BIOACID II – Consortium 1: Pelagic ecosystems under ocean Acidification: Ecological, Biogeochemical and Evolutionary Responses
Lead Proponent: Prof. Dr. Ulrich Sommer (GEOMAR, Kiel)
The importance of the ocean’s pelagic system is beyond doubt. It covers 70 per cent of the Earth’s surface and contributes ca. 50 per cent to global primary productivity. The human interest in the pelagic system rests on the following ecosystem services:
- Biological carbon pump: The production of organic matter in the sunlit surface ocean and its export to depth is the main process sequestering atmospheric CO2 in the deep ocean.
- Fisheries: Pelagic production forms the nutritional basis of pelagic fisheries.
- Recreational value of the sea: A malfunctioning of the pelagic ecosystem may lead to harmful algal blooms (HABs) which might seriously impair the recreational value of the sea.
Until now, ample evidence has been found for the impact of ocean acidification on individual planktonic species. As hypothesized before, the calcifying groups being the most sensitive ones. In addition, there is scattered evidence that acidification insensitive phytoplankton species might respond with biochemical changes reducing their nutritional value for zooplankton and thus impairing the upward transmission of matter and energy in the food web. From the perspective of ecosystem services this is an amplification of effects. It is not known, to which extent this effect can be compensated by alternative trophic pathways, e.g. by feeding of mesozooplankton on heterotrophic protists (“trophic upgrading”; Klein Breteler 1999) which could dampen the effect. Overall, we do not yet know to which extent the interactions and feed-back mechanism within the pelagic ecosystem act as amplifiers or shock absorbers of acidification effects.
While the effects of acidification and other kinds of environmental change (e.g. climate warming, eutrophication) have mostly been studied in isolation from each other, there is a lack of studies how these factors interact. In theory, additive, synergistic (more than additive) and antagonistic (less than additive, i.e. compensatory) interactions of effects are possible, but a priori knowledge is not possible without studying the effects in a crossed, factorial design. A high priority should be given to the factors CO2 and warming, because both are inevitably linked by the greenhouse effect. Recent research in evolutionary ecology suggests that evolutionary adaptation of species could be a potential dampening mechanism (Hoffmann and Sgro Nature 2011). It could permit the persistence of species in spite of environmental conditions moving outside the present niche of a species. When dealing with short-generation time microbes, i.e. unicellular phytoplankton, the ‘gold-standard’ of demonstrating evolvability are experimental evolution experiments (Collins and Bell 2004), yet most of these have so far been conducted with single species subjected to a single and constant selection factor. It is unknown, whether such rapid evolution will also occur in natural ecosystems, where interactions with other species impose additional, and temporally variable selection factors beyond the change in the physico-chemical environment. Preliminary modelling results suggest that diversity at the species level may prevent adaptive evolution (De Mazancourt et al. 2008) but empirical data are entirely lacking.
Recent research in evolutionary ecology suggests that evolutionary adaptation of species could be a potential dampening mechanism. It could permit the persistence of species in spite of environmental conditions moving outside the present niche of a species. However, rapid evolution at ecological time scales, i.e. the time scale of species replacements has so far been shown only in simplified experimental systems with single species subjects to a single and constant selection factor. It is unknown, whether such rapid evolution will also occur in natural ecosystems, where interactions with other species impose additional, and temporally variable selection factors beyond the change in the physico-chemical environment.
In summary, we need an ecosystem perspective and a combination of the factors CO2 and warming in BIOACID II.
In order to predict the impact of ocean acidification and its combination with warming on the ecosystem services provided by the pelagic system, we have to understand how it will affect the distribution of energy and matter fixed by primary production into the different competing channels of the pelagic system: grazing food chain, microbial loop, export production and production of refractory DOM. We have to know to which extent ocean acidification and its combination with warming will change the incidence, magnitude and toxicity of HABs. All these changes might be dampened by evolutionary adaptation which has to be understood before reliable predictions are possible.
We prose the following overarching research hypotheses:
- Ocean acidification and the combination of ocean acidification with warming will reduce carbon sequestration via changes in vertical flux and/or partitioning between dissolved and particulate organic matter.
- Ocean acidification and the combination of ocean acidification with warming will increase the role of the microbial loop relative to the grazing food chain.
- Ocean acidification and the combination of ocean acidification with warming will enhance microbial turnover of organic matter and enhance the natural release of CO2 from the ocean.
- Ocean acidification and the combination of ocean acidification with warming will change the production of bio-resistant DOM.
- Ocean acidification and the combination of ocean acidification with warming will deteriorate the quality of microbial food (share of poorly edible algae, stoichiometry, fatty acids) for mesozooplankton, in particular copepods, and thus deteriorate the nutritional base of pelagic fish.
- Ocean acidification and the combination of ocean acidification with warming will increase the incidence, magnitude and toxicity of harmful algal blooms.
- The response at higher aggregation levels (trophic levels, entire ecosystem) will be less pronounced than the response of single species found in BIOACID I.
- Short-lived species will be able to track environmental change via adaptive evolution and prevent local extinction, while long-lived species will not.
- Evolutionary adaptation embedded in experimental ecosystems may modulate all of the above short term changes. Adaptation will however be slower in an ecosystem context than in single species-single factor experiments.
We are aware, that several of the predicted responses might depend on specific features of regional ecosystems, e.g. presence/absence of calcifying plankton, N2-fixers or toxic dinoflagellates. Therefore, we propose to perform our experimental studies in three different ecosystems, the subtropical North Atlantic Ocean near Gran Canaria representing the oligotrophic ocean, the North Sea as representing of coastal, eutrophic seas and the Baltic Sea, representing coastal, eutrophic seas with a reduced alkalinity and prone to nuisance blooms of N2-fixing cyanobacteria.
As a consequence of our ecosystem-oriented approach, the core activities of the consortium will consist of mesocosm experiments where the response of the plankton part of the pelagic food web and the biogeochemical processes can be studied in response to the experimental manipulation.
We will use two types of mesocosm systems: The large systems (KOSMOS: Kiel Offshore Mesocosms for Future Ocean Simulation) have a volume of up to 80 cubic metres and will be deployed in situ off Gran Canaria (subtropical Atlantic Ocean) and in Gullmar Fjord near Kristineberg, Sweden (North Sea water).
The small systems (Kiel indoor mesocosms, Fig. 1.2) have a volume of 1.4 m3 and are installed in four temperature-controlled rooms, thus permitting a factorial combination of CO2 and temperature change. They will be used for two Baltic Sea experiments (one in the cyanobacteria-season, one in a diatom-dominated season). Each ecological and biogeochemical WP of the consortium will participate in two or more of the mesocosm experiments, while the evolutionary WPs will participate in at least one mesocosm experiment and will in parallel conduct single species experiments to compare the evolutionary velocity of their target species in isolation with the evolutionary velocity embedded in a stressed ecosystem.
The KOSMOS experiment at Gullmar Fjord (January to July 2013) will comprise nine mesocosms with pCO2 levels ranging from ambient to ca. 1800 μatm. The unusually long duration will permit for a succession of phytoplankton blooms and is particularly attractive for the evolutionary WPs. The plankton community at Gullmar Fjord contains several dinoflagellates with the potential to form harmful blooms.
The KOSMOS experiment off Gran Canaria (tentatively scheduled for spring 2014) will comprise nine mesocosms with pCO2 levels ranging from ambient to ca. 1800 μatm. It will be conducted in a season when nitrogen fixing cyanobacteria and calcifying phytoplankton (coccolithophores) contribute substantially to the phytoplankton community.
The Baltic Sea fall experiment (September/October 2012) will comprise 12 mesocosms combining three pCO2 levels of 390, 750 and 1200 μatm and two temperature levels, each replicated twice. The experiment falls into a season when diatoms and dinoflagellates dominate the phytoplankton community.
The Baltic Sea summer experiment (July/August 2013) will comprise 12 mesocosms combining three CO2 levels of 390, 750 and 1200 μatm and two temperature levels, each replicated twice. The experiment falls into a season, when diazotrophic cyanobacteria can for nuisance blooms.
The evolutionary WPs will use bacteria, phytoplankton and a copepod as model organisms, thus having a gradient in generation times (from <1 d to several weeks) and from asexually reproducing unicellular species to metazoans with obligate sexuality.