Benthic inputs as predictors of seagrass (Posidonia oceanica) fish farm-induced decline

3 Fish farms represent a growing source of disturbance to shallow benthic ecosystems like 4 seagrass meadows. Despite some existing insights on the mechanisms underlying decline, efficient tools 5 to quantitatively predict the response of benthic communities to fish farm effluents have not yet been 6 developed. We explored relationships of fish farm organic and nutrient input rates to the sediments with 7 population dynamics of the key seagrass species (Posidonia oceanica) in deep meadows growing 8 around four Mediterranean Sea bream and Sea bass fish farms. We performed 2 annual shoot censuses 9 on permanent plots at increasing distance from cages. Before each census we measured sedimentation 10 rates adjacent to the plots using benthic sediment traps. High shoot mortality rates were recorded near 11 the cages, up to 20 times greater than at control sites. Recruitment rates remained similar to undisturbed 12 meadows and could not compensate mortality, leading to rapid seagrass decline within the first 100 13 meters from cages. Seagrass mortality increased with total (R= 0.47, p< 0.0002), organic matter (R= 14 0.36, p= 0.001), nitrogen (R= 0.34, p= 0.002) and phosphorus (R= 0.58, p< 3⋅10) sedimentation 15 rates. P. oceanica decline accelerated above a phosphorus loading threshold of 50 mg m day. 16 Benthic sedimentation rates seem a powerful predictor of seagrass mortality from fish farming, 17 integrating local hydrodynamics, waste effluents variability and several environmental mechanisms, 18 fuelled by organic inputs and leading to seagrass loss. Coupling direct measurements of benthic 19 sedimentation rates with dynamics of key species is proposed as an efficient way to predict and 20 minimize fish farm impacts to benthic communities. 21 22