1. ECOLOGICAL STOICHIOMETRY OF FOOD WEBS

 

In the last decades, the biogeochemistry of major mineral nutrients has changed dramatically on global and regional spatial scales. Marine coastal ecosystems are strongly affected by these changes, which essentially create a new abiotic environment for primary production and trophic transfer. Recent biogeochemical changes of the North Sea include a strong increase in the available N:P ratio, shifting the ecosystem towards P-limitation. Shifts in the identity of primarily limiting nutrients alter the chemical food quality of primary producers and the regeneration of nutrients by consumers and the microbial food web. Ecological stoichiometry (ES) has proven to be a suitable framework to predict consequences of changed nutrient availability and ratios for trophic interactions. These consequences of altered biogeochemistry will on top be strongly affected by global change induced altered temperature and atmospheric CO2 concentrations. Higher temperatures will increase respiration, but will affect photosynthesis only under light saturation; thereby global warming will increase the heterotroph:autotroph ratio in pelagic food webs. Elevated CO2 concentrations will lead to higher C-at lower P-availability, which will increase phytoplankton C:P and thus decrease its nutritional value for primary consumers. Such synergistic effects have hardly ever been investigated together so far and not at all in a realistic experimental framework.

Current Projects

“ELSER” EcoLogical Stoichiometry in aquatic food webs (2009-2012)

funded by: DFG Hi 848/7-1 (in Kooperation mit dem AWI/BAH, DFG Ma4501/1-1)

The ecological stoichiometry theory has been successful in enhancing our understanding of trophic interactions between consumer and prey species. Consumer and prey dynamics depend on the nutrient composition of the prey relative to the nutrient demand of the consumer. However, most experiments on this topic used a single consumer species and very simple prey communities. Therefore, little is known about the validity of stoichiometric constraints on trophic interactions in more natural food webs. This project seeks to enhance our understanding of nutritional constraints in marine pelagic food webs by testing the consumer’s ability to select high quality food, by addressing the importance of food quality in multispecies zooplankton-phytoplankton interactions and by investigating the propagation of different phytoplankton quality via herbivores to predatory zooplankton and fish larvae. These aspects will be experimentally analysed in microcosms and mesocosms and fundamentally enhance our ability to predict the consequences of anthropogenically altered biogeochemistry in coastal waters on trophic transfer in ecosystems.

Involved scientists: Helmut Hillebrand, N.N.

Collaborators: Arne Malzahn, Maarten Boersma (Alfred Wegener Institute, Helgoland)

Former projects

2005-2008        The role of the microbial loop in benthic communities – combining food web theory and ecological stoichiometry” (DFG: Hi 848 4-1; 4-2)

2005-2008        Comparing trophic structure across ecosystems, National Center of Environmental Analysis & Synthesis, NCEAS, Santa Barbara, Ca, USA, together with Jonathan B. Shurin and Daniel Gruner

2001-2002        Structure and stoichiometry of microbenthic communities Marie Curie Individual Fellowship (MCFI-CT-2000-00912)

1999-2001        Simultaneous effects of top-down and bottom-up regulating forces on periphyton
 

2. COASTAL ECOSYSTEMS AND CLIMATE CHANGE

 

Observed changes in global climate will have dramatic consequences for coastal ecosystems. These include direct effects of increased temperature and CO2 concentrations on, e.g., seasonality, trohic mismatch as well as indirect effects mediated by changed currents or resuspension. These indirect and direct effects will change biotic communties. The potential consequences of global warming on the biota in terrestrial and aquatic ecosystems represent one of the predominant questions of recent ecological research.
The response of species composition and biological diversity to warming warrants special attention for two major reasons. On one hand, global warming is expected to become a major driver of future species extinctions and diversity loss (Millenium Ecosystem Assessment). On the other hand, diversity itself is supposed to work as an insurance against changing conditions, i.e., more diverse communities are supposed to show faster recovery due to higher response diversity. Thus, the link between global change and biodiversity is important from two perspectives: how does global change drive diversity and how does diversity affect ecosystem responses to global change.

Current Projects

Long- and short-term effects of climate variability and physical forcing on the diversity of aquatic organisms (2005-2009)

funded by: DFG Hi 848/3-1; 3-2 (Priority program Aquashift)

Climate variability and climate shifts have been shown to affect the temporal dynamics and abundance of populations in many ecosystems. Few climate studies, however, have dealt with entire communities and the question how biodiversity responds to changes in abiotic forcing. Here we propose the continuation of a unique study analysing the effects of both long-term shifts and short-term variability of abiotic forcing on the diversity of aquatic communities. This final phase of the project will synthesize the existing knowledge in the form of meta-analyses of experimental and observational data. To achieve this goal, we will use three pathways. First, we will address direct effects of temperature on diversity by creating a database with temperature manipulation experiments, testing for an overall effect of temperature increase on assemblage diversity. Second, we will amend existing databases summarizing the effects of various factors (nutrient supply, consumer presence) on diversity by adding temperature data. These data will be used to test whether temperature indirectly alters diversity by affecting coexistence mechanisms. Third, we will use observational time series data for phytoplankton and zooplankton, which allow diversity assessments. We will use generalized linear models to disentangle relationships between diversity (richness, evenness and species turnover) and temperature from other factors (total productivity, top down effects). By using both experimental and observational data we will finally be able to estimate whether diversity responses to experimental temperature manipulations actually reflect patterns in situ.

Former projects

2005-2008         Comparing trophic structure across ecosystems, National Center of Environmental Analysis & Synthesis, NCEAS, Santa Barbara, Ca, USA, together with Jonathan B. Shurin and Daniel Gruner

2009                    Effects of global change on carbon sequestration and food web structure across ecosystems. Symposium funded by Deutsche Forschungsgemeinschaft (DFG Hi848/10-1)

3. CONSEQUENCES OF DIVERSITY FOR ECOSYSTEM FUNCTIONING

 

The impact of diversity on ecosystem processes (productivity) and properties (stability) is one of the most debated questions in ecology right now. Experimental tests of this impact are mainly confined to terrestrial environments. Special characteristics of aquatic ecosystems such as fairly open propagule and material exchange, steeper physical and chemical gradients, often more rapid biological processes (such as short generation times of primary producers), and higher phylogenetic diversity (especially in marine systems) limit the ability to transfer these conclusions. In turn, however, these characteristics provide excellent opportunities for hypothesis testing through experimental manipulations.

Current Projects

Relevance of functional diversity in microbial food webs (2008-2011)

funded by: DFG Mo 1931/1-1

Biodiversity studies are a major focus in ecological research due to the increasing species loss through anthropogenic impacts and environmental changes. Multitrophic effects, however, have only been analysed recently. In this project, the relevance of functional diversity will be investigated in microbial food webs. Consumer diversity and composition (generalists and specialists) will be manipulated in microbial microcosms in the laboratory and in mesocosms in a small lake (Accumer See, Schortens, Niedersachsen) to investigate the effect on prey diversity and composition (artificially assembled prey in the laboratory, natural prey community in lake mesocosms). This research question has neither been investigated experimentally so far with complex natural communities nor with differently specialized consumers and will therefore considerably enhance our understanding of multitrophic biodiversity effects. Furthermore, diversity will be analysed with rDNA based molecular methods, allowing to determine the genetic diversity in addition to the morphological diversity of the prey community.

Involved: Stefanie Moorthi, Joanna Filip, Helmut Hillebrand 

Collaborations: Prof. Dr. U.G. Berninger und Dr. Steve Wickham, Universität Salzburg (A), Prof. Dr. David A. Caron, University of Southern California, USA

Resilience and diversity in aquatic metacommunities: Effects of dispersal and the spatial scale of disturbance (2009-2012)

funded by: DFG Hi 848/8-1

Recent years have seen stringent advances in the analysis of spatial ecological dynamics embedded in the concept of metacommunities, in which species coexistence is mediated by local interactions and regional dispersal. However, we lack information on how metacommunity dynamics alter responses of diversity and resilience to disturbance and environmental fluctuations, especially if these disturbances differ in their spatial extent. To test this idea, we will establish experimental metacommunities consisting of connected microcosms inhabited by phytoplankton (and their consumers). The metacommunities differ in their dispersal rates (which affect regional and local diversity) and their coexistence mechanism (patch dynamics versus source-sink dynamics). In these communities, we will alter mortality by removing biomass in local or regional extent and measure the recovery of community composition and important ecosystem functions (primary production, nutrient retention) after this disturbance. We except resilience to be a function of dispersal rate, local richness and regional richness, but we expect different relative importance of these factors at different spatial scales of mortality. In a second set of experiments, we will address the importance of spatial coexistence for the stability of ecosystem processes over time under undisturbed but fluctuating environmental conditions. The results from this project will allow addressing the importance of spatial insurance mechanisms in fragmented landscapes in cases of habitat destruction, which are considered to be major causes of biodiversity loss in future environments.

Involved: Helmut Hillebrand, Nils Gülzow

Collaboration: Dr. Birte Matthiessen, IfM-GEOMAR Kiel; Dr. Bradley J. Cardinale, UCSB

 

Effects of plant diversity on the ecological stoichiometry of ecosystem functioning (2010-2013)

funded by DFG  Hi848/11-1 within the research group Jena Experiment

This subproject will analyze the relationship between plant diversity and the ecological stoichiometry of multiple ecosystem processes and trophic interactions. Ecological stoichiometry is a theoretical framework allowing the prediction of competitive and trophic interactions based on the balance between resource supply and resource needs of interacting species. The balance of multiple elements (C, N, P, and K) in the soil, plants and their consumers will be investigated across the gradient of plant diversity and species composition in the JenaExperiment. In the main experiment, we will track the stoichiometry of plant and insect chemical composition along the experimental gradients of species richness and functional diversity. We will test (i) whether plant diversity enhances the uptake and thus content of multiple elements on the community level, and (ii) whether differential stoichiometry will change consumption patterns by insect herbivores. In the trait-based experiment, we will address stoichiometry of elements on a species-specific basis, asking whether plant diversity alters the resource content and ratios of plant species depending on their traits and the traits of their co-occurring species. We will test whether (iii) higher coverage of the resource use traits increases resource use efficiency for multiple elements, and (iv) whether plant stoichiometry responds plastically to the competitive environment. Thereby we will address the question how species diversity affects ecosystem multifunctionality, i.e., whether more species are required to maintain the processing of different elements. Moreover, we will analyze whether functional turnover occurs, i.e., whether different species carry function in resource use efficiency and trophic transfer across time.

Involved: Helmut Hillebrand, Robet Ptacnik

Collaboration: Prof. Dr. M. Kleyer (Univ. Oldenbrug), Prof. Elizabeth T. Borer (University of Minnesota) und Prof. Stan Harpole (Iowa State University)

Former projects

2004-2005       Diversity of marine invertebrates enhances the functioning of the ecosystem” Deutscher Akademischer Austauschdienst (DAAD: D04/04378)

2003-2006       Experimental test of biodiversity effects on ecosystem functioning in marine environments (DFG: Hi 848 1-1/-2)

 

4. REGULATION OF DIVERSITY IN AQUATIC HABITATS

 

 

Concerning the mechanisms regulating diversity in local assemblages, a variety of factors increasing, decreasing and maintaining diversity have been identified. These mechanisms operate on different scales, from short-term ecological interactions to regional fa

ctors such as history of climate, evolution and migration. Conceptually, the assembly of a local community can be visualised as species passing through a series of filters. These represent historical (e.g. dispersal, speciation) and ecological (e.g. competition, predation, disturbance, abiotic environmental factors) constraints on the arrival and survival of organisms at a certain locality. Several contributions tried to disentangle the importance of regional and local constraints on local species richness. Generally, these studies agree on an important influence of both, regional and local factors, but the relative importance of these factors is still uncertain since they act on different temporal and spatial scales. We will use experimental and analytical tools to address local and regional cosntraints of species coexistence. A common feature of diversity studies is that most of them deal with the local number of species (or another diversity measure), i.e. the standing alpha diversity at a certain time. However, this focus may actually miss more important aspects of anthropogenic changes in diversity. There is reason to hypothesize that before the local number of species is affected by a gradual change in, e.g., temperature, we might see shifts in local species dominance and temporal turnover. Especially the temporal sequence of species occurrence (i.e. beta-diversity over time) is prone to be accelerated by increasing temperature, which might go unnoticed as the observed diversity at any point in time will not change significantly. Species-time relationships (STR) have been introduced as one analytical tool to describe the increase of species richness over cumulative sampling time. The slope of the STR, which is an indicator of temporal turnover, has been shown to depend on the spatial scale of sampling, the organism group and ecosystem, as well as the sample and cumulative diversity.

Current Projects

Temporal and spatial turnover in phytoplankton communities (2008-2011)

funded by start-up money

This projects combines observational and experimetnal approaches to test the factors regulating spatial and temporal turnover of plankton spcies composition. We will use a field survey and experimental metacommunities to address changes in species composition across spatial, temporal and environmental gradients.

Involved: Helmut Hillebrand, Sandra Meier

Collaboration: Dr. Janne Soininen, University of Helsinki (FI)

 

Former projects

2002-2003        Quantification of global patterns of biodiversity Vetenskapsrådet (VR: 621-2002-215)
 

5. Mixotrophy in marine ecosystems

Current Projects

Role of light for consumer-dynamics and nutrient turnover in microbial food webs of the marine pelagial (2010-2013)

funded by: DFG Pt 5/ 3-1

The bulk of marine bacterial and primary production is consumed by unicellular grazers (‘protists’). Recent research revealed that pigmented grazers (‘mixotrophs’) represent an important consumer group of bacteria and phytoplankton. Especially in oligotrophic waters, mixotrophs play a pivotal role as consumers of the pico-plankton. These mixotrophs combine autotroph photosynthesis with ingestion of particulate food items. This dual mode of nutrition provides clear advantages under limitation of either energy or essential nutrients. However, the combination of multiple strategies exerts additional costs for building dual cell machineries compared to specialized competitors. Given the tremendous importance of mixotrophs for marine ecosystems, it is mandatory to develop a mechanistic model predicting the relative importance of mixotrophs in the environment, as well as their implications for food webs. Based on preliminary results and existing data, it is postulated that light intensity and grazing pressure represent major factors controlling the relative importance of mixotrophy in microbial food webs. We will study the competition between mixotrophic and heterotrophic grazers systematically in artificial food webs, applying gradients of light and loss rate. The collected data will eventually be the basis for a stoichiometric model of the microbial food web, taking the specific role of mixotrophy into account.

Involved: Robert Ptacnik, N.N.

Collaboration: Prof. Tom Andersen, Univ. of Oslo, Norway, and Prof. Herwig Stibor, IUEM, Technopôle Brest-Iroise, France

LightDynaMix - Dynamics of microbial food webs and mixotrophy in a light gradient (2010)

funded by: EU Mesoaqua

Pigmented grazers (‘mixotrophs’) contribute significantly to microbial grazing, especially in oligotrophic lakes and oceans. Recent studies show that the bulk of bacterivory in the ‘blue ocean’ can in fact be attributed to pigmented grazers. The type of grazing (mixotrophy vs. heterotrophy) is expected to have major implications on the resulting remineralisation rates of nutrients in the microbial food web. Heterotrophic protists are limited by the amount of energy in their prey, and function as net-remineralizers in the food web (Azam et al. 1983). Conversely, mixotrophs may cover their energy demand by light and utilize all nutrients ingested with their prey for synthesis of new biomass. However, the competitive edge of mixotrophs over their heterotrophic competitors is bound to the availability of light. We thus hypothesize that light is a major agent in determining the relative importance of mixotrophic vs. heterotrophic grazing, and hence the relative nutrient regeneration rates within microbial food webs. We will test this hypothesis in a mesocosm experiment at the HCMR facility, where the natural plankton community will be exposed to a light gradient. The experimental design includes tracer studies for measurement of nutrient regeneration rates, molecular probes for the identification of the picoplankton and estimation of grazing rates, and specific measurement of photosynthetic yield. Finally, by relating zooplankton growth to seston stoichiometry, we also aim at testing the L:N hypothesis in this ultra-oligotrophic environment.

Involved: Robert Ptacnik, Stefanie Moorthi, Radka Ptacnikova

Collaboration: Paraskevi Pitta at Hellenic Centre for Marine Research (HCMR), Maren Striebel (WasserCluster Lunz, A)