Stressed but Stable: Canopy Loss Decreased Species Synchrony and Metabolic Variability in an Intertidal Hard-Bottom Community

The temporal stability of aggregate community properties depends on the dynamics of the component species. Since species growth can compensate for the decline of other species, synchronous species dynamics can maintain stability (i.e. invariability) in aggregate properties such as community abundance and metabolism. In field experiments we tested the separate and interactive effects of two stressors associated with storminess–loss of a canopy-forming species and mechanical disturbances–on species synchrony and community respiration of intertidal hard-bottom communities on Helgoland Island, NE Atlantic. Treatments consisted of regular removal of the canopy-forming seaweed Fucus serratus and a mechanical disturbance applied once at the onset of the experiment in March 2006. The level of synchrony in species abundances was assessed from estimates of species percentage cover every three months until September 2007. Experiments at two sites consistently showed that canopy loss significantly reduced species synchrony. Mechanical disturbance had neither separate nor interactive effects on species synchrony. Accordingly, in situ measurements of CO2-fluxes showed that canopy loss, but not mechanical disturbances, significantly reduced net primary productivity and temporal variation in community respiration during emersion periods. Our results support the idea that compensatory dynamics may stabilise aggregate properties. They further suggest that the ecological consequences of the loss of a single structurally important species may be stronger than those derived from smaller-scale mechanical disturbances in natural ecosystems

Fonctionnement (métabolisme et réseau trophique) d'un système intertidal rocheux abrité, la zone à Ascophyllum nodosum, relation avec la biodiversité algale et animale

This PhD work aimed at studying the functioning of the Ascophyllum nodosum zone, a macroalgal belt characteristic of the midintertital zone of sheltered rocky shores from the North Atlantic coasts. This study allowed to establish (1) the stocks of A. nodosum available for harvesting along the Brittany coast (France) together with the algal and animal communities; (2) a heterotrophic annual carbon budget for the A. nodosum zone, dominated by the fluxes due to the canopy; (3) the general trophic web architecture of the zone, characterized by the use of most of the sources of organic matter instead of being only based on the main primary producers of the zone; (4) the effect of partially or totally harvesting the canopy on the specific and functional diversity and the metabolism of the zone. It was also possible to compare the matabolism of the zone to that of other neighbouring canopy covered zone and to the metabolism of a zone without canopy. This study allowed better identifying the functional role of the canopy on the regulation of the biodiversity and on the functioning of rocky shores.L'objectif de ce travail de thèse était l'étude du fonctionnement de la zone à Ascophyllum nodosum, une ceinture algale caractéristique de la zone médiolittorale des estrans rocheux abrités des côtes de l'Atlantique Nord. Cette étude a permis d'établir (1) l'état des stocks d'A. nodosum disponibles à la récolte le long des côtes de Bretagne et des communautés algales et animales associées ; (2) un bilan annuel de carbone de la zone à A. nodosum hétérotrophe, dominé par le flux dus à la canopée ; (3) l'architecture générale du réseau trophique de la zone, basée sur l'utilisation de la plupart des sources de matière organique et pas seulement sur celle des principaux producteurs primaires ; (4) l'effet de la récolte partielle ou totale de la canopée sur la diversité spécifique, fonctionnelle et le métabolisme de la zone. Il a également été possible de comparer le métabolisme de la zone à celui d'autres ceintures algales voisines et à une zone dépourvue de canopée. Cette étude a permis de mieux préciser le rôle fonctionnel joué par la canopée sur la régulation de la biodiversité et sur le fonctionnement des estrans rocheux

Effect of canopy removal on a rocky shore community metabolism and structure

International audienceThe effects of the permanent removal of the canopy-forming alga Fucus serratus was studied in terms of both functioning and diversity on a mid–low rocky shore ecosystem in the south-west English Channel (48°N 43.686′, 3°W 59.282′). Ecosystem functioning was examined as net or gross primary productivity (NPP or GPP) and respiration (Resp) measured through CO2 fluxes. Diversity was examined as number and composition of species. Measurements were performed in situ, during emersion times, without altering target assemblages. The experiment was designed with two treatments [control (C) or canopy removed (CR)] and five replicates, and was conducted over an 18-month period (from February 2006 to August 2007) to integrate the seasonal variability. The mean GPP and Resp were severely reduced in CR treatment compared to control throughout the survey. The mean NPP was not affected at first, due to the development of opportunistic green algae, but was drastically reduced after 9 months of experiment. The canopy removal affected neither the number of species nor their distribution among trophic groups, and the algal community was only slightly affected. The abundance and biomass of mobile invertebrates, however, were greatly reduced in the absence of canopy. This indicates an important effect of the dominant alga on the higher trophic levels of the community. At this tidal level, the canopy did not seem to affect the community by dampening the environmental stress but by providing food, habitat or both

Functioning of a canopy-dominated intertidal community during emersion: highly productive but heterotrophic at the annual scale

In order to address lack of data regarding coastal carbon budgets, we estimated the annual metabolic carbon budget of an intertidal rocky reef macroalgal community during emersion. This budget is based on direct in situ measurements during emersion and establishes the seasonal variations of the photosynthetic parameters of such a community. CO2 fluxes were measured hourly to study the response of community gross primary productivity (GPP) to irradiance and the variation of GPP and community respiration (CR) over the emersion period at different times of the year. These were combined together with existing monthly measures of GPP and CR hourly rates to model the variations of these fluxes as a function of irradiance and the tidal cycle throughout an entire calendar year. Daily, monthly and annual values of GPP, CR and net primary productivity (NPP) were calculated with a relatively low sensitivity to any of the parameters used. While GPP fluxes show comparable orders of magnitude to those measured in other systems, higher CR fluxes lead to a heterotrophic system during emersion, both under measured (NPP = −299 gC m−2 year−1) and theoretical irradiances (NPP = −119 gC m−2 year−1). This heterotrophy is directly linked to the light availability, varying according to combined daily, tidal and seasonal cycles, and to temperature at the seasonal scale. Measurements performed in situ at the community scale integrated interactions that are otherwise absent at the individual scale. This gives access to aspects of the functioning that cannot be otherwise identified

The health of mangroves in Mayotte Island (Indian Ocean): Comparison of stakeholders and scientific knowledge

International audienceMayotte Island in the Mozambique Channel holds 735 hectares of mangroves, occupying 29% of the coastline. In 2011, the island became a French overseas department meaning adopting the same legal and social system used in France. This change in status has consequences for the management of the mangrove ecosystems there. The National Forestry Authority has established itself in Mayotte and is drafting a management plan of the island's mangroves for the next 10 years.In this context, we conducted a campaign of interviews with stakeholders influencing the preservation and management of the island's mangroves in order to better understand their knowledge, representations and opinions. Overall, we carried out 25 semi-structured interviews with 12 from the central and local government, 5 from associations, 5 from the tourist sector and 3 from experts. Here, we focus on the knowledge of stakeholders and analyse it by two experts in the geomorphology and the ecology of the mangroves of Mayotte. The knowledge assessed from open-ended questions relates to the biodiversity and the specificity of Mayotte’s mangroves relative to mangroves in other territories; the nature and relative importance of impacts was evaluated from classifying and commenting a set of photographs; knowledge of the health status was assessed with a cognitive map exercise.Results show disparity in stakeholder’s knowledge depending on the topic. For instance, regarding the biodiversity, only some stakeholders think this ecosystem is particularly rich. Regarding the impacts on mangroves, all stakeholders agree that embankments, roads across mangroves and rubbish are very impacting but few are aware of the consequences of other impacts mentioned. This is the case with paths crossing the mangrove seaward: the majority of stakeholders didn’t know that this has been shown to contribute to the erosion of mangroves. These results are useful to know what kind of knowledge should be transmitted to stakeholders

In situ primary productivity measurements on rocky shores

<p>These pictures show the adaptations made to the chambers described in Migné et al 2002 and Golléty et al 2008 to perform in situ measurements of CO2 fluxes on rocky shores as a function of light. In addition to the benthic chamber, a frame holding various light filters allows for the control of the amount of light reaching the chamber.</p> <p>Results from this work contribute to the first carbon budget of a canopy-dominated shore basedon in situ metabolism measurements performed at low tide (work in prep).</p

Local Ecological Knowledge on Mangroves in Mayotte Island (Indian Ocean) and Influencing Factors

International audienceThe majority of studies on local ecological knowledge (LEK) relate to communities orgroups relying on ecosystem(s) for their livelihood. In our case study, Mayotte Island, a Frenchoverseas department, very few people rely on mangrove ecosystem for natural resources but mostof them are attached to it because of leisure activities and beliefs. The questions on mangrove LEKgenerally deal with a single aspect of ecological knowledge of surveyed people and is mixed withother information such as harvesting practices, anthropogenic impacts, and management issues. Theaim of our study is to better understand the level of ecological knowledge of surveyed inhabitantsof Mayotte and to assess whether factors linked to the profile of respondents have an influenceon it. For this purpose, we carried out two main survey campaigns in three villages fringing twostable mangroves of Mayotte: the first one consisted of qualitative interviews and the second one,questionnaires lending quantitative results. Cross tabulations and Chi square tests of independencewere carried out to determine the link between LEK and influencing factors. Results show that someLEK implying localized observation, such as the identification of mangrove trees and the knowledgeof the coastal protection role of the mangrove, are well shared by surveyed people whereas others,such as the number and the name of mangrove tree species, are poorly known. The results alsohighlight the difficulty of questions implying observation at the landscape level and interpretation ofobservation. All the influencing factors selected have a significant influence on, at least, one LEKvariable. The results highlight differences in LEK of villages bordering two nearby mangroves callingfor a local management of these system

Narisome muhoko, a citizen science project to monitor the mangroves of Mayotte Island (France) at the time of the initial stages of institutional management

International audienceResearchers in geography and ecology together with an environmentalist association of Mayotte have recently conceived the citizen science project ‘Narisome muhoko’, meaning ‘We study the mangrove’. This project emerged at the end of an interdisciplinary research project on the resilience of two mangrove social-ecological systems (2018-2021) and at the time of the first Mangrove Management Plan (2019-2028) of the island. For each mangrove of Mayotte, the Plan is expected to be enforced by local authorities and/or associations. These stakeholders are particularly struggling with one of the tasks asking to “monitor the evolution of the ecological state and threats” in the absence of any instruction.The ‘Narisome muhoko’ project has two aims: 1. To operationalize this monitoring action by proposing relevant but easy to implement indicators that could be applied by citizens and/or technical agents; 2. To engage Mahore citizens in the management of the mangrove neighbouring their village using these indicators. It is planned to develop a set of indicators aimed at monitoring the evolution of the mangroves health state in response to different environmental pressures. Our experience allowed us to design a project based on the local ecological knowledge of inhabitants (Longépée et al., 2021) in a context where not all citizens have access to recent technologies or is a French speaker. The project first focuses on two mangroves of Mayotte, already studied by the team. In 2022, a team of trained citizens has started monitoring indicators of the phenology of mangrove trees and the regeneration of trees in several areas of the mangrove. At the same time, other indicators will be devised to assess the influence of waste and/or waste water on the mangrove ecosystem and to follow the practise of mangrove tree cutting. This project is in line with the complex systems thinking, aiming to encourage an adaptive management in the spirit of learning-by-doing (Berkes et al., 2003)

Narisome muhoko, a citizen science project to monitor the mangroves of Mayotte Island (France) at the time of the initial stages of institutional management

International audienceResearchers in geography and ecology together with an environmentalist association of Mayotte have recently conceived the citizen science project ‘Narisome muhoko’, meaning ‘We study the mangrove’. This project emerged at the end of an interdisciplinary research project on the resilience of two mangrove social-ecological systems (2018-2021) and at the time of the first Mangrove Management Plan (2019-2028) of the island. For each mangrove of Mayotte, the Plan is expected to be enforced by local authorities and/or associations. These stakeholders are particularly struggling with one of the tasks asking to “monitor the evolution of the ecological state and threats” in the absence of any instruction.The ‘Narisome muhoko’ project has two aims: 1. To operationalize this monitoring action by proposing relevant but easy to implement indicators that could be applied by citizens and/or technical agents; 2. To engage Mahore citizens in the management of the mangrove neighbouring their village using these indicators. It is planned to develop a set of indicators aimed at monitoring the evolution of the mangroves health state in response to different environmental pressures. Our experience allowed us to design a project based on the local ecological knowledge of inhabitants (Longépée et al., 2021) in a context where not all citizens have access to recent technologies or is a French speaker. The project first focuses on two mangroves of Mayotte, already studied by the team. In 2022, a team of trained citizens has started monitoring indicators of the phenology of mangrove trees and the regeneration of trees in several areas of the mangrove. At the same time, other indicators will be devised to assess the influence of waste and/or waste water on the mangrove ecosystem and to follow the practise of mangrove tree cutting. This project is in line with the complex systems thinking, aiming to encourage an adaptive management in the spirit of learning-by-doing (Berkes et al., 2003)