  Fish farms represent a growing source of disturbance to shallow benthic
ecosystems like seagrass meadows. Despite some existing insights on the
mechanisms underlying decline, efficient tools to quantitatively predict the
response of benthic communities to fish farm effluents have not yet been
developed. We explored relationships of fish farm organic and nutrient input
rates to the sediments with population dynamics of the key seagrass species
(Posidonia oceanica) in deep meadows growing around four Mediterranean Sea
bream and Sea bass fish farms. We performed 2 annual shoot censuses on
permanent plots at increasing distance from cages. Before each census we
measured sedimentation rates adjacent to the plots using benthic sediment
traps. High shoot mortality rates were recorded near the cages, up to 20 times
greater than at control sites. Recruitment rates remained similar to
undisturbed meadows and could not compensate mortality, leading to rapid
seagrass decline within the first 100 meters from cages. Seagrass mortality
increased with total (R2= 0.47, p< 0.0002), organic matter (R2= 0.36, p=
0.001), nitrogen (R2= 0.34, p= 0.002) and phosphorus (R2= 0.58, p< 3 x 10-5)
sedimentation rates. P. oceanica decline accelerated above a phosphorus loading
threshold of 50 mg m-2 day-1. Benthic sedimentation rates seem a powerful
predictor of seagrass mortality from fish farming, integrating local
hydrodynamics, waste effluents variability and several environmental
mechanisms, fuelled by organic inputs and leading to seagrass loss. Coupling
direct measurements of benthic sedimentation rates with dynamics of key species
is proposed as an efficient way to predict and minimize fish farm impacts to
benthic communities.
