Årsovervåking med FerryBox - Indre Oslofjord 2019

Summary

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.