The objective of this research action is to characterize the trophic functioning of coastal ecosystems. The habitats of interest are the interconnected ecosystems along the continuum « catchment basin/marshes/intertidal mudflats/semi-closed coastal bays/continental shelf ».
Action 1 : Benthic and planktonic microbial processes : structuring, control and fate
Section 1 : Structuring and control of the benthic microbial biofilm
This section consists in (1) describing and understanding the processes the actors of the benthic microbial biofilm use to interact and (2) how these direct and indirect interactions control in fine the primary production of intertidal mudflats and it dynamics (figure 2).
This work is performed via an experimental approach from the community to the species. The questions to be answered are :
- 1) The seasonal dynamics of the diversity and photosynthetic productivity of microphytobenthic (MPB) : how the primary production is regulated by the main environmental parameters : light and nitrogen ;
- 2) Determinism of the bacterial production and of the prokaryotic diversity associated to MPB : a) what is the role of exopolysaccharides (EPS) from MPB in controlling the bacterial production (collaboration AMES), b) what is the genetic diversity of the prokaryotes associated with MPB in different light conditions ;
- 3) The role of prokaryotes in the nitrogen cycle as an environmental forcing of MPB production : a) to characterize the remineralization activity of the organic matter (including EPS) of prokaryotes, b) to understand the impact of the subsequent availability of nitrogen on the regulation of the MPB photosynthetic activity in different light conditions ;
- 4) The regulation of the biomass, diversity and production of prokaryotes and MPB by the benthic viruses : a) dynamics of the virus diversity as a function of environmental conditions, b) quantification of the biomass loses and of the changes in the prokaryotic and MPB diversity through cellular lysis, c) impact of viral infection on the MPB photosynthetic productivity in different light conditions ;
- 5) Interactions between the microorganisms of the biofilm : identification and characterization of the molecules involved in the positive and negative interactions between isolated microorganisms isolated from the benthic biofilm or from other bacterial biofilm models involved in the bio-corrosion of marine metal structures (collaboration LEMMA) ;
- 6) Regulation of the development of the biofilm by the mudflat foraminifera : how these heterotrophic protists control the biofilm along a sediment gradient.
Section 2 : Benthic-pelagos coupling : the fate of benthic microorganisms
The objective of this section is to deepen the studies of the preceding 4 year project. First, the microbial planktonic network will be deciphered by studying the processes (1) of bacterial mortality due to viruses or protist predation and (2) of predation of the mesozooplankton on the autotrophic and heterotrophic microbial compartments. Second, the benthic-pelagos coupling will be studied following two tracks : (1) hydrodynamics action (collaboration DPL) and (2) biological action, on the fate of the biofilm.
- Point (1) : Resuspension of the biofilm at high tide : how the microorganisms from the benthic biofilm impact on the planktonic trophic web : a) physical characterization of the resuspension with an erodimetry experimental system ; modification of the nutrient quality of the water column ‘enriched’ in benthic organic matter (EPS, DOM, nutrients) (collaboration AMES), b) impact on the relationship viruses/bacteria/planktonic flagellates.
- Point (2) : The role of biotic vectors in the trophic transfer of the microorganisms from the biofilm : a) The shorebirds at low tide: the food preferences of Calidris alpina (Pertuis-Charentais) and of C. semipalmata (French Guyana) : quantification of the ingested biofilm, characterization of the associated biofilm and meiofauna in order to estimate the availability and the abundance of the resource (collaboration AMARE), b) Fish at high tide : ingestion and assimilation rates of the biofilm by the mullet (collaboration AMARE) and characterization of its digestive system microflora (bacteria and archeae). The phylogenic and functional diversity of the microflora will allow to understand how the mullet efficiently feeds on the biofilm.
Action 2 : Integrative approaches and comparison between trophic functionings
The objective of this research action is to study the structure and the functioning of coastal ecosystems in order to decipher the main types of trophic webs and their main properties. The experimental tools will be a mix of the monitoring of the dynamics of the main trophic compartments, the quantification of the trophic processes with approaches coupling experiments and monitoring of markers, and the use of numerical tools to decipher organization shapes and farcing variables (multivariate analysis and inverse analysis). Among the studied ecosystems, a large knowledge has been already acquired during the preceding 4 year project : part of the dataset is currently being analyzed, another part is already modeled and will be exploited through an inter-ecosystem comparison.
Section 1 : Planktonic systems along the continuum marshes-estuaries-semi-enclosed bays-continental shelf
The ecologic functioning of the plankton will be studied with the following objectives (1) to determine what are the types of functioning (based on Legendre et Rassoulzadegan (1995)) for different conditions (microbial loop or network, herbivorous or omnivorous network), and (2) to complete this typology. For the marshes, the objective is to determine the environmental forcings (climate- and human-driven) that define the different types of functioning, the consequences regarding exportation, recycling and trophic efficiency, in order to ultimately search for health indices of a human-impacted habitat (water resources, soil occupation, etc.). The effect of a rare perturbation (like the Xynthia storm of 2010) will be analyzed in the framework of the theories of resistance, resilience and return to an original state/change to a new state. For estuaries, the objective will be to characterize the ecological borders based on the zooplankton compartment and its integration in the trophic web : are the zooplanktonic communities of the Charente, Gironde and Escault estuaries characterized by a gradual (ecocline) or a brutal (ecotone) transition ? For the semi-closed bays (Marennes-Oléron, Arcachon), the Spring phytoplanktonic bloom will be characterized. Ory et al. (2010) distinguished between these two bays during the bloom period based on the microbial dynamics, and especially the virus dynamics (herbivorous vs omnivorous). Nevertheless, the role of the viruses in the matter fluxes, and especially in the properties of the system (recycling, etc.), remain to be elucidated.
These different observations of the link between the web structure and its stability will be discussed in the context of known theories (for instance those linking the recycling, the complexity of trophic paths, or in contrast the specialization of these tracks to a higher resistance). Finally, on the overall of the continuum from the marshes to the continental shelf, a multivariate analysis will evaluate the importance of quantifying the major processes together with monitoring the biomasses of the different compartments, in order to define the types of functioning. Some dataset are already available, for instance the biomass of all planktonic compartments, the primary and bacterial productions, and the trophic fluxes between all compartments for the continental-shelf of the bay of Biscay, the bay of Bizerte and the Charente-Maritime marshes. For other systems (Marennes-Oléron bay and the polyhalines areas of the Charente and Gironde estuaries), the dataset is incomplete and especially the processes need to be measured.
Section 2 : Benthic-planktonic : influence of the resuspension on the trophic functioning and comparison between intertidal flat habitats
Bare mudflats (Brouage): during the preceding 4 year project, the characterization of the benthic trophic web and quantification of the matter fluxes, especially during the resuspension of the biofilm, interaction of the benthic biofilm with the planktonic trophic web, were studied. They are currently being published (program Vasiremi, collaboration AMARE, AMES, DPL) and part of the dataset is being used in an inverse analysis model in order to characterize the properties of the system and to calculate the indices for the Ecological Network Analysis (ENA), with and without resuspension as a function of weather conditions. The main question is: does the resuspension constitute a weak link between two sub-systems which a priori show strong links ? If yes, the biofilm resuspension would then be a stability vector according to the meta-ecosystem scheme as defined by Loreau et al (2003). The collaboration with the team DPL will allow to relate the climate scenario, the influence of the biofilm resuspension and the consequences on the benthic-planktonic coupled trophic web.
During the preceding 4 year project, a large dataset has been acquired on other ecosystems than bare intertidal mudflats, especially the zoostera intertidal sandflats, which is a habitat with a strong environmental interest. The differences of vertical profiles of the sediment erosion/settling speeds and fluxes between bare mudflats and zoostera sandflats will be measured during mesocosms experiments (benthic artifical canals). This approach will allow exploring i) the modification of hydro-sedimentary forcings related to the presence of the zoostera, ii) the modulation of this modification due to the seasonal density of the zoostera biomass. The structuring parameters that explain the functioning differences between these two habitats (bare mudflat: strong microalgal production with an important physical exportation; zoostera sandflat: strong microalgal and detritic biomasses locally used, exportation via biotic vectors) will be deciphered by i) completing the dataset with a refined analysis (GC-IRMS coupling) of the contribution of some food sources (diatoms vs. bacteria, planktonic vs. benthic diatoms), ii) modeling and characterizing the properties of the systems with the ENA indices. The comparison between these indices will be broadened to diverse sites (Mondego estuary, Sylt bay, etc.) where models for coupled benthic-planktonic trophic webs already exist. Here, the objective is to characterize the physical forcings and the influence of diversity loses related to the disappearance of zoostera on the functioning of the ecosystem. This approach will allow to judge about the ecosystem health indices produced by the analysis of food webs and their link with ecological theories relating the structure of food webs and their stability.