Figure 5Figure 5 :Hovmoller diagrams showing monthly chlorophyll concentration [mg/ m3] during the simulated year at the 3 stations A, B, C respectively.

PRELIMINARY APPLICATIONS OF THE ROMS BIOGEOCHEMICAL MODEL TO THE NORTHERN ADRIATIC SEA

I. Iermano,a A. Russo,a S. Carniel,b J. Chiggiato,c A. Coluccelli,a R. P. Signell,d

a DiSMar, Polytechnic University of Marche, Ancona, Italy, b National Research Council-ISMAR, Venice, Itlay, cARPA-SIM , Bologna, Italy, d U.S. Geological Survey, Woods Hole, MA, Usa

AbstractThe Adriatic Sea (Figure 1) is a continental basin of the Mediterranean Sea, and its northern part is particularly shallow (average depth about 35 m) and influenced by large river runoff (Po is the main river, with an average runoff of 1500 m3 s-1). This portion of the Adriatic basin is affected by phenomena such as eutrophication, mucilage, and bottom water anoxia, which have negative impacts on the environment, tourism and fisheries. Despite the buoyancy gain due to the freshwater input, very dense waters (up to 30 in sigma-theta) may form in the northern Adriatic during winter due to Bora (a strong, dry north-easterly wind) events. These characteristics make the area challenging to model.To address the environmental concerns about the northern Adriatic basin, the biogeochemical module of ROMS has been used for realistic simulations with the general aim to improve understanding of the Adriatic physical-biogeochemical phenomenology. Some of the current specific objectives are evaluation of the model ability to provide short term forecast of hypoxic events and evaluation of the influence of nutrient input from different rivers on the eutrophication and hypoxic/anoxic phenomena.

ROMS model descriptionThe Regional Ocean Modeling System (ROMS) is a 3-D, primitive-equation, hydrostatic, finite difference hydrodynamic model. In this application, ROMS model is implemented on a curvilinear-orthogonal grid with a horizontal resolution of about 2-3 km in the northern Adriatic, while 20 levels are used in the vertical. Surface fluxes are derived from the LAMI model, in particular heat fluxes are interactively computed using bulk formulae. 26 rivers are included as mass and biogeochemical sources, with daily runoff measured values where available (Po river) and climatological values elsewhere (biogeochemical values were deduced from literature).The Fasham-type biogeochemical model [Fasham et al., 1990] is a representation of nitrogen cycling processes in the water column and organic matter remineralization at the water-sediment interface that explicitly accounts for sediment denitrification (see also Fennel et al., 2006).The simulation is initialized in September 2002 and runs for one year to represent the seasonal cycle of biological properties in northern Adriatic sea. Model initialization is derived by MEDAR/MEDATLAS climatological data set for the biological variables and by measured field data collected during oceanographic cruises in mid September 2002 for hydrological parameters.

Model resultsA typical seasonal biogeochemical cycle in the water column is evident in the northern Adriatic. In general, Dissolved Inorganic Nitrogen and Chlorophyll follow typical distributions (i.e. values decreasing from coast to open sea and from surface to bottom). For DIN and Chlorophyll, the higher concentration at station B compared to the stations A and C is likely due to proximity to the Po Delta (note the different value scales between the three stations).Nutrient profiles (Figure 4) are generally characterized by high surface concentrations, vertically decreasing down to a depth of about 5-10m, a trend opposite to that of the vertical distribution of salinity (Figure 6), which clearly indicates the influence of the river input on the nutrient concentrations. For DIN, the autumn-winter season shows the highest content in the whole water column, with a peak in December. From winter to summer, concentrations show a general decrease, with a minimum in autumn before recovery toward winter values. This behavior is mainly related to the different nutrient assimilation by phytoplankton (maximum in spring, minimum in autumn).Chlorophyll (Figure 5) shows a well-defined seasonal cycle. Highest phytoplanktonic biomass are found in autumn, primarily in the western areas, which are under direct freshwater influence. A secondary peak is found in February at station A and C, reaching the bottom: increased light availability and water column vertical homogeneity are clear factors. A spring bloom is observed at station B and A, clearly related to freshwater input. As can be seen in figure 5, this limited phytoplankton bloom is not supported by the same increments in the nutrient concentrations (Figure 4) in the same period; this is due to the already mentioned fast nutrient assimilation in this season and to the anomalously low Po river runoff (figure 7 evidences the particularly dry season of Po river runoff in 2003 compared to the historical one).The described characteristics are in good agreement with the known nutrient and phytoplankton dynamics in the Adriatic Sea.

REFERENCESCushman-Roisin,B.,M.Gacic,P.M.Poulain,and A.Artegiani (Eds) (2001), Physical Oceanography of the Adriatic Sea. Springer, New York

Degobbis,D. et al. (2000),Long-term changes in the northern Adriatic ecosystem related to anthropogenic eutrophication.Int.J.Environment and Pollution, Vol.13,Nos. 1-6 Fasham et al,1990.A nitrogen-based model of plankton dynamics in the oceanic mixed layer.J.Mar.Res.,48.591-639Fennel,K. Et al. (2006), Nutrient cycling in the Middle Atlantic Bight:Results from a three-dimensional model and implementation for the North Atlantic nitrogen budget.Global biogeochemical cycle,Vol.20,GB3007Shchepetkin, A. F., and J. C. McWilliams (2005), The Regional Ocean Modeling System: A split-explicit, free-surface, topography following coordinates ocean model, Ocean Modelling, 9, 347-404.

ACKNOWLEDGEMENTS

We wish to thank Katia Fennel (IMCS-Rutgers University, USA) and Emanuele Di Lorenzo (Georgia Institute of Technology, USA) for useful discussion and suggestions

Study areaNorthern Adriatic (NA) can be defined as that region with relatively homogeneous physical water properties, extending up to the 100m isobaths in the south (Artegiani et at. 1997). Even though the NA sea represents only a portion of the total area of the Adriatic Sea, it is the most productive region because it receives nutrient rich freshwaters from several rivers and in particular from the Po river. It is particularly susceptible to eutrophication by increases in the anthropogenic nutrient load to the Po river watershed, leading to hypoxic and anoxic conditions.

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Figure 8Figure 8 : shows a sequence of dissolved oxygen bottom distribution [ml/l] in the NA basin.(a)measured data used to initialized ROMS model (16-19 September 2002). (b-c-d) 3 daily modeled snapshots from September 21nd till 23nd 2002 (e)measured data interpolated from 22 to 25 September 2002.

The sequence (Fig. 8) is an example of model/data comparison; simulated bottom distribution of dissolved oxygen (ml l-1) compared to measured data. The sequence shows that the hypoxic area (the shallower western coastal one in particular) received an important supply of oxygen due to the vertical mixing generated by Bora, and is also particularly evident the horizontal displacement of hypoxic waters due to resulting intensified bottom currents. The simulation shows that the Bora blowing was able in few hours to re-oxygenate fully the shallower hypoxic area, and partially the deeper one; moreover, Bora enhanced the bottom water circulation, and this in turn caused an horizontal displacement of the hypoxic area. From the qualitative point of view, dissolved oxygen simulated patterns resemble the observed ones. However, it should be remembered that observations have not been collected sinoptically, since they started in the southern area the morning of 22 September (before Bora onset) and ended in the northern area on 25 September (during the Bora blowing).

ConclusionsFirst results of the biogeochemical fluxes model implementation in the Adriatic Sea appear very encouraging and are a powerful tool to study interections between the physical and ecosystem dynamic at high spatial and temporal resolution. Based on this success, an operational forecasting system at short time (2-3 days) for the evolution of hypoxic events in the south of Po area is being developed.

The impact of Bora wind on a hypoxic event in the Northern Adriatic SeaDuring summer-autumn season, the NA basin often presents a very stable and stratified water column; the bottom waters, if interested by a relevant deposition of organic matter produced in the upper layer, can be affected by hypoxic and anoxic events (negatively influencing environment, tourism and fisheries). The Bora wind, which blows from the NE sector, is particularly important in determining changes in the vertical structure of this shallow water system. This ROMS simulation allows a more detailed description of this hypoxic event evolution, in particular, the response of dissolved oxygen to a relatively strong Bora wind event (September 23 to 25, 2002).

Figure 9Figure 9:: shows two daily snapshots of (A) surface ammonium concentration [mmol/ m3] on 13th July 2003 and on 15th July 2003 (B).

Figure 10Figure 10:: shows two daily snapshots of (A) surface temperature[°C] distribution on 13th July 2003 and on 15th July 2003 (B).

An unusual upwelling eventAn unusual upwelling event along the western Adriatic coast happened in summer 2003 between 11 and 23 July, documented by SST and drifter data (Poulain et al. 2004). This peculiar situation happened as a combination of dominant sirocco winds (blowing from S-E in the Adriatic Sea) and reduced river discharge rates due to a prolonged dry season. The ROMS biogeochemical model is able to reproduce the upwelling event with a very high resolution. Surface temperature and nutrient concentration are correlated, indicating that the cold-upwelled water is richer in nutrients.

Figure 3Figure 3 : time series plots of surface salinity [PSU], dissolved inorganic nitrogen [mmol/ m3] and chlorophyll [mg/ m3] from mid September 2002 to August 2003 in the three stations.

Figure 2Figure 2 : Northern Adriatic bathymetry with the 3 chosen stations. Station A can be considered as representative of the whole NA basin because it is less influenced by riverine discharge (Zavatarelli et al., 1998). Station B is chosen very close to the Po plume (under its direct influence) and station C close to Istria area to show east to west trophic gradient.

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Figure 6Figure 6: Hovmoller diagrams showing monthly salinity distribution during the simulated year at the stations A and B.

Figure 4Figure 4 : Hovmoller diagrams showing monthly dissolved inorganic nitrogen concentration [mmol/ m3] during the simulated year at the 3 stations A, B, C respectively

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Figure 7Figure 7 : Time series plot of monthly Po river runoff [m3 /s] for 2003 (red line) and from climatological data set (blue line)

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