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We introduce a trait-based description of diatom functional diversity to an existing plankton functional type (PFT) model, implemented for the eutrophied coastal ecosystem in the Southern Bight of the North Sea. The trait-based description represents a continuum of diatom species, each characterized by a distinct cell volume, and includes size dependence of four diatom traits: the maximum growth rate, the half-saturation constants for nutrient uptake, the photosynthetic efficiency, and the relative affinity of copepods for diatoms. Through competition under seasonally varying forcing, the fitness of each diatom varies throughout time, and the outcome of competition results in a changing community structure. The predicted seasonal change in mean cell volume of the community is supported by field observations: smaller diatoms, which are more competitive in terms of resource acquisition, prevail during the first spring bloom, whereas the summer bloom is dominated by larger species which better resist grazing. The size-based model is used to determine the ecological niche of diatoms in the area and identifies a range of viable sizes that matches observations. The general trade-off between small, competitive diatoms and large, grazing-resistant species is a convenient framework to study patterns in diatom functional diversity. PFT models and trait-based approaches constitute promising complementary tools to study community structure in marine ecosystems.

A mechanistic model has been developed to explore the factors controlling the production of domoic acid (DA) by the pennate diatom Pseudo-nitzschia. The idealized model allows consideration of the uncoupling between photosynthesis and growth, while DA production has been set as a secondary metabolism sharing common precursors with growth. Under growth limitation, these precursors can accumulate, resulting in an increased DA production. The model was first evaluated based on its ability to simulate the observed DA production by either silicon (Si) or phosphorus (P) limited batch cultures of Pseudo-nitzschia available in the literature. Sensitivity tests were further performed to explore how the ambient nutrients and the light regime (intensity and photoperiod length) are possibly directing the Pseudo-nitzschia toxicity. The general pattern that emerged is that excess light, in combination with Si or P limitation, favours DA production, provided nitrogen (N) is sufficient. Model simulations with varying nutrient stocks supporting Pseudo-nitzschia blooms under non-limiting light suggest two potential ways for nutrients to control DA production. First, N excess in comparison to available Si and P relieves DA production from its limitation by N, an absolute requirement of the DA molecule. Second, increased nutrient stocks amplify the DA production phase of the blooms (in addition to enhancing Pseudo-nitzschia biomass) which leads to an even more toxigenic bloom. Simulations investigating the light regime suggest a light threshold below which an important delay in DA production could be expected in Pseudo-nitzschia cultures. In the natural environment, the monitoring of light conditions during Pseudo-nitzschia blooms might help to anticipate the magnitude of the toxic event. Pseudo-nitzschia toxicity is indeed linked to the excess of primary carbon that accumulates during photosynthesis under growth limitation by nutrients.

The freshwater (RIVE) and the marine (MIRO) biogeochemical models were coupled to a 1D hydro-sedimentary model to describe contemporary phytoplankton successions and nutrient transfers in the macrotidal Scheldt estuary (BE/NL) affected by anthropogenic nutrient loads. The 1D-RIVE-MIRO model simulations are performed between Ghent and Vlissingen and the longitudinal estuarine profiles are validated with physico-chemical and phytoplankton observations available for the year 2006. Results show the occurrence of two distinct phytoplankton blooms in the upper and lower estuary, suggesting that neither the freshwater nor the marine phytoplankton is crossing the maximum turbidity zone (MTZ) at the saline transition. Sensitivity tests performed to understand how changing conditions (salinity, turbidity and nutrients) along the estuary are controlling this bimodal spatial phytoplankton distribution point salinity and light availability as key drivers while the grazing pressure and nutrient limitations are negligible. In the absence of species-specific salinity control or in low light limitation, freshwater phytoplankton is able to cross the MTZ and grow in the lower estuary where they compete with marine phytoplankton for nutrients. Additional tests with varying salinity-resistant (euryhaline) species in the freshwater assemblage conclude that the presence (or absence) of euryhalines determines the magnitude and the extension of freshwater and marine phytoplankton blooms in the estuary. Annual nutrient budgets estimated from 1D-RIVE-MIRO simulations show that biological activities have a negligible impact on nutrient export but modify the speciation of nutrients exported to the coastal zone towards inorganic forms.

Nutrient reduction measures have been already taken by wealthier countries to decrease nutrient loads to coastal waters, in most cases however, prior to having properly assessed their ecological effectiveness and their economic costs. In this paper we describe an original integrated impact assessment methodology to estimate the direct cost and the ecological performance of realistic nutrient reduction options to be applied in the Southern North Sea watershed to decrease eutrophication, visible as Phaeocystis blooms and foam deposits on the beaches. The mathematical tool couples the idealized biogeochemical GIS-based model of the river system (SENEQUE-RIVERSTRAHLER) implemented in the Eastern Channel/Southern North Sea watershed to the biogeochemical MIRO model describing Phaeocystis blooms in the marine domain. Model simulations explore how nutrient reduction options regarding diffuse and/or point sources in the watershed would affect the Phaeocystis colony spreading in the coastal area. The reference and prospective simulations are performed for the year 2000 characterized by mean meteorological conditions, and nutrient reduction scenarios include and compare upgrading of wastewater treatment plants and changes in agricultural practices including an idealized shift towards organic farming. A direct cost assessment is performed for each realistic nutrient reduction scenario. Further the reduction obtained for Phaeocystis blooms is assessed by comparison with ecological indicators (bloom magnitude and duration) and the cost for reducing foam events on the beaches is estimated. Uncertainty brought by the added effect of meteorological conditions (rainfall) on coastal eutrophication is discussed. It is concluded that the reduction obtained by implementing realistic environmental measures on the short-term is costly and insufficient to restore well-balanced nutrient conditions in the coastal area while the replacement of conventional agriculture by organic farming might be an option to consider in the nearby future.

A 3D coupled biogeochemical–hydrodynamic model (MIRO-CO2&CO) is implemented in the English Channel (ECH) and the Southern Bight of the North Sea (SBNS) to estimate the present-day spatio-temporal distribution of air–sea CO2 fluxes, surface water partial pressure of CO2 (pCO2) and other components of the carbonate system (pH, saturation state of calcite (Oca) and of aragonite (Oar)), and the main drivers of their variability. Over the 1994–2004 period, air–sea CO2 fluxes show significant inter-annual variability, with oscillations between net annual CO2 sinks and sources. The inter-annual variability of air–sea CO2 fluxes simulated in the SBNS is controlled primarily by river loads and changes of biological activities (net autotrophy in spring and early summer, and net heterotrophy in winter and autumn), while in areas less influenced by river inputs such as the ECH, the inter-annual variations of air–sea CO2 fluxes are mainly due to changes in sea surface temperature and in near-surface wind strength and direction. In the ECH, the decrease of pH, of Oca and of Oar follows the one expected from the increase of atmospheric CO2 (ocean acidification), but the decrease of these quantities in the SBNS during the considered time period is faster than the one expected from ocean acidification alone. This seems to be related to a general pattern of decreasing nutrient river loads and net ecosystem production (NEP) in the SBNS. Annually, the combined effect of carbon and nutrient loads leads to an increase of the sink of CO2 in the ECH and the SBNS, but the impact of the river loads varies spatially and is stronger in river plumes and nearshore waters than in offshore waters. The impact of organic and inorganic carbon (C) inputs is mainly confined to the coast and generates a source of CO2 to the atmosphere and low pH, of Oca and of Oar values in estuarine plumes, while the impact of nutrient loads, highest than the effect of C inputs in coastal nearshore waters, also propagates offshore and, by stimulating primary production, drives a sink of atmospheric CO2 and higher values of pH, of Oca and of Oar.

The accumulation of anthropogenic CO2 in the ocean has altered carbonate chemistry in surface waters since preindustrial times and is expected to continue to do so in the coming centuries. Changes in carbonate chemistry can modify the rates and fates of marine primary production and calcification. These modifications can in turn lead to feedback on increasing atmospheric CO2. We show, using a numerical model, that in highly productive nearshore coastal marine environments, the effect of eutrophication on carbon cycling can counter the effect of ocean acidification on the carbonate chemistry of surface waters. Also, changes in river nutrient delivery due to management regulation policies can lead to stronger changes in carbonate chemistry than ocean acidification. Whether antagonistic or synergistic, the response of carbonate chemistry to changes of nutrient delivery to the coastal zone (increase or decrease, respectively) is stronger than ocean acidification.

Recent field observations of Phaeocystis globosa colonies (number, size and cell content) and historical ecosystem model simulations have been used to define and diagnose undesirable eutrophication in the Belgian coastal zone (BCZ) as recommended by the EU Water Framework Directive and OSPAR. The proposed methodology included an initial definition of a reference abundance of Phaeocystis cells from which the disturbance could be scaled. This number was based on the Phaeocystis attribute that creates ecosystem disturbance: the ability of Phaeocystis colonies to grow and reach sizes unmanageable for the current copepods. The threshold was set at 4 × 106 cells L- 1 and corresponded to the maximum Phaeocystis cells contained in the grazable colonies (Phaeocystis colony cells simulated for pristine time by the coupled RIVERSTRAHLER-MIRO (R-MIRO) model, suggesting that natural Phaeocystis ecosystems are well balanced and efficiently transfer their production to higher trophic levels. However, both the field observations obtained during the 1988–2000 period and the 1950–2000 historical reconstruction of eutrophication in the BCZ simulated by the R-MIRO model showed Phaeocystis colony cell maxima well above the threshold value.Considering that a return to the pristine reference value is not achievable, another methodology was then proposed based on historical R-MIRO simulations of annual primary (PP) and copepod (CP) productions where the CP:PP ratio can be considered an indicator of trophic efficiency. The analysis of changing CP:PP ratios in relationship with simulated winter concentrations and annual inputs of nutrients showed that the decrease in trophic efficiency was related to the imbalanced DIN and DIP inputs (N:P > 25) explained here by the reduction in DIP loads after 1985 while DIN remained elevated. On this basis, a critical DIN load estimated at 60 kT y- 1 was proposed to re-equilibrate the N:P balance in the BCZ and to favour diatoms over Phaeocystis colonies. More generally we conclude that N:P ratios in nutrient loads (or winter stocks in the coastal area) in excess of 25 in Phaeocystis ecosystems are indicative of Phaeocystis colony dominance and weak trophic efficiency.

AMORE (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 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 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.

Massive blooms of Phaeocystis colonies usually occur in the Belgian coastal zone (BCZ) between spring and summer diatom blooms but their relative magnitude varies between years. In order to understand this interannual variability, we used the biogeochemical MIRO model to explore the link between diatom and Phaeocystis blooms and changing nutrient loads and meteorological conditions over the last decade. For this application, MIRO was implemented in a simplified 3-box representation of the domain between the Baie de Seine and the BCZ. MIRO was run over the 1989-2003 period using actual photosynthetic active radiation (PAR), seawater temperature and riverine nutrient loads as forcing. The water mass residence time was calculated for each box based on a monthly water budget estimated from 1993-2003 water flow simulations of the three-dimensional hydrodynamical model COHSNS-3D. Overall MIRO simulations compare fairly well with nutrient and phytoplankton data collected in the central BCZ but indicate the importance of the hydrodynamical resolution frame for correctly describing the extremely high nutrient concentrations and biomass observed in the BCZ. Analysis of model results suggests that while interannual variability in diatom biomass depends on both meteorological conditions (light and temperature) and nutrient loads, Phaeocystis blooms are mainly controlled by nutrients. Further sensitivity tests with varying N and P loads suggest that only N reduction will result in significantly decreased Phaeocystis blooms without negative affects on diatoms, while P reduction will negatively affect diatoms. Moreover, Atlantic nutrient loads play such a great role in BCZ enrichment that reduction of Scheldt nutrient loads only is not sufficient to significantly decrease phytoplankton blooms in the BCZ. It is concluded that future nutrient reduction policies aimed to decrease Phaeocystis blooms in the BCZ without impacting diatoms should target the decrease of N loads in both the Seine and the Scheldt rivers.

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system has been coupled to MIRO, a complex biogeochemical model describing diatom and Phaeocystis blooms in the marine domain, to investigate the link between changing human activities over the past 50 years and coastal eutrophication in the eastern Channel and Southern Bight of the North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt) is invaded each spring by massive blooms of Phaeocystis colonies which impact on the structure and functioning of the coastal ecosystem. For the present application MIRO is implemented in a simplified multi-box description of the coastal zone and is run over the last 50 years using average 1989–1999 meteorological conditions (global solar radiance and temperature) and RIVERSTRAHLER simulations of Seine and Scheldt nutrient loads as forcing conditions. Model validation is obtained by visual and statistical comparison of nutrients, Chl a, diatom and Phaeocystis simulations with data collected in the central BCZ (51°26.05 N; 02°48.50 E) over the 1992–1998 period.The simulated Phaeocystis and diatom trends (yearly average and maxima reached) in BCZ, over the past 50 years are discussed with respect to changing nutrient loads, especially NO3 and PO4. Historical reconstruction back to the fifties suggests that Phaeocystis colonies were already blooming in BCZ in the early 1950's and were sharing almost equally with diatoms, the bulk of annual primary production. Over the 1960–1992 period, the increased loads of both N and P were beneficial to both Phaeocystis colonies and summer diatoms with however a little advantage to the latter. Since 1989, the decrease in P loads subsequent to the removal of PO4 in washing powders and the maintenance of elevated NO3 loads had a positive feedback to both diatoms and Phaeocytis production with however a significantly larger impact on diatoms than on Phaeocytis. This is due to the fact that Phaeocystis has strong affinity for low PO4. Consequently Phaeocystis development is mainly controlled by NO3 loads. We conclude that future management of nutrient emission reduction aiming at decreasing Phaeocystis blooms in BCZ without impacting on diatoms would target decrease of NO3 loads by both the rivers Seine and Scheldt.

Nowadays, there is still a great uncertainty on the sink versus source role of the coastal zone for atmospheric CO2. As a first step to reduce this uncertainty we constructed a complex biogeochemical model to investigate the impacts of anthropogenic and terrestrial organic carbon and nutrients from the Scheldt estuary on the functioning of the Phaeocystis-dominated ecosystem of the Southern Bight of the North Sea and the related air to sea CO2 exchange. The model results of the coupling of a physicochemical module describing the seawater carbonate system and the air-sea exchange of CO2 to the existing biogeochemical model MIRO (Lancelot et al., 1997). The upgraded MIRO was implemented in the Southern Bight of the North Sea and simulations were run over the 1993-1999 period. The prediction capability of the upgraded MIRO model is assessed by comparing modelled fCO2 with existing field data in the Southern Bight of the North Sea. MIRO simulations are further used to analyse the seasonal dynamics of CO2 exchange at the air-sea interface and their controlling factors. Particular attention is given to wind speed, seawater temperature and biological processes, in particular those associated to the Phaeocystis bloom. Finally an annual budget based on carbon flows simulations allows to estimate the role of the Southern Bight of the North Sea as a source or sinking for atmospheric CO2.

[This corrects the article DOI: 10.1016/j.ecolmodel.2021.109690.]., (© 2023 The Authors.)