No abstract

Biogenic, seasonal, and stochastic fluctuations at various scales characterize coastal marine habitats and modulate environmental stress. The relevance of most past studies into climate change impacts is weakened by the usually intentional exclusion of fluctuations from the experimental design. We describe a new outdoor mesocosm system for benthic research (“benthocosms”) which permit the control and manipulation of several environmental variables while admitting all natural in situ fluctuations. This is achieved by continuously measuring the relevant variables (e.g., temperature, pH, O2, CO2) in situ, defining these in real time as reference values in the control software and simulating target climates by delta treatments. The latter constitute the manipulative addition of predefined changes (e.g., “warming”, “acidification”) to the reference values. We illustrate the performance of the system by presenting the environmental data of four seasonal experiments which together represent an entire year. The “Kiel Outdoor Benthocosms” allow realizing near‐natural climate change experiments on complex benthic communities under controlled scenarios.

Ocean acidification (OA) is generally assumed to negatively impact calcification rates of marine organisms. At a local scale however, biological activity of macrophytes may generate pH fluctuations with rates of change that are orders of magnitude larger than the long-term trend predicted for the open ocean. These fluctuations may in turn impact benthic calcifiers in the vicinity. Combining laboratory, mesocosm and field studies, such interactions between OA, the brown alga Fucus vesiculosus, the sea grass Zostera marina and the blue mussel Mytilus edulis were investigated at spatial scales from decimetres to 100s of meters in the western Baltic. Macrophytes increased the overall mean pH of the habitat by up to 0.3 units relative to macrophytefree, but otherwise similar, habitats and imposed diurnal pH fluctuations with amplitudes ranging from 0.3 to more than 1 pH unit. These amplitudes and their impact on mussel calcification tended to increase with increasing macrophyte biomass to bulk water ratio. At the laboratory and mesocosm scales, biogenic pH fluctuations allowed mussels to maintain calcification even under acidified conditions by shifting most of their calcification activity into the daytime when biogenic fluctuations caused by macrophyte activity offered temporal refuge from OA stress. In natural habitats with a low biomass to water body ratio, the impact of biogenic pH fluctuations on mean calcification rates of M. edulis was less pronounced. Thus, in dense algae or seagrass habitats, macrophytes may mitigate OA impact on mussel calcification by raising mean pH and providing temporal refuge from acidification stress.

Anthropogenic CO 2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity (A T ) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such A T dynamics, due to its direct link to terrestrial processes, short water residence time and long history of A T measurements dating back to the early 20 th century. We compiled an extensive A T data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water A T levels increased throughout the Baltic Sea. The rates of change were highest in the low-saline, northern areas and decreased gradually toward constant levels in the North Sea. The A T increase observed in the Central Baltic Sea (13.4 mmol kg 21 yr 21 ) and the Gulf of Bothnia (17 mmol kg 21 yr 21 ) has compensated CO 2 -induced acidification by almost 50% and 100%, respectively. Further, the A T trends enhanced the CO 2 storage capacity and stabilized the CaCO 3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the A T trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO 2 , agricultural liming and internal A T sources.

No abstract

No abstract

Spectrophotometric pH measurements allow for an accurate quantification of acid-base equilibria in natural waters, provided that the physico-chemical properties of the indicator dye are well known. Here we present the first characterization of purified m-Cresol Purple (mCP) directly linked to a primary pH standard in the salinity range 5-20. Results were obtained from mCP absorption measurements in TRIS buffer solutions. The pH T of identical buffer solutions was previously determined by Harned cell measurements in a coordinated series of experiments. The contribution of the TRIS/HCl component to the ionic strength of the buffer solutions increases toward lower salinity: This was taken into account by extrapolating the determined pK 2 e 2 to zero buffer concentration, thereby establishing access to a true hydrogen ion concentration scale for the first time. The results of this study were extended with previous determinations of pK 2 e 2 at higher and lower salinity and a pK 2 e 2 model was fitted to the combined data set. For future investigations that include measurements in the salinity range 5-20, pH T should be calculated according to this pK 2 e 2 model, which can also be used without shortcomings for salinities 0-40 and temperatures from 278.15 to 308.15 K. It should be noted that conceptual limitations and methodical uncertainties are not yet adequately addressed for pH T determinations at very low ionic strength.

A multi-proxy approach study (cladocerans, diatoms, geochemistry, plant macrofossils, pollen), was performed on a sediment core from Lake Vrana (Vransko Jezero), a large and deep karstic lake on the northern Adriatic island of Cres, Croatia. Considerable lake-level changes occurred during the last approx. 16,000 years. The stratigraphic evidence suggests that periods of enhanced precipitation and the post-LGM rise in sea level were the main driving forces. The lake records indicate early human impacts. Sediment echo-sounding indicated that >25 m of lake sediments lies within the site, from which 5 m have been cored. Shallow lake stages occurred from 14.4 14C ky BP to early Holocene. Prior to Alleröd, interglacial sediments were redeposited, reflecting the influences of rising sea-level (forming a local groundwater barrier), a temporary increase in precipitation, and lake-level changes. There appears to be a hiatus in the sequence, for no sediments assignable to the Alleröd chronozone could be found. A discordance in the echo profile at the appropriate horizon in the sequence supports this interpretation. Groundwater level increased again at 10.6 ky BP (during Younger Dryas chronozone), a swamp vegetation formed, which gave way to a shallow lake. During the Preboreal chronozone, this freshwater lake persisted with fluctuating levels. The establishment and subsequent persistence of the present deep water lake at about 8.5 ky BP, correspond with findings of a pluvial period at the Dalmatian coast, which lasted from 8.4 to 6 ky BP. First human catchment disturbances were related to settlements of Neolithic or Bronze Age. The increase in summer drought, coupled with forest clearance during Illyrian times, are assumed to be responsible for the change towards present evergreen oak vegetation in the lake catchment. The intensification in land-use during Roman and post-Roman settlements caused a slight increase in the lake trophic level