A Budget Model to Scale Nutrient Biogeochemical Cycles in Two Semienclosed Gulfs

Transcription

1 Environ Model Assess (2009) 14:59 72 DOI /s A Budget Model to Scale Nutrient Biogeochemical Cycles in Two Semienclosed s Georgios K. Sylaios & Vassilios A. Tsihrintzis Received: 15 September 2006 / Accepted: 12 March 2008 / ublished online: 26 April 2008 # Springer Science + Business Media B.V Abstract The nutrient dynamics of the Strymonikos and Ierissos s, two semienclosed coastal water bodies, are studied using a simple steady-state budget model, according to the Land Ocean Interaction in the Coastal Zone modeling guidelines. Strymon river plume dynamics prevailed in the area of the Inner and Outer Srymonikos s, comprising two layers, while the Ierissos was defined as a one-box system. Seasonal and mean annual model input data for river discharge, precipitation, evaporation, and concentrations of salt, phosphorus, and nitrogen were obtained during four field campaigns. Results from the nitrogen and phosphorus cycling revealed the importance of river discharge in the horizontal and vertical transport of these substances within the system. Furthermore, it occurred that the major biogeochemical transformation of nitrogen and phosphorus takes place in the immediate nearshore zone (Inner Strymonikos ), while the outer system sustains its nutrient dependence on oceanic exchanges. Therefore, under the summer low flow conditions, the river-influenced inner system acts as a net source of nitrogen and phosphorus, while under increased Strymon River discharge, phosphorus is transferred to the biological material (and the sediments), and the system moves to an autotrophic state. The outer system showed an opposite behavior being autotrophic throughout the year and heterotrophic in February. The Ierissos, a system not directly influenced by significant river discharge, experienced a seasonally independent behavior with net heterotrophic and denitrification processes prevailing. G. K. Sylaios (*) : V. A. Tsihrintzis Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, Xanthi, Greece gsylaios@env.duth.gr Model scenarios demonstrated that phosphate concentration increases, even under low river flow conditions and stimulates primary production in excess of respiration, resulting in nitrogen fixation prevalence in the Inner Strymonikos. Keywords Box model. LOICZ. Model scenarios. Nutrient fluxes. Strymonikos 1 Introduction Semienclosed shallow seas and estuaries are regions where intense and complex biogeochemical processes, such as nutrient cycling, organic matter decomposition, and/or carbon fixation, take place. Moreover, coastal shallow seas and estuaries serve as buffer zones between terrestrial and marine systems, involving the exchange of water and salt, flows of dissolved nutrients and organic matter, and net fluxes of living or dead material and suspended sediments [6]., salt, and nutrient fluxes between an estuary and the adjacent offshore waters are regulated by the supply of water and nutrients from the seasonally variable river, the advective and diffusive processes established between the two systems, and the internal processes such as nutrient accumulation, remineralization, and burial of particulate organic matter and nutrient release and transformation. Quantification of nutrient and organic matter exchanges between estuaries and the coastal waters has been conducted in several ecosystems using simple box-model approaches, applied on seasonal and/or annual basis (e.g., [2, 8, 9, 13, 14]). Generally, the seasonal variability of freshwater flow determines largely the estuarine biogeochemical processes, since, during the wet period, the greater influx of freshwater results in enhanced inflow of landderived nutrients into the receiving coastal/oceanic waters

2 60 G.K. Sylaios, V.A. Tsihrintzis via the estuarine zone. The Land Ocean Interaction in the Coastal Zone (LOICZ) study of the International Geosphere Biosphere rogram has presented a series of guidelines to assess and quantify the fluxes of the biogeochemically active components across the land ocean boundary using steady-state budgeting methods [4, 12]. The particular objective has been to estimate the capacity of selected coastal water bodies to store and transform dissolved and particulate-bound matter of nitrogen and phosphorus, with the view to identify the factors controlling their distribution on a global scale. The present work comprises an elaborated attempt to utilize detailed seasonal hydrographic data, aiming at understanding the biogeochemical processes functioning in two semienclosed coastal water bodies, the Strymonikos and Ierissos s, North Greece. 2 Materials and Methods 2.1 Study Area The Strymonikos (surface km 2, volume m 3 ) and Ierissos (surface 116 km 2, volume m 3 ) are two of the larger semienclosed water bodies in the Thracian Sea, North Greece (Fig. 1). The Strymonikos is the final recipient for the catchments of River Strymon (drainage area=18,329 km 2 ) and River Richios (drainage area=2,090 km 2 ), together with smaller basins scattered along the coastal zone (drainage area=598 km 2 ; Fig. 1). River Strymon has a mean annual discharge of 59.5 m 3 s 1, supplying freshwater and domestic, agricultural, and industrial effluents. The discharge pattern of Strymon River shows strong seasonal variability, ranging Fig. 1 Map of Strymonikos and Ierissos s (Northern Greece), showing the sampling stations and the plan of the three main boxes used for budgetary analysis 4 RIVER STRYMON 40 48' 00'' N 24 01' 00'' E 5 ASROVALTA A RIVER STAVROS RICHIOS N STRYMONIKOS GULF Inner Outer Open Sea 10 m 50 m km GREECE AEGEAN SEA OLYMIAS TURKEY 22 22A STRATONI 32 GULF OF IERISSOS IERISSOS NEA RODA 40 21' 00'' N 23 38' 00'' E

3 Nutrient biogeochemical budget 61 Table 1 Background data for estimating the seasonal and annual budget models DIN Dissolved inorganic nitrogen, DI dissolved inorganic phosphorus June 1997 September 1997 November 1997 February 1998 Annual recipitation (mm) Evaporation (mm) River discharge (m 3 s 1 ) River DIN (mmol m 3 ) River DI (mmol m 3 ) on average from 18 m 3 s 1 in August to 122 m 3 s 1 in April [11]. River Richios shows a mean annual discharge of 0.90 m 3 s 1 maintained at a relatively constant rate throughout the year. Moreover, a number of seasonal streams and torrents outflow in the Strymonikos and Ierissos s, having mean annual flows of approximately 1.70 m 3 s 1 [11]. Mean annual evaporation (approximately 900 mm) exceeds mean annual precipitation (approximately 500 mm), producing a net water deficit. However, this budget is also seasonally variable, usually being negative from the summer to the end of the winter and slightly positive during the spring. Table 1 presents the background data used by the seasonal and annual budget models of the Strymonikos and Ierissos s. 2.2 Data Availability A series of collected data on water salinity, density, and nutrient distribution exists for the Strymonikos and Ierissos s obtained during four seasonal surveys conducted in June, September, and November 1997 and February A hydrographic grid of 36 stations (28 in the Srymonikos and eight in the Ierissos ) was sampled to study the spatial distribution of the physical and chemical water parameters (temperature, salinity, dissolved oxygen, and nutrients) (Fig. 1). hysical parameters were measured with a conductivity temperature pressure instrument (CTD) at a 0.1-m vertical interval. Dissolved nutrient determinations (nitrate, nitrite, ammonium, and phosphate) were made according to Strickland and arsons [10] from water samples collected in each station at a 10-m vertical interval. CTD measurement errors were as follows: 0.01 C for temperature, 0.5 dbar for pressure, 0.01 ms cm 1 for conductivity, and psu for salinity. Quality control for nutrient determination showed that precision of all analyses was better than 5%. Figure 2 presents a longitudinal transect of water density (σ t ) in the Strymonikos revealing vertical stratification in the region of freshwater influence (Inner ), with freshwater covering the first 1 to 4 m of the water column, over a mean area of 96.5 km 2, ranging from 70 km 2 during the summer to 150 km 2 during the winter. The remaining Strymonikos and the of Ierissos were vertically well mixed throughout the year. (a) St. 4 St. 10 St. 15 St. 17 St. 4 St. 10 St. 15 St (b) St. 4 St. 10 St. 15 St. 17 St. 4 St. 10 St. 15 St (c) (d) Fig. 2 Longitudinal transect of water density (σ t ) from Strymon River mouth (station 4) to the open boundary (station 17), revealing the freshwater influence in the Strymonikos, in a June 1997, b September 1997, c November 1997, and d February Density 60 contours influenced by river discharge appear at the vicinity of station 10, located at the boundary between the Inner and Outer Strymonikos

4 62 G.K. Sylaios, V.A. Tsihrintzis Following these spatial water density patterns, the system was divided into three main boxes for the purposes of the subsequent budgetary analysis: the Inner Strymonikos, Outer Strymonikos, and Ierissos (Fig. 1). The Inner Strymonikos (mean area= 96.5 km 2 ) represents the region directly affected by the freshwater flow of Strymon and Richios rivers and was represented as a stratified box system, divided into an upper low-salinity and a lower high-salinity layer, with their interface defined by the σ t =26.5 isopleth. The Outer Strymonikos (mean area=292.9 km 2 ) is not directly influenced by the riverine freshwater inflow, but it is also represented by a two-layer system (defined by the σ t =26.5 isopleth), since the vertical salinity and density gradients in the area are also important. The Ierissos is represented by a vertically well-mixed one-box system, exchanging water, salt, and nutrients with the adjacent open sea. The typical values of water salinity, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DI), used as input data for the seasonal and the annual budgets of the Strymonikos and Ierissos s are shown in Table and Salt Budgets The principle of conservation of water and salt mass is applied to estimate the water and salt fluxes, which have not been measured directly. Assuming a steady-state (i.e., no significant water volume variation over the budgeted period, season or year, dv/dt=0), the water balance for the surface layer of the Inner Strymonikos is given by: V R IN ¼ V Q IN þ V IN V E IN þ V G IN ð1þ where V R-IN (m 3 day 1 ) is the compensating residual outflow to balance the freshwater volume entering the system, such as surface runoff (V Q-IN,m 3 day 1 ), precipitation (V -IN,m 3 day 1 ), groundwater (V G-IN,m 3 day 1 ), and the evaporative losses (V E-IN,m 3 day 1 ; Fig. 3). The combined outflow from the surface layer of the Inner Strymonikos is V -IN, defined as: Table 2 Input data for the seasonal and the annual budget models of the Strymonikos and Ierissos s Salinity (psu) DIN (mmol m 3 ) DI (mmol m 3 ) June 1997 Inner Strymonikos Upper layer Lower layer Outer Strymonikos Upper layer Lower layer Ierissos September 1997 Inner Strymonikos Upper layer Lower layer Outer Strymonikos Upper layer Lower layer Ierissos November 1997 Inner Strymonikos Upper layer Lower layer Outer Strymonikos Upper layer Lower layer Ierissos February 1998 Inner Strymonikos Upper layer Lower layer Outer Strymonikos Upper layer Lower layer Ierissos Annual Model Inner Strymonikos Upper layer Lower layer Outer Strymonikos Upper layer Lower layer Ierissos V IN ¼ V IN þ V R IN ð2þ where V -IN (m 3 d -1 ) is the volume of the deep water entrained into the surface layer. Combining equations (1) and (2) one gets: V Q IN þ V IN V E IN þ V G IN þ V IN V IN ¼ 0 ð3þ The terms V R-IN (m 3 day 1 ), V -IN (m 3 day 1 ) and V E-IN (m 3 day 1 ) represent losses, pushing water out of this box; thus, in the presentation of results, these parameters appear with negative signs. However, their absolute value enters Eqs. 1 to 3 and the following equations. The water balance for the lower layer is given by: V IN ¼ V ' IN ð4þ meaning that the water inflow at the lower layer of the Inner (V ' IN ) is balanced by the vertical flow to the surface layer (V -IN ).

5 Nutrient biogeochemical budget 63 Fig. 3 volume and salt fluxes through the various model compartments of the Strymonikos and Ierissos s V G-IN V Q-IN Upper Layer V -IN V E-IN V -OUT V E-OUT V Layer1-IN S Layer1-IN V G-OUT V Q-OUT V-IN V Layer1-OUT S Layer1-OUT V -OUT V Z-IN V -IN V Z-OUT V -OUT Lower Layer V Layer2-IN S Layer2-IN V '-IN V Layer2-OUT S Layer2-OUT V '-OUT Inner Strymonikos Outer Strymonikos Open Sea V-I VE-I V G-I V Q-I V R-I VIerissos S Ierissos S OCEAN V X-I Ierissos Open Sea Assuming that the freshwater inflows (V -IN and V Q-IN ) and evaporative losses (V E-IN ) do not carry salt and taking into account Eq. 4, the salt balance for the upper layer of the Inner Strymonikos is given by: V IN S Layer1 IN þ V IN S Layer2 IN þ V Z IN S Layer2 IN S Layer1 IN ¼ 0 ð5þ meaning that the salt flowing to the outer Strymonikos is balanced by the vertical advective flux and the exchange flow. V Z-IN (m 3 day 1 ) expresses this vertical exchange between the surface and the bottom layer of the Inner Strymonikos. All salt fluxes through the upper layer box boundaries are expressed in psu m 3 day 1. The salt balance for the lower layer, taking into account Eq. 4, is given by: V 0 IN S Layer2 OUT V IN S Layer2 IN ð6þ V Z IN S Layer2 IN S Layer1 IN ¼ 0 showing that salt inflow from the Outer maintains the vertical advective flux and the exchange flow. Combining Eqs. 2, 4, 5, and 6 produces the following relations: V R IN S Layer1 IN V IN ¼ S Layer2 OUT S Layer1 IN and V Z IN ¼ V IN S Layer2 OUT S Layer2 IN S Layer2 IN S Layer1 IN ð7þ ð8þ The residual water flow from the upper layer of the Outer Strymonikos, taking into account Eq. 1, is given by: V R OUT ¼ V Q OUT þ V OUT V E OUT þ V G OUT ð9þ The combined outflow from the surface layer of the Outer Strymonikos is V -OUT, defined as: V OUT ¼ V OUT þ V R OUT þ V IN ð10þ where V -OUT (m 3 day 1 ) is the volume of the deep water entrained into the surface layer of the outer box. The residual outflow term V R-OUT will appear in the results with a negative sign, but its absolute value is used in Eqs. 9, 10,

6 64 G.K. Sylaios, V.A. Tsihrintzis and other equations. The salt balance for the upper layer of the Outer is given by: V IN S Layer1 IN V OUT S R OUT þ V OUT S Layer2 OUT þv Z OUT S Layer2 OUT S Layer1 OUT ¼ 0 ð11þ where it is assumed that the salinity of outflowing water (S R-OUT ) is the average of salinity between the budgeted water body and the oceanic salinity [4]: S R OUT ¼ S Layer1 OUT þ S Ocean 2 ð12þ In a similar manner, the water balance for the bottom layer is: V ' OUT V OUT V ' IN ¼ 0 and the salt balance is: V ' OUT S Ocean V OUT S Layer2 OUT ð13þ V S ' IN Layer2 OUT V Z OUT S Layer2 OUT S Layer1 OUT ¼ 0 ð14þ Combining the above Eqs. 9 to 14, we derive the following equations for the Outer Strymonikos : V ' OUT ¼ V OUT S R OUT S Ocean V OUT ¼ V ' OUT V IN and V Z OUT ¼ V ' OUT S Ocean S Layer2 OUT S Layer2 OUT S Layer1 OUT ð15þ ð16þ ð17þ The water and salt budgets for Ierissos were also developed in a manner similar to the above Eqs. 9 and 11, without the inclusion of the V R-IN term in Eq. 9 and the V S Layer1 terms in Eq. 11, following Gordon et al. [4] for well-mixed systems (Fig. 3). 2.4 Nitrogen and hosphorus Budgets The nitrogen and phosphorus budget for each system were defined following the salt budget for the various budget model compartments, as follows: For the upper layer of the Inner Strymonikos : V IN Y Layer1 IN þ V IN Y Layer2 IN þv Z IN Y Layer2 IN Y Layer1 IN þ $YLayer1 IN ¼ 0 ð18þ For the lower layer of the Inner Strymonikos : V 00 IN Y Layer2 OUT V IN Y Layer2 IN V Z IN Y Layer2 IN Y Layer1 IN þ $YLayer2 IN ¼ 0 For the upper layer of the Outer Strymonikos : ð19þ V IN Y Layer1 IN V OUT Y R OUT þ V OUT Y Layer2 OUT þv Z OUT Y Layer2 OUT Y Layer1 OUT þ $YLayer1 OUT ¼ 0 and for the lower layer of the Outer Strymonikos : ð20þ V 00 OUT Y Ocean V OUT Y Layer2 OUT V 00 IN Y Layer2 OUT V Z IN Y Layer2 IN Y Layer1 IN þ $YLayer2 OUT ¼ 0 ð21þ The inclusion of term ΔY represents the change in the concentration of DIN (Y=DIN) or phosphorus (Y=DI), which is positive in case of constituent Y addition and negative in case of constituent Y reduction, due to physical, abiotic chemical, or biotic chemical processes that take place within the system. In Eqs. 18 to 21, concentration of DI or DIN, denoted by Y, is expressed in mmol m 3, and the rate of change in concentration terms ΔY in mmol day 1. Using stoichiometric linkages between the fluxes, the developed nitrogen and phosphorus budgets can be used for calculations of the net rate of primary production minus respiration (p r) and nitrogen fixation minus denitrification [nfix denitr], respectively [4]. The term ΔDI obtained from the phosphorus budget can be used to provide an estimate of the net rate value of primary production minus respiration for the system: ðp rþ ¼ ΔDI ðc=þ art ð22þ where (C/) art is given by the Redfield ratio as C/=106:1 for plankton. Therefore, if the difference in the concentration of DI (ΔDI) is positive, then DI is moving from the organic material to the water column, (p r) is negative, and the system is a net producer of dissolved inorganic carbon (DIC) through respiration. If, on the other hand, ΔDI is negative, then DI is moving to the organic materials from the water column, (p r) is positive, and the system is a net consumer of DIC through net organic production. The nitrogen budget is complicated, since, during the measurements of DIN concentrations in the system, nitrogen fixation converts nitrogen gas to organic and inorganic nitrogen, and denitrification converts nitrate to nitrogen gas. Both of these processes require bacterial/

7 Nutrient biogeochemical budget 65 cyanophyte presence and usually require anaerobic conditions to proceed in aqueous ecosystems, especially in sediments. The net effect of this transfer [nfix denitr] is the difference between the measured DIN flux and that expected from the production and decomposition of organic matter, as given by: ½nfix denitrš ¼ ΔN Obs ΔN Exp ¼ ΔN Obs ΔDI ðn=þ art ð23þ where (N/) art is given by the Redfield ratio as N/=16:1 for plankton. Following the above analysis, Eqs. 1, 2, 7, and 8 were solved to determine the volume fluxes of residual outflow (V R-IN ), deep water entrainment (V -IN ), combined surface outflow (V -IN ) and vertical diffusive exchange flow (V Z-IN ) for the Inner Strymonikos, when background information is known on the seasonal or annual basis of surface runoff (V Q-IN ), precipitation (V -IN ), evaporation (V E-IN ) and mean upper- (S Layer1-IN ) and lower- (S Layer2-IN ) layer salinities of the Inner Strymonikos. Similarly, Eqs. 9, 15, 16, and17 were solved to determine the residual surface water outflow (V R-OUT ), the vertical deep water entrainment (V -OUT ), the horizontal deep water exchange (V ' OUT ), and the vertical diffusive exchange (V Z-OUT ) terms for the Outer Strymonikos. Finally, Eqs. 18 to 21 were used to determine the rate of change of nitrogen and phosphorus in the upper layer (ΔY Layer1-IN, ΔY Layer1-OUT ) and the lower layer (ΔY Layer2-IN, ΔY Layer2-OUT ) of the Inner Strymonikos and the Outer Strymonikos. Then, (p r) and [nfix denitr] were estimated from Eqs. 22 and 23. The convention for presentation of the model results is that whenever a flux moves materials (water, salt, nitrogen, and phosphorus) out of a model compartment, a negative sign is used in front of its absolute value. 2.5 Flushing Time Estimations The freshwater flushing time of semienclosed coastal water bodies can be calculated by various techniques, with the combination of the fraction of the freshwater method [3] producing the most reliable results. The seasonal variability of the total freshwater input volume (V Q ) in the Strymonikos was derived by applying the salinity deficit method to the salinity distribution data. A reference salinity level (S o ) was considered for each sampling period as appropriate to describe the sea water free from any freshwater influence. Since Black Sea modifies seasonally the mean system s salinity, the reference salinity level (S o ) was defined as a seasonally varying parameter, Table 3 and salt circulation volumes (V, V E, V Q, V, V Z ), residual flow (V R ), and exchange water flow (V ) calculated from the water and salt seasonal and annual budgets for the Strymonikos and Ierissos s. Volumes are 10 3 m 3 day 1 V Q V V E V R V V V Z June 1997 Inner 1, ,393 12, Outer ,269 12,365 5,582 17, Ierissos ,307 September 1997 Inner ,205 1, Outer ,525 3, Ierissos ,234 November 1997 Inner 3, ,683 79,695 76,012 32,764 Outer ,770 66,267 13,503 24,332 Ierissos ,425 February 1998 Inner 7, ,936 85,756 10,940 Outer , ,831 24,998 23,693 Ierissos ,122 Annual Model Inner 5, ,905 71,913 67,008 7,238 Outer ,851 68,970 2,881 12,235 Ierissos ,124 The convention is that positive volumes enter a box and negative volumes exit a box.

8 66 G.K. Sylaios, V.A. Tsihrintzis = 0.2 Q = 0.0 Q = 0.6 Q = 0.0 Q = 7.2 tn = +0.2 = 2.0 = 19 tn = 0.8 = -1.1 = 24.2 tn = +0.1 = 0.9 = 22.7 tn = -1.0 = 0.6 (a) = 0.05 Z = 1.5 = 0.03 Z = 1.7 (b) = 0.03 Z = 0.2 = 0.02 Z = 0.5 = 5.5 tn = +0.3 ' = 1.2 = 31.6 tn = +0.1 = 0.6 '' = 11.4 tn = = 0.2 ' = 54 tn = +0.2 = 0.5 '' = +0.6 System = -1.0 System = System = -0.8 System Q = 3.8 Inner Strymonikos Q = 0.0 Outer Strymonikos = 5.6 Q Inner Strymonikos = 0.0 Q Outer Strymonikos = 19.1 tn = = 23.7 = 45 tn = -4.2 = 15.4 = 17.2 tn = -1.4 = 20.0 = 41 tn = -5.6 = 19.9 (c) = 7.3 Z = 5.7 Z = 2.3 = 1.8 (d) = 0.3 Z = 16.0 = 0.7 Z = 4.9 = 2.2 tn = ' = 10.6 = 41 tn = -1.1 = 7.6 '' = 2.4 tn = -1.1 = 16.7 ' = 47 tn = +7.9 = 14.4 '' = +9.3 System = -5.3 System System = -2.5 = +2.3 System Fig. 4 Monthly phosphorus transport (in tonnes) between the various model compartments in the Strymonikos, in a June 1997, b September 1997, c November 1997, and d February 1998 with values 34.5, 33.8, 34.0, and 34.5 psu, for June, September, November, and February, respectively [11]. The freshwater flushing time (τ) was calculated as [3]: t ¼ Fresh Volume Fresh Input ¼ n i¼1 h S i S o S o V Q V i i ð24þ where n the number of estuary segments (n=36), S i salinity of the ith volume segment, V i volume of the ith segment (m 3 ), and V Q the seasonally variable river water volume influx (m 3 day 1 ). The estuarine system renewal time was derived from the water and salt budget terms as the ratio of the system s total volume to the sum of the mixing exchange flux (V X ) and the absolute value of the residual flow (V R )[4]. 3 Model Results 3.1 Seasonal Budgets The water and salt circulation volumes produced from the application of the seasonal and annual water and salt budgets for the Strymonikos and Ierissos s are given in Table 3. Values for groundwater volume fluxes V G were not available but can be considered negligible compared to the other quantities, on an order of magnitude analysis. Indeed, the area suffers from seawater intrusion due to groundwater overpumping, especially in the summer (low river discharge), when irrigation demand from groundwater sources is increased. As expected, river water discharge (V Q ) at the head of the upper layer of Inner Strymonikos induces an outward water flux, varying seasonally between m 3 day 1 in September and 7, m 3 day 1 in February. This residual outward flux from the inner box (V R ) is added to the inward fluxes of the upper layer box of the Outer Strymonikos. In the Inner Strymonikos, net vertical entrainment is seen to transfer water volumes upward (V ), while at the same time, this net vertical entrainment volume flux is removed from the lower box of the Outer Strymonikos. Net vertical diffusive exchange (V Z ) was found of the order and 32, m 3 day 1 in September and November, respectively. In the upper layer of the Outer Strymonikos, the excess of evaporation over local surface runoff and precipitation during the summer period (June and September) induces an inward surface transport (V ) from the open sea, with values 5, and 2, m 3 day 1, respectively. Horizontal mixing volumes were positive under low flow conditions, due to the salinity gradient developed in the estuary ocean system. volume fluxes in the Ierissos showed similar variability to those in the Strymonikos

9 Nutrient biogeochemical budget 67 N Q = N Q = 0.0 N = 10.5 tn N = 2.9 Ν = N = 32 tn Ν = -0.4 N = 1.2 N = 3.1 Q -4.3 N = 43 tn Ν = -3.2 N = 0.0 Q N = 1.6 N +6.1 Ν = -1.5 = 70 tn N = 1.8 (a) N = 1.6 N = 3.0 N = 7.8 N = 3.8 (b) N = 1.0 N = 1.7 N = 7.6 N = 2.0 N = 11.0 tn Ν = +0.4 N = 2.7 ' N = 73 tn Ν = 0.0 N = 1.1 '' N = 98 tn Ν = +1.1 N = 0.6 ' N = 227 tn N = 1.1 Ν = -0.2 '' release burial release burial Inner Strymonikos Outer Strymonikos Inner Strymonikos Outer Strymonikos N = 12.7 Q N = 0.1 Q N = 57.2 Q N Q = N = 121 tn Ν = +145 N = 150 N = 215 tn Ν = N = 73 N = 511 tn Ν = +525 N = 591 N = 658 tn Ν = N = (c) N = 45.4 Z N = 156 N Z = 17.2 N = 37.4 N = 5.3 N = 30.2 (d) N Z = 60.7 N = 9.9 N = 69.2 N Z = 34.5 N = 31 N = N = 14.3 tn Ν = release N = 30 ' N = 114 tn Ν = burial N = 26.3 '' N = 10.4 tn Ν = burial N ' = N = 300 tn Ν = -214 N = '' 16.2 release Fig. 5 Monthly nitrogen transport (in tonnes) between the various model compartments in the Strymonikos, in a June 1997, b September 1997, c November 1997, and d February 1998, even though this system is not affected by direct river inflow. Nevertheless, there is freshwater inflow from runoff from the drainage areas surrounding the gulf. Residual water flux was directed inwards in September ( m 3 day 1 ) and outwards in the winter ( m 3 day 1 ). The amounts (in tonnes) of phosphorus and nitrogen transported between the various model compartments of the Strymonikos, calculated on a monthly basis, are shown schematically in Figs. 4 and 5. The high variability in the horizontal and vertical transport of nitrogen and phosphorus within the system is controlled by the river influx. Therefore, under low flow conditions, the horizontal and vertical advective transport of nitrogen and phosphorus is reduced, compared to the strong horizontal and vertical fluxes induced by a high river discharge. The terms Δ illustrate the net transfer of phosphorus to/from the biological materials (and sediments) for each model compartment. During low and moderate phosphorus river fluxes (June, September, and November) the Inner Strymonikos shows Δ System terms to be positive indicating a net release of phosphorus from biological materials (and sediments) ranging from 0.12 t of phosphorus in September to 9.3 t of phosphorus in November. Within the same period, the Outer Strymonikos appears with negative Δ System values, meaning that phosphorus burial in this system varies from 0.8 to 5.3 t. Under increased river loads (5.6 t in February), the Inner transfers phosphorus to the biological materials (and ultimately to the sediments) while the Outer system releases phosphorus from the bottom. In the nitrogen budget (Fig. 5), the terms [nfix denit] balance the remaining nonconservative DIN fluxes in the surface boxes, while the terms ΔN represent the net amounts of nitrogen biologically converted by the nitrification and denitrification processes. In the deeper boxes, the remainder of the nitrogen flux, gains or losses, is balanced by the [nfix denit] terms, while the nitrogen burial/release terms are inferred from the dissolved phosphorus fluxes. The net ecosystem metabolism of the estuary bay system is summarized in Table 4. ΔDI and ΔDIN fluxes for the Inner Strymonikos refer to the combined effect of the upper and lower layer. Calculated fluxes demonstrate the wide variability in all nutrients and their fluxes in the system. In June 1997, a deficit in DI balance was computed for the upper and the lower layers of the Inner Strymonikos (0.2 and 0.4 t, respectively), producing a net deficit of 0.6 t for the whole system. This means that phosphorus was delivered dominantly by respiratory processes at both layers of the water column of the Inner. In June, the Inner Strymonikos was considered as slightly heterotrophic, since it is a net producer of organic matter through respiration at an estimated rate of 8.13 mg C

10 68 G.K. Sylaios, V.A. Tsihrintzis Table 4 Net ecosystem metabolism calculated from the nitrogen and phosphorus seasonal and annual budgets for the Strymonikos and Ierissos s Inner Outer Ierissos June 1997 ΔDI (mg m 2 day 1 ) ΔDIN (mg m 2 day 1 ) (p r) (mg C m 2 day 1 ) [nfix denitr] (mg N m 2 day 1 ) September 1997 ΔDI (mg m 2 day 1 ) ΔDIN (mg m 2 day 1 ) (p r) (mg C m 2 day 1 ) [nfix denitr] (mg N m 2 day 1 ) November 1997 ΔDI (mg m 2 day 1 ) ΔDIN (mg m 2 day 1 ) (p r) (mg C m 2 day 1 ) [nfix denitr] (mg N m 2 day 1 ) February 1998 ΔDI (mg m 2 day 1 ) ΔDIN (mg m 2 day 1 ) (p r) (mg C m 2 day 1 ) [nfix denitr] (mg N m 2 day 1 ) Annual budget ΔDI (mg m 2 day 1 ) ΔDIN (mg m 2 day 1 ) (p r) (mg C m 2 day 1 ) [nfix denitr] (mg N m 2 day 1 ) m 2 day 1. DIN appeared in excess in the surface layer of the system by 0.8 t N and in deficit at the bottom layer by 0.4 t N; therefore, net denitrification processes converted DIN to nitrogen gas at rate of mg N m 2 day 1. In September, the DI deficit in both layers produced a net biological release of 0.12 t in the Inner Strymonikos. This behavior was mostly attributed to the considerable reduction in the residual and the deep water volume fluxes, occurring under very low flow conditions of the Strymon River. Respiratory processes exceeded primary production at a rate of mg C m 2 day 1, and thus, the system appeared as a net consumer of organic matter. An excess of 2.12 t N was computed in the balance of the system, and therefore, net denitrification processes converted DIN to nitrogen gas at a rate of mg N m 2 day 1. Under moderate river flow conditions, prevailing in November, the low salinity surface layer exhibited a DI deficit of 21.6 t. This deficit was not balanced by the excess of DI at the bottom layer (12.3 t), and therefore, 9.3 t was released from organic materials and sediments to the water column. The (p r) term was strongly negative during this period, and the system was considered as strongly heterotrophic, producing organic matter through respiration at a rate of 132 mg C m 2 day 1. Similarly, a surface layer DIN deficit of 145 t and a bottom layer surplus of 38 t were estimated for November, leading to nitrogen fixation conditions ( mg N m 2 day 1 ) during this period. Under the very high river flow regime of February, the Inner system shows net autotrophy (+35 mg C m 2 day 1 ) and net nitrogen fixation conditions ( mg N m 2 day 1 ). It is the only period that the net ecosystem metabolism of the Inner Strymonikos showed this autotrophic behavior. Outer Strymonikos showed a constant behavior in its net ecosystem metabolism, being net autotrophic in all seasons, apart from February when very strong residual and exchange flows prevailed between the inner and outer systems (boxes). The excess of primary production over respiration in this system ranged from mg C m 2 day 1 in September to mg C m 2 day 1 in November. Nitrogen fixation exceeded denitrification during the summer period (June and September) with values of 0.75 and 0.50 mg N m 2 day 1, respectively, while the opposite behavior was shown in the winter period (November and February). The Ierissos was estimated to be net heterotrophic and net denitrifying in all seasons. The production of organic matter through respiration in the Ierissos ranged from mg C m 2 day 1 in November to 64 mg C m 2 day 1 in February. Net DIN conversion to nitrogen gas occurred at rates ranging between 0.7 mg N m 2 day 1 in November and 23.2 mg m 2 day 1 in February. 3.2 Annual Budget The annual balance model was obtained by considering the average input values of the seasonal models. The dependence of model results to model inputs was tested on the annual budget, showing that for ±25% change to the considered surface salinity gradient, a limited variation of ±3 6% occurred in the model output values of ΔDI, ΔDIN, (p r), and [nfix denitr]. However, model results appeared sensitive to variations in phosphorus and nitrogen horizontal gradients; thus, input changes of ±10% resulted in 10 33% changes in model outputs. In the Strymonikos, a residual water volume flux of 4,905 and 71, m 3 day 1 was estimated for the Inner and Outer gulfs, respectively. A DI and DIN deficit occurred in the surface and the bottom layers of the Inner Strymonikos, leading to an overall deficit of 128 t and 1,280 t N. The phosphorus deficit was balanced by its return to the water column from the oxidation of biological materials (and

11 Nutrient biogeochemical budget 69 Net Metabolism (mmol C or N m -2 d -1 ) (p-r) Outer Strymonikos from sediments) at a rate of 10.6 t year 1. The Inner Strymonikos appears to act as a strongly heterotrophic system oxidizing organic matter through respiration at a net rate of 150 mg C m 2 day 1. Nitrogen deficit in the inner system was balanced by the excess of nitrogen fixation over denitrification (10 mg N m 2 day 1 ). The Outer Strymonikos shows a net autotrophic behavior, with the production of organic matter through primary production at a high net rate (56.7 mg C m 2 day 1 ). Denitrification exceeded slightly nitrogen fixation at a net rate of 0.04 mg Nm 2 day 1. Overall, the Strymonikos (Inner and Outer) is a net autotrophic system producing through primary production 832 t C year 1 (5.86 mg C m 2 day 1 ). The system is a net sink for nitrogen converting 347 t N year 1 (2.46 mg N m 2 day 1 ) of nitrogen gas to organic nitrogen. The Ierissos appears heterotrophic producing 3.1 t C day 1 and denitrifying releasing 0.5 t N day 1 to the atmosphere. 3.3 Testing of Scenarios with the Model (r) Inner Strymonikos (r) Outer Strymonikos (p-r) Inner Strymonikos Strymon River Discharge (m 3 s -1 ) Fig. 6 Scenario 1 model output results for (p r) and [nfix denitr], in relation to different Strymon River flow conditions. Strymon River phosphorus concentration was kept constant to the annual mean value (1.16 mmol m 3 ) Using previous calculations as a reference, three different scenarios were developed and examined. Investigations were restricted to phosphorus because its flux pathways appear relatively well defined, although the basic assumption of this model is that phosphorus exists only in two states, dissolved or part of organic particulates [4]. All scenarios examined the variation of (p r) and [nfix denitr] terms in the Inner and Outer Strymonikos s, under variable river discharge and constant river concentrations of dissolved phosphorus (scenario 1), under constant river discharge and variable river phosphorus concentration (scenario 2), and under variable river discharge and constant riverine input of phosphorus (scenario 3). In scenario 1, the annual budget model examined the influence of river inflow (varying from 1.2 to 78.8 m 3 s 1 ), while maintaining the Strymon River phosphorus concentration constant at a mean annual value of 1.16 mmol m 3. Figure 6 presents the variability of model output terms of the Inner and Outer Strymonikos s, in relation to river discharge. The Inner Strymonikos is an autotrophic system under almost zero river flow conditions (Q<2.4 m 3 s 1 ), which could result from Strymon River water retention by Kerkini Lake. The lake is a controlled artificial reservoir located 77 km upstream from the river mouth, which meets the great demand for irrigation water during spring and summer. Heterotrophic conditions prevail in the Inner part of the gulf under moderate and high river discharge. Net primary production continuously increased for the Outer Strymonikos as a function of river flow increase. Nitrogen fixation exceeded denitrification as river discharge increased in the Inner, while an almost constant behavior occurred in the Outer. In scenario 2, the mean annual model was used to study the effect of river phosphate concentration increase (from 0.05 to 1.58 mmol m 3 ) while maintaining Strymon River discharge to its mean annual value (58 m 3 s 1 ). Such a situation could result from considerable increases in fertilizer applications and an unchanged hydrological regime. The influence of phosphate concentration increases in the Strymon River flowing under annual mean flow conditions is shown in Fig. 7. The Inner Strymonikos is a strong net heterotrophic system under low DI concentrations. An increase in DI diminished the difference between respiratory processes over primary production. Denitrification prevailed in the Inner under low DI values (DI<0.32 mmol m 3 ) but yielded nitrogen fixation excess under higher DI values. The Outer Net Metabolism (mmol Cor N m -2 d -1 ) (p-r) Outer Strymonikos (r) Inner Strymonikos (r) Outer Strymonikos (p-r) Inner Strymonikos DI River Concentration (mmol m -3 ) Fig. 7 Scenario 2 model output results for (p r) and [nfix denitr], in relation to different Strymon River phosphorus concentrations. Strymon River discharge was kept constant to the annual mean value (58 m 3 s 1 )

12 70 G.K. Sylaios, V.A. Tsihrintzis Net Metabolism (mmol Cor N m -2 d -1 ) (p-r) Outer Strymonikos (r) Outer Strymonikos (p-r) Inner Strymonikos remained unaffected in all model output terms under various DI concentrations. In scenario 3, the product of riverine dissolved phosphorus outflux (V Q-IN DI Q-IN ) was assumed to remain constant to the mean annual value (5,800 mol day 1 ), while river discharge and DI Q-IN values varied accordingly. Model results show that a phosphate concentration increase (DI Q-IN >4.1 mmol m 3 ), even under low river flow (V Q <1, m 3 day 1 ), would move the Inner toward net autotrophy and increased net nitrogen fixation (Fig. 8). 3.4 Flushing and Renewal Time Estimates (r) Inner Strymonikos DI River Concentration (mmol m -3 ) Fig. 8 Scenario 3 model output results for (p r) and [nfix denitr], in relation to different Strymon River phosphorus concentrations. Strymon River import of phosphorus to the system was kept constant to the annual value (5,800 mol day 1 ) Results from the seasonal water and salt budget models may also be used to estimate the freshwater flushing time and the estuarine renewal time. Estuarine flushing time is the average amount of time that freshwater remains in the system, while estuarine renewal time is the time needed to renew the entire volume of the estuary [17]. Both time scales are important in terms of the biological and chemical processes occurring in estuaries and determine the rate of most biogeochemical transformations. Following this analysis, we estimate that the river water remained in the Strymonikos for a period of 86 days in June, 41 days in September, 4 days in November, and for only 2 days in February (Table 5). As expected, freshwater residence time is shorter during the period of high river discharge, compared to low flow conditions. Furthermore, it occurs that the freshwater flushing time varies considerably with river flow, which implies that under reduced river discharge, there exists sufficient time for nitrogen and phosphorus recycling to take place. Similarly, a wide range of estuarine renewal time for the different compartments of the Strymonikos and Ierissos s was computed using the water and salt budget terms. The log log relationship between surface runoff flux and system s renewal time illustrates the sensitivity of the biogeochemical cycles scaling for each system, under variable river discharge conditions (Fig. 9). This sensitivity is expressed by the following regression equations: For the Inner Strymonikos : log ðrenewal TimeÞ ¼ 6:73 1:33 log acerunoffv Q ; R 2 ¼ 0:96 For the Outer Strymonikos : log ð Renewal TimeÞ ¼ 6:01 1:30 log ace Runoff V Q ; R 2 ¼ 0:98 For the Ierissos : ð25þ ð26þ log ðrenewal TimeÞ ¼ 5:43 1:32 log ace Runoff V Q ; R 2 ð27þ ¼ 0:96 The similar slope of all lines implies that a potential doubling of surface runoff results in a decrease in the entire system s renewal time by approximately 60%. 4 Discussion A simple steady-state budget model according to LOICZ modeling guidelines was applied in two semienclosed coastal water bodies, aiming at scaling the net nutrient biogeochemical processes taking place in these systems. Such steady-state box models have been used successfully to elucidate the interactions in deep ocean water masses [16], large-scale marine basins [2, 8], coastal seas [1, 5, 7], and mangrove areas [15]. The nitrogen and phosphorus budgets examined constructively the biogeochemical cycles of the Strymonikos System Renewal Time (d) Inner Strymonikos Outer Strymonikos 10 6 Ierissos ace Runoff (m 3 d -1 ) Fig. 9 Log log relationship between surface runoff flux and systems renewal time, under variable river discharge conditions

13 Nutrient biogeochemical budget 71 Table 5 Freshwater flushing time (in days) and estuarine renewal time (in days) for Strymonikos and Ierissos s, as derived from the seasonal and annual water and salt budgets June 1997 September 1997 November 1997 February 1998 Annual Freshwater flushing time Strymonikos Estuarine renewal time Inner Strymonikos Outer Strymonikos Ierissos 2,953 1, and Ierissos s, scaled to rates per unit area. The main point emerging from this analysis is that the budgets of individual sub-basins may be tied together (or nested) in such a manner that the fluxes between these sub-basins are expressed in total mass units. On an annual basis, it has been estimated that 65.8 t enter the Inner through river flow. Horizontal surface flow and vertical diffusion remove from the upper layer 310 and 18.6 t, respectively, while vertical entrainment adds t. Hence, the annual phosphate budget suggests a small excess of uptake over release (146.5 t ) for the upper layer and a reverse process for the bottom layer of the Inner Strymonikos (18.6 t ). In the surface of the Outer, horizontal surface transport removes t year 1, vertical advective flux removes 5.0 t year 1, while the vertical diffusive flux imports 7.2 t year 1, and the residual transport from the Inner imports 310 t year 1, leading to the burial of t year 1. In a similar manner, the bottom layer of the Outer Strymonikos produces an excess of 11.0 t year 1, leading to an overall net transfer of t year 1, to the organic materials of the Outer Strymonikos. Similarly, river inflow and vertical advection add and 640 t N, respectively. Horizontal surface flow removes a significant amount of nitrogen from the upper layer of the Inner Strymonikos (2,142 t) and vertical exchange flow t. Nitrogen deficit in the inner system was balanced by the excess of nitrogen fixation over denitrification (358 t). In the surface of the Outer Strymonikos, residual transport from inner gulf imports 2,142 t N year 1, deep water entrainment removes 21.2 t N year 1, vertical diffusion exports 85.2 t N year 1, and surface horizontal transport removes 991 t N year 1, producing a net denitrifying effect of 1.8 t N year 1 for the outer system. Overall, 970 and 1,301 t C are being produced and released to the atmosphere as CO 2 from the Inner Strymonikos and Ierissos s, respectively. The Strymonikos is a nitrogen sink of 350 t annually, to be transferred due to nitrogen fixation surplus, while a release of 180 t annually was computed to be transferred through the denitrification-dominant processes from the Ierissos. Uncertainties on the above analysis arise as a result of the derivation of the spatially averaged nutrient concentrations (±1%), the computation of the river-derived N and annually averaged loads (±10%), and the fact that organic nutrients were not considered (±1%). Such errors imply that the ΔDI is subject to uncertainty of the order of ±6%, while ΔDIN was estimated by a rather wide uncertainty level of 12%. From the three tested scenarios, it was demonstrated that an increase in phosphorus load from increasing river flow or river DI concentration could stimulate net primary production in the Inner Strymonikos, turning the system toward an autotrophic balance. However, since the outer part is directly related to the processes of the inner box, the Outer Strymonikos under these conditions would exhibit an opposite behavior moving toward heterotrophy. Given that the Outer Strymonikos covers an area three times larger than that of the Inner, it seems that the whole system would retain a heterotrophic balance even under conditions of significantly increased phosphorus. 5 Conclusions An estuarine-type two-layer, two-box budget model was developed to study the internal circulation dynamics and the transport of conservative (salt) and nonconservative (nutrients) constituents in two semienclosed coastal regions. These budgets followed the methodology developed by LOICZ that has been proved capable of assessing the nutrient status of coastal areas using limited hydrologic, hydrographic, and water quality data. Based on this study, the following conclusions can be formulated: (1) The major biogeochemical transformation of nitrogen and phosphorus takes place in the immediate nearshore zone (Inner Strymonikos ), while the outer system sustains its nutrient dependence on oceanic exchanges; (2) under the summer low to moderate river flow conditions, the riverinfluenced inner part of the Strymonikos acts as a net source for nitrogen and phosphorus, being a net producer of DIC through respiration; (3) during the winter high river flow conditions, phosphorus is transferred (buried) to the biological material and the bottom sediments in the inner part, and the system moves to an autotrophic state consuming organic matter through net organic production;

14 72 G.K. Sylaios, V.A. Tsihrintzis (4) the area of the system not directly influenced by river discharge, such as the Outer Strymonikos, shows an opposite net metabolic behavior throughout the year, with the net autotrophic and denitrification processes prevailing; and (5) the investigated scenarios reflected realistic future developments of these systems in terms of river discharge and river nutrient concentrations. Acknowledgments Comments on the manuscript by Dr. C.J. Crossland are greatly appreciated. References 1. Cao, W., Hong, H., & Yue, S. (2005). Modelling agricultural nitrogen contributions to the Jiulong River estuary and coastal water. Global and lanetary Change, 47(2 4), Chen, C-T. A., Wang, S-L., Wang, B-J., & ai, S-C. (2001). Nutrients budgets for the South China Sea basin. Marine Chemistry, 75, Dyer, K. R. (1973). Estuaries: A physical introduction p London: Wiley. 4. Gordon, D. C., Boudreau,. R., Mann, H., Ong, J. E., Sivert, W. L., Smith, S. V., Wattayakorn, G., Wulff,. and Yanagi, T. (1996). LOICZ Biogeochemical modeling guidelines. In: LOICZ reports and studies: no. 5 (p. 96). LOICZ, Texel, The Netherlands. 5. Hart, B. T., van Dok, W., & Djuangsih, N. (2001). Nutrient budget for Saguling Reservoir, West Java, Indonesia. Research, 36 (8), Nixon, S. W. (1981). Remineralisation and nutrient cycling in coastal marine ecosystems. In B. J. Nielson, & L. E. Cronin (Eds.) Estuaries and nutrients (pp ). Totowa, NJ: Humana. 7. Savchuk, O.. (2002). Nutrient biogeochemical cycles in the of Riga: scaling up field studies with a mathematical model. Journal of Marine Systems, 32(4), Simpson, J. H., & Rippeth, T.. (1998). Non-conservative nutrient fluxes from budgets for the Irish Sea. Estuarine, Coastal and Shelf Science, 47, Smith, S. V., Hollibaugh, J. T., Dollar, S. J., & Vink, S. (1991). Tomales Bay metabolism: C N stoichiometry and ecosystem heterotrophy at the land sea interface. Estuarine, Coastal and Shelf Science, 33, Strickland, J. D. H., & arsons, T. R. (1965). A manual of sea water analysis (2nd ed.). Ottawa, Canada: Fish Research Board. 11. Sylaios, G., Koutrakis, E., & Kallianiotis, A. (2006). Hydrographic variability, nutrient distribution and water mass dynamics in Strymonikos (N. Greece). Continental Shelf Research, 26(2), Talaue-McManus, L., Smith, S. V., & Buddemeier, R. W. (2003). Biophysical and socio-economic assessments of the coastal zone: the LOICZ approach. Ocean & Coastal Management, 46(3 4), van Beusekom, J. E. E., & de Jonge, V. N. (1998). Retention of phosphorus and nitrogen in the Ems estuary. Estuaries, 21, Wattayakorn, G., rapong,., & Noichareon, D. (2001). Biogeochemical budgets and processes in Bandon Bay, Suratthani, Thailand. Journal of Sea Research, 46, Wosten, J. H. M., de Willigen,., Tri, N. H., Lien, T. V., & Smith, S. (2003). Nutrient dynamics in mangrove areas of the Red River Estuary in Vietnam. Estuarine, Coastal and Shelf Science, 57, Wright, R. (1969). water movement in the western Atlantic as determined by use of a box model. -Sea Research, 16, Zimmerman, J. T. F. (1976). Mixing and flushing of tidal embayments in the Western Dutch Wadden Sea. art I: Distribution of salinity and calculation of mixing time scales. Netherlands Journal of Sea Research, 10(2),