Pitta P, Tsapakis M, Apostolaki ET, Tsagaraki T, Holmer M, Karakassis I

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

Although fish farms release substantial amounts of nutrients into the water column, various studies have failed to detect significant increases in planktonic chl a in the vicinity of fish farms. To trace the fate of these disappearing ('ghost') nutrients, Pitta and co-workers conducted experiments with dialysis bags in gradients of nutrient enrichment at 2 locations in the Eastern Mediterranean. The productivity of the water column decreased with distance from the nutrient discharge at the farms. However, bioassays with and without grazer exclusion showed that, at least in oligotrophic systems, grazing plays a key role in regulating phytoplankton biomass, keeping chl a at very low levels, and effectively transferring nutrients up the food web.

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