A comprehensive reconstruction of the Baltic Sea state from 1850 to 2006 is presented: driving forces are reconstructed and the evolution of the hydrography and biogeochemical cycles is simulated using the model BALTSEM. Driven by high resolution atmospheric forcing fields (HiResAFF), BALTSEM reproduces dynamics of salinity, temperature, and maximum ice extent. Nutrient loads have been increasing with a noteworthy acceleration from the 1950s until peak values around 1980 followed by a decrease continuing up to present. BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.

Benthic-pelagic coupling is manifested as the exchange of energy, mass, or nutrients between benthic and pelagic habitats. It plays a prominent role in aquatic ecosystems, and it is crucial to functions from nutrient cycling to energy transfer in food webs. Coastal and estuarine ecosystem structure and function are strongly affected by anthropogenic pressures; however, there are large gaps in our understanding of the responses of inorganic nutrient and organic matter fluxes between benthic habitats and the water column. We illustrate the varied nature of physical and biological benthic-pelagic coupling processes and their potential sensitivity to three anthropogenic pressures -climate change, nutrient loading, and fishing -using the Baltic Sea as a case study and summarize current knowledge on the exchange of inorganic nutrients and organic material between habitats. Traditionally measured benthic-pelagic coupling processes (e.g., nutrient exchange and sedimentation of organic material) are to some extent quantifiable, but the magnitude and variability of biological processes are rarely assessed, preventing quantitative comparisons. Changing oxygen conditions will continue to have widespread effects on the processes that govern inorganic and organic matter exchange among habitats while climate change and nutrient load reductions may have large effects on organic matter sedimentation. Many biological processes (predation, bioturbation) are expected to be sensitive to anthropogenic drivers, but the outcomes for ecosystem function are largely unknown. We emphasize how improved empirical and experimental understanding of benthic-pelagic coupling processes and their variability are necessary to inform models that can quantify the feedbacks among processes and ecosystem responses to a changing world.

Abstract-Changes in the timing, composition, and intensity of freshwater phytoplankton blooms are known to have an impact on water quality and aquatic ecosystem functions. Factors provoking these changes are, therefore, of major importance. In Lake Erken in southeastern Sweden considerable changes in the timing and large variations in the composition of phytoplankton spring peaks have been observed during the past 45 yr. Here we show that long-term changes and variations in Lake Erken are strongly related to a single global parameter-the North Atlantic Oscillation (NAO). Even regional parameters that are known to have most influence on the spring development of phytoplankton such as ice break-up and nutrient concentrations could not provide a more conclusive explanation of the observed changes in spring phytoplankton, making the NAO a very powerful and simple tool in determining the timing and composition of phytoplankton spring peaks in a temperate lake.

We tested the hypothesis that CO 2 supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized-additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO 2 in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg 2 1 had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg 2 1 for streams compared with TOC. Both DSi and TOC explained 22-46% of the pCO 2 variability in various lake classes (0.01-4100 km 2 ) and Mg 2 1 and TOC explained 11-41% of the pCO 2 variability in the various stream orders. This suggests that aquatic pCO 2 has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO 2 efflux. pCO 2 and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO 2 with the atmosphere and inlake mineralization of TOC. For streams, we observed a decreasing trend in pCO 2 with stream orders between 3 and 6. We calculated the total CO 2 efflux from all Swedish lakes and streams to be 2.58 Tg C yr À1 . Our analyses also demonstrated that 0.70 Tg C yr À1 are exported to the ocean by Swedish watersheds as HCO 3 À and CO 3 2À of which about 0.56 Tg C yr À1 is also a residual from terrestrial respiration and constitute a long-term sink for atmospheric CO 2 . Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr À1 , which corresponds to 10% of the NEE in Sweden.