Controlled artificial upwelling in a fjord to stimulate non-toxic algae

Abstract

During the summer, primary production in the surface layers of some fjords depletes the nutrients to the degree that some species of toxic algae can dominate. We describe field experiments employing a bubble curtain and a submerged freshwater outlet to lift significant amounts of nutrient-rich seawater to the light zone to provide an environment in which non-toxic algae can bloom. The motivation for the experiment is to provide a local region with stimulated growth of non-toxic phytoplankton and thereby creating a possibility for mussels to be cleansed from the effects of toxic algae. In the first experiment, a 100-m long bubble curtain, using three perforated pipes submerged to 40 m depth, was operated in the Arnafjord, a side arm of Sognefjorden in western Norway. An air supply of 44 Nm 3 each minute lifted 65 m 3/s of deeper seawater to the upper layer with intense mixing during a period of 3 weeks. The mixed water flowed from the mixing region at depths from 5 to 15 m. Within a few days, the mixture of nutrient-rich water covered most of the inner portion of Arnafjord. In the second experiment, the 40 m deep, 26 m 3/s discharge of freshwater from the Jostedal hydropower plant to Gaupnefjord, another side arm of Sognefjorden, was manipulated to enhance the upwelling of seawater by using a diffuser plate. The increased entrainment of seawater to the buoyant plume led to an intrusion of the discharge into the compensation current at 5–10 m depth and a longer residence time in the local fjord arm. The field experiment showed an entrainment of 117 m 3/s of nutrient-rich seawater to the rising plume compared with 140 m 3/s obtained in a small-scale laboratory simulation, implying a sub-optimal placement of the plate over the outlet plume. This, however, was still more energy-efficient than the bubble curtain. In both experiments (bubbles and freshwater discharge) the increased nutrient inputs to the light zone resulted in increased growth of phytoplankton with a relative reduction of toxic algae.