ALBERT

Description: Concerns about increasing atmospheric CO2 concentrations and global warming have initiated studies on the consequences of multiple-stressor interactions on marine organisms and ecosystems. We present a fully-crossed factorial mesocosm study and assess how warming and acidification affect the abundance, body size, and fatty acid composition of copepods as a measure of nutritional quality. The experimental set-up allowed us to determine whether the effects of warming and acidification act additively, synergistically, or antagonistically on the abundance, body size, and fatty acid content of copepods, a major group of lower level consumers in marine food webs. Copepodite (developmental stages 1-5) and nauplii abundance were antagonistically affected by warming and acidification. Higher temperature decreased copepodite and nauplii abundance, while acidification partially compensated for the temperature effect. The abundance of adult copepods was negatively affected by warming. The prosome length of copepods was significantly reduced by warming, and the interaction of warming and CO2 antagonistically affected prosome length. Fatty acid composition was also significantly affected by warming. The content of saturated fatty acids increased, and the ratios of the polyunsaturated essential fatty acids docosahexaenoic- (DHA) and arachidonic acid (ARA) to total fatty acid content increased with higher temperatures. Additionally, here was a significant additive interaction effect of both parameters on arachidonic acid. Our results indicate that in a future ocean scenario, acidification might partially counteract some observed effects of increased temperature on zooplankton, while adding to others. These may be results of a fertilizing effect on phytoplankton as a copepod food source. In summary, copepod populations will be more strongly affected by warming rather than by acidifying oceans, but ocean acidification effects can modify some temperature impacts

Description: Marine copepods provide the major food-web link between primary producers and higher trophic levels, and their feeding ecology is of acute interest in light of global change impacts on food-web functioning. Recently, quantitative polymerase chain reaction (qPCR) protocols have been developed, which can complement classic diet quantification methods, such as stable isotope or fatty acid analyses tools. Here, we present first results of feeding experiments assessing sex- and stage-specific food intake by the ubiquitous calanoid copepod Acartia tonsa by 18S targeted qPCR and microscopic grazing assessment. In triplicated mixed-diet feeding treatments, three suitable A. tonsa diets, the cryptophyte Rhodomonas balthica, the haptophyte Isochrysis galbana, and the diatom Thalassiosira weissflogii, were offered in equal biomass proportions under constant conditions. Prey uptake substantially varied between different algal species, as did the extent of sex- and stage-specificity of prey uptake. Male adult copepods had higher R. balthica gut contents than females, and nauplii contained more of this prey source than copepodites or adult copepods in mixed treatments. A trend towards higher amounts of ingested T. weissflogii in adult females than in males and in nauplii than in other stages was detected. Genetic gut content quantifications indicated low feeding on I. galbana, and no consistent sex- or stage-specific differences of I. galbana content in A. tonsa. Our results highlight diet-specific feeding differences between Acartia life stages and sexes, which can have implications on food-web dynamics and specific nutrient transfer to higher trophic levels in copepod populations of varying age composition under changing environmental parameters, such as rising temperatures and increasing ocean acidification.

Description: Global warming and ocean acidification are among the most important stressors for aquatic ecosystems in the future. To investigate their direct and indirect effects on a near-natural plankton community, a multiple-stressor approach is needed. Hence, we set up mesocosms in a full-factorial design to study the effects of both warming and high CO2 on a Baltic Sea autumn plankton community, concentrating on the impacts on microzooplankton (MZP). MZP abundance, biomass, and species composition were analysed over the course of the experiment. We observed that warming led to a reduced time-lag between the phytoplankton bloom and an MZP biomass maximum. MZP showed a significantly higher growth rate and an earlier biomass peak in the warm treatments while the biomass maximum was not affected. Increased pCO2 did not result in any significant effects on MZP biomass, growth rate, or species composition irrespective of the temperature, nor did we observe any significant interactions between CO2 and temperature. We attribute this to the high tolerance of this estuarine plankton community to fluctuations in pCO2, often resulting in CO2 concentrations higher than the predicted end-of-century concentration for open oceans. In contrast, warming can be expected to directly affect MZP and strengthen its coupling with phytoplankton by enhancing its grazing pressure.

Description: Over broad thermal gradients, the effect of temperature on aerobic respiration and photosynthesis rates explains variation in community structure and function. Yet for local communities, temperature dependent trophic interactions may dominate effects of warming. We tested the hypothesis that food chain length modifies the temperature-dependence of ecosystem fluxes and community structure. In a multi-generation aquatic food web experiment, increasing temperature strengthened a trophic cascade, altering the effect of temperature on estimated mass-corrected ecosystem fluxes. Compared to consumer-free and 3-level food chains, grazer-algae (2-level) food chains responded most strongly to the temperature gradient. Temperature altered community structure, shifting species composition and reducing zooplankton density and body size. Still, food chain length did not alter the temperature dependence of net ecosystem fluxes. We conclude that locally, food chain length interacts with temperature to modify community structure, but only temperature, not food chain length influenced net ecosystem fluxes.

Description: Near-surface seawater containing the natural mix of phytoplankton, protozoa and bacteria was pumped from Kiel Fjord, Baltic Sea, on 31 October 2016 and distributed through a hose system into 9 mesocosms (volume 1500 L, 1.5m in diameter and 1m high) which were evenly placed in three walk-in climate chambers. Light phase: the mesocosms were exposed to three different constant irradiance levels at a 9:15 hrs light:dark cycle. Light was supplied by computer controlled (GHL, Prometheus) LED light units. Each light treatment was triplicated and evenly distributed among the climate chambers. The three different light intensities were assumed to represent low light stress (low: 31 µmol quanta m-2 s-1), moderately limiting to saturating conditions (medium: 72 µmol quanta m-2 s-1), and saturating to slightly inhibiting conditions (high: 139 µmol quanta m-2 s-1). The experimental temperature was 11°C, representing in situ conditions. Fluorescence measurements were done every day in the lab at the same time using a subsample from the mesocosms. The different light treatments were applied until fluorescence measurements indicated stationary phase of growth (day 7 for the medium and high light treatments, day 11 for the low light treatment). An automatic propeller was used to mix the mesocosms gently, representing natural water waving to avoid sedimentation and achieve homogeneity of the plankton community. Nutrient phase: after reaching stationary phase, the light pre-conditioned communities were split into two nutrient treatments. One of them received a saturating nutrient pulse and the other was not modified as it served as control. For the new treatments 120 L-1 volume floating plastic bags were used (0.5m in diameter and 0.6m high), set in pairs (one nutrient and one control treatment) inside the bigger mesocosms and under each light unit (maximum distance from light: 1m). The plastic bags containing the new treatments were filled gradually using a 10 L bucket to achieve the same initial plankton assemblage. The content of the bags was stirred manually twice per day. Light intensity for the second experimental phase was set at the moderately limiting to saturating conditions (medium light: 72 µmol quanta m-2 s-1) for all treatments. The split into nutrient treatments and controls took place at day 11 for low light and at day 8 for medium light and high light, respectively. The nutrient pulse targeted final concentrations of 2 µmol L-1 phosphate, 32 µmol L-1 nitrate, and 32 µmol L-1 silicate in each mesocosm. Growth was followed until the nutrient augmented treatments had reached the stationary phase. Microscopic phytoplankton counts were performed according to Utermöhl's (1958) sedimentation method. Geometric proxies according to Hillebrand et al. (1999) were used to calculate cell volumes (V; in µm3). Samples for dissolved inorganic nutrients were taken together with the phytoplankton samples. The samples were filtered through pre-washed (10% HCl) cellulose acetate filters and frozen immediately until analysis. Samples for POC, PON and POP measurements were filtered onto prewashed (in 5-10% HCl) and precombusted (6 h, 550 °C) Whatman GF/F filters. POC and PON was determined by gas chromatography (Sharp, 1974) in an elemental analyser (Thermo Flash 2001, Thermo Fisher Scientific Inc., Schwerte, Germany). POP was converted to orthophosphate and determined colorimetrically (Hansen and Koroleff, 2007).

Description: Increasing seawater temperature and CO2 concentrations both are expected to increase coastal phytoplankton biomass and carbon to nutrient ratios in nutrient limited seasonally stratified summer conditions. This is because temperature enhances phytoplankton growth while grazing is suggested to be reduced during such bottom-up controlled situations. In addition, enhanced CO2 concentrations potentially favor phytoplankton species, that otherwise depend on costly carbon concentrating mechanisms (CCM). The trophic consequences for consumers under such conditions, however, remain little understood. We set out to experimentally explore the combined effects of increasing temperature and CO2 concentration for phytoplankton biomass and stoichiometry and the consequences for trophic transfer (here for copepods) on a natural nutrient limited Baltic Sea summer plankton community. The results show, that warming effects were translated to the next trophic level by switching the system from a bottom-up controlled to a mainly top-down controlled one. This was reflected in significantly down-grazed phytoplankton and increased zooplankton abundance in the warm temperature treatment (22.5°C). Additionally, at low temperature (16.5°C) rising CO2 concentrations significantly increased phytoplankton biomass. The latter effect however, was due to direct negative impact of CO2 on copepod nauplii which released phytoplankton from grazing in the cold but not in the warm treatments. Our results suggest that future seawater warming has the potential to switch trophic relations between phytoplankton and their grazers under nutrient limited conditions with the consequence of potentially disguising CO2 effects on coastal phytoplankton biomass.