Use of stable isotopes to investigate the dispersion of fish farming waste as a function of hydrodynamics.

Stable isotopes were used to examine differential effects of fish farm waste on the water column and sediments. To achieve this objective, we chose 3 marine fish farms located along the coast of Sicily (Mediterranean Sea) as point-source disturbances, and a control area. The hypothesis that carbon and nitrogen isotope composition of particulate (POM) and sedimentary (SOM) organic matter varied with increasing distance (from cages to 1000 m) was tested at 3 levels of hydrodynamics: low (mean velocity of current [MVC] ~12 cm s–1), intermediate (MVC ~22 cm s–1), and high (MVC ~40 cm s–1). Different isotopic signals from allochthonous (fish waste) over natural (phytoplankton, terrigenous, and sand microflora) inputs allowed identification of the ‘spatial effect regime’ of fish farming. The increasing water current velocities seem to proportionally enlarge the relative area of influence of the cages, particularly on sediments. At low hydrodynamics, an increasing contribution of terrigenous signals was inferred: POM and SOM showing a depleted gradient of C (ranging from –22.0 to –24.0‰) and N (from 5.0 to 2.0‰). At an intermediate hydrodynamic level, C and N showed a slight increase in waste contribution, particularly in POM (δ15N from 2.6 to ~4.0‰). At high hydrodynamics,an enriching isotopic gradient (δ15NPOM-SOM from 1.8 to 4.6‰) suggested a notable contribution of fish waste. Accordingly, the dispersal of waste from the cages seemed to be related to movements at the bottom of the water column, confirming the recently identified role played by resuspension movements.