High winter respiration has been observed in a subarctic estuary with high levels of organic matter inputs, while winter is generally thought to be a non-productive season. We constructed an oxygen and carbon budget of the system to validate the high respiration rate, including the resulting low production-to-respiration ratio, and to identify important carbon and energy sources. Measurement data of production and respiration parameters from running monitoring programs were used. Furthermore, we set up a microcosm experiment in order to study effects of iron increases by riverine organic matter inputs. The carbon balance of this subarctic estuary shows a small deficiency of carbon on an annual basis, but is able to explain how winter respiration is fueled by carbon fixed in the autumnal season and inputs of riverine material. Also, the balance calculation was able to predict oxygen deficiencies on a seasonal basis. The effect of riverine organic matter on biological activity was clearly present, but iron did not appear to affect responses in primary or secondary producers. Additional studies will be needed to fully understand the role of iron additions to marine microbial communities, particularly focusing on fractioning of iron and organic matter species.

Temperature dependence of marine plankton respiration is an important factor in understanding the function and changes in the ecosystem of the ocean. The aim of this study is to test the temperature sensitivity (Q10) of plankton respiration. The oxygen optode method was used to measure plankton respiration. Natural water samples from the Baltic Sea was incubated at short (in situ +1, +2, +3°C) and long (in situ +5, +10, +20°C) temperature intervals with influence of dissolved organic matter (DOC). The Arrhenius equation and Q10-model was used to determine the temperature dependence (Q10) of respiration at different temperatures. There was a significant difference in Q10 between short temperature intervals at low temperatures (p=0,008) and long temperature intervals at higher temperatures. There was no significant difference between long and short temperature intervals when DOC was added (p=0,094). A significant effect could be seen with the DOC enrichment at low temperatures, where the Q10-values became significantly lower (p=0,002) after DOC addition. This effect could, however, not be seen at higher temperatures (p=0,117). Together with results from earlier studies it was concluded that the difference in temperature depends on the actual temperature and not the length of the interval. Lowered temperature dependence at raised DOC concentration, was the opposite of what was expected. The results suggest that the importance of temperature for CO2 emissions and development of hypoxia in the sea may have been underestimated.