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Summer growth near salmon sea cages and nitrogen uptake of kelp Ahn, Ok-Hyun


The purpose of this study was to test the hypothesis: a fish farm can fertilize kelp during summer when only nitrogen is a limiting factor for the growth of kelp. Temperature (10.0-14.7°C), salinity (22.3-30.0%o), water visibility (3.5-9.5 m) and daytime-irradiance data at 2 m depth in summers of 1995 and 1996 indicated suitable levels for the growth of kelp. Three kelp species, Laminaria saccharina, L. groenlandica and Nereocystis luetkeana, were collected from salmon fish farm nets and cultivated adjacent to and far from a salmon farm. Distal sites as control sites did not receive effluent from salmon farms. During the summer of 1995, the blade growth rates in length at the sites ranged from 2.50-5.4% d⁻¹ for L. saccharina and 5.15-6.57% d⁻¹ for N. luetkeana. Significant differences in the blade growth between L. saccharina control and experimental plants were observed (p > 0.05), and C/N ratio in the plant tissues increased with distance from the salmon farm. At both the farm and control sites, ambient NO₃⁻ concentrations were similar (above 12 μM). Ammonium concentration adjacent to the farm varied from 1.7-13.3 μM, while at the control site it was < 2 μM. For two consecutive 4-weeks experiments from May-July in 1996, in general, significant differences in growth rates were recorded between three species of control and experimental plants ranging from 2.70-3.79% d⁻¹ from May-June and 2.16-4.79% d⁻¹ from June-July. The ambient concentrations of NO₃⁻ near the fish farm and control sites were low (< 5 μM). The concentrations of NH⁴⁺ next to sea cages varied over time (1-34 μM) and were associated with fish feeding events. In the nitrogen uptake experiments involving various combinations of NH⁴⁺ and NO₃⁻ concentrations, the uptakes of NO₃⁻ by both L. saccharina and Nereocystis increased linearly with dissolved inorganic nitrogen (up to 70 μM). In contrast, NH⁴⁺ uptake by the kelps was saturated at approximately 11-13 μM. Only Nereocystis exhibited preference of NO₃⁻ over NH⁴⁺ when they were present equally in medium of 20 μM NO₃⁻ plus 20 μM NH⁴⁺ (p > 0.05). Any site where ambient concentration of ammonium is high (near 15 μM) during summer may be suitable for both L. saccharina and Nereocystis. However, it seems more efficient to fertilize kelp by nitrate than ammonium since nitrate uptake by both species increased linearly without the effect of external concentration of ammonium in the medium.

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