| dc.description.abstract | This investigation focuses on the phytoplankton-bloom in in the Sogndalsfjord in western-Norway. Hydrography data consisting of salinity, density, temperature, oxygen and chlorophyll-a from 2007, 2013/2014 and 2016 from different geographical locations within the fjord have been used to obtain a better understanding of the process of a phytoplankton spring bloom in the Sogndalsfjord.
The bloom start date (BSD) for three time series (2007, 2013/2014 and 2016) have been calculated to see if the bloom generally started earlier in the last ten years suing the BSD threshold method. The BSD have been calculated to be respectively February 24th, February 5th and February 17th, this shows that the onset of the bloom as approximately may vary within two/three weeks.
Some yearly measurements (2007, 2013/2014 and 2016) showed that the peak in concentrations are found in the same period of time (March 4-18), being approximately two weeks. It has not been possible to explain the mechanisms responsible for variation. Phytoplankton blooms when a combination of favorable conditions is met: light, stable water mass and low wind conditions.
Density and salinity data from 2016 show that phytoplankton bloom in weakly stratified waters, indicating that strongly stratified water masses is of lesser importance in concerning phytoplankton bloom conditions in the Sogndalsfjord.
The phytoplankton spring bloom in 2016 consisted of roughly 5 phases, the first stage started on February 17th in the top most two meters of the water column. The depth of the highest found concentrations generally deepens over time, the second phase consists of 0-5 meter of the water column. The highest concentrations of measured chlorophyll-a are found on March 2nd and 9th (third stage of the bloom), these concentrations are found between 2-10 m. The fourth stage is a stage with relatively lower concentrations, found between the two major peaks in concentrations of the bloom. The last stage of the bloom are the increased measured concentrations found at the last 2 measurement dates. The depth of this last stage is approximately 15-25 m.
Different geographical locations (station A: Loftenes bridge, station B: Barsnesfjord, station C: Sogndalsfjord and station D: Solhov) showed similarities in the general trend of the measured chlorophyll-a time series. This general trend concerns a very similar pattern with respect to concentrations and development over time. Two peaks in concentrations were found for all four different stations.
The highest growth rates found at Skjernes in 2007 were 2.44 d Double-1 between February 22nd and March 5th found at a depth of two meter. Highest growth rates found in the Sogndalsfjord in 2016 were 2.37 d Double-1 between February 24th and March 2nd found at a depth of 15 meter.
The maximum chlorophyll-a concentrations of the blooms varied with approximately a factor of 2.5 (peak concentrations of 16 μg/l measured at Skjernes in 2007 and a peak concentration of 6 μg/l measured in the Sogndalsfjord in 2016). It is speculated that this may be the related to the depth of the annual winter vertical convection. In this case, one possible effect from hydroelectric power production (HPP) could be that primary production is reduced in regulated fjords (fjords with an outlet of HPP) because of shallower winter vertical mixing. | nb_NO |
