, whether the response of the different species to the experimental treatment could also be explained by their phylogenetic position (diatoms vs. dinoflagellates). Compared to the effect of cell size, the effect of phylogenetic position turned out to be minor. As demonstrated in Chapter 2 & 3 the nutrient (nitrogen) effect on phytoplankton cell size is dominant over direct temperature effects. However, the question remained, whether the nutrient effect would be similar for other potentially limiting nutrients. Therefore, a further experiment (Chapter 4) was carried out where temperature, nitrogen limitation and phosphorus limitation were employed as independent variables. The results indicated that the effect of N –limitation effect is stronger than the effect of P-limitation but both nutrient effects dominated over direct temperature effects. In conclusion, direct temperature, nutrient, and grazing effects as explanations for temperature dependent size trends are not mutually exclusive, while general results indicate strongly nutrient effect dominance over direct temperature effect. However, the effect of grazing is expected to be less consistent, because phytoplankton groups of different size are grazed by different groups of grazers.

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