Effects of Nutrient Management Scenarios on Marine Eutrophication Indicators: A Pan-European, Multi-Model Assessment in Support of the Marine Strategy Framework Directive

A novel pan-European marine model ensemble was established, covering nearly all seas under the regulation of the Marine Strategy Framework Directive (MSFD), with the aim of providing a consistent assessment of the potential impacts of riverine nutrient reduction scenarios on marine eutrophication indicators. For each sea region, up to five coupled biogeochemical models from institutes all over Europe were brought together for the first time. All model systems followed a harmonised scenario approach and ran two simulations, which varied only in the riverine nutrient inputs. The load reductions were evaluated with the catchment model GREEN and represented the impacts due to improved management of agriculture and wastewater treatment in all European river systems. The model ensemble, comprising 15 members, was used to assess changes to the core eutrophication indicators as defined within MSFD Descriptor 5. In nearly all marine regions, riverine load reductions led to reduced nutrient concentrations in the marine environment. However, regionally the nutrient input reductions led to an increase in the non-limiting nutrient in the water, especially in the case of phosphate concentrations in the Black Sea. Further core eutrophication indicators, such as chlorophyll-a, bottom oxygen and the Trophic Index TRIX, improved nearly everywhere, but the changes were less pronounced than for the inorganic nutrients. The model ensemble displayed strong consistency and robustness, as most if not all models indicated improvements in the same areas. There were substantial differences between the individual seas in the speed of response to the reduced nutrient loads. In the North Sea ensemble, a stable plateau was reached after only three years, while the simulation period of eight years was too short to obtain steady model results in the Baltic Sea. The ensemble exercise confirmed the importance of improved management of agriculture and wastewater treatments in the river catchments to reduce marine eutrophication. Several shortcomings were identified, the outcome of different approaches to compute the mean change was estimated and potential improvements are discussed to enhance policy support. Applying a model ensemble enabled us to obtain highly robust and consistent model results, substantially decreasing uncertainties in the scenario outcom

Revisiting the Estimate of the North Sea Air-Sea Flux of CO2 in 2001/2002: The Dominant Role of Different Wind Data Products

Abstract For the North Sea, a semienclosed shelf sea in the northeastern North Atlantic, the seasonal and annual CO2 air-sea fluxes (ASF) had been estimated for 2001 and 2002 in earlier work. The underlying observations, pCO(2), salinity, and temperature had been combined with 6-hourly wind data derived from ERA40 reanalysis. In order to assess the impact of different wind data products on the computation of CO2 ASF, we compared ERA40 wind data with coastDat data derived from the nonhydrostatic regional climate model COSMO-CLM. From the four observational months September, November, February, and May all but the May data show higher wind speeds for coastDat than for ERA40, especially off the Norwegian, UK, and continental coasts. Largest differences occur in the northern offshore areas. The comparison with observed wind data supports this feature generally: At Helgoland, an island in the German Bight, and at the Belgium pile Westhinder the ERA40 data underestimate both, the coastDat data and the observations. Wind observations for two Norwegian North Sea platforms were available: At the northern station Troll off the Norwegian coast the coastDat data overestimate the observations in winter. At Ekofisk in the central North Sea the ERA40 data fit the observations well, while the coastDat data slightly overestimate the observational data in all months but in May. The corresponding CO2 ASF estimates show strongest deviations off the Norwegian coast. Using different bulk formulas for determining the net annual ASF resulted in differences due to different wind products of up to 34%. Plain Language Summary Climate change is induced by gases like carbon dioxide, which are added to the atmosphere. The increase of the concentration in the atmosphere is dampened by the uptake of this gas by land and ocean. Especially, the coastal ocean is able to efficiently absorb CO2. To calculate the North Sea-wide uptake of CO2, simulated wind speed data were used. The formerly used model data cover the total Earth and thus have a less fine resolution. Especially near the coast this effect becomes dominant, as wind over land is more efficiently retarded than over sea. A new wind product (coastDat) with a refined grid was established especially for coastal applications. We compare the old and the new data with observational data sets. It has shown that the coastDat data are closer to observations near the coast. The old data set significantly underestimates the observational data there. At the open sea the new data set slightly overestimates the observations. The comparison of the mean flux of CO2 from the atmosphere into the ocean revealed an increase of 34% when using the new wind data instead of the old one

The ocean response to volcanic iron fertilisation after the eruption of Kasatochi volcano: a regional-scale biogeochemical ocean model study

In high-nutrient–low-chlorophyll regions, phytoplankton growth is limited by the availability of water-soluble iron. The eruption of Kasatochi volcano in August 2008 led to ash deposition into the iron-limited NE Pacific Ocean. Volcanic ash released iron upon contact with seawater and generated a massive phytoplankton bloom. Here we investigate this event with a one-dimensional ocean biogeochemical column model to illuminate the ocean response to iron fertilisation by volcanic ash. The results indicate that the added iron triggered a phytoplankton bloom in the summer of 2008. Associated with this bloom, macronutrient concentrations such as nitrate and silicate decline and zooplankton biomass is enhanced in the ocean mixed layer. The simulated development of the drawdown of carbon dioxide and increase of pH in surface seawater is in good agreement with available observations. Sensitivity studies with different supply dates of iron to the ocean emphasise the favourable oceanic conditions in the NE Pacific to generate massive phytoplankton blooms in particular during July and August in comparison to other months. By varying the amount of volcanic ash and associated bio-available iron supplied to the ocean, model results demonstrate that the NE Pacific Ocean has higher, but limited capabilities to consume CO₂ after iron fertilisation than those observed after the volcanic eruption of Kasatochi