BIOACID II – Consortium 2: Responses of benthic assemblages to interactive stress

Lead Proponent: Prof. Dr. Martin Wahl (GEOMAR, Kiel)

The geographical distribution of species are the result of ecological and evolutionary (adaptation, niche differentiation), geological (e.g. distributional barriers) and oceanographic or climatic settings (e.g. salinity, temperature, light, and their fluctuations) (e.g. Kearney et al. 2010). The overlap of species’ distributional areas in a given region together with habitat conditions (depth, substratum, biotic interactions) determines the composition of local benthic communities. Finally, species identities and relative abundances in an assemblage determine its ecosystem services, their fluctuations in time and the community’s sensitivity to stress and disturbance (Wahl 2009 and articles therein). When the abiotic setting is shifting in the course of climate change, it can be expected that the composition and functioning of local communities change as well (e.g. Harley et al. 2006). During the coming decades many environmental variables will shift simultaneously both at the global scale (e.g. temperature, pH) and at the regional scale (e.g. salinity, stratification, eutrophication, oxygen concentration). In addition, the rate of re-structuring of benthic communities is further accelerated by other features of Global Change, such as bioinvasions, overfishing, pollution and coastal constructions (e.g. IPCC 2007). Numerous studies have shown the impact of single stressors on single species, but the combined effect of multiple variables on a multi-species assemblage – the natural scenario – is virtually uninvestigated (e.g. Wahl et al. 2011). The extent to which community diversity and biotic interactions modulate the impact is vividly debated (e.g. Mooney et al. 2002, Raffaelli 2006).

The most wide-spread and perhaps most severe effects of climate change at a global level are expected from a rise in temperature and a decrease of pH (e.g. Kroeker et al. 2010). Regionally, however, other shifts like the increasingly severe and/or frequent occurrence of eutrophication, hyposalinity and hypoxia may equal or surpass the importance of the former factors (e.g. BACC author team 2010, Schernewski et al, 2011).

The lack of investigations in near natural scenarios (multiple stressors, multi-species communities) creates the unfortunate situation that we know that our marine communities will change, but not in which direction, to which extent and with which functional consequences. Consequently, the motivation for this consortiums’ project was to improve our understanding for the ongoing and future re-structuring and re-functioning of an essential part of the marine ecosystem, macrophyte communities. This project will focus on the impact of multi-factorial climate change scenarios on the structure and functioning of macrophyte communities of the Baltic and the North Sea, composed of a metaorganism (macrophyte plus epibionts) and its consumers. Focal species of these communities are the sea grass Zostera marina/Z. noltii, living on soft bottom, and the brown macroalga Fucus vesiculosus, living on hard substrata. Seagrass and wrack communities are widely distributed in both seas, and play an important ecological and economic role as primary producers, carbon sinks, water purifiers, stabilizers of sediments, energy sources for microbes and herbivores, and providers of substratum and structure of epibionts and juvenile fishes (e.g. Mangi et al. 2011).

General objectives

We intend to study the direct impact of single and combined stresses on the performance of the macrophytes (tolerance width, physiological plasticity, survival, productivity, reproduction), as well as on similar metrics of their associated fauna and flora: epibiotic communities (bacteria, diatoms, macroepibionts) and consumers (snails, crustaceans). We will establish the capacity of the macroorganisms to adapt to the future climate scenarios by phenotypic plasticity and selective mortality, and we will assess how the microbial and macrobial communities re-structure under the applied environmental pressure. Finally, we will assess how the genetic and/or taxonomic re- structuring at the population and the community levels affect major services of the assemblage such as oxygen production, carbon fixation, productivity, uptake of nutrients and more. Thus, the project will span biogeochemical, genetic, physiological, biological, ecological and economic aspects, and will include a wide range of phylogenetic groups such as bacteria, diatoms, bryozoa, barnacles, polychaetes, crustaceans, various macroalgae and sea grasses. Obviously, this challenging scope requires a wide array of techniques and expertise. This critical mass will be achieved by clustering all sub-projects and their PhD students around a series of core experiments run in the benthocosm facilities of Kiel (Baltic Sea) and Sylt (North Sea).

We will ask the following main research questions:

How will environmental stress affect

  • the physiology of macrophytes?
  • the genetic composition of the macrophyte populations and their sensitivity to further 
  • the interaction among macrophytes and their epibionts and consumers?
  • the composition and functioning of epibiotic bacterial communities?
  • the composition and functioning of microepiphytic communities?
  • the fluxes of energy and matter across the macrophyte communities?
  • the ecological and economic“ value” of the macrophyte communities?

Research approaches

The entire research will concentrate around a series of core experiments with macrophyte communities run in the Kiel and Sylt benthocosms. The Kiel infrastructure consists of 4000 litres experimental units, sub-dividable into two subunits each of which are independent of each other in every aspect except temperature. A similar facility will be constructed at the Sylt site in summer 2012. Important environmental variables such as temperature, pH, oxygen and salinity are continuously logged and automatically controlled. Additional variables such as light, pCO2, nutrients, DOC, POC, Chl a, alkalinity, DIC will be “manually” monitored and controlled (see WP 6).

We will run several consecutive experiments with Zostera and Fucus, each of 3-4 months duration. This time span allows for physiological and genetic (selective mortality of recruits) responses of macroorganisms, re-structuring of microbial communities, and a re-functioning in the sense of biotic shifts and biogeochemical signals of all.
The consecutive experiments will permit to investigate the single and interactive impacts of a variety of potential stresses at the species and community level. As acidification will be part of all experiments, the project will also allow assessing the relative importance of this factor as compared to other aspects of Global Change (warming, hyposalinity, hypoxia, eutrophication). The various components of the communities passing though different stages of their life cycle and different physiological states in the course of a year, we will repeat the first experiment (acidification x warming) in all four seasons.

Importantly, we will take into account the natural fluctuations of all environmental variables and superimpose our treatment factors onto these. We, thus, work with delta-treatments in all seasons, i.e. ambient Kiel Fjord temperature plus the predicted 3-5°C of warming by the end of the century (e.g. Schernewski et al. 2011) or the ambient Baltic or North Sea pH (or pCO2) minus (plus) the predicted change for the year 2100 (e.g. BACC author team, 2010). A moderate flow-through with unfiltered sea water (ca one re-fill per week) will “connect” the benthocosm units to the natural fluctuations in nutrients, salinity, or plankton (including potential recruits) composition in the adjacent sea. In summary, the “non-stress” treatments will always be similar to the in situ conditions, while the stress treatments consist of the delta (Global Change) values for 2100 added to the in situ conditions.

Seagrass communities will be investigated primarily in the Sylt benthocosms, while Fucus communities are investigated primarily in the Kiel benthocosms. This reflects the expertise of the local groups and the relative regional importance of the two macrophyte groups. At the same time, at both locations, comparative experiments will be run with primary target group of the other location, i.e. Fucus in Sylt and Zostera in Kiel, allowing to compare stress impact among populations. The experimental approach of this consortium will, thus, cover the two large groups of macrophytes (algae and seagrasses) and two major habitat types (hard bottom and sediments).

The Benthocosm Core Experiments (CE):

Four major CEs will be run.

  • (1) In a first phase, we will orthogonally cross the two factors acidification and warming and study their impact on the Baltic and North Sea Fucus community subsequently in all four seasons. This design allows disentangling single and interactive stress effects and the influence of season. On Sylt, in parallel tanks the same treatment combinations will be applied to North Sea Zostera communities. Replication is 3.
  • (2) Then we submit Fucus communities to a 3-factorial treatment of acidification, warming and eutrophication both on Sylt and in Kiel using the respective populations. This experiment will run in a “climate simulation mode”, i.e. we do not orthogonally cross the 3 factors (for lack of experimental units) but rather run single benthocosm units under a future climate setting with regard to these three factors. This approach allows evaluating the impact of complex global change in two seasons (winter, summer) with a replication of 6.
  • (3) In a third CE (intercalated into CE2) the same experiment as CE2 will be run with Zostera communities in spring and autumn (Kiel and Sylt) and summer (Sylt).
  • (4) The last experiment is similar to CE2, but the third factor is the predicted decrease in salinity. It runs in a season (spring) not covered by CE2 and lasts a month longer. Target communities are Fucus (Kiel) and Zostera (Sylt). Replication is 6. Response data for all WPs will be taken regularly (see WP descriptions). Two weeks before the end of a CE a pulse of unfavourable conditions (hypoxia, heat wave) will be applied to assess how communities stressed by the simulated climate differ in their sensitivity to an additional disturbance relative to unstressed communities.

The PhDs of the six WPs will closely cooperate on these core experiments and take all their samples and data from the same system. This ensures a maximal comparability. Additional small experiments for questions complementary to the core experiment will be run in the lab or the climate chambers of the institutes. To keep the workload and costs realistic, WPs 1 and 6 will do comparative work on Zostera and Fucus in Kiel and Sylt, WP 4 will put major emphasis on Fucus (Kiel) while not disregarding Zostera, whereas WPs 2,3, and 5 will limit their efforts to the Kiel CEs on Fucus.

Thus, WP 4 provides the information how much and in which regard the focal species (Fucus, Zostera) are affected by the treatment regime(s). The interaction of early life-stage stress sensitivity to these scenarios and intraspecific genetic diversity is treated by WP 5. The influence of these physiological responses of the substratum organisms, the direct treatment impact on the biofilms and the interaction among biofilm components will be disentangled by a close cooperation among WPs 2, 3 and 4. The results of these WPs will explain a major portion of the interaction shifts studied by WP 1, which in turn will provide insight how the impact of abiotic stresses are modulated by biotic shifts. At the community level, finally, the shifting fluxes among components will be investigated by WP 6, thus quantifying the re-functioning with regard to the ecosystem services. The added value of this close and complementary cooperation on a focal meta-organism in common core experiments is a complete picture of community level responses to single and interactive stresses.

All data obtained by the WPs will be fed into two independent modelling approaches. (1) A mechanistic community model will simulate the influence of all abiotic and biotic interactions of Fucus vesiculosus and the impact of the interacting factors on its performance (growth, maximal depth distribution). (2) An Ecological Network Analysis will synthesize the results for each of the treated community modules used as experimental units. This allows a comparison of system behaviour of stressed and unstressed systems and indicates changes by altering specific system indices as well as the cycling structure of material and energy flow, particularly between primary and secondary producers. It also allows comparing the provision of ecosystem services for each of the analysed systems and characterises their global system properties.