'Ghost nutrients' from fish farms are transferred up the food web by phytoplankton grazers

Several studies have failed to detect significant changes in chl a content of the water column in the vicinity of fish farms despite the large amount of nutrient wastes discharged into the marine environment. To trace the fate of these disappearing (or 'ghost') nutrients, experiments using dialysis bags deployed in situ along gradients of nutrient enrichment (at different distances from cage fish farms) were carried out at 2 coastal locations in the eastern Mediterranean. The productivity of the water column decreased with distance from the nutrient discharge point, as indicated by the chl a content found in the dialysis bags. However, comparison of the results from bioassays with and without grazer exclusion showed that grazing plays a key role in regulating phytoplankton biomass, keeping chl a at very low levels and effectively transferring nutrients up the food web. The fact that this type of response was found in otherwise oligotrophic conditions is probably clue to the structure of the phytoplankton community in such areas, which is based on small cell-size primary producers that can be easily grazed on by planktonic ciliates.