Effects of dissolved humic matter on phytoplankton

Sammanfattning: Water colour is currently increasing in thousands of lakes in the northern hemisphere due to an increased input of terrestrial dissolved organic carbon (DOC), and more specifically coloured disscolved humic matter (DHM). I studied how water colour affects the light climate in the epilimnion and, as a consequence, phytoplankton biomass, species composition and production in lakes in southern Sweden. In my first study, I found that epilimnion depth (ze) did not, contrary to my expectations, decrease with increasing water colour. Instead, ze increased when fetch (the distance wind can blow uninterrupted over a lake, calculated as the square root of lake area) increased, independently of water colour. Consequently, light intensities in the epilimnion were in general lowest in large lakes with high water colour. Values for average light intensities in the epilimnion (Ē%) suggested that light may be a limiting factor for phytoplankton in some lakes. This was confirmed in an experimental study, where small volumes of lake water were incubated at different depths in lakes. Decreasing light intensities, i.e. increasing incubation depths, led to decreasing phytoplankton biomass (chlorophyll a) even when nutrient availability was sufficient to support higher biomasses. However, effects of water colour on phytoplankton species composition in this study were minor. In my third study, I investigated specifically differences between phytoplankton communities in lakes in a water colour gradient using phytoplankton pigments to identify groups of phytoplankton. Only one phytoplankton pigment, chlorophyll c2 (affiliated with dinoflagellates, diatoms and chrysophytes), was affected directly by water colour, but Ē % and ze influenced the phytoplankton communities of the lakes. Finally, a mesocosm study revealed that primary production decreased with increasing water colour. In this study, increasing water colour was not accompanied by an increase in DOC, which explains why (bacterial) respiration did not increase as expected. If both production decreases and respiration increases as a consequence of increasing water colour in the future, lakes will become more heterotroph and thus emit more CO2 to the atmosphere. This effect may be enhanced by increasing temperature. In my mesocosm study, I found that respiration increased with increasing temperature but production did not. There were also no interaction effects of warming and browning on production and respiration. In summary, the results from these studies imply that increasing water colour is likely to lead to lower phytoplankton biomass and production in lakes in southern Sweden in the future.