ALBERT

Description: In order to assess the effects of ocean acidification (OA) on natural plankton communities, a large-scale mesocosm experiment was conducted in Gullmar Fjord (North Sea) from a spring bloom to a mid-summer situation. Here, the emphasis was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. Due to its high specific growth and grazing rates, MZP has a strong impact on phytoplankton standing stocks which can lead to dietary competition between MZP and mesozooplankton. Furthermore, the ability of MZP to act as trophic upgraders for mesozooplankton by buffering nutritional imbalances of algae might gain importance given the expected decrease in algal food quality with OA. We present data on MZP communities (abundance, biomass, taxonomic composition) with emphasis on phytoplankton-MZP-mesozooplankton interactions. Overall, two phytoplankton peaks occurred (1st peak: around day 30, 2nd peak: around day 50). While there was no direct numerical response of MZP biomass to increases in phytoplankton biomass during the 1st peak, a clear numerical response of MZP to the 2nd peak was observed. However, no significant differences in MZP biomass with regard to the different CO2 scenarios occurred. In general, the ciliate community was dominated by small Strombidiids and no clear CO2 impacts on the ciliate community composition could be detected. Additional grazing experiments conducted during the 1st phytoplankton peak showed negative phytoplankton growth rates throughout. MZP grazing could only be detected in some of the low pCO2 treatments, pointing towards complex responses of MZP communities to OA.

Description: Microzooplankton (MZP) plays an important role as intermediary between the microbial loop and higher trophic levels. Due to high growth and grazing rates, MZP has a strong impact on phytoplankton biomass and community composition thus leading to dietary competition with mesozooplankton. Ocean acidification (OA) is anticipated to cause food quality alterations by changing algal stoichiometry in disfavor of grazers and the ability of MZP to buffer nutritional imbalances for higher trophic levels is expected to gain importance. Within the framework of an outdoor mesocosm experiment in the North Sea (BIOACID II), we investigated the effects of elevated CO2 on natural plankton communities. From a spring to a mid-summer situation, MZP abundance, biomass and species composition were analyzed. Grazing experiments provided additional information on the MZP grazing potential. Results show a predominance of small Strombidiids and a lower MZP biomass at high CO2 during the bloom. This finding is contradictory to the hypothesis that MZP growth will increase at high CO2 due to enhanced phytoplankton growth, thus pointing at more complex responses of MZP to ocean acidification.

Description: This study elucidates the interspecific interactions between competing unicellular predators in an intraguild predation system. The organisms studied were two microzooplankton (MZP) predators competing for the phototrophic dinoflagellate prey Scrippsiella trochoidea. Since the smaller dinoflagellate predator Gyrodinium dominans was also potential prey for the larger predator, the tintinnid ciliate Favella ehrenbergii, the experimental system included the probability of intraguild predation (IGP). The development of the three species was studied in set-ups containing either one of the two predators or both together with their prey. The IG predator F. ehrenbergii grew at a mean rate of 0.77 d-1 independent of the presence of the IG prey G. dominans. High grazing of the IG predator on the smaller IG prey was detected in treatments containing only the two predators. However, when all three species were present, the IG prey displayed significantly higher growth rates (0.42 d-1) compared to treatments containing only the IG prey as predator (0.32 d-1). The results of further experiments allowed the exclusion of mechanical or chemical signals induced by the IG predator being responsible for the observed increase in growth rate of IG prey. Live observations revealed that the IG predator rejected a significant proportion of its S. trochoidea catch after initial uptake. This behavior led to an immobilization of around 26% of the caught cells. We tested if this prey immobilization by the IG predator facilitated prey uptake by the IG prey and thus could be potentially responsible for the higher growth rates of the IG prey. Indeed, the smaller predator selected positively for immobilized prey and reacted with higher grazing and growth rates. Consequently, the IG prey benefitted from this commensalism between IG predator and IG prey and the strength of this pattern predominated IGP in our model system. As both predators co-occur in the same environment their feeding relationship could increase exploitation efficiency of common mobile prey items. Furthermore, such commensalism potentially opens a loophole for a stable coexistence of MZP predators despite their competition.

Description: Within the BIOACID framework, the effects of elevated CO2 levels on microzooplankton were investigated. Apart from its importance as intermediary between the microbial loop and the traditional food web, its high specific growth and feeding rates makes microzooplankton an important competitor for larger mesozooplankton. The ability of microzooplankton to buffer nutritional imbalances of phytoplankton for higher trophic levels will gain even more importance with the anticipated ocean acidification, as this is expected to render the phytoplankton into low quality food by changing the carbon-to-nutrient ratio in disfavour of the grazers. The microzooplankton community was analysed both during indoor and outdoor mesocosm experiments. At the KOSMOS 2013 Gullmar Fjord field campaign, different CO2 levels were applied to the eutrophic North Sea water throughout the entire spring bloom period. During the indoor mesocosm studies at the GEOMAR, different temperature and CO2 scenarios were simulated. The BIOACID Autumn 2012 and Summer 2013 experiments allowed closer insights into the dynamics and seasonality pattern of Baltic Sea plankton communities from the Kiel Bight. Within all three studies, microzooplankton species composition and abundance, biomass and size classes were investigated. Apart from regular samplings which aimed at providing insights into the succession of the natural plankton communities as a whole, modified Landry-Hassett dilution series were performed to assess microzooplankton grazing activities. By choosing the most abundant copepod species as top predator, these small-scale experiments allowed the differentiation between micro- and mesozooplankton grazing impact and to evaluate food preferences of microzooplankton.

Description: Plankton organisms play a crucial role in the functioning of aquatic ecosystems as they serve as a basis in terms of primary production and the transfer of matter and energy from one trophic level to the next. Thus, the majority of benthic and pelagic flux processes are fuelled by energy derived from the plankton. From an anthropocentric point of view, top-predators (e.g. fish, birds, mammals) at the uppermost trophic levels might be considered as more relevant compared to those organisms at the base of aquatic food webs. However, from an ecosystems’ perspective, lower food web dynamics are pivotal, as the baseline is inevitably and constantly subject to changes as a result from external drivers. Accordingly, changes at the basis are directly transferred up the food web. In order to fully understand processes in the plankton, to enable accurate food web models and to generate more realistic energy budgets, it is essential to consider external drivers that act directly or indirectly at the base of food webs. The present habilitation provides a comprehensive study on how external drivers affect lower food web dynamics by including both, bottom-up and top-down control processes. CHAPTER 1 addresses the role of micro-zooplankton (MZP) as a trophic link between phytoplankton and mesozooplankton. It shows that MZP can be considered as a key component of the zooplankton throughout the year and points at its relevance particularly during diatom blooms in spring. MZP was proven to play a substantial role as phytoplankton grazers showing an even higher grazing efficiency when compared to copepods. Further MZP was shown to play a crucial role in modulating phytoplankton blooms and in providing a high quality food source for copepods. Commensalism was proven to be an efficient strategy when different MZP species compete for resources. Climate-change is considered as a major threat of aquatic ecosystems. The degree to which plankton communities will be affected strongly depends on the responses of plankton organisms to climate change related external drivers (e.g. global warming and ocean acidification). CHAPTER 2 addresses the responses of micro- and mesozooplankton communities to changes in external drivers under future greenhouse conditions. While the timing of phytoplankton was shown to be only slightly affected by warming, the growth of MZP and copepod nauplii showed a strong acceleration by temperature. Further, higher grazing rates and a strong dietary overlap between MZP and copepods could be observed in relation to warming as well as a strong suppression of MZP by overwintering copepod densities. Analyses across all experiments within a series of indoor mesocosm experiments demonstrated some general, warming-induced trends e.g. reduced time-lags between phytoplankton and MZP, earlier bloom timings of almost all functional groups in the plankton, changes in peak biomass and size structure of phytoplankton as well as enhanced grazing. Further, changes in the light climate proved to affect the timing, peak biomass and cell size of phytoplankton. The consideration of indirect, trophic cascading effects with warming resulted in e.g. enhanced copepod grazing on medium-sized phytoplankton. Investigations on the vulnerability of MZP to ocean acidification demonstrated a high tolerance of Arctic MZP communities to OA as neither direct nor indirect effects on MZP composition and diversity was observed. Nutrient-limitations and the consequences associated with nutritional imbalances in the plankton are presented in CHAPTER 3. Stable isotopes (SI) were used to trace nutrient limitations and relate these to the trophic position of consumers. A strong variation in SI was found between different algal species and in relation to different nutrient regimes. Further, the SI enrichment of consumers was significantly affected by their diets’ nutrient composition. Further, a strong contribution of ’new’ nitrogen to the pelagic food web in a meso-oligotrophic system was found which was associated with the utilization of aerosol nitrate by unicellular cyanobacteria. This signal was directly transferred to higher trophic thus providing new insights into the food webs and nitrogen budgets of sub-tropical and tropical oceans. A series of food chain experiments using 3- to 4-trophic levels to investigate the effects of nutrient limitations on different trophic levels showed a rather weak homeostatic ability of herbivores with regard to their nutrient content, a finding which enabled new views in the field of ecological stoichiometry. Further, it was shown that nutrient-limitations of primary producers propagate up the food web. Nutrient-limited diets affected herbivores by reducing growth and reproduction rates of consumers thus resulting in lower densities and lower condition of the latter. When a trophic intermediary (e.g. a protist grazer) was added to the food chain, nutritional imbalances were buffered thus improving the food quality for higher trophic levels. The studies presented here show that changes occurring at the base of the food web are directly transferred to higher trophic levels. Thus, changes in environmental parameters (nutrients, temperature and CO2) will have far-reaching consequences for aquatic ecosystems. The complexity of trophic interactions shows that the predictability of implications associated to e.g. eutrophication and climate change is still limited. Therefore, it is inevitable to increase efforts to combine experimental and modeling approaches thus enabling the simulation of complex interactions and setting existing and future data sets into a broader context.

Description: Young larval stages of many organisms represent bottlenecks in the life-history of many species. The high mortality commonly observed in, for example, decapod larvae has often been linked to poor nutrition, with most studies focussing on food quantity. Here, we focus instead on the effects of quality and have investigated its effects on the nutritional condition of lobster larvae. We established a tri-trophic food chain consisting of the cryptophyte Rhodomonas salina, the calanoid copepod Acartia tonsa and larvae of the European lobster Homarus gammarus. In a set of experiments, we manipulated the C:N:P stoichiometry of the primary producers, and accordingly those of the primary consumer. In a first experiment, R. salina was grown under N- and P-limitation and the nutrient content of the algae was manipulated by addition of the limiting nutrient to create a food quality gradient. In a second experiment, the effect on lobster larvae of long- and short-term exposure to food of varying quality during ontogenetic development was investigated. The condition of the lobster larvae was negatively affected even by subtle N- and P-nutrient limitations of the algae. Furthermore, younger lobster larvae were more vulnerable to nutrient limitation than older ones, suggesting an ontogenetic shift in the capacity of lobster larvae to cope with low quality food. The results presented here might have substantial consequences for the survival of lobster larvae in the field, as, in the light of future climate change and re-oligotrophication of the North Sea, lobster larvae might face marked changes in temperature and nutrient conditions, thus significantly altering their condition and growth.