Monitoring of the outer Oslofjord. Investigations in 2016.

Summary

The report describes the inputs of nutrients to the fjord in 2015 and the investigations and results for phytoplankton, hydro-physical and hydro-chemical conditions and hard bottom communities in 2016. There has not been a major replacement of the bottom water in the side fjords to the Oslofjord since the winter in 2010. However, a moderate replacement in the Hvaler area was registered in November 2016, which resulted in improved conditions in the bottom water all the way to Iddefjorden. A similar replacement did not take place in the Drammensfjord and the Frierfjord. Most stations showed no changes in overall environmental conditions between 2015 and 2016. There was a reduction in the environmental conditions at the mid-fjord stations OF-2 and OF-4 due to increased total phosphate and in the Sandefjordsfjord due to increase in nitrate concentration and reduction in oxygen. In general, total phosphate conditions are improved on many stations, while stations in inner Hvaler area showed an improvement environmental condition based on the amount of nitrate. Based on salinity data, there was a lower supply of freshwater to the Oslo fjord and the side fjords in 2016 compared with previous years. This has an impact on nutrient concentrations and phytoplankton biomass in the fjord system. Generally, dinoflagellates dominated the phytoplankton in 2016. In the Hvaler area, significant summer blooms of diatoms have been observed previous years. This was not the case in 2016. Benthic investigations in Outer Oslo fjord included in 2016 surveys of hard bottom flora and fauna in the littoral zone (quantitative quadrat registrations) at 15 stations and surveys of lower growth limit for a selection of macroalgae at 12 stations. Semi-quantitative registrations of hard bottom flora and fauna in the littoral zone, financed by Borregaard AS, was conducted at 4 stations. In 2014 the introduced species pacific oyster (Crassostrea gigas) was registered at 6 station, in 2016 it was registered at 11 of the 15 “quadrat-stations” and 2 of the 12 littoral zone stations. High prevalence of diatoms- and blue-green algae and the opportunistic greenalgae, Ulva intestinalis was observedin the littoral zone at 3 stations (G9 by Sandefjord, G10 by Stavern and G21 by Hvaler) andindicateeutrophication. However, there are no obvious correlation between species composition at the stations, and nutrients measured in the surface water at nearby stations. Using the Multi Species Macroalgae Depth Index (MSMDI), 6 stations were classified to be in “good” ecological condition, 5 stations in “moderate” condition and one station (G14) was classified to be in “poor” ecological condition.