Effects of high CO2 and warming on a Baltic Sea microzooplankton community

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

Trophic interactions modify the temperature dependence of community biomass and ecosystem function

Aquatic ecosystems worldwide continue to experience unprecedented warming and ecological change. Warming increases metabolic rates of animals, plants, and microbes, accelerating their use of energy and materials, their population growth, and interaction rates. At a much larger biological scale, warming accelerates ecosystem-level processes, elevating fluxes of carbon and oxygen between biota and the atmosphere. Although these general effects of temperature at finer and broader biological scales are widely observed, they can lead to contradictory predictions for how warming affects the structure and function of ecological communities at the intermediate scale of biological organization. We experimentally tested the hypothesis that the presence of predators and their associated species interactions modify the temperature dependence of net ecosystem oxygen production and respiration. We tracked a series of independent freshwater ecosystems (370 L) over 9 weeks, and we found that at higher temperatures, cascading effects of predators on zooplankton prey and algae were stronger than at lower temperatures. When grazing was weak or absent, standing phytoplankton biomass declined by 85%–95% (<1-fold) over the temperature gradient (19–30 °C), and by 3-fold when grazers were present and lacked predators. These temperature-dependent species interactions and consequent community biomass shifts occurred without signs of species loss or community collapse, and only modestly affected the temperature dependence of net ecosystem oxygen fluxes. The exponential increases in net ecosystem oxygen production and consumption were relatively insensitive to differences in trophic interactions among ecosystems. Furthermore, monotonic declines in phytoplankton standing stock suggested no threshold effects of warming across systems. We conclude that local changes in community structure, including temperature-dependent trophic cascades, may be compatible with prevailing and predictable effects of temperature on ecosystem functions related to fundamental effects of temperature on metabolism

Freshwater mesocosm experiment 2012 in Vancouver, Canada on effects of increased temperature and food chain length on oxygen fluxes and plankton community structure

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

Mesocosm experiment on warming and acidification effects in 2012: Microzooplankton abundance

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2015) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2016-10-12

Life in the future ocean: effects of end-of-century warming and acidification conditions on microzooplankton

Among the most important stressors for aquatic ecosystems in the future are global warming and ocean acidification. To investigate their direct and indirect effects on a near-natural plankton community, a large-scale approach is needed. Therefore, four mesocosm experiments were conducted within the BIOACID II framework, using plankton communities from the Baltic Sea, North Sea and Atlantic Ocean. The focus of this project was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. At the base of the food web, MZP has a strong impact on phytoplankton standing stocks due to high growth and grazing rates, leading to dietary competition with mesozooplankton. Thus, data on MZP abundance, biomass and taxonomic composition was analysed with emphasis on phytoplankton-MZP-mesozooplankton interactions. In conclusion, warming can be expected to directly affect MZP communities and enhance their growth and grazing pressure. In contrast, the results point at more complex responses of MZP to an increase in pCO2. While direct effects on the MZP community could not be observed, the present data points at predominately indirect effects via e.g. changes in phytoplankton community composition and/or standing stocks. Such indirect alterations might, however, be compensated on an ecosystem level

Geotechnical investigation of the "Titanic" wreck site

Recent marine forensic investigations have largely unravelled the sequence of events concerning the sinking of the R.M.S. Titanic and its descent through nearly 3800 m of water to the seafloor on the morning of 15 April 1912. In particular, the velocity and attitude of the Titanic's bow section (at present lying upright, reasonably intact, and embedded by ~12 m at the prow) as it hit the bottom are of general interest to marine accident investigators. During the 1998 Titanic Science Expedition, a single sediment sample was retrieved from the seafloor (depth 20-30 cm) near the wreck by the deep water submersible, Nautile. Published geological studies suggest the seafloor in this area has remained largely undisturbed since 1912. Geotechnical analysis of the sediment sample reveals that the impact was probably a substantially undrained event and that the characteristic undrained shear strength of the sediment is ~25kPa within 10-16 m below the seafloor. A simple analytical model was used to calculate the embedment of a cuboid with dimensions and mass of the water-filled bow as a function of impact velocity, impact angle, and the undrained shear strength of the sediment. The results indicate the impossibility of a steep angle of impact and fast velocity. The most likely scenario is an impact velocity of 5-10 m/s at a fairly shallow angle (&lt;40°), which corroborates the results of hydrodynamic investigations