BENTHIC NUTRIENT FLUXES ACROSS A PRODUCTIVE SHELF ADJACENT TO AN OLIGOTROPHIC BASIN: CASE OF THE NORTHEASTERN MEDITERRANEAN SEA

The Coastal ecosystem of the Northeastern (NE) Mediterranean has been affected by nutrient inputs originated from regional rivers and wastewater discharges leading to development of eutrophication. Atmospheric nutrient inputs have also remarkable contribution to marine nutrient pool in the NE Mediterranean, especially in dry periods. Sediment porewater nutrient fluxes into the deep waters are strongly associated with eutrophic and suboxic/anoxic conditions. There was only limited number of studies performed on the porewater and solidstate sediment biogeochemistry in the NE Mediterranean Sea having oxic conditions in the deep waters. In this study, therefore, sediment porewater (PW) nutrient (Si, N, P) and sediment organic matter biogeochemistry were studied. The study results indicated a series of redox reactions (oxic respiration, denitrification, iron reduction) as well as remarkable contribution of porewater diffusive nutrient fluxes to the total nutrient budget in the NE Mediterranean Sea. Lower Si/N and higher N/P molar ratios in the total nutrient inputs are very likely to modify phytoplankton composition and abundance in the phosphorus deficient NE Mediterranean productive shelf waters leading to development of mesotrophic/eutrophic conditions in the NE Mediterranean Sea.


Introduction
The Mediterranean Sea is an oligotrophic sea due to limited nutrient inputs to its sunlit surface from internal and external sources (UNEP, 1989;Yılmaz and Tugrul, 1998;Kress and Herut, 2001;Krom et al., 2004;Tugrul et al., 2016;. Surface dissolved inorganic nutrient concentrations in offshore waters of the Northeastern Mediterranean (NE) Sea ( Figure 1) are measured as low as for inorganic phosphate and nitrate during the springautumn period (Yılmaz and Tugrul, 1998) and increased slightly in winter due to vertical mixing (Yılmaz and Tugrul, 1998;Dogan-Saglamtimur andTugrul, 2004, Tugrul et al., 2016 and references therein). Furthermore, atmospheric nutrient inputs, both dry and wet deposition, have remarkable contribution to sustain primary productivity in the Northeastern Mediterranean (Kocak et al., 2010;Kocak, 2015). For example, contribution of P fluxes to the algal production was calculated as high as 0.9 % in offshore waters of the Cilician Basin of the Northeastern Mediterranean whereas N fluxes would sustain 8.4 % of primary production in offshore waters and the contribution of atmospheric nutrient fluxes would become drastically high during the stratified summer-autumn period (Kocak, 2015). In the lownutrient upper layer waters of the Mediterranean Sea, primary production is mainly limited by phosphorus due to unusually high NO 3 /PO 4 ratios in the deep waters as ~28:1 (Krom et al., 1991;Yılmaz and Tugrul, 1998) and in atmospheric and regional rivers (Kocak at al., 2010) as also reported recently by the 14 C bioassay experiments conducted in the Turkish coastal waters of the Mediterranean and Aegean Seas (Tufekci et al, 2013). Though low nutrient concentrations in offshore waters of the NE Mediterranean ( Figure 1) are recorded in the upper layer, its coastal ecosystem is highly fueled by terrestrial nutrient and organic matter inputs from the major rivers and wastewater discharges leading to development of coastal eutrophication in the inner bay waters (Dogan-Saglamtimur and Tugrul 2004;Tugrul et al., 2009;2016;. In the offshore waters of the NE Mediterranean, for example, concentrations of total phosphorous (TP) and Chlorophyll-a were as low as 0.05-0.07 µM for TP and 0.02-0.05 µg/L for Chl-a, reaching to peak values in eutrophic coastal waters (Tugrul et al., 2011;2016;. Organic matter geochemistry of the sediments has been affected by a series of redox reactions (Jørgensen, 1996) as well as terrestrial inputs of the particulate inorganic/organic matter that might be transported to offshore regions of the continental seas (Middelburg et al., 1993;Yemenicioglu and Tunc, 2013;Erdogan, 2014;Akcay, 2015, Ermiş, 2017Deininger and Frigstad, 2019;Katz et al., 2020). In the Eastern Mediterranean Sea, surface sediment geochemistry and grain size distributions have been studied extensively during the last decades (Eijsink et al., 2000;Yemenicioglu and Tunc, 2013;Erdogan, 2014, Akcay, 2015Ermiş, 2017) with the limited number of studies based on sediment core samples to understand organic matter geochemistry (sedimentation, degradation, accumulation and burial of organic carbon) of the upper 25-40 cm sedimentary column (Van Santvoort et al., 2002;Katz et al., 2020).  Tugrul et al., 2016).
Biogeochemical cycling of key nutrients (N, P) is highly coupled to oxygen concentrations and metal (Fe, Mn) cycles (Williams, 1987;Jørgensen, 1996). The increase in dissolved inorganic phosphorous and iron concentrations in the Oxygen Minimum Zones (OMZs) is highly related to anoxic conditions in the marine environment as the role of sediments in the OMZs is important for dissolved iron and inorganic phosphate sources to the bottom water (Noffke et al., 2012). Furthermore, in the eutrophic and hypoxic/anoxic Baltic Sea, the intrusion of oxygen-rich North Sea waters into the Eastern Gotland Basin decreased to releases of inorganic phosphate and ammonium from deep sediments (Sommer et al., 2017).
Since the cycles of oxygen, phosphorous and nitrogen are highly coupled, ongoing eutrophication in the Baltic Sea induced hypoxia resulting in internal phosphorous loading due to changing redox state of the seafloor (Vahtera et al., 2007;Ferreira et al., 2011;Malmaeus et al., 2012;Noffke et al., 2012). Seafloor can, therefore, play a key role in rapid degradation of labile organic matter and nitrification/denitrification processes as well as other abiotic processes in the uppermost millimeters of sediment layer resulting in nutrient releases from the sediments studied extensively during the last decades (Christensen et al., 1988;Ignatieva, 1999;Rasheed, 2004;Al-Rousan et al., 2004;Hille et al., 2005;Rasheed et al., 2006;Rydin et al., 2011;Cheng et al., 2014;Mu et al., 2017). However, there was only limited number of studies performed on the sediment biogeochemistry and porewater nutrient dynamics in the oligotrophic NE Mediterranean Sea having oxic conditions in the deep waters though redox-dependent benthic nutrient fluxes would enhance algal production in the oligotrophic marine basins (Christensen et al., 1988;Ignatieva, 1999;Rasheed et al., 2006).
For example, in a study conducted in the oligotrophic Eastern Mediterranean continental shelf, the phosphate and nitrate fluxes into the water column supported as much as 11.7 % and 2 % of the phytoplankton P and N demand, respectively (Christensen et al., 1988). Therefore, studying both external (riverine, wastewater, atmospheric inputs) and internal (sediment, submarine groundwater) nutrient fluxes into the oligotrophic and eutrophic marine environments is critical to understand biogeochemical cycling of key elements for further use in biogeochemical modeling and eutrophication management efforts.
The objectives of this study are, therefore, i) to generate a first-time dataset resolving vertical downcore porewater nutrients (Si, DIN, DIP) and solid state sediment geochemical (Carbon; C, nitrogen; N, reactive iron; r-Fe) properties, ii) based on this novel dataset, to estimate sediment porewater diffusive nutrient fluxes to the bottom waters and iii) to determine riverine and wastewater nutrient fluxes, v) to compare both external (riverine, wastewater and atmospheric (Kocak et. al., 2010) inputs) and internal (sediment PW diffusive inputs) nutrient fluxes in the NE Mediterranean shelf waters.

Methodology
The study area is located at the NE Mediterranean Sea (Figure 2). Field studies were carried out using R/V Bilim-2 of METU-IMS. Sediment core samples were obtained from the five stations (Table 1) Table 1).
Porewater extraction procedure of the collected samples was performed as described by the studies of Christensen et al. (1988), Sundby et al. (1992), Rasheed et al. (2006), Gao et al. (2008), Noffke et al. (2012), Cheng et al. (2014). Each sediment core was sliced on board under minimum oxygen conditions using inert nitrogen gas and sediment horizons were determined as 0-1 cm, 1-2 cm, 2-3 cm, 3-4 cm, 4-6 cm, 6-8 cm, 8-10 cm, 10-15 cm, 15-20 cm, 20-25 cm, 25-30 cm, 30-35 cm, 35-40 Table 2. Annual mean volume fluxes of the regional rivers (Kocak et al. 2010) and local facilities (Tugrul et al., 2009)  analysis, nearly 30 mg of dry and homogeneous sediment samples were put into the precombusted silver cups. Then, 5-10 µL of distilled water was added into each silver cup to wet the samples. After distilled water addition, 10 µL of 20 % HCl (vol/vol) was added to remove inorganic carbon from the sediment samples. The HCl additions were continued until all the inorganic carbon was removed in the form of CO 2 . Then, the carbonate-free samples were dried at 60 ˚C for one day. After drying the sediment samples, silver cups were compacted and put into autosampler of the CHN analyzer. TN concentrations were also measured on HCl-added samples and there was no significant difference between HCl-treated and untreated samples. Therefore, TC and TN analyses in sediments were performed by the same method as for the TOC analysis, but without acid addition. The reactive iron (r-Fe) concentrations were determined by colorimetric ferrozine method (Stookey, 1970;Jeitner, 2014) after dithionite (0.3 M solution in a buffer containing 0.35 M sodium acetate and 0.2 M sodium citrate (pH=4.8)) extraction of freeze-dried sediments (Kostka and Luther, 1994;Raiswell et al., 1994, Yücel et al., 2010. In this extraction technique, r-Fe refers to iron oxy(hydr)oxides, but it should be noted that dithionite can also dissolve acid volatile sulfides (ASV)-bound Fe(II) (Yücel et al., 2010).
Porewater dissolved inorganic nutrients (nitrate+nitrite, ammonium, phosphate and silicate) were determined as described method above (Grasshoff et al., 1983). Diffusive nutrient fluxes were calculated based on Fick's First Law of Diffusion as the following equation (Al-Rousan et al., 2004;Cheng et al., 2014;Mu et al., 2017): Where F corresponds to the diffusion flux across sediment-water interface, dC/dz to the concentration gradient of nutrients across sediment-water interface, ϕ to the porosity of sediment, D s to the actual molecular diffusion coefficient corrected for the sediment tortuosity. Ullman and Aller (1982) proposed an empirical formula related to the actual diffusion coefficient, D s , and porosity, ϕ: where D 0 is the self diffusion coefficient of ions at infinite dilution. For phosphate, nitrate, nitrite and ammonium D 0 values are used from Li and Gregory (1974) and for Si, D 0 values were used from Rebreanu et al. (2008) as 7.34x10 -6 cm 2 s -1 for PO 4 , 19.0x10 -6 cm 2 s -1 for NO 3 , 19.1x10 -6 cm 2 s -1 for NO 2 , 19.8x10 -6 cm 2 s -1 for NH 4 and 11.7x10 -6 cm 2 s -1 for Si, at 25 ˚C respectively. Porosity of surface sediments was determined by the displacement method as described by Mu et al. (2017).
Porewater diffusive, riverine and wastewater nutrient fluxes (normalized to NE Mediterranean shelf area) with atmospheric deposition (Kocak et al., 2010) were evaluated to determine internal and external nutrient fluxes to the NE Mediterranean shelf waters for further use in biogeochemical modeling and eutrophication management efforts of the oligotrophic Mediterranean Sea.

Porewater nutrient dynamics in the NE Mediterranean Sea
The core samples collected from the selected sites of the NE Mediterranean Sea (Figure 2) appeared to represent undisturbed particulate accumulations. Except for the Göksu River influenced region (St. 4), the upper parts of the sediment columns were generally light brown in color. However, at a depth of 8-10 cm and below this depth, the sediment profile became This paper is a non-peer reviewed preprint submitted to EarthArXiv. 8 darker. The sediment core sample obtained from St. 4 had yellowish brown color throughout the sediment column having coarse-grained sediment texture.
In marine sediments, a number of organisms, such as bacteria, fungi, micro-or macro-fauna, are responsible for the aerobic degradation of organic matter in the uppermost sedimentary column where oxygen is used as electron acceptor (Jørgensen, 1996;Fenchel et al., 1998;Kristensen, 2000). In general, in this redox zone, aerobic respiration and nitrification reactions take place. In the suboxic zone and anoxic zone, anaerobic degradation of organic matter occurs stepwise by different functional types of bacteria (Fenchel et al., 1998;Kristensen, 2000). In these redox zones, denitrification, manganese oxide reduction, iron oxide reduction reactions in the suboxic zone and sulfate reduction, hydrolysis/fermentation, and carbon dioxide reduction reactions in the anoxic zone take place, respectively. Biogeochemical cycling of key nutrients (N, P) is highly coupled to oxygen concentrations and metal (Fe, Mn) cycles (Williams, 1987;Jørgensen, 1996 (Yılmaz and Tugrul., 1998) but comparable with the riverine nutrient concentrations in the study region which will further be discussed in this section. Seafloor can play a key role in rapid degradation of labile organic matter and nitrification/denitrification processes as well as other abiotic processes in the uppermost millimeters of sediment column resulting in nutrient releases from the sediments (Christensen et al., 1988;Ignatieva, 1999;Rasheed, 2004;Al-Rousan et al., 2004;Hille et al., 2005;Rasheed et al., 2006;Rydin et al., 2011;Cheng et al., 2014;Mu et al., 2017). Expectedly, PW nutrient concentrations (PO 4 , NH 4 , Si) increased with depth throughout the sediment column indicating that organic matter degradation processes took place in the first 3-10 centimeters of the sediment column in the NE Mediterranean Sea.

Solid-phase sediment geochemistry in the NE Mediterranean Sea
Organic matter geochemistry of the sediments has been affected by a series of redox reactions (Jørgensen, 1996) as well as terrestrial inputs of the particulate inorganic/organic matter that might be transported to offshore regions of the continental seas (Middelburg et al., 1993;Yemenicioglu and Tunc, 2013;Erdogan, 2014;Akcay, 2015, Ermiş, 2017Deininger and Frigstad, 2019;Katz et al., 2020). In the Eastern Mediterranean Sea, surface sediment geochemistry and grain size distributions and have been studied extensively during the last decades (Eijsink et al., 2000;Yemenicioglu and Tunc, 2013;Erdogan, 2014, Akcay, 2015Ermiş, 2017) (Yemenicioglu and Tunc, 2013;Akcay, 2015). Moreover, differences in the sedimentation rates and currents and wave energy affect distributions of geochemical properties (De Falco et al., 2004;Katz et al., 2020). According to results of the five major river data, maximum concentrations of suspended matter (TSS) were recorded in the Göksu River among the other regional rivers (Tugrul et al., 2009) and fine-grained sediments were carried to the central and offshore regions due to regional wave actions and currents (De Falco et al., 2004;Katz et al., 2020) resulting in carbonate-rich sediment column in the Göksu River influenced coastal area ( Figure 4).
The TOC and TN concentrations measured in this study were in agreement with the organic matter geochemical properties from the previous studies performed in the oligotrophic Eastern Mediterranean Sea (Eijsink et al., 2000;Erdogan, 2014, Akcay, 2015Ermiş, 2017;Katz et al., 2020). The TOC concentrations of the sediment column (0.26-0.70 mmol/g dw in the 1-45 cmbs) recorded in this study were in agreement with the TOC contents obtained from South Pacific Gyre sediment core samples (0.14-0.43 mmol/g dw in the 3 cmbs-

Riverine, wastewater, atmospheric and sediment porewater nutrient concentrations in the NE Mediterranean Sea
The seasonal and regional averages of nutrient concentrations measured in the five regional rivers (Figure 2) for the 2008-2015 period are presented in Table 3, showing remarkable seasonal and regional variations with the peak values reached in the wet winter-spring seasons. Rivers often transport nutrients that can enrich coastal plankton communities (Farrow et al., 2019). The development of eutrophication in the coastal sites of the NE Mediterranean shelf waters has been experienced due to riverine and wastewater discharges (Dogan-Saglamtimur and Tugrul 2004;Tugrul et al., 2009;2016;. Atmospheric nutrient inputs, both dry and wet deposition, have also remarkable contribution to sustain primary production in the Northeastern Mediterranean Sea (Kocak et al., 2010;Kocak, 2015), especially in dry summer-autumn period when the volume fluxes of the regional rivers  Wastewater nutrient concentrations were much greater than measured in the regional rivers and showed marked spatial variability (Table 4) (Table 2), but still resulting in local pollution and eutrophication in the coastal areas of the NE Mediterranean as experienced in İskenderun and Mersin inner bays (Tugrul et al., 2009;2016). Atmospheric nutrient concentrations in aerosols determined from a long-term observation (Kocak et al., 2010) showed that water soluble reactive PO 4 and Si concentrations ranged from 0.03 to 6.40 nmol m -3 for PO 4 and 0.04 to 26.27 nmol m -3 for Si. Water soluble NO 3 and NH 4 concentrations varied between 0.2-258.8 nmol m -3 and 0.1-473.2 nmol m -3 , respectively.
For rainwater samples obtained from the same study, the volume weighted mean values for PO 4 , Si, NO 3 and NH 4 were calculated as 0.7, 1.9, 44 and 46 µM, respectively. comparison of atmospheric deposition (dry+wet) (Kocak et al., 2010) showed that aerosol and rainwater reactive Si concentrations measured in the NE Mediterranean Sea were much lower than measured Si concentrations in the riverine and wastewater concentrations, but PO 4 and NO 3 concentrations were comparable to measured in the rivers. Aerosol and wastewater samples were highly enriched by NH 4 compared to riverine NH 4 concentrations.

Comparison of internal and external nutrient fluxes and their elemental compositions (Si/N/P) in the Northeastern Mediterranean Sea
In this study, the quantification of all source terms of nutrients (rivers, wastewaters, PW diffusive nutrient fluxes, atmospheric inputs (based on the study performed by Kocak et al. (2010)) was made to assess nutrient loads for the NE Mediterranean Sea having oligotrophic properties in its offshore waters (UNEP, 1989;Yılmaz and Tugrul, 1998;Kress and Herut, 2001;Krom et al., 2004;Tugrul et al., 2016;. Since riverine and wastewater nutrient inputs have highly affected coastal zones of the NE Mediterranean, fluxes were normalized to the area of shelf regions of the NE Mediterranean. It should be noted that the total nutrient fluxes were assumed to be greater than diffusive nutrient fluxes due to other processes at sediment-water interface such as bioturbation, bioirrigation (Barbanti et al., 1992) and also biotic/abiotic processes for the organic matter degradation in the uppermost millimeters of sediment column (Christensen et al., 1988;Ignatieva, 1999;Rasheed, 2004;Al-Rousan et al., 2004;Hille et al., 2005;Rasheed et al., 2006;Rydin et al., 2011;Cheng et al., 2014;Mu et al., 2017). In this study, calculated diffusive nutrient fluxes based on the Fick's First Law of Diffusion presented in Table 5 indicated remarkably high nutrient fluxes from the sediment into the deep waters of the NE Mediterranean Sea. Maximum nutrient fluxes were calculated in the river-influenced coastal region indicated that human-induced pressures not only affect the coastal surface waters, but also affect sediment porewater nutrient dynamics due to increase in primary productivity and hence sedimentation in the eutrophic coastal areas. Expectedly, lower PW diffusive nutrient fluxes were calculated in the oligotrophic offshore regions of the NE Mediterranean (Table 5). Redox-dependent benthic nutrient fluxes would enhance algal production (Christensen et al., 1988;Ignatieva, 1999;Rasheed et al., 2006). In a study conducted in the oligotrophic Eastern Mediterranean continental shelf, the phosphate and nitrate fluxes into the water column supported as much as 11.7% and 2% of the phytoplankton P and N demand, respectively (Christensen et al., 1988). Furthermore, it was observed that porewater nutrient concentrations revealed reduced conditions in these shelf sediments where nitrate depletion was seen in the uppermost centimeters of the collected sediment cores (Jørgensen, 1996) as also experienced in NE Mediterranean shelf region by this study. In this study, calculated diffusive nutrient fluxes were very similar to the study of Christensen et al. (1988) and Rasheed et al. (2006) performing their studies in the oligotrophic regions, but much lower than the studies by Ignatieva (1999) and Noffke et al. (2012) where the studies were conducted in the highly eutrophic regions having suboxic and anoxic/sulfidic deep waters.
Mean annual nutrient fluxes in the NE Mediterranean shelf region were calculated (Table 6) for both external and internal sources and the contribution of each source term were quantified in the NE Mediterranean Sea (Figure 5) Figure 5). The increase in dissolved inorganic phosphorous and iron concentrations in the eutrophic and suboxic/anoxic/sulfidic regions is highly related to redox conditions in the marine environment as the role of sediments in these regions has important for dissolved iron, inorganic phosphate and ammonium sources to the bottom water (Noffke et al., 2012). Though dissolved oxygen concentrations were at saturation levels in the NE Mediterranean upper layer (Tugrul et al., 2016; with highly oxygenated deep waters, contribution of PW diffusive nutrient fluxes were markedly high. Contribution of diffusive nutrient inputs was much greater than wastewater nutrient inputs for nitrate, ammonium and reactive silicate, but lower than phosphate due probably to adsorption of phosphate onto metal oxides (Mortimer, 1942 as referenced in Nteziryayo andDanielsson, 2018) in the oxygenated surface sediments of the NE Mediterranean Sea.  The eastern Mediterranean deep water is characterized by high N/P molar ratios (Krom et al., 1991, Yılmaz andTugrul, 1998) due to internal and external sources with high N:P molar ratios (Krom et al., 2004;Kocak et al., 2010) as well as lack of feedback mechanisms for bioavailable N through denitrification in the sediment and water column (Krom et al., 2004).
Positive nutrients (Si, N, P) fluxes indicated sediment in the NE Mediterranean shelf acts as a source for nutrients. The N/P molar ratios calculated for the regional rivers, atmospheric deposition and sediment porewaters (Table 6, Figure 6), except for wastewaters, were greater than classical Redfield Ratio (N/P=16), reflecting high N/P molar ratios in the Mediterranean deep waters (Krom et al., 1991, Yılmaz andTugrul, 1998). As a result, it can be concluded benthic nutrient fluxes with high N/P molar ratios and the nutrient dynamics at the sedimentwater interface in the NE Mediterranean shelf region might be also one of the reasons addressing why the eastern Mediterranean is P-limited as also experienced in the previous studies (Krom et al., 1991;2004;Yılmaz and Tugrul, 1998;Tufekci et al., 2013;Tugrul et al., 2011;2016;. It should be also noted that low Si/N (Si/NO 3 =0.73) and high N/P Redfield Ratio are very likely to modify phytoplankton composition and abundance in the phosphorus deficient NE Mediterranean productive shelf waters leading to development of mesotrophic/eutrophic conditions in the NE Mediterranean Sea. Therefore, quantification of both internal and external nutrient inputs is needed for further use in biogeochemical modeling of the oligotrophic Mediterranean Sea and its ecosystem dynamics which are of critical importance to attain Good Environmental Status (GES) for the Eastern Mediterranean Sea.

Conclusions
Sediment porewater nutrient (Si, N, P) and sediment organic matter biogeochemistry were studied in the NE Mediterranean Sea. Porewater diffusive nutrient fluxes were also calculated for the comparison with the external nutrient inputs in the NE Mediterranean shelf region.
The study results indicated a series of redox reactions; organic matter mineralization was occurred by oxic/aerobic respiration in the upper 30-45 cmbs in the offshore regions of the NE Mediterranean Sea having low surface water chlorophyll concentrations and low sedimentation rates which are typical for (ultra-)oligotrophic and deep-sea environments whilst denitrification process was occurred in the river influenced coastal region. Vertical profiles of the reactive iron concentrations were also suggested microbial iron reduction in the NE Mediterranean subseafloor. Determination of redox-dependent benthic nutrient fluxes is critical for the management of eutrophication status of coastal marine ecosystems since high nutrient inputs from the surface sediment can support benthic community and productivity in highly polluted sites of the inner bays. In this study, therefore, redox-dependent benthic nutrient fluxes as an internal nutrient input were determined in the NE Mediterranean shelf waters. Riverine and wastewater nutrient fluxes were also determined in the NE Mediterranean Sea and using atmospheric (wet+dry deposition) nutrient fluxes, all the external inputs were quantified and compared with the benthic nutrient fluxes. Results of this study indicated remarkable contribution of porewater diffusive nutrient fluxes to the total nutrient budget in the NE Mediterranean Sea. Lower Si/N and higher N/P molar ratios in the total nutrient inputs are very likely to modify phytoplankton composition and abundance in the phosphorus deficient NE Mediterranean productive shelf waters leading to development of mesotrophic/eutrophic conditions. Another important source term of allochthonous nutrients is the submarine groundwater discharge which is not determined in the scope of this study though its contribution was markedly high in the Mediterranean Sea (Rodellas et al., 2015). The magnitude of volume fluxes, the nutrient concentrations and composition of the submarine groundwater discharge in the NE Mediterranean Sea located on one of the widest shelf areas should be needed for further use in biogeochemical modeling of the oligotrophic Mediterranean Sea and its ecosystem dynamics. It is also important that sediment porewater nutrients and solid-phase geochemical parameters should be determined and integrated to marine monitoring programmes to attain Good Environmental Status (GES) of the Eastern Mediterranean Sea.