Nutrient and plankton development in Rosfjorden and enclosed ecosystems captured from changing water bodies during POSER

Abstract

During POSER (Plankton Observations with Sinlultaneous Enclosures in Rosfjorden) in spring 1979, nutrient and plankton development were investigated in an open south Norwegian fjord and in different enclosures. Enclosures were plastic tubes, 1 m in diameter and up to 40 m long, fastened to a central float. The water in the fjord exchanged several times. This could be well documented also by changes in nutrient, phyto-and zooplankton concentrations. Water with different plankton and nutrient concentrations was enclosed in order to exclude advective processes. The experiment was divided into 3 main periods. In the first period the water in the fjord was haline. nutrient-rich, and slightly stratified, due to a bloom of Skeletonema costaturn. During this phase the phytoplankton disappeared in the fjord and in the enclosures within 6 d, likely due to low light intensities. In the second period, following a water exchange, the water was less haline, poorer in nutrients, and richer in phytoplankton (mainly Thalassiosira nordenskioeldii) and in zooplankton. Part of this water was enclosed until silicate and phytoplankton concentrations became depleted and the major zooplankton species. Calanus finmarchicus, had developed to the third copepodite-stage. In the third period, following a new upwelling of haline and nutrient-rich water up to 10 m depth, 1 enclosure was filled with well-stratified water, nutrient-poor in the upper layer, but nutrient-rich below. In spite of sufficient nutrients the phytoplankton biomass decreased in this enclosure as well as in the fjord. This was partly caused by increasing numbers of copepodites, which reached more than 50 '10 of the phytoplankton biomass during the last 8 d of our observations.

Total dissolved carbohydrates in an enclosure experiment with unialgal Skeletonema costatum culture

“…8) were recorded in the filtered fjordwater enclosed after a diatom bloom, consisting largely of 73alassiosira nordenskioeldii, Thalassionema nitzschioides and Chaetoceros debilis (Brockmam et al, 1981;Kattner et al, 1983). The initial pool of dissolved carbohydrates may have consisted mainly of material originating in walls of dead cells.…”

“…Because of horizontal advection, measurements in the open sea do not seem an appropriate O Inter-Research/Printed in F. R. Germany means for such investigations. Enclosures containing unfiltered sea water do not provide much better conditions, since normally in such experiments many plankters develop simultaneously (Brockmann et al, 1977(Brockmann et al, b, 1979Kattner et al, 1983). A bloom of one particular species is achieved more easily when sea water is filtered, enriched with inorganic nutrients and inoculated with a monoculture of phytoplankton.…”

Samplers for enclosed stratified water columns

“…The boundary was at a depth of 14 m. Water temperatures varied from 0.5 to 5.5OC, and the resultant density differences were between c, = 2.2 at the beginning to c, = 1 at the end of experiment. This stratification became weaker, but was not destroyed (although the pycnocline was penetrated more than 360 times with the opened and closed sampler), as noted also from frequent nutrient by flexible rubber into a groove around the central rod measurements (Brockmann et al, 1982;Kattner et al, 1983, enclosure CC). Only about 15 % of nitrate concentration in the nutrient-rich lower layer were lost from the 20 m level to the nutrient-poor upper layer by diffusion and frequent sampling.…”

“…In this way sampling would be representative with respect to phytoplankton but not to larger zooplankton. This sampling procedure has been used in enclosures with monocultures of diatoms (Brockmann et al, 1977a;Eberlein et al, 1983). Using silicone tubes, which can be sterilized from time to time, bacteria can also be sampled representatively.…”

Influences of bag dimensions on the development of enclosed plankton communities during POSER

“…The development in bags ranging from 1.5 to 30 m3 appears to be quite natural. The factors initiating the bloom probably can be found in the increasing water temperature and the increasing light conditions Kattner et al, 1983).…”