Phytoplankton size classes competitions at sub-mesoscale in a frontal oceanic region

The effects of mesoscale and sub-mesoscale dynamics on the competition between two different phytoplankton size classes are investigated with a 3D primitive equations model. The model reproduces realistic simulations of mesoscale turbulence generated by a westward current in the southern hemisphere at statistical equilibrium in a summer situation. Effects of two different grazing pressures on phytoplankton competitions are compared and the role of eddy variability is quantified comparing high and low resolution simulations.High resolution simulations reveal a filamentary distribution of biomass and nutrients induced by the combination of vertical advection and horizontal stirring. This fine scale variability is observed not only on the horizontal but also on the vertical into the subsurface chlorophyll maximum.One of the key results is that such a dynamics induces a spatial segregation of the phytoplankton in the southern part of the frontal region that is mainly filamentary. This spatial segregation consists in biomass maxima for large phytoplankton in rich nutrients filaments and maxima for small phytoplankton outside these filaments. This anti-correlation is particularly strong when grazing pressure is low and is confirmed by statistical analysis. In the central frontal region, dominated by mesoscale dynamics, the two phytoplankton classes are strongly correlated together and biomass maxima are located close to downwelling regions that are poor in nutrients.It is shown that the effect of grazing is significantly amplified by the fine scale dynamics and that the combination of these two mechanisms is responsible of a switch of the ecosystem dominance in the surface layers.In addition, the effect of frontal dynamics on the detritus export is very sensitive to grazing pressure: increasing grazing induces a significant decrease of the export in the presence of frontal dynamics whereas it induces an increase of the export without small-scale variability.     

Validation of the 3D biogeochemical model MIRO&CO with field nutrient and phytoplankton data and MERIS-derived surface chlorophyll a images

This paper presents results obtained with MIRO&CO-3D, a biogeochemical model dedicated to the study of eutrophication and applied to the Channel and Southern Bight of the North Sea (48.5°N–52.5°N). The model results from coupling of the COHERENS-3D hydrodynamic model and the biogeochemical model MIRO, which was previously calibrated in a multi-box implementation. MIRO&CO-3D is run to simulate the annual cycle of inorganic and organic carbon and nutrients (nitrogen, phosphorus and silica), phytoplankton (diatoms, nanoflagellates and Phaeocystis), bacteria and zooplankton (microzooplankton and copepods) with realistic forcing (meteorological conditions and river loads) for the period 1991–2003. Model validation is first shown by comparing time series of model concentrations of nutrients, chlorophyll a, diatom and Phaeocystis with in situ data from station 330 (51°26.00′N, 2°48.50′E) located in the centre of the Belgian coastal zone. This comparison shows the model's ability to represent the seasonal dynamics of nutrients and phytoplankton in Belgian waters. However the model fails to simulate correctly the dissolved silica cycle, especially during the beginning of spring, due to the late onset (in the model) of the early spring diatom bloom. As a general trend the chlorophyll a spring maximum is underestimated in simulations. A comparison between the seasonal average of surface winter nutrients and spring chlorophyll a concentrations simulated with in situ data for different stations is used to assess the accuracy of the simulated spatial distribution. At a seasonal scale, the spatial distribution of surface winter nutrients is in general well reproduced by the model with nevertheless a small overestimation for a few stations close to the Rhine/Meuse mouth and a tendency to underestimation in the coastal zone from Belgium to France. PO₄ was simulated best; silica was simulated with less success. Spring chlorophyll a concentration is in general underestimated by the model. The accuracy of the simulated phytoplankton spatial distribution is further evaluated by comparing simulated surface chlorophyll a with that derived from the satellite sensor MERIS for the year 2003. Reasonable agreement is found between simulated and satellite-derived regions of high chlorophyll a with nevertheless discrepancies close to the boundaries.     

Impact of phytoplankton community size on a linked global ocean optical and ecosystem model

We isolated the effect phytoplankton cell size has on varying remote sensing reflectance spectra (R_rs(λ)) in the presence of optically active constituents by using optical and radiative transfer models linked in an offline diagnostic calculation to a global biogeochemical/ecosystem/circulation model with explicit phytoplankton size classes. Two case studies were carried out, each with several scenarios to isolate the effects of chlorophyll concentration, phytoplankton cell size, and size-varying phytoplankton absorption on R_rs(λ). The goal of the study was to determine the relative contribution of phytoplankton cell size and chlorophyll to overall R_rs(λ) and to understand where a standard band ratio algorithm (OC4) may under/overestimate chlorophyll due to R_rs(λ) being significantly affected by phytoplankton size. Phytoplankton cell size was found to contribute secondarily to R_rs(λ) variability and to amplify or dampen the seasonal cycle in R_rs(λ), driven by chlorophyll. Size and chlorophyll were found to change in phase at low to mid-latitudes, but were anti-correlated or poorly correlated at high latitudes. Phytoplankton size effects increased model calculated R_rs(443) in the subtropical ocean during local spring through early fall months in both hemispheres and decreased R_rs(443) in the Northern Hemisphere high latitude regions during local summer to fall months. This study attempts to tease apart when/where variability about the OC4 relationship may be associated with cell size variability. The OC4 algorithm may underestimate [Chl] when the fraction of microplankton is elevated, which occurs in the model simulations during local spring/summer months at high latitudes in both hemispheres.     

A spatially explicit ecosystem model of seston depletion in dense mussel culture

A fully-coupled biological–physical–chemical model of a coastal ecosystem was constructed to examine the impact of suspended mussel culture on phytoplankton biomass in Tracadie Bay, Prince Edward Island, Canada. Due to the extent of mussel culture there, we hypothesised that shellfish filtration would control the concentration and distribution of phytoplankton and other suspended particles in the bay. Circulation was delineated with a tidally-driven 2D numerical model and used to drive an ecosystem model with a focus on pelagic components including phytoplankton production, nutrients, detritus, and mussels. The benthos were treated as a sink. Nutrients and seston were forced by tidal exchange and river input, with phytoplankton additionally forced by light. Boundary conditions of seston and nutrients were derived from field studies with an emphasis on the contrast between spring (high river nutrients, low temperature) and summer (low river inputs and high temperatures). Model output was used to map phytoplankton carbon over the bay for each season and in the presence of mussels and river nutrient input. Results indicate severe depletion effects of mussel culture on overall phytoplankton biomass, but no spatial pattern that can be attributed to grazing alone. Primary production generated by nutrient-rich river water created a mid-bay spike in phytoplankton that dominated the spatial pattern of chlorophyll-based carbon. Model results were validated with surveys from a towed sensor array (Acrobat) that confirmed the river influence and indicated bay-wide depletion of 29% between high and low water. Our model results indicate that the farm-scale depletion emphasised in previous studies cannot simply be extrapolated to seston limitation at the ecosystem level.     

Simulation of the oil spill processes in the Sea of Japan with regional ocean circulation model

A simulation of the movement of spilled oil after the incident of the Russian tanker Nakhodka in the Sea of Japan, in January 1997, was performed by a particle tracking model incorporating advection by currents, random diffusion, the buoyancy effect, the parameterization of oil evaporation, biodegradation, and beaching. The currents advecting spilled oil were defined by surface wind drift superposed on the three-dimensional ocean currents obtained by the Geophysical Fluid Dynamics Laboratory modular ocean model (GFDL MOM), which was forced by the climatological monthly mean meteorological data, or by the European Center for Medium Range Weather Forecasts (ECMWF) daily meteorological data, and assimilated sea surface topography detected by satellite altimeter. A number of experiments with different parameters and situations showed that the wide geographical spread of oil observed is not explained by wind drift alone, and that including the simulated climatological currents gives better results. The combination of surface wind drift and daily ocean currents shows the best agreement between the model and observations except in some coastal areas. The daily meteorological effect on the ocean circulation model results in a stronger variability of currents that closely simulates some features of the nonlinear large-scale horizontal turbulent diffusion of oil. The effect of different parameterizations for the size distribution of model oil particles is discussed. Received for publication on July 26, 1999; accepted on Nov. 17, 1999     

Sequential data assimilation in an upwelling influenced estuary

We have evaluated the impact of assimilating chlorophyll, nitrate, phosphate, silicate and ammonium into a coupled 1D hydrodynamic ecosystem model (GOTM-ERSEM) in an upwelling influenced estuary. The assimilation method chosen is the Ensemble Kalman Filter (EnKF), which has been demonstrated to improve field estimates of key variables (chlorophyll, nutrients) for bulk algal bloom prediction. The 1D model has been set up for a central station inside the Ría de Vigo (Spain). Data from bi-weekly surveys are used to constrain the model. Temperature and salinity profiles are used to ensure the correct representation of the water structure through a relaxation scheme. Chlorophyll extracts and nutrients at three depths are assimilated sequentially during 1 year simulation (1991). The assimilation period includes episodes of active upwelling and downwelling. All five assimilated variables are successfully constrained and represent a large improvement on the reference simulation (without assimilation). Small divergences can be related to poorly resolved physical processes in the model. The assimilation was further evaluated by comparing observed biomass partitioning with model results. Diatoms accounted for the largest biomass update and the largest improvement in terms of percentage of variance explained (R²). This is particularly significant as they represent the 46% of the yearly integrated observed biomass of the planktonic autotrophs. Nonetheless the R² value was low for all phytoplankton groups. Bacteria and nanoflagellates showed an improvement with respect to their yearly Root Mean Square (RMS), while the other functional groups worsen or remained unaffected. Chlorophyll assimilation was responsible for most of the impact on the phytoplankton biomass with small contributions from the silicate. It had minor impact on the updates of nutrients which in turn corrected the state variables related to the detrital pool. In this current setting, combined assimilation of chlorophyll and nutrients is not sufficient to produce a skillful simulation of the phytoplankton succession.     

Analysis of the contribution of ice algae to the ice-covered ecosystem in Lake Saroma by means of a coupled ice–ocean ecosystem model

A new coupled ice–ocean ecosystem model that links the pelagic and ice ecosystems was used to clarify the role of ice algae in ice-covered ocean ecosystems. The model was applied to Lake Saroma (Hokkaido, Japan) in 1992. Comparison of the model's results with observational data confirmed that the model reproduced the behavior of the ecosystem with acceptable accuracy during the period from winter to spring. The primary production of the ice algae is effectively transported into the pelagic system by means of physical releasing effects: brine convection, ice melting and freezing, and diffusion generated at the bottom of the ice. Ice algae released from the ice are rapidly exported because of their high sinking speed and the shallow depth of Lake Saroma. For this reason, the zooplankton in Lake Saroma cannot graze these released algae. However, zooplankton actively graze the ice algae living along the bottom of the ice. These results show that, before their release, ice algae play an important role as a food source for overwintering zooplankton. A sensitivity analysis revealed a positive correlation between the sinking speed of the released ice algae and the magnitude of the spring bloom by pelagic phytoplankton, and that the time when secondary production becomes active is an important factor in the linkage between these two algal populations.     

Spatial and temporal variability in the pelagic ecosystem of the East Sea (Sea of Japan): A review

The East Sea (Sea of Japan) is a unique marginal sea because it exhibits features of oceanic dynamics of much larger ocean basins. This semi-enclosed basin may be considered as a model or microcosm for understanding of how biological processes and distributions in pelagic ecosystem are interacting with physical processes in highly dynamic ocean regions. This overview summarizes the recent progresses concerning spatial and temporal variability of pelagic ecosystem components form an interdisciplinary point of view. Spatial characteristics of physical environments and biogeography in the region are distinguished mainly by the subpolar front. It was also found that long-term changes in biomass and community structure as well as those in the physical and biological environments are associated with climate variability in the region. We conclude by identifying main needs for the information and researches, particularly regular and long-term sampling, and permanent monitoring if possible.     

A generalized model of pelagic biogeochemistry for the global ocean ecosystem. Part I: Theory

The set of equations for global ocean biogeochemistry deterministic models have been formulated in a comprehensive and unified form in order to use them in numerical simulations of the marine ecosystem for climate change studies (PELAGOS, PELAgic biogeochemistry for Global Ocean Simulations). The fundamental approach stems from the representation of marine trophic interactions and major biogeochemical cycles introduced in the European Regional Seas Ecosystem Model (ERSEM). Our theoretical formulation revisits and generalizes the stoichiometric approach of ERSEM by defining the state variables as Chemical Functional Families (CFF). CFFs are further subdivided into living, non-living and inorganic components. Living CFFs are the basis for the definition of Living Functional Groups, the biomass-based functional prototype of the real organisms. Both CFFs and LFGs are theoretical constructs which allow us to relate measurable properties of marine biogeochemistry to the state variables used in deterministic models. This approach is sufficiently generic that may be used to describe other existing biomass-based ecosystem model.     

Quantitative estimation of the influence of surface thermal fronts over chlorophyll concentration at the Patagonian shelf

Eighteen-year (1985–2002) mean monthly SST Pathfinder data with 9 km spatial resolution have been used to estimate surface gradients by finite differences. Then the seasonal climatological means have been calculated from the intensity of these gradients, and surface thermal fronts present in the Patagonian Continental Shelf (PCS) have been located. Moreover, 6 years (1998–2003) of SeaWiFS data with approximately 4 km spatial resolution have been used to estimate monthly composite images of surface chlorophyll concentration, after which seasonal climatological means distributions have been generated. Both seasonal distributions have been analyzed together and by combining the knowledge of oceanographic processes and phytoplankton responses to light and nutrient availability, regions where the presence of a thermal front affects photosynthetic activity have been identified. Subjective criteria have been applied to define eighteen areas where phytoplankton biomass is influenced by the presence of a thermal front. In these areas, the surface chlorophyll (spatial mean and total), its relationship with the surface chlorophyll of the whole region, and the seasonal evolution of this relationship have been calculated. All frontal areas cover less than 15% of the total surface, but they contribute with over 23% of the phytoplankton annual mean biomass. Considered as a group, during summer they show high chlorophyll values very similar to those in spring. During the cold period, when the water column is vertically mixed in practically the whole of PCS, the influence of physical fronts over the biological production is minimum. The frontal zone image remains clearly defined during summer, when approximately 85% of the area will have a determined mean chlorophyll concentration, while the other 15% has a 2.45 times larger value. While three pattern trends have been identified in the frontal areas, only two of them condition the pattern of the group, due to their horizontal extension.     

Why is the southwest the most productive region of the East Sea/Sea of Japan?

The East Sea/Sea of Japan is a moderately productive sea that supports a wealth of living marine resources. Of the East Sea subregions, the southwest has the highest productivity. Various authors have proposed coastal upwelling, the Tsushima Current, the Changjiang Dilute Water, eddies, or discharge from the Nagdong River as potential sources of additional nutrients. In this paper, we propose, using satellite data from 1998 to 2006, that the biological productivity of the southwestern region is enhanced mainly by wind-driven upwelling along the Korean coast. Firstly, the climatology of seasonal patterns suggests that the enhanced chlorophyll a along the Korean coast is of local origin. Secondly, coastal upwelling is frequent in all seasons except winter. For example, along the coast of the Ulgi region, enhanced chlorophyll a due to coastal upwelling was observed for 25–92% of the time between Jun and Sep in the period 1998–2006. Thirdly, the advection of upwelled water through various pathways to the deeper basin was observed. Fourthly, there appeared to be a strong correlation between the interannual chlorophyll a variations of the coastal upwelling regions and the Ulleung Basin. The chlorophyll a patterns of both regions were closely related to the wind pattern in the upwelling regions, but not to that in the Ulleung Basin. Finally, changes in advection pathways also appeared to affect the productivity of the Ulleung Basin. Since 2004, there has been a shift in the pathways of upwelled water, and consequent increases in chlorophyll a in the Ulleung Basin were observed. This last observation requires further investigation.     

Modelling the ice-ocean-plankton interactions in the Souther Ocean

Our goal is the study of interactions between sea ice and ocean and of their influence on planktonic communities. We use a physical model which includes explicitly melting dynamics and mixed-layer physics. A one-dimensional model of the water column with a k-1 turbulent closure is applied. The sea-ice model is the one proposed by Semtner (1976); we add a parameterization of leads. We enlighten the importance, in this kind of model, of the sharing of the energy between lateral and basal meltings. The biological model comprises two state variables: phytoplankton and zooplankton biomasses. Melting induces a persistent shallow mixed layer and thus appropriate conditions for primary production. If ice melting is present, high biomasses are possible even with high losses. The absence of ice nearly forbids a massive bloom to form. Some sensitivity studies have shown that grazing pressure is a key factor governing the evolution of biomasses. The biomasses are also sensitive to little modifications of the photosynthetic production. The initial amount of phytoplankton or the presence of ice algae seems to be of lesser importance.     

Top-down effects and carbon flux in the ocean: A hypothesis

The close relationships between mesozooplankton, chlorophyll, and primary production normally observed across latitudes may be regarded as a bottom-up effect of increased productivity or, conversely, as a top-down effect of mesozooplankton on microzooplankton, releasing phytoplankton growth. The time-course of primary producers and mesozooplankton observed during the Late Winter Bloom in subtropical waters suggests a top-down effect, similar to the results found in lakes and mesocosm experiments. Two scenarios were observed during the development of the bloom in the Canary Island waters. The first was the increase in mesozooplankton as the effect of higher primary production due to mixing. The second scenario was the decrease in mesozooplankton due to the top-down effect of predation by diel vertical migrants upon epipelagic mesozooplankton. The structure of the food web in the euphotic zone was different depending on the presence or not of mesozooplankton, suggesting important consequences for the transport of carbon in those large areas of the ocean.     

Seasonal and interannual variation of physical and biological processes during 1994–2001 in the Sea of Japan/East Sea: A three-dimensional physical–biogeochemical modeling study

A Pacific basin-wide physical–biogeochemical model has been used to investigate the seasonal and interannual variation of physical and biological fields with analyses focusing on the Sea of Japan/East Sea (JES). The physical model is based on the Regional Ocean Model System (ROMS), and the biogeochemical model is based on the Carbon, Si(OH)₄, Nitrogen Ecosystem (CoSiNE) model. The coupled ROMS–CoSiNE model is forced with the daily air–sea fluxes derived from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis for the period of 1994 to 2001, and the model results are used to evaluate climate impact on nutrient transport in Mixed Layer Depth (MLD) and phytoplankton spring bloom dynamics in the JES.The model reproduces several key features of sea surface temperature (SST) and surface currents, which are consistent with the previous modeling and observational results in the JES. The calculated volume transports through the three major straits show that the Korea Strait (KS) dominates the inflow to the JES with 2.46 Sv annually, and the Tsugaru Strait (TS) and the Soya Strait (SS) are major outflows with 1.85 Sv and 0.64 Sv, respectively. Domain-averaged phytoplankton biomass in the JES reaches its spring peak 1.8 mmol N m^− 3 in May and shows a relatively weak autumn increase in November. Strong summer stratification and intense consumption of nitrate by phytoplankton during the spring result in very low nitrate concentration at the upper layer, which limits phytoplankton growth in the JES during the summer. On the other hand, the higher grazer abundance likely contributes to the strong suppression of phytoplankton biomass after the spring bloom in the JES. The model results show strong interannual variability of SST, nutrients, and phytoplankton biomass with sudden changes in 1998, which correspond to large-scale changes of the Pacific Decadal Oscillation (PDO). Regional comparisons of interannual variations in springtime were made for the southern and northern JES. Variations of nutrients and phytoplankton biomass related to the PDO warm/cold phase changes were detected in both the southern and northern JES, and there were regional differences with respect to the mechanisms and timing. During the warm PDO, the nutrients integrated in the MLD increased in the south and decreased in the north in winter. Conversely, during the cold PDO, the nutrients integrated in the MLD decreased in the south and increased in the north. Wind divergence/convergence likely drives the differences in the southern and northern regions when northerly and northwesterly monsoon dominates in winter in the JES. Subjected to the nutrient change, the growth of phytoplankton biomass appears to be limited neither by nutrient nor by light consistently both in the southern and northern regions. Namely, the JES is at the transition zone of the lower trophic-level ecosystem between light-limited and nutrient-limited zones.     

Nutrient status of the Northeast Water Polynya

The nutrient distribution in the Northeast Water Polynya (NEW) was investigated intensively between the end of May and the beginning of August 1993 during the R/V Polarstern cruise ARK IX. The major characteristics were low initial nitrate concentrations (ca. 4 μM) in the surface mixed layer of the East Greenland Shelf Water, accompanied by high silicate values (ca. 10–14 μM). These concentrations were not reduced by phytoplankton growth. Silicate was rather homogeneously distributed in the entire water column, whereas nitrate increased continuously with depth to about 13 μM. Phosphate concentrations were about 1.1 μM and had a similar distribution to that of silicate. During the course of the summer, nutrients became depleted, and nitrate was exhausted in large parts of the NEW. Silicate was reduced to values of less than 2 μM at some stations which implies that diatom growth continued despite nitrate depletion, ammonium serving as a nitrogen source. The polynya is fertilised by water with the initial nutrient concentrations downstream of the Norske Øer Ice Shelf. This process continuously supplies nutrients to the surface throughout the year and these are transported northward by the anticyclonic surface circulation following the topography of the trough system. The northern boundary of this tongue of relatively nutrient-rich water is controlled by the uptake of nutrients by phytoplankton in summer. Its extemsion is variable due to interactions between biological processes, circulation and ice cover. In the Ob Bank region the nutrient distribution can be altered by the inflow of Polar Water from the north when strong northerly winds prevail as happened during the first part of the study.     

Intrusions of eastern North Atlantic central waters and phytoplankton in the north and northwestern Iberian shelf during spring

The influence of intrusions of eastern North Atlantic central water (ENACW) in the north and northwestern Iberian shelf on phytoplankton composition and abundance and on particle-size distributions of seston was analyzed using data collected on three extensive cruises during spring 1991 and 1992. Water with temperature and salinity values between 12.20 and 13.86 °C and between 35.66 and 35.98 psu, respectively, characteristics of the subtropical type of ENACW (ENACW_t), was detected in the upper 100 m of the water-column in all cruises, but particularly in the western coast in 1992. The highest salinity values of this water were found near the surface (0–100-m depth) and in early spring 1992, while minimum salinity values, and also minimum geographical extension, were found in late spring in both years. Phytoplankton blooms concentrated in frontal areas between different water types, with maximum intensity and extension in early spring.Using temperature and salinity characteristics, samples were classified in four groups corresponding to the major water types found in the region: Bay of Biscay central water (BBCW), two segments of ENACW of different salinity and surface water influenced by continental runoff. This classification was significantly confirmed by three independent discriminant analyses using hydrographic and chemical (dissolved nutrients and chlorophyll) variables, phytoplankton species abundance variables and particle-size concentration of seston variables. Phytoplankton blooms related to the presence of saline waters were characterized by the dominance of either chain-forming diatoms or a mixture of diatoms and phytoflagellates and high concentrations of seston. The diatom species dominating in saline waters were typical of upwelling-induced blooms occurring generally during summer. Blooms occurring in waters influenced by runoff also contained diatoms but in lower numbers than those of saline waters. Nutrients were not exhausted in the region, suggesting that phytoplankton populations were still in active growth. These results are interpreted taking into account the known variability in water-mass formation and in the poleward current driving ENACW_t along the shelf, and indicate that saline intrusions are a major feature affecting the distribution and composition of plankton in the spring in the southern Bay of Biscay, thus enlarging to a wider spatial scale their reported influence on the pelagic ecosystem.     

Study of the seasonal cycle of the biogeochemical processes in the Ligurian Sea using a 1D interdisciplinary model

A one-dimensional coupled physical–biogeochemical model has been built to study the pelagic food web of the Ligurian Sea (NW Mediterranean Sea). The physical model is the turbulent closure model (version 1D) developed at the GeoHydrodynamics and Environmental Laboratory (GHER) of the University of Liège. The ecosystem model contains 19 state variables describing the carbon and nitrogen cycles of the pelagic food web. Phytoplankton and zooplankton are both divided in three size-based compartments and the model includes an explicit representation of the microbial loop including bacteria, dissolved organic matter, nano-, and microzooplankton. The internal carbon/nitrogen ratio is assumed variable for phytoplankton and detritus, and constant for zooplankton and bacteria. Silicate is considered as a potential limiting nutrient of phytoplankton's growth. The aggregation model described by Kriest and Evans in (Proc. Ind. Acad. Sci., Earth Planet. Sci. 109 (4) (2000) 453) is used to evaluate the sinking rate of particulate detritus. The model is forced at the air–sea interface by meteorological data coming from the “Côte d'Azur” Meteorological Buoy. The dynamics of atmospheric fluxes in the Mediterranean Sea (DYFAMED) time-series data obtained during the year 2000 are used to calibrate and validate the biological model. The comparison of model results within in situ DYFAMED data shows that although some processes are not represented by the model, such as horizontal and vertical advections, model results are overall in agreement with observations and differences observed can be explained with environmental conditions.     

An eddy-permitting, coupled ecosystem-circulation model of the Arabian Sea: comparison with observations

A nitrogen-based, pelagic ecosystem model has been coupled with an eddy-permitting ocean general circulation model of the Arabian Sea, and the results are compared with observations. The seasonal variability simulated by the model is in good agreement with observations: during the southwest monsoon season, phytoplankton increases in the western Arabian Sea due to upwelling along the coast; during the northeast monsoon season, phytoplankton abundance is large in the northern Arabian Sea because of the enhanced nitrate entrained by relatively deep vertical mixing. Two major differences are, however, found in the basin-wide comparison between model results and observations: an unrealistic nitrate maximum in the subsurface layer of the northern Arabian Sea and too low primary production in oligotrophic regimes. The former may be attributed to the lack of denitrification in the model. Possible causes for the latter include the present model's underestimation of fast nutrient recycling, the neglect of carbon fixation decoupled from nitrogen uptake and of nitrogen fixation, and inadequate nitrate entrainment by mixed layer deepening. The rate at which simulated nitrate increases in the northern Arabian Sea is 11–24 TgN/year, and should correspond to the denitrification rate integrated over the northern Arabian Sea assuming that the loss of nitrogen through denitrification is balanced by advective input. The model does not reproduce the observed phytoplankton bloom in the late southwest monsoon season. Possible causes are that the mixed layer may be too shallow in summer and that the horizontal transport of nitrate from the coast of Oman may be too weak. Sensitivity experiments demonstrate a strong dependence of the simulated primary productivity on the vertical mixing scheme and on the inclusion of a fast recycling loop in the ecosystem model.