Controlled ecosystem experiment in Lindåspollene, western Norway, June 1979: Comparisons between the natural and two enclosed water columns

Abstract

The vertical structure and development of phyto- and zooplankton were studied in the natural water column and in two naturally stratified water columns enclosed in plastic bags, 1 m in diameter and 13 m deep. More than half of the primary production occurred in the uppermost 4.5 m. In contrast, the dominant copepods (Oncaea spp., Temora longicornis, and Oithona spp.) were distributed mainly below this upper layer. There were no marked differences between the day and night distributions for any of the zooplankton groups. It is argued that the high production in the upper layer must reflect a high mineralization activity, presumably by smaller organisms such as bacteria and protozoans. The phytoplankton biomass and primary production were somewhat lower inside than outside the bags. The zooplankton distributions were initially similar, but during the two weeks of observation the development within the bags diverged somewhat from that outside. Copepod nauplii, Oncaea spp., and Temora longicornis tended to be displaced upwards within the bags, especially in one of them. In contrast to the populations outside, the populations within the bags of Oithona spp. and Evadne nordmanni decreased and the population of Fritillaria borealis increased. Bivalve larvae, except of Mytilidae, showed a less pronounced decrease within than outside the bags. Multivariate analysis revealed that the enclosed water columns remained more similar to each other than to the surrounding water. The differences notwithstanding, it is concluded that the enclosed water columns retained their basic ecological characteristics, and that they therefore can be applied for realistic short-term experiments (weeks) on effects of manipulations, such as in pollution studies.