Photosynthesis, mixotrophy and microbial plankton dynamics in two high Arctic lakes during summer

Photosynthesis and microbial plankton dynamics of two lakes in the Kongsfjorden catchment of Spitzbergen (Svalbard Archipelago, 78degreesN) were investigated during the summer of 2000. One of the lakes (Tvillingvatnet) served as the water supply for the village of Ny Angstromlesund. The other was a deeper, larger unnamed lake in the Ossian Sarsfjella reserve-named Lake OS in this study. Both lakes can be classified as oligotrophic on the basis of chlorophyll a and inorganic nutrient concentrations. Chlorophyll a concentrations ranged between 0.33 and 1.65 mug l(-1) in Tvillingvatnet and 0.2 and 0.3 mug l(-1) in Lake OS. The phytoplankton was dominated by chrysophytes and cryptophytes, with the diatom Rhizosolenia and a number of dinoflagellate species. Rates of photosynthesis were typically low, between 24.5 mug and 1.0 mg l(-1) day(-1) in Tvillingvatnet (photosynthetic efficiency 0.004-0.26), and between 3.1 and 29.5 mug l(-1) day (-1) in Lake OS (photosynthetic efficiency 0.0019-0.0085). Among the phytoflagellates (PNAN) there were a number of mixotrophs which reached their peaks of abundance before other PNAN. Mixotrophy appeared to provide a competitive advantage. Grazing rates for the mixotroph Dinobryon ranged between 0.063 and 1.12 pg C cell(-1) day(-1) in Lake OS and between 0.22 and 1.11 pg cell(-1) day(-1) in Tvillingvatnet, with rates increasing between July and August. However, Dinobryon removed less than 1% of bacterial biomass day(-1), while the heterotrophic nanoflagellates (HNAN) removed up to 28% of bacterial biomass day(-1). Bacterial concentrations were low, with a maximum of 28.8x10(8) l(-1) in Tvillingvatnet and 23.6x10(8) l(-1) in Lake OS. Ciliated protozoan and rotifer diversity in the plankton was low.