Nutrients as controlling factors of phytoplankton. Production in the Balsfjord - Northern Norway

Abstract

Fisheries management research is mostly centered on providing the needed knowledge about the fish stock in order to influence fishermen’s behavior at sea for stock sustainability. Management should also integrate knowledge acquired from primary producers (particularly phytoplankton) which form the foundations for fish production in implementing fishing controlling measures. Phytoplankton production therefore was studied in the Balsfjord from March to December in 2008. The study focused on nutrients as controlling factors. Water samples were collected at 5, 10 and 50 m from the Balsfjord from early March to early December. Nitrate, phosphate, silicate, Biogenic silica, chlorophyll a concentrations and phytoplankton abundance were analyzed throughout this sampling period. Seawater temperature and pH were also measured during sampling. The peak of the spring bloom occurred on 1st April at 5 and 10 m. Nitrate depletion and phosphate reduction occurred at 5, 10 and 50 m during summer and autumn while silicate showed major reduction during the spring bloom. Change in concentrations of silicate caused a shift in phytoplankton composition and abundance from diatoms to flagellates. The dominant phytoplankton genus and abundance was Chaetoceros during the spring bloom and flagellates after the bloom. Phytoplankton production in this fjord is controlled by nitrate.