• Energy Research

Significance Oxygen-deficient waters are expanding globally in response to warming and coastal eutrophication. Coastal ecosystems provide valuable services to humans, but these services are severely reduced with decreasing oxygen conditions. In the Baltic Sea, oxygen-deficient waters have expanded from 5,000 to over 60,000 km 2 with large decadal fluctuations over the last century, reducing the potential fish yield and favoring noxious algal blooms. This increase is due to the imbalance between oxygen supply from physical processes and oxygen demand from consumption of organic material, enhanced by nutrient inputs and temperature increases. Further nutrient reductions will be necessary to restore a healthier Baltic Sea and counteract effects from warming.

AbstractA sediment record from a small lake in the north-eastern part of the Kamchatka Peninsula has been investigated in a multi-proxy study to gain knowledge of Holocene climatic and environmental change. Pollen, diatoms, chironomids and selected geochemical parameters were analysed and the sediment record was dated with radiocarbon. The study shows Holocene changes in the terrestrial vegetation as well as responses of the lake ecosystem to catchment maturity and multiple stressors, such as climate change and volcanic eruptions. Climate change is the major driving force resulting in the recorded environmental changes in the lake, although recurrent tephra deposition events also contributed. The sediment record has an age at the base of about 10,000calyrs BP, and during the first 400years the climate was cold and the lake exhibited extensive ice-cover during winter and relatively low primary production. Soils in the catchment were poor with shrub alder and birches dominating the vegetation surrounding the lake. At about 9600–8900calyrs BP the climate was cold and moist, and strong seasonal wind stress resulted in reduced ice-cover and increased primary production. After ca. 8900calyrs BP the forest density increased around the lake, runoff decreased in a generally drier climate resulting in decreased primary production in the lake until ca. 7000calyrs BP. This generally dry climate was interrupted by a brief climatic perturbation, possibly attributed to the 8.2ka event, indicating increasingly windy conditions with thick snow cover, reduced ice-cover and slightly elevated primary production in the lake. The diatom record shows maximum thermal stratification at ca. 6300–5800calyrs BP and indicates together with the geochemical proxies a dry and slightly warmer climate resulting in a high productive lake. The most remarkably change in the catchment vegetation occurred at ca. 4200calyrs BP in the form of a conspicuous increase in Siberian dwarf pine (Pinus pumila), indicating a shift to a cooler climate with a thicker and more long-lasting snow cover. This vegetational change was accompanied by marked shifts in the diatom and chironomid stratigraphies, which are also indicative of colder climate and more extensive ice-cover.

Abstract The coastal zone of the Baltic Sea is diverse with strong regional differences in the physico-chemical setting. This diversity is also reflected in the importance of different biogeochemical processes altering nutrient and organic matter fluxes on the passage from land to sea. This review investigates the most important processes for removal of nutrients and organic matter, and the factors that regulate the efficiency of the coastal filter. Nitrogen removal through denitrification is high in lagoons receiving large inputs of nitrate and organic matter. Phosphorus burial is high in archipelagos with substantial sedimentation, but the stability of different burial forms varies across the Baltic Sea. Organic matter processes are tightly linked to the nitrogen and phosphorus cycles. Moreover, these processes are strongly modulated depending on composition of vegetation and fauna. Managing coastal ecosystems to improve the effectiveness of the coastal filter can reduce eutrophication in the open Baltic Sea.

Regime shifts in lake ecosystems can occur in response to both abrupt and continuous climate change, and the imprints they leave in paleolimnological records allow us to investigate and better understand patterns and processes governing ecological changes on geological time scales. This synthesis investigates paleolimnological records that display apparent regime shifts and characterizes the shifts as either smooth, threshold-like or bistable. The main drivers behind the shifts are also explored: direct climate influence on lakes, climate influence mediated through the catchment, lake ontogenetic processes and/or anthropogenic forcing. This framework helps to elucidate the relationship between driver and regime shift dynamics and the type of imprint that the associated regime shifts leaves in sediment records. Our analysis of the limited sites available (22 sites) show that smooth regime shifts are characterized with forcing and response variables acting on similar time scales, whereas regime shifts that demonstrate a threshold like response or a hysteresis response occur on shorter time scales than changes in drivers. The temporal resolution of the record, a common concern in paleo records, limits identification of the timing and rate of the regime shifts. When detected, past regime shifts offer rich opportunities to understand ecosystem responses to climate and other changes and to evaluate the mean state and natural variability of lake ecosystems on time scales of decades to millennia. There are a number of remaining challenges in understanding regime shifts and ecosystem dynamics in a paleolimnological perspective including lack of an appropriate temporal resolution and ecosystem feedback mechanisms. Combining paleoecology with contemporary studies can help clarify the scale of regime shifts and to distinguish patterns in ecosystem changes from natural variability.

This paper summarises the results of the EU funded MEAD project, an interdisciplinary study of the effects of atmospheric nitrogen deposition on the Kattegat Sea between Denmark and Sweden. The study considers emissions of reactive nitrogen gases, their transport, transformations, deposition and effects on algal growth together with management options to reduce these effects. We conclude that atmospheric deposition is an important source of fixed nitrogen to the region particularly in summer, when nitrogen is the limiting nutrient for phytoplankton growth, and contributes to the overall eutrophication pressures in this region. However, we also conclude that it is unlikely that atmospheric deposition can, on its own, induce algal blooms in this region. A reduction of atmospheric nitrogen loads to this region will require strategies to reduce emissions of ammonia from local agriculture and Europe wide reductions in nitrous oxide emissions.

AbstractIn terrestrial ecosystems, a large portion (20–80%) of the dissolved Si (DSi) in soil solution has passed through vegetation. While the importance of this “terrestrial Si filter” is generally accepted, few data exist on the pools and fluxes of Si in forest vegetation and the rate of release of Si from decomposing plant tissues. We quantified the pools and fluxes of Si through vegetation and coarse woody debris (CWD) in a northern hardwood forest ecosystem (Watershed 6, W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire,USA. Previous work suggested that the decomposition ofCWDmay have significantly contributed to an excess ofDSi reported in stream‐waters following experimental deforestation of Watershed 2 (W2) at theHBEF. We found that woody biomass (wood + bark) and foliage account for approximately 65% and 31%, respectively, of the total Si in biomass at theHBEF. During the decay of American beech (Fagus grandifolia) boles, Si loss tracked the whole‐bole mass loss, while yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum) decomposition resulted in a preferential Si retention of up to 30% after 16 yr. A power‐law model for the changes in wood and bark Si concentrations during decomposition, in combination with an exponential model for whole‐bole mass loss, successfully reproduced Si dynamics in decaying boles. Our data suggest that a minimum of 50% of theDSi annually produced in the soil of a biogeochemical reference watershed (W6) derives from biogenic Si (BSi) dissolution. The major source is fresh litter, whereas only ~2% comes from the decay ofCWD. Decay of tree boles could only account for 9% of the excessDSi release observed following the experimental deforestation of W2. Therefore, elevatedDSi concentrations after forest disturbance are largely derived from other sources (e.g., dissolution ofBSi from forest floor soils and/or mineral weathering).