Towards a framework for assessment and management of cumulative human impacts on marine food webs.

Effective ecosystem-based management requires understanding ecosystem responses to multiple human threats, rather than focusing on single threats. To understand ecosystem responses to anthropogenic threats holistically, it is necessary to know how threats affect different components within ecosystems and ultimately alter ecosystem functioning. We used a case study of a Mediterranean seagrass (Posidonia oceanica) food web and expert knowledge elicitation in an application of the initial steps of a framework for assessment of cumulative human impacts on food webs. We produced a conceptual seagrass food web model, determined the main trophic relationships, identified the main threats to the food web components, and assessed the components' vulnerability to those threats. Some threats had high (e.g., coastal infrastructure) or low impacts (e.g., agricultural runoff) on all food web components, whereas others (e.g., introduced carnivores) had very different impacts on each component. Partitioning the ecosystem into its components enabled us to identify threats previously overlooked and to reevaluate the importance of threats commonly perceived as major. By incorporating this understanding of system vulnerability with data on changes in the state of each threat (e.g., decreasing domestic pollution and increasing fishing) into a food web model, managers may be better able to estimate and predict cumulative human impacts on ecosystems and to prioritize conservation actions.

Hacia un Marco de Trabajo para la Evaluación y el Manejo de los Impactos Humanos Acumulativos sobre las Redes Alimenticias Marinas Resumen El manejo efectivo con base en los ecosistemas requiere entender la respuesta de los ecosistemas a múltiples amenazas humanas en lugar de enfocarse en amenazas individuales. Para entender holísticamente la respuesta de los ecosistemas a las múltiples amenazas antropogénicas es necesario saber cómo estas amenazas afectan a los diferentes componentes dentro de los ecosistemas y cómo alteran finalmente el funcionamiento de los ecosistemas. Usamos el estudio de caso de la red alimenticia del pasto marino del Mediterráneo (Posidonia oceanica) y la obtención de conocimiento de expertos en una aplicación de los pasos iniciales de un método para la evaluación de los impactos humanos acumulativos sobre las redes alimenticias. Produjimos un modelo de red alimenticia de pastos marinos, determinamos las principales relaciones tróficas, identificamos a las principales amenazas para los componentes de la red y evaluamos la vulnerabilidad de los componentes a esas amenazas. Algunas amenazas tuvieron impactos altos (p. ej.: infraestructura costera) o bajos (p. ej.: escorrentía agrícola) sobre todos los componentes de la red, mientras que otros (p. ej.: carnívoros introducidos) tuvieron impactos muy diferentes sobre cada componente. Partir al ecosistema en sus componentes nos permitió identificar amenazas no vistas previamente y reevaluar la importancia de las amenazas percibidas comúnmente como mayores. Al incorporar este entendimiento de la vulnerabilidad del sistema con datos sobre los cambios en el estado de cada amenaza (p. ej.: disminución de la contaminación doméstica e incremento de la pesca) al modelo de red alimenticia, los manejadores pueden ser capaces de estimar y predecir de mejor manera los impactos humanos acumulativos sobre los ecosistemas y priorizar las acciones de conservación.

A multi-approach inventory of the blue carbon stocks of Posidonia oceanica seagrass meadows: Large scale application in Calvi Bay (Corsica, NW Mediterranean).

In Mediterranean, Posidonia oceanica develops a belowground complex structure ('matte') able to store large amounts of carbon over thousands of years. The inventory of blue carbon stocks requires the coupling of mapping techniques and in situ sediment sampling to assess the size and the variability of these stocks. This study aims to quantify the organic (Corg) and inorganic (Cinorg) carbon stocks in the P. oceanica matte of the Calvi Bay (Corsica) using sub-bottom profiler imagery and biogeochemical analysis of sediment cores. The matte thicknesses map (average ± SD: 2.2 m ± 0.4 m) coupled with marine benthic habitat cartography allows to estimate matte volume at 12 473 352 m3. The cumulative stocks were assessed at 20.2-50.3 kg Corg m-2 and 26.6-58.7 kg Cinorg m-2 within the first meter of depth on matte (3632 ± 486 cal yr BP). The data contributed to estimate the overall carbon stocks at 389 994 t Corg and 615 558 t Cinorg, offering a new insight of the heterogeneity of blue carbon stocks in seagrass meadows. Variability of carbon storage capacity of matte influenced by substrate is discussed.

From mechanical to chemical impact of anchoring in seagrasses: The premises of anthropogenic patch generation in Posidonia oceanica meadows.

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20m depth (42.6µM and 18.8µM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures.