Production and food web interactions of Arctic freshwater plankton and responses to increased DOC

A gradient of dissolved organic carbon (DOC) was added to enclosures in a high Arctic lake (Svalbard, 79 degreesN). The aim was to simulate the effect of increased concentrations of DOC that will be a predicted effect of increased temperature and precipitation. The study aimed to provide information on the overall effects of such increased levels of DOC on the pelagic food-web, as well as the increased attenuation of UV-radiation (UV-R) caused by increased DOC. The biomass development of phytoplankton, heterotrophic bacteria and ciliates from 15. July (shortly after ice-off) to 8. August revealed a consistent pattern across all enclosures. Initial phytoplankton biomass decreased from maxima around 600 mug Cl-1 towards 50 mug Cl-1 by the end of July. Similarly, ciliate biomass decreased from similar to100 to 5 mug Cl-1, while heterotrophic bacteria decreased from initially 280 mug Cl-1 to biomasses near 100 mug Cl-1. Over the same period, zooplankton biomass (almost a monoculture of Daphnia tenebrosa) increased from < 40 to some 170 mug Cl-1. These patterns were reflected also in decreasing absolute production of bacteria and phytoplankton, while production: biomass ratio remained fairly constant. In general, the study demonstrated a very dynamic system over the brief ice-free season, where Daphnia grazing had a strong impact on the unicellular biota. Additions of DOC initially stimulated planktonic production, yet this effect was soon overruled by Daphnia grazing. This stimulating effect could be a result both of increased UV-R attenuation and some growth stimulating effect. Since no corresponding stimulating effect was observed in the bag shielded with Mylar filter to screen off UV-B, the latter cause seems most likely.