Sammendrag
This report presents the results from “Årsovervåkingen med FerryBox”, the yearly monitoring
undertaken for “Fagrådet for indre Oslofjord” in 2019 with the use of NIVA FerryBox system for
continuous measurements and sampling.
The temperature in Vestfjorden varied during the year as a result of heating during the summer and
cooling during the winter. There were warmer surface temperatures during winter and colder
summer temperatures compared to the recent years. The fjord was from January to the end of April
characterized by relatively high salinity. This gradually declined throughout the summer season with
the lowest salinities in May and June. 2019 was comparably higher in salinity than recent years due
to less rain. High fDOM and DOC in June and autumn correspond to lower salinities. The spring
bloom started in April with Chlorophyll a at about 4 μg/L.
The plankton abundance was low until April when the spring bloom started with chlorophyll-a
abundance of 4 microgram/L and dominated by Skeletonema cf. marinoi og Chaetoceros-species.
Most of May had low biomass, but at the end of the month the biomass increased in a new bloom
due to Dictyocha speculum flagellates. During the bloom in June the chlorophyll-a increased again
due to an increased number of diatoms. This bloom was strongest in the inner part of Vestfjorden
with gradually less biomass towards the Drøbaksund. During summer and autumn, the plankton
biomass decreased, but in late autumn a new diatom period occurred. Dinoflagellates were not as
abundant as the diatoms during 2019, but they were present all year and at times they provided a
substantial contribution to the biomass. A diverse community of dinoflagellates occurred in April and
May. Several Tripos-species were registered during spring and summer. These are large cells that
contribute significantly to the biomass even at low abundances.
The high-resolution data that is achieved by FerryBox sensors illustrate the frequency and intensity of
algal blooms at a time scale of less than the discrete samplings at every 2 weeks. This will be
important in the assessments of the fjord's water quality based on the use of the 90th percentile.
With monthly discrete sampling, information on blooms can be lost. Sensor data also show the
development of environmental and climate variables.
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