Distribution of nutrients for seaweed cultivation around salmon cages at farm sites in north–west Scotland

The aim of this study was to determine nutrient concentrations in the vicinity of fish farm cages to maximise potential utilization by cultured macroalgae. At 4 m depth, nitrate and phosphate concentrations did not vary significantly with distance from fish farm cages. Elevated levels of ammonium were detected for extended periods of time during the day for distances that exceed 200 m. Principal enhancements were within 50 m in the direction of currents, and no increases were detected in the direction from which the current impacted the cages. Ammonium concentrations measured at 4 m depth within 50 m of the cages reached a maximum of 8 μM but most ranged from 2 to 4 μM and 2–3 μM above background. While current measurements gave an indication of ammonium distributions, the geography of the site and the physical presence of the fish farm cages were influencing factors. The highest concentrations of ammonium found at these fish farm sites were on the lee sides of cages in shallow water. Concentrations of ammonium throughout a relatively small-enclosed bay with fish farms were higher than in a similar bay without fish farms consistent with broader scale effects of nutrient enhancement. Multiple drogues used to follow currents were found to be a valuable tool for prediction of enhanced ammonium concentrations. The data suggest that there are elevated levels of ammonium at appropriate depths for potential utilization by marine macroalgae (i.e. in the photic zone) for an extended period of time at distances from the fish farm cages greater than 50 m. Careful assessment of current patterns around farm cages should be made if the objective is to maximise exposure of cultured algae to farm-derived nutrients.