Food webs and diversity under global and regional change

We identify key relationships between ecosystem structure and function and improve our understanding of the response of coastal food webs to global and regional change.

Objectives and challenges

Over-exploitation and climate change cause substantial modifications in coastal ecosystems: biodiversity changes, and food web structure and energy flow patterns change accordingly. Combined with habitat alterations, such shifts not only threaten marine ecosystem stability and recovery potential, but also will lead to significant and potentially irreversible changes in coastal ecosystem characteristics and services. The main objectives of this WP are to generate fundamental understanding of the structure and function of coastal ecosystems, to assess coastal biodiversity, and reveal both its dependence and its impact on ecosystem functioning. In pursuing these objectives, we face the following challenges:

• To develop and establish the tools and expertise for assessing diversity both within and among species, and to link this to functional biodiversity;
• To understand the implications of changing coastal habitats for individual key species, biodiversity and specific ecosystem processes;
• To determine the role of food web interactions and the implications of changing players, for the energy and matter flow through coastal and shelf systems.

Implementation

Facing the challenges identified above requires a suite of complementary approaches. We will study communities, populations, individuals and processes within individuals both in an analytical and an experimental manner in the laboratory and in the field. We will link observed processes and patterns with environmental conditions and manipulations. We will assimilate our results into ecosystem models, which will serve in hypothesis testing, in putting adaptive properties into the context of coastal food-webs (link to WP2), in long-term hind-casting (link to WP3) which will also be integrated into an operational monitoring and simulation system (link to WP4).

On the basis of our challenges we have established four organisational foci:

Changing diversity
The shifts in species composition in many coastal ecosystems have been considerable in recent years, and even within species, intrinsic adaptations to changing conditions might be substantial. Biodiversity plays a pivotal role for ecosystem function in a changing environment. Increasing species richness can enhance ecosystem productivity and stability; genetic diversity within species might have equivalent effects. Unfortunately, our current knowledge on such intra-specific variation and their consequence for ecosystem functioning, as well as the identity and diversity, particularly of organisms, which are too small or delicate to study using "classic" taxonomical methods, is still very limited. Hence, we intend to shed light on both diversity aspects, studying the "hidden" intra-specific diversity of known species (e.g. microplankton), as well as the "cryptic" diversity of organisms in the pico- and nano-meter size range. By means of molecular biological tools we will analyze all phylogenetic levels from communities to populations, from species to ecotypes. Because species diversity is directly linked to functional diversity, genomic and expression (EST) studies will be included for species that might be particularly sensitive indicators of environmental change. We will study the consequences of the introduction/arrival as well as the disappearance of species on the overall diversity and functioning of coastal systems. These alterations in the species pool will presumably have cascading effects in coastal marine food webs from prey-predator relationships down to microbial communities. Examples are bacterial communities associated with new organisms in the system, the invasive Sargasso weed and its associated species assemblage, the effects of the neozoan ctenophore Mnemiopsis leidyi on plankton diversity and hidden changes in microalgal communities.

Habitat structuring species interactions
Coastal systems comprise a variety of habitats. Local physical, chemical and biological properties determine which consortia of organisms live in particular habitats. Moreover, invaders such as the pacific oyster can change habitat characteristics substantially, and, consequently, modify species composition. For selected habitats, we will study the effects of variation within and among habitats on species composition and interactions. For example, if extreme weather events become more frequent, we expect a shift from macroalgal domination to filter feeders such as mussels, oysters or barnacles in exposed habitats. Studies on the persistence and success of organisms found in different habitats will complement work on adaptation and will provide substantial insight into the inter-dependency between diversity and ecosystem function. Interactions between habitats, for example via exchange of biomass by means of drifting benthic organisms or their meroplanktonic larval stages, as well as the active migration of mobile organisms will be studied. Methods for a continuous assessment of the distributions of highly mobile predatory species, such as cod and the competitive invader red mullet in these habitats will be developed. By artificially manipulating the habitats and the composition of habitat-forming species (oysters, mussels and settling space) experiments will elucidate the way how individual (key-) organisms modify their environment as well as the associated community on a short- and long term scale. Studies will include the comparison of oyster dominated versus mussel dominated habitats, the role of seagrass meadows as a fish refuge, the effects of invaders on the fish community, and macroalgae as habitats for epiphytic species.

Foodweb structure & shifts: trophic interactions in a changing food web
Dramatic changes in species composition have occurred in many shelf seas. In the North Sea, for example, loss or reduction of dominant species (e.g., cod or blue mussel) as well as the appearance of invaders (e.g., red mullet, Pacific oyster) are prominent harbingers of change. Less visible modifications, such as changes in the seasonal presence of certain fish species, can be of equal or even greater significance for ecosystem function. We will define characteristic properties of the food web which will facilitate the identification of control mechanisms (e.g. imbalances in nutrient stoichiometry or presence of toxins) and their changes. Such structural shifts will be linked to energy flow patterns as well as to exchange rates and pathways of specific substances. In the pelagic compartment, emphasis will be placed on higher trophic levels, in particular, on the shifting balance between gelatinous plankton and fish and on small sized predators such as ciliates and heterotrophic dinoflagellates. In benthic habitats we will focus on the impact of species gain and loss on overall food web dynamics. We will analyse spatio-temporal dynamics of both pelagic and benthic communities with respect to seasonality in abiotic parameters and in hydrodynamics. Experimental approaches will simulate the loss or gain of particular species and will tackle trophic relations, behaviour and competition of key species. An existing energy flow model, based on network analysis, of the coastal North Sea that describes functional processes and properties of benthic and pelagic sub-systems will be refined and further developed to incorporate effects of seasonal and spatial heterogeneity. Particularly significant tasks are the mechanisms of key species mediated shifts within functional groups, the incorporation of metabolic scaling (size dependence of metabolic processes) into size-based models of plankton interactions, and the significance of cell size and stoichiometry for food preferences. Our over-arching task is to provide modular components for future ecosystem models, which will be developed.

Organismal physiology and success
Links between ecosystem structure and function will be studied by incorporating organism information of key species into our conceptual framework. This will enable us to consolidate our understanding of the individual species response and extrapolate to changes on the community level. Key species will be used in integrated studies on life-history reactions in response to the changing environment by combining laboratory and field experiments. Comparisons between organisms from different climatic regions will be carried out thus linking WP1 and WP2 tasks. Planned and ongoing research in this topic includes studies on dietary adaptations of brown shrimp (a species central to the North Sea fishing industry) to different resources, the external (temperature, feeding conditions) and internal (ontogenic) factors that influence the toxicity of selected jellyfish species, and the effects of changes in toxicity on the pathways of energy and matter through the food web. Other studies will target the effects of changed food sources and changing salinity or temperature on crustaceans, ciliates and dinoflagellates, and effects of different food sources on fish and sea bird behaviour.

Milestones

• Inventory of ongoing and recent change in biodiversity in significant coastal habitats (year 2)
• Models of adaptive shifts in coastal food webs (year 2)
• Analysis of the significant biotic factors that determine the transfer of energy and matter through a food chain (year 3)
• Trophic models of significant coastal habitats and simulation of future scenarios (year 4)

Integration and deliverables

This work package will generate knowledge on structures and processes pivotal to the maintenance and functioning of different habitats and associated species in changing coastal ecosystems. We will provide models that incorporate the aspects of metabolic scaling, variable stoichiometry and size-based trophic interactions. Given data on shifts in plankton diversity, these models can be applied to investigate sensitivities of food webs and their vulnerability to anthropogenic and natural changes. We will exemplify how the adaptive capacity of key organisms is related to their habitats and potential change, leading to the following deliverables:

• Trophic level evaluation of key species and corresponding models of adaptive shifts in food web functional properties
• Analysis of changes in biodiversity in coastal regions, and its impact on ecosystem function
• Models coupling food web dynamics and key organisms response to the physico-chemical environment
• Sensitivity atlas of change in key habitats, with focus on habitat-related biodiversity