Publication

Summary

This report presents the results from “Årsovervåkingen med FerryBox”, the yearly monitoring undertaken for “Fagrådet for indre Oslofjord” in 2018 with the use of NIVA FerryBox system for continuous measurements and sampling. The temperature in the Vestfjord varied during the year due to seasonal heating in the summer months and cooling in the winter months, with the lowest temperatures in winter (March) with a drop in temperature down to 0 °C, and the highest temperatures (around 22 ° C) during summer (August). From January until end of April 2018, the inner fjord was characterized by relatively salty water, sometimes as high as 28 in salinity. The salinity was low (down to 13) in the period May to July, which was quite a long period compared to previous years. cDOM was highest in May and June for the measured period, coinciding with the low salinity. In this data report, no classification of nutrients was performed, but the data were discussed. In the surface layer, during the winter, there were some high values for total phosphorus (μg P/L) recorded, corresponding to a moderate condition, while the condition was good for total nitrogen. In winter, there were varying concentrations of nitrate (+nitrite) (μg N/L) and phosphate (μg P/L), with values corresponding to a good or moderate state. For total nitrogen and total phosphorus, there were good to very good conditions during summer. DOC was around 1.8-2.0 mg / L in the winter and then increased during the summer with a maximum of 2.8 mg / L in June, during the period with lower salinity. There was a spring bloom at the end of March with a maximum of 10 μg chlorophyll a/ liter which cooccured with a diatoms bloom, dominated by Skeletonema cf. marinoi. After March there was little algae until May-June when both diatoms and the group of undetermined flagellates and monads increased in numbers. The diatom genus Chaetoceros dominated, and C. minimus was common. After June, the number of algae decreased. In October, diatoms increased again together with an increase in chlorophyll a concentration, with species such as Dactyliosolen fragilissimus and Leptocylindrus danicus. There were modest occurrences of toxic algae in 2018. The shellfish poison producing dinoflagellates in the Dinophysis genus, including D. norvegica was observed in the spring, and Alexandrium species occurred occasionally throughout the year. Fish-toxin producing algae were also recorded in the spring, such as the silicoflagellate Pseudochattonella spp, and the dinoflagellate Karlodinium veneficum.