Advanced modeling and research on eutrophication AMORE II: final report
Daro, Marie Hermande
KeywordsEnvironnement et pollution
Océanographie physique et chimique
Modèles mathématiques d'aide à la décision
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AbstractAMORE (Advanced Modeling and Research on Eutrophication) is an interdisciplinary consortium composed of biologists and physical and ecological modellers focusing their research activities on coastal eutrophication in the eastern Channel and Southern Bight of the North Sea with special interest in harmful Phaeocystis colony blooms. The long-term objective of AMORE is to develop a a tri-dimensional ecological model - MIRO&CO - able to predict the magnitude and geographical extent of Phaeocystis colony blooms in the Eastern Channel and Southern Bight of the North Sea with focus on the Belgian Coastal Zone (BCZ) and in response to varying short-term climate conditions and riverine nutrient (N, P, Si) loads. To achieve this objective, AMORE has developed an integrated research methodology that involves and combines in an interactive way the collection of historical and new field data, process-level studies, statistical analysis, mathematical modelling and data assimilation. In this approach, the ecological model plays a central role as integrator of new knowledge gained from experimental studies and as tool for eutrophication assessment and prediction as well as decision support. Between 2002 and 2006, AMORE research focused on mechanisms determining Phaeocystis colony bloom inception and development, their dominance over diatoms in spring and their trophic fate. Particular attention was paid to the role of gelatinous organisms, especially Noctiluca which blooms at Phaeocystis colony decline. The relevant knowledge gained was synthesized for integration in the existing ecological MIRO which in turn was coupled with the 3D COHSNS hydrodynamical model developed for describing water transport in the studied domain. Progress achieved in our understanding of eutrophication mechanisms in the BCZ as well as our present ability to predict Phaeocystis spreading and magnitude in response to riverine nutrient delivery are discussed in the present report. For the first time in the region, molecular tools identified as P. globosa the Phaeocystis species blooming in the Southern Bight of the North Sea. Experimental studies on Phaeocystis life cycle evidenced a haploid-diploid cycle in which haploid flagellates persist in the water column between blooms of diploid colonial cells, suggesting that colony bloom initiation and termination involve sexual processes. The recurrent diatom-Phaeocystis succession observed in the area suggests that early spring diatoms could play a triggering role in syngamy. Statistical analysis of diatom and Phaeocystis data collected between 1992 and 2000 in BCZ showed that their relative contribution to the spring community was regulated by both river nutrient loads (mankind) and hydro-climatic conditions themselves under the influence of the North Atlantic Oscillation (NAO). Years with elevated Phaeocystis blooms in central BCZ were identified as those characterized by a medium NAO index, i.e. when a maximum of NO3 delivered by the Scheldt is spread Project EV/19 – “Advanced modeling and research on eutrophication - AMORE II” SPSD II – Part 2 – Global change, ecosystems and biodiversity – North Sea 6 over BCZ. This observation, also supported by 0D multi-box MIRO runs, suggests that excess NO3 (but low PO4) sustain the growth of Phaeocystis colonies. For the first time, PO4 limitation was demonstrated in the BCZ via the detection of alkaline phosphatase activity in spring. One major result is that this enzymatic activity is associated mainly to large particles including phytoplankton cells and their attached bacteria. Specific grazing experiments on Phaeocystis ultimately concluded that healthy Phaeocystis colonies are not grazed by either copepods or gelatinous organisms but are mostly degraded in the water column rather than the sediment. Numerical experiments included 0D and 3D modelling. The published version of 0D multi-box MIRO was further tested and upgraded. Results of the sensitivity analysis (variatonal adjoint method and sensitivity factor computation) identified processes associated to the microbial loop dynamics as main controls of the time evolution of all the MIRO state variables in BCZ. More specific sensitivity tests were showing in addition that the timing of the spring bloom of both diatom and Phaeocystis was determined by the light availability and their maximum photosynthetic specific rate. New developments and applications of 0D multi-box MIRO included (i) the addition and test of a CO2 module pointing the role of riverine nutrient loads in stimulating the uptake of atmospheric CO2 in BCZ, (ii) the upgrading of the sediment diagenetic module and its further simplified parameterization for integration in MIRO&CO-3D and, (iii) the exploration of causes of diatom-Phaeocystis bloom variability in the Belgian waters over the last decade. The latter study concluded that while the diatom variability was depending on both meteorological conditions (light and temperature) and nutrient loads, Phaeocystis blooms were mainly controlled by nutrients, especially NO3. The MIRO&CO-3D was implemented by coupling the published version of MIRO to the COHSNS-3D hydrodynamical model. The geographical domain extended between 4° W (48.5°N) and 52.5°N (4.5°E) with a grid resolution of 5.6 x 4.6 km and included inputs from the main rivers within this domain (Rhine/Meuse, Scheldt/Leie/Ijzer, Thames, Seine). Model simulations were performed for the period 1993-2003 and validated based on a successful comparison of simulated salinity and in situ measurements and by comparing biogeochemical results [nutrient and phytoplankton (Chl a)] with field measurements (time series at fixed stations, surface seasonal mean, monthly mean MERIS-derived Chl a). MIRO&CO-3D sensitivity tests with decreasing (1%) river nitrogen and phosphorus inputs from different origins explored the relative impact of the different rivers (Scheldt/Rhine/Seine) and Channel waters on the nutrient availability in the Belgian waters. Results showed that Channel nutrient inflow had a direct effect on the surface nutrients over the whole domain. The comparison between the three rivers suggested that the effect of the Seine River is the most important for the whole domain considered except in BCZ areas where eutrophication is severe due to Scheldt loads. Interestingly enough, the effect of nutrient reduction from all rivers was higher for DIN than that for PO4, expressing a stronger sensitivity of DIN to riverine nutrient reduction because rivers supply relatively more DIN than PO4 as compared to the Atlantic inflow. Finally, these results in agreement with mitigation scenarios conducted with 0D multi-box MIRO, concluded that an integrated management plan involving riparian countries of the eastern Channel and Southern Bight of the North Sea and targeting NO3 delivery is needed to decrease eutrophication and Phaeocystis blooms in BCZ.