IMPACT

MODELING OF COASTAL ANTHROPOGENIC PRESSURES AND VULNERABILITY THRESHOLDS

The IMPACT network is operated since 2013 by Andromède océanologie with the support of the Rhone Mediterranean and Corsica Water Agency. Human activities induce significant pressures on the marine environment, and in particular on coastal ecosystems. The spatial models available here aim to improve knowledge of the links between human pressures and the status of coastal ecosystems. By spatially correlating these data, it becomes possible to carry out a large-scale model (French Mediterranean) of the extent and interaction of anthropogenic pressures on the marine environment.

This model could be used in various fields :

(1) Decision support by targeting areas with high environmental stakes

(2) Basis for the development of ecological indicators

(3) Predictive tool for the management of the coastal zone.

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In 2015 ten anthropogenic pressures were considered for their origin and intensity : urbanization, coastal developments, boat mooring, population, coastal erosion/aggradation, agriculture, industrial effluents, aquaculture, fisheries and urban discharges.

In 2018, an update of the pressure maps was completed.

Thirteen anthropogenic pressures are now considered for their origin and intensity : urbanization/population, coastal developments, small-scale recreational anchoring, large-scale anchoring, coastal erosion/aggradation, agriculture, industrial effluents, aquaculture, non-trawl professional fishing, seaside tourism, maritime traffic, pollution by waterways, and urban discharges. Their spatialization is constructed according to a grid of 20 x 20 m cells by applying a negative exponential curve according to the distance (horizontal and depth) to the source varying from 1 (origin) to 0 (no more pressure).

Spatial data of pressures and eleven different marine habitats (DONIA expert project) are then assembled and cumulative impact scores (sum of pressures and their impacts according to the dominant habitat of the pixel) are calculated per pixel according to the methodology of Micheli et al (2013) and Halpern et al (2008).

In order to better understand the relative impact of the different pressures on the decline of Posidonia meadows, the data are transformed to a 50 x 50 m grid (the initial grid and a 100 x 100 m cell grid giving worse results). The rate of decline (=% of dead matte, null values excluded from the analysis) is then modeled as a function of pressure and depth using the “Random Forests” method (Breiman, 2001 ; Richard Cutler et al., 2007). A threshold effect (non-linear relationship) is sought for each pressure and the distance to the threshold is calculated in each pixel and then scaled -1 to 1 (threshold = 0). A combination of the layers of the different threshold distances according to the pressures is constructed and visualized in four equal categories (-1 to 0.5 ; 0.5 to 0 ; 0 to 0.5 and 0.5 to 1).

Anchorage pressure was updated : two maps of anchorage pressure were produced : one map for small pleasure craft (vessels up to 24 m in length), and one map for the anchorage of large units. The pressures from coastal population and urbanization were combined into a single pressure “urbanization and population”. Three new pressures were added to the project, namely beach tourism, marine traffic and river pollution.

The methodology used in this project for the calculation of pressure maps, cumulative pressures and cumulative impacts is detailed in the scientific publication : Fine-Scale Cartography of Human Impacts along French Mediterranean Coasts: A Relevant Map for the Management of Marine Ecosystems (Holon et al, 2015).

Indeed, the cumulative impacts layer corresponds in terms of methodology to the Ic layer of the Holon et al 2015 publication :

–> Where “Di” is the value of an anthropogenic pressure “i” ;