ALBERT

Description: Ecosystem models often rely on heuristic descriptions of autotroph growth that fail to reproduce various stationary and dynamic states of phytoplankton cellular composition observed in laboratory experiments. Here, we present the integration of an advanced phytoplankton growth model within a coupled 3-dimensional physical-biogeochemical model, and the implementation of the model system to the Southern North Sea (SNS) defined on a relatively high resolution (~ 1.5–4.5 km) curvilinear grid. The autotrophic growth model, recently introduced by Wirtz and Kerimoglu (2016), is built up on a set of novel concepts for the allocation of internal resources and operation of cellular metabolism. The coupled model system consists of the general estuarine transport model (GETM) as the hydrodynamical driver, a lower trophic level model and a simple sediment diagenesis model. We force the model system with realistic atmospheric and riverine fluxes, background turbidity caused by suspended particulate matter and open ocean boundary conditions. For a simulation for the period 2000–2010, we show that the model system satisfactorily reproduces the physical and biogeochemical states of the system, as inferred from comparisons against data from long-term monitoring stations, sparse measurements, continuous transects, and remote sensing data. In particular, the model shows high skill both in coastal and off shore waters, and captures the steep gradients in nutrient and chlorophyll concentrations observed prevalently across the coastal transition zone. We show that the cellular chlorophyll to carbon ratio show significant seasonal and lateral variability, the latter amplifying the steepness of the transitional chlorophyll gradient, thus, pointing to the relevance of resolving the physiological acclimation processes for an accurate description of biogeochemical fluxes.

Description: Marine coastal ecosystem functioning is crucially linked to the transport and fate of suspended particulate matter (SPM). Transport of SPM is, amongst others, controlled by sinking velocity ws. Since ws of cohesive SPM aggregates varies significantly with size and composition of mineral and organic origin, ws probably exhibits large spatial variability along gradients of turbulence, SPM concentration and SPM composition. In this study, we retrieved ws for the German Bight, North Sea, by combining measured vertical turbidity profiles with simulation results for turbulent eddy diffusivity. Analyzed with 5 respect to modeled prevailing energy dissipation rates &epsilon,, mean ws were significantly enhanced around log10(ε (m2s−3)) ≈ −5.5. This ε region is typically found at water depths of approximately 15 m to 20 m on a cross-shore transect. Across this zone, SPM concentration declines drastically towards the offshore and a change in particle composition occurs. This characterizes a transition zone with potentially enhanced vertical fluxes. Our findings contribute to the conceptual understanding of nutrient cycling in the coastal region which is as follows: Previous studies identified an estuarine circulation. Its residual landward-oriented bottom currents are likely loaded with SPM particularly within the transition zone. This retains and traps fine sediments and particulate-bound nutrients in coastal waters where organic components of SPM become re-mineralized. Residual surface currents transport dissolved nutrients towards the off-shore, where they are again consumed by phytoplankton. Algae excrete extracellular polymeric substances which are known to mediate mineral aggregation and thus sedimentation. This probably takes place particularly in the transition zone and completes the coastal nutrient cycle. The efficiency of the transition zone for retention is thus suggested as an important mechanism that underlies the often observed nutrient gradients towards the coast.

Description: The study addresses the nitrogen cycling in Elbe estuary. Observations of salinity, nutrients and oxygen from moored stations, ship casts and helicopter surveys are presented. Observations are complemented by simulations obtained from a coupled physical-biogeochemical 3D unstructured model, applied for the first time to the estuarine environment. Model simulations reproduce the temporal variability of nutrients and oxygen along the estuarine salinity gradient. Both, observations and model results, demonstrate mostly conservative mixing of nitrate and non-conservative behavior of ammonium. Model hind-casts of the years 2012 and 2013 provide a detailed reconstruction of nitrogen recycling with ammonium appearing as the key species of the remineralisation process. Estuarine turnover processes are fueled by inputs of diatoms and organic nitrogen at the tidal weir with intense primary production manifest in the shallow river section downstream of the weir. The harbor area is the hot spot of heterotrophic decay associated with growth of meso-zooplankton, sedimentation of degradable material, remineralisation, oxygen depletion, denitrification and ammonium production. In the harbor, biochemistry shows strong vertical gradients while hydrodynamics demonstrate connectivity between the main channel and the harbor. At the estuary bed nitrogen is deposited during spring and early summer. Resuspension leads to nearly closed budget by the end of the year. During the Elbe flood in June 2013, estuarine biogeochemistry is significantly disturbed with the harbor being deactivated as hot spot of heterotrophic decay. Plankton and organic matter are flushed towards the outer estuary which in consequence sees high abundance of grazers, oxygen depletion and elevated release of ammonium.