How does the complexity observed in nature persist? How does it contribute to the functioning of ecosystems? In addition to natural causes, global changes are currently the main factor impacting the structure and functioning of marine ecosystems. Populations and communities, both benthic and pelagic, animal and plant, integrate into their structure and functioning the effects of these spatial and temporal variations in the environment. Understanding these different processes is essential to predict and mitigate potential degradation of these ecosystems.
The research carried out within DISCOVERY is motivated by the need to better describe, understand and model the responses of populations, communities and ecosystems to changes in environmental conditions and to separate the effect of natural and anthropogenic forcings. It is based on observation and experimentation in the present and the recent past, paleo-ecology, and a reflection on the construction of predictive models.
Research Axis (RA) 1 focuses on describing biodiversity and habitats in various ecosystems in order to be able to decide on their health status. The AR2 seeks to describe and understand how environmental forcings influence the transfer of matter and energy within marine populations and communities across a wide range of spatial and temporal systems and scales. AR3 focuses on spatial and temporal variations in recruitment in a wide range of biological models with different environmental constraints. AR4 combines paleo-ecology and in situ observation to better understand the structure, functioning and productivity of systems in the recent past. The AR5 works to build conceptual models of medium- and long-term changes in coastal ecosystems.
Research Axis 1: Describe and analyze diversity (taxonomic, functional and ecosystem)
The main factors structuring marine ecosystem communities are currently global changes and increasing anthropogenic pressures. These communities, whether benthic or pelagic, integrate into their structures the effects of environmental changes over time, whether natural or anthropogenic. For example, dredging, organic enrichment related to urban development and agricultural activities, aggregate extraction, fishing, climate change or eutrophication (Birchenough et al., 2010, 2010, 2010, 2006; Foden et al., 2009; Frid et al., 1999; Rachor, 1990; Rees et al., 2006).
Today, some communities are widely studied because the species that make up them play an essential functional role in coastal areas: primary benthic production, sediment reworking, stimulation of organic matter degradation and mineralization, recycling of nutrient salts (Ragueneau et al., 2000), nurseries, feeding of higher trophic levels. Others are much less so, in coastal or offshore environments: macro-zooplankton and micro-nekton, although constituting the trophic environment of top predators, are not easily accessible.
In this context, the objective of this RA is twofold:
- First, to develop our ability to describe biodiversity and habitats in a variety of ecosystems (tropical, polar and temperate), whether or not they are free of direct anthropogenic impacts. The main objective is to increase our ability to describe these systems at a given time t (i.e. by integrating short-term variations, ~over a year). Wildlife and botanical tools are considered as tools allowing the construction and use of descriptors of health status and system functioning. Their use and control are nevertheless likely to evolve with the generalization of molecular tools (metabarcoding);
- understand how diversity is structured in response to environmental forcings at the taxonomic (alpha, beta, gamma diversities), functional (life traits, trophic functional group, bioturbation functional group) and ecosystem levels.
In particular, it is necessary to provide the means to answer, based on very specific examples, the following questions:
- How do the structural parameters of communities vary spatially? This question is asked at several scales and is of interest both for the comparison of biocenoses on an ecosystem or regional scale and also on a façade scale. Are there any short-term cycles (tide, nycthemeral, season)?
- What is the origin of the variations observed: internal (natural community dynamics, neutral processes) and/or external (physico-chemical conditions, climate, extreme weather events, anthropogenic pressures)?
- From a biogeographic point of view, do the communities of the different habitats have homogeneous patterns, or do they each respond in a specific way?
- Are the variations in biodiversity of the same habitat type only taxonomic or do these differences underlie significant functional differences?
- What is a community in good ecological status for a given habitat? Is the concept of good ecological status (as defined by the DCSMM) relevant to the functioning of communities and ecosystems?
- How are the spatial distribution and structuring of the different components of a community articulated?
Research Axis 2: Describe and analyze the transfer of matter and energy within populations and communities
Locally, biodiversity (genetic, specific, or functional) and the dynamics of marine communities can be modulated by various mechanisms, including the dispersion of organisms and the environmental pressures acting on them. In AR2, the objective is to describe and understand how environmental pressures – whether biotic, abiotic, naturally occurring or anthropogenic – affect the transfer of matter and energy within marine populations and communities. Recruitment, dispersal and connectivity mechanisms between ecosystems and populations will be more specifically studied in AR3.
In a population and/or community-environment system, environmental pressures are likely to include:
- Physico-chemical variables of water and sediment such as temperature, salinity, pH, O2 and nutrient concentrations, light. The determinism of their variability can have a local (hypoxia etc.) or more global (climate change) dimension.
- Interactions between organisms; this component may include various aspects such as determining factors in access to reproduction, prey-predator, host-parasite relationships, the functional role of biodiversity (complementarity in resource use, facilitation, competition, redundancy, idiosyncrasy…) on material and energy flows…
In this context, this research axis proposes to study both pelagic (phytoplankton, zooplankton and fish) and benthic (microphytobenthos, meiofauna, macroalgae, macrofauna, megafauna) communities, but also to focus on pelago-benthos coupling. These different study objects will be addressed by combining in-situ observation, experimentation and modelling approaches. This research axis also has a transversal character in terms of spatial and temporal scale. It aims to better understand how environmental pressures influence the transfer of matter and energy within organisms, focusing in particular on benthic molluscs and pelagic fish through bioenergy approaches; but also how environmental pressures (“bottom-up” and “top-down”) or biodiversity (specific and functional wealth) influence ecosystem functioning (food webs, material and energy flows: primary, secondary, respiration, nutrient and carbon flows, etc.).
Research Axis 3: Recruitment Determinism under Environmental Constraints
The majority of marine species are bentho-pelagic life cycle organisms, i. e. they have a complex life cycle with a larval phase of more or less long dispersion, during which the mortality rate is much higher (by several orders of magnitude) than in the subsequent life stages. At the end of this dispersal phase, larval abundance is therefore often highly variable both spatially and temporally (Krebs, 1972; May and McLean, 2007), which completely affects recruitment and consequently population renewal.
The complexity of studying population recruitment and dynamics therefore lies in the large and cumulative number of different processes. These processes are generally approached in 4 complementary steps: (1) adult fertility and spawning period (2) larval abundance and survival, (3) larval transport and connectivity, and (4) factors controlling installation or fixation. In addition, these processes are non-linear, interact more or less (factor chain) and operate at different levels of spatial and temporal scale. Without neglecting this complexity, it appears that if recruitment fails several times in a row, the population declines and may disappear, especially if other adverse factors are added, for example, factors related to multiple and varied anthropogenic pressures (e.g. Sale, 1991). In addition, in the particular case of ecosystem engineering species, the repercussions also strongly affect the distribution, community structure, biodiversity and sometimes the entire ecosystem: the hollow oyster, the flat oyster, the crepidula are all examples of our laboratory that perfectly illustrate this problem. However, as soon as conditions become favourable for larval life again, the high individual fertility often characteristic of these species may be sufficient to quickly rebuild new populations (e.g. Hughes, 2000).
In the current context of climate change, this theme of recruitment determinism is once again becoming topical (e.g. Beukema et al., 2009; Feehan et al., 2009; Parmesan, 2006). Although still rare, some studies also tend to show that climate change-related disturbances are far from being as trivial as demonstrating the simple one-dimensional effect of warming or acidification. Some authors do demonstrate the existence of complex cross effects, based for example on match-mismatch theory (Donnelly et al., 2011; Menge et al., 2009; Toupoint et al., 2012).
This AR proposes to further study the determinism of recruitment using the model species of our laboratory (hollow and flat oysters, pectinids, crepidules…) and in different environments (Rade de Brest, Baie de Quiberon, Baie de Saint Brieuc). The specific actions already identified through ongoing projects are as follows:
- Analysis of long-term recruitment variability through indices available in different biological series (growth, fertility, laying dates, larval abundance and lifespan, recruitment index)
- Development of scenarios for the evolution of the life history traits of these model species under different climate change constraints (i.e. warming, recurrence of time regimes)
- Improvement of larval dispersal and connectivity models of different populations of interest.
- Scientific contribution to certain restocking, restoration and environmental, fisheries or aquaculture management operations and assessment of impacts in terms of recruitment dynamics (hollow oysters, flat oysters, scallops, scallops, pearl mules).
Research Axis 4: Paleoecology and medium and long term observation
While it is now accepted that human activities impact to varying degrees on the structure and functioning of the coastal zones of the global ocean, this global vision of human impact conceals many temporal and regional disparities. This RA will be based on medium- and long-term observation and paleoecological reconstruction from biogeochemical records. These two complementary approaches will make it possible to describe temporal variations in the biological characteristics of ecosystems, and to analyse them in terms of natural variation and dynamics on the one hand, and response to disturbances on the other.
The first objective of this RA will be to describe and analyze the diversity patterns of communities at the habitat level over the short and medium term. The IUEM observatory maintains series of faunal-flora observations covering a wide range of sedimentary and rocky coastal habitats throughout the Breton coast. The longest series extend over the last 25 years. Analysis of the taxonomic and functional diversity of communities will provide a better understanding of their functioning, health status and, more generally, explore the relationships that may exist between diversity and stability.
The second objective will be to reconstruct current and past variability in coastal ecosystems through sclerochronological and sclerochemical analysis of bivalve shells and rhodolites. The study of long times will be based on long-lived species (> 100 years) or specimens extracted from ancient deposits (e.g. archaeological shellfish piles). The study of high temporal frequency will use short-lived organisms (e.g. Pectinidae) to characterize processes ranging from day to a few weeks (e.g. phytoplankton dynamics).
Finally, this RA offers the opportunity to link observational and paleo-ecological data. Indeed, if we have series of observations of benthic fauna, environmental data measured in the immediate vicinity of the benthos, at medium and high frequency, are generally non-existent. However, they are necessary to understand the variations in benthic communities. Paleo-ecological reconstructions from organisms collected from study sites should provide a better understanding of the patterns and processes that have occurred over the periods covered by the observation series, and beyond.
Research Axis 5: Horizon 2050: Analyze / evaluate the structure and functioning of ecosystems and develop scenarios of responses to global change
There is a consensus on the need to promote an integrated approach that will aim to provide answers on the state, functioning and vulnerability of coastal ecosystems, build scenarios and provide the necessary elements for sustainable management and adaptation of societies. The awareness of users/actors of the sea and coasts, and more generally by all citizens, to have a “changing sea” in front of them leads to the legitimate formulation of questions such as: What will we fish tomorrow along our coasts? What anticipation for aquaculture? How can we think of a conservation policy when a global movement of marine ecosystems is already visible?
Develop the necessary conceptual basis before any attempt is made, not as an answer but as a scientific project designed to approach the answer to societal questions. The aim is to work on the construction of conceptual models of coastal ecosystem change. This is to allow a better understanding of the concept of an ecosystem approach, one of the objectives of which is the sustainable use of natural resources. The progress towards this conceptualization should lead us to prioritize the mechanisms underlying the change and to propose/construct the scientific issues that we believe must be addressed.