Nutrient enrichment experiments in plastic cylinders and the implications of enhanced primary production in Lindåspollene, western Norway

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Abstract
Water from 34 m depth was introduced in a continuous flow rate of 0.8 and 0.4 litre/min to the surface layer of two plastic cylinders being 1 m in diameter and 18 m deep. The nitrate content of the deep water ranged from 4.0 to 9.0 µM (probably underestimated nitrogen content), phosphate from 1.1 to 2.5 µM, and silicate from 8.7 to 21.9 µM during the course of the experiment. From 10 to 12 July the primary production in the cylinders increased with a factor of 5 to about 300 mg C/m2 per hour and the highest primary production. 340 mg C/m2 per hour or 3.23 g C/m2 per day, was reached on 23 July. Inside the cylinders the highest chlorophyll content (127.1 and 127.7 mg Chi µ/m2) was measured on 18 July in cylinder 1 (CI) and on 14 July in cylinder 2 (C2). Later there was a distinct decrease in the chlorophyll content, reaching about 70 mg Chl µ/m2 in both cylinders on 23 July and coinciding with a drastic increase in phaeopigments. From 8 to 23 July the mean number of cells per litre increased by a factor of about 5, reaching about 0.9 × 106 cells outside, and 9.3 × 106 and 8.0 × 106 cells per litre in the two cylinders, with the highest number in the cylinder with the highest flow-rate. The phytoplankton community inside the cylinders was dominated by Leptocylindrus danicus, Skelelonema costalum. and Chaetoceros spp., whereas Nitzschia delicatissima and Emiliania huxleyi dominated outside. The expected ecological consequences of nutrient enrichment to the natural system in Lindaspollene is discussed.