Primary phytoplankton productivity in the Gullmar Fjord, Sweden : An evaluation of the 1985 – 2008 time series

Primary phytoplankton productivity has since 1985 been measured in the moutharea of the Gullmar Fjord, as a part of the Swedish marine monitoring program.Results from earlier evaluations of this primary production time series suggestedthe presence of an indirect link between NAO, supply of nutrients, wind, and thephytoplankton primary production and revealed the importance of climatic andenvironmental forces among the factors responsible for the observed developmentand fluctuations.An analysis of five-year running means of the primary production time-seriesfrom 1985 to 2008 revealed that the primary production in the Gullmar Fjordincreased and peaked during the five-year period 1992 – 1996, followed again by adecrease in production. The increase in production, as calculated from the firstannual running mean and compared with the maximal mean, was 20 % and thedecrease of the last annual mean was 25 %. The total result was that the meanannual primary production was 3.8 % lower when the difference between the firstand the last 5-year running means compared to the over-all mean production for thewhole period of time.The present evaluation concludes that there is a direct link between primaryproduction and nitrate concentrations in the mouth area of the Gullmar Fjord.Further, there is no influence from local runoff on the long term development ofthe primary production. Any long-term co-variation in primary production of theGullmar Fjord and the adjacent Kattegat and Belt Sea areas is not evident,.The overall results suggested that the primary production in the mouth area ofthe Gullmar Fjord during 1985 – 2008 has been controlled by the coupling of largeclimatic decadal patterns such as the NAO and the subsequent changes in thenutrient regime at a regional scale including the Skagerrak. It may further beconcluded that the anomaly of an exceptional flux of nitrate-rich ocean water ontothe NW European shelf most likely extended its distribution also to the Swedishwest coast and the mouth area of the Gullmar Fjord, which in turn triggered the 19% rise in annual primary production during the mid 1990s.