The Annual Phytoplankton Cycle of a Landlocked Fjord near Bergen (Nordåsvatn)

Abstract

1. Quantitative phytoplankton records for 1941 and 1942 from Nordåsvatn near Bergen were used for a discussion of the annual phytoplankton cycle of this land-locked fjord. 2. Ice covered the surface till late in April 1942 and retarded the onset of the spring diatom vegetation, which reached its maximum in early May, while in the coastal waters outside it occurs in March. 3. Between the spring diatom maximum and an autumnal increase in the diatom population in the middle of September, irregular changes in the phytoplankton population took place. These were due partly to succession, partly to influx of water from the outside. 4. The summer plankton consisted of a few diatoms, coccolithophorids in increasing numbers from June (mainly Coccolithus Huxleyi and Calypirosphaera oblonga) and dinoflagellates. The ceratia had their maximum in the middle of September, while other dinoflagellates (Peridinium trochoideum, Prorocentrum micans and Oxytoxum mucronatum) occurred in considerable quantities at various times. 5. The causes of the heterogeneous composition of the phytoplankton as to ecological types are discussed. 6. The main factors influencing phytoplankton abundance in the fjord are enumerated, including local factors which are effective in Nordåsvatn, such as pollution by sewage, ice cover and currents.