Water and Nutrient Budgets of the Gulf of Riga

O. P. Savchuk and D. P. Swaney

The Gulf of Riga is a large bay on the Baltic Sea which straddles the border of Estonia and Latvia. Its surface area is 16330 km2 (3.9% of the total area of the Baltic Sea) and volume is 424 km3 (2.1% of the total volume of the Baltic Sea).  Though the mean depth of the Gulf is about 26 meters (HELCOM, 1996), its maximum depth exceeds 60 m (see bathymetric map).The Gulf is connected to the Baltic by two straits separated by the island of Saaremaa. The larger outlet to the west, the Irbe strait, is 27 km wide.  It has a cross-sectional area of 0.37 km2 at its minimum, where the depth of the sill is around 21 m, and frequently exhibits bi-directional flow to and from the Baltic. The other, northern outlet is a series of straits, called the Vainameri ("sea of straits" in Estonian). The southernmost strait of this series, Suur Strait, has a cross-sectional area of about 10% of the Irben strait (Otsmann et al., 1997).

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Map 1.  Bathymetry of the Gulf of Riga

The drainage area of the Gulf, about 134000 km2 (8% of the total drainage area of the Baltic Sea), is approximately 40% agricultural land, and 38% forested. About 4.6 million people inhabit the watershed of the Gulf, two-thirds of them in urban areas (Laznik et al. 1997). More information on the drainage area of the Gulf and other areas surrounding the Baltic Sea may be found at the Ballerina website.

Below, we first present an analysis of water and nutrient budgets for the Gulf of Riga based on long-term average data (1977-95).  Next, we discuss the question of the interannual variability of this system by looking at nutrient budgets for the years 1993-95.  Finally, more detailed model-based ecological and biogeochemical analyses can be found on the Gulf of Riga project website (here).

The steady-state water budget of the Gulf of Riga

Rivers draining to the Gulf, dominated by the Daugava and Lielupe, contribute a flow of approximately 36.2 km3 yr-1 (Laznik et al. 1997). Direct precipitation on the surface of the Gulf, averaging about 59 cm yr-1 adds approximately 11.2 km3 yr-1. Evaporation from the Gulf of around 50 cm yr-1 results in a loss of 9.5 km3 annually (HELCOM, 1986). The resulting annual net freshwater flow to the Gulf of 37.9 km3 yr-1 has a strong seasonal component which typically peaks in April (Laznik et al. 1997).

Compared to the freshwater inflows, the flows through the two straits of the Gulf are enormous and highly variable. The flows through the Suur and Irben straits ‘act together’, i.e. when water enters the Gulf through the Irben strait, water flows out through the Suur, and vice versa. No long-term circulatory patterns exist; flows are largely wind-driven. For this reason, the Gulf experiences larger flows through the straits during mild winters with low ice-cover, in which the sea surface is not protected from the wind. (Otsmann et al., 1997; Suursaar and Astok, 1996; Suursaar et al., 1995).

While the circulation of the Gulf of Riga is complex and time-varying, a steady-state analysis (figure 1) gives reasonably good agreement with existing estimates of exchange with the Baltic (Berzinsh, 1995; Suursaar et al., 1996; Håkansson et al., 1998). Salinity of  waters flowing into the Gulf from the Baltic are average values of the top 30 m of waters of the Eastern Gotland Basin. Annual median vertical profiles of salinity for this area (56º30'-58º10'N, 19º15'-20º50'E) were extracted from the Baltic Environmental Database (BED) with the SwingStations tool for 1977-1995 and averaged for the 0-30 m layer.  Corresponding data for the Gulf of Riga (56º57'-58º30'N, 22º09'-24º34'E) were taken from the same sources and averaged over the entire available depth profiles.  Average salinity of the Gulf is 5.7 psu,  and of the Baltic, 7.4 psu.  Taking the above values of VQ (riverine inflows) + VP (precipitation) - VE (evaporation from the Gulf) yields an estimate of VR,  the net freshwater inflow (37.9 km3 yr-1). Other analysts have assumed that VE equals VP, which makes little difference in the estimate.  Using a ‘Knudsen analysis’ (see salt budget page) to balance the salt budget yields an ‘exchange flow ‘ of around 148 km3 yr-1.  While we use average salinity figures of 7.4 psu and 5.7 psu for the Baltic and the Gulf respectively, it is worth noting that over the period from the mid '70s to the mid '90s, the average salinities of both systems have decreased slightly (~ 0.7 psu), at about the same rate (so that the effect of this trend on the 'exchange flow' calculation is negligible).

 

 

 Figure 1. Average annual water and salinity budgets for the Gulf of Riga (LOICZ notation)

 

    The steady-state water and salinity budget gives a general sense of the magnitudes of these flows, but can be misleading in a given year, particularly regarding exchange with the Baltic proper.   The forces driving the circulation and salinity of the Gulf and Baltic vary both seasonally and interannually (Gulf of Riga Project estimates of Vx for the period 1993-1995 indicated a variability of 50%).  This point is discussed further under Interannual Variability, below.

 Steady-state Nutrient Budgets for the Gulf of Riga

Previous work on the Gulf of Riga (e.g. Yurkovskis et al., 1993)  has suggested that the Gulf serves as a major sink of both nitrogen and phosphorus, in effect buffering the Baltic proper from the effects of terrestrial and atmospheric nutrient loads.  Newer information on nutrient loads (Laznik et al. 1997) allows us to elaborate on these conclusions below.  Nutrient load from riverine sources and exchange with the Baltic are estimated as a product of monthly water flows and nutrient concentrations.

Nitrogen

Nutrient concentrations in  waters flowing into the Gulf from the Baltic and within the Gulf for 1977-1995 are determined from data in the same region, and using the same database, as indicated above. Annual average concentrations were calculated by averaging all available vertical profiles of the median values of total nitrogen (TN), total phosphorus (TP), dissolved inorganic nitrogen (DIN = [NO3]+[NO2]+[NH4]), and dissolved inorganic phosphorus (DIP = [PO4]) over the top 30 m of the Baltic, and over the entire depth of the Gulf, for all available dates.  Some bias may be expected from this procedure because more profiles are sampled from research vessels during the summer months than the winter months (when the Gulf may be ice-covered), and because the total area of the water column of the Gulf decreases with depth (we give equal weight to samples at each depth).  Resulting average concentrations for period in the Gulf and Eastern Gotland Basin region of the Baltic are 36.6 mM and 20.7 mM, respectively (figure 2).   The average of these two values is taken to be the concentration of waters at the boundary of the Gulf, which is used to calculate the loss of nutrients from the Gulf to the Baltic with 'residual' flow of 1.1 x 109 moles N yr-1.   Because the  average TN concentration in the local Baltic is roughly half that of the Gulf, a large loss of nitrogen corresponding to this gradient is also seen, associated with 'exchange flow', amounting to 2.4 x 109 moles N yr-1

 

tnbud2.gif (7293 bytes) 

Figure 2. Average annual total nitrogen budget for the Gulf of Riga (LOICZ notation).

The dominant source of nitrogen to the Gulf is from riverine transport.  An estimate of this value, averaging 113189 metric tons N yr-1 (8.1x 109 moles N yr-1) for the same period (1977-1995) is taken from (Laznik et al. 1997).   Annual nutrient input from point sources for 1990 amounting to 5341 metric tons N yr-1(0.4 x 109 moles N yr-1) is taken from (HELCOM 1993). Nitrogen deposition from the atmosphere was estimated by Yurkovskis et al. (1993) at 16000-17000 metric tons N yr-1 (1.1-1.2 x 109 moles N yr-1) based on a combined wet- and dry-deposition value of 60 mmol N  m-2 yr-1.

An estimate of nitrogen loss to burial in sediments is obtained as the product of an average sedimentation rate of TN (77 mmol m-2 yr-1) and the area of accumulation bottom (i.e. the area of the Gulf in which sediment accumulation is observed, equal to 5290 km2) (Carman et al., 1996).  This value, around 5700 metric tons N yr-1 (0.4 x 109 moles N yr-1), does not come close to accounting for the discrepancy between the total load to the Gulf and the loss from the Gulf to the Baltic, a difference of 6.2 x 109 moles N yr-1.

To gain a little more insight into the N budget, we turn to the budget of dissolved inorganic N (figure 3).  Riverine load of DIN for the period (Laznik et al. 1997) remains the dominant source.  DIN amounts to half the total nitrogen load from point sources, following the assumption of Yurkovskis et al. (1993).  Atmospheric sources of nitrogen are assumed to be entirely inorganic and dissolved or readily soluble, so that the atmospheric DIN load is assumed to be the same as the TN load.  Residual and exchange flows to the Baltic are calculated as with TN, using average DIN concentrations of 7.1 and 0.8 mM, for the Gulf and Eastern Gotland Basin, respectively (figure 3).  The discrepancy between loads and losses of DIN amounts to around 4.8 x 109 moles N yr-1.  About 4.2 x 109 moles N yr-1 of this is accounted for by net denitrification, as described  below.  The remainder we attributed to limitations of the steady-state assumption for this system.  We discuss this further under Interannual Variability, below.

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 Figure 3. Average annual dissolved inorganic nitrogen budget for the Gulf of Riga (LOICZ notation).

Phosphorus

 The major terms of the total phosphorus budget of the Gulf of Riga are shown in figure 4.  Calculations are made using the same assumptions as in the N budget.  As before,  the largest source term for phosphorus is from riverine inflows, about 66 x 106 moles P yr-1.  In contrast to total N,  coastal point sources of phosphorus account for around 1/3 of the riverine sources (22 x 106 moles P yr-1).   Atmospheric deposition provides a relatively small source to the Gulf (9.7 x 106 moles P yr-1).  The Gulf exports approximately the same amount of phosphorus to the Baltic as enters it from the above three sources; an average of 96 x 106 moles P yr-1 leaves with outflowing waters and exchange flow.   However, making the same type of estimate of phosphorus burial in sediments as for nitrogen results in a burial loss of 35 x 106 moles P yr-1. As a result, in the the total P budget about 34 x 106 moles P yr-1or 8% of the total P pool is "missing".  No estimate of P remineralization is available.

 

 Figure 4. Average annual total phosphorus budget for the Gulf of Riga (LOICZ notation)

Again, considering the dissolved inorganic budget is helpful.  DIP load from riverine sources accounts for about 49 x 106 moles P yr-1.   The combined atmospheric and point source loads together account for another 21x 106 moles P yr-1.  Losses to the Baltic associated with residual and exchange flows together account for 34 x 106 moles P yr-1, thereby leaving an excess of DIP of 36 x 106 moles DIP yr-1 unaccounted for, or roughly half the total DIP load.  Following the LOICZ stoichiometric methodology, this corresponds to an annual  net ecosystem production of 230 mmol C m-2 yr-1 (assuming a molar  Redfield ratio of 106:1).  The uptake of DIP associated with this estimate of production is the same as the estimate of burial flux of phosphorus.  Assuming a Redfield ratio of 16:1 N:P, the corresponding burial flux of N is 560 x 106 moles N yr-1, which is in rough agreement with the estimate made in the total N budget.  The difference between the discrepancy in the dissolved N budget and that associated with biological uptake amounts to about 4.19 x 109 moles N yr-1,  which can be attributed to net denitrification (i.e. denitrification - N fixation).   This amount of denitrification (0.6 mmol m-2 yr-1) is close to the average of the range of values compiled by Seitzinger and Giblin (1996).   These considerations reduce the 'excessice' value of total N to around 1.6 x 109 moles N yr-1, or 10% of the total N pool of the Gulf.

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 Figure 5. Average annual dissolved inorganic phosphorus budget for the Gulf of Riga (LOICZ notation)

Silica

The average silica budget (figure 6) indicates that the Gulf of Riga is a large silica sink.  Approximately 2.3x 109 moles SiO2 yr-1 enter the system from riverine sources alone (Laznik et al.,1997).  Riverine sources of any biogenic silica (diatoms) are not included.  Atmospheric and point sources of Si are unknown, but assumed negligible.  The standard calculation of Si export, assuming a concentration equal to the average of that of the local Baltic and the Gulf, yields a loss of  0.3 x 109 moles Si yr-1 from the system, but because the average concentration in the system is below that of the Baltic, the 'exchange flow' with the Baltic results in an Si flux into the Gulf of 0.5 x 109 moles Si yr-1.  As a result, the Gulf sees a net flux from the Baltic of 0.2 x 109 moles Si yr-1 , which together with riverine fluxes give a value of 2.5 x 109 moles Si yr-1, which is close to the annual silica loss to burial in sediments of 1.8 x 109 moles Si yr-1 (50 300 metric tons Si according to Carman and Aigars, 1997). However,an excess of 0.7 x 109 moles Si yr-1 or 29% of total silica water pool in the Gulf of Riga still remains unaccounted for. 

 

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 Figure 6. Average annual silica budget for the Gulf of Riga (LOICZ notation)

An accumulation of 1.6 x 109 N moles yr-1 could be related to a clear long-term increase of nitrate concentration during '70s - '80s, which lasted until the beginning of '90s (Yurkovskis and Mazmachs, 1996 ). However, a similar and uninterrupted increase of total phosphorus contradicts to "missing" 34 x 106 moles P yr-1 , while an opposite is true for silicate, which concentration decreased two- to three-fold during 80s - 90s contrary to "excessive" 0.7 x 109 moles Si yr-1 calculated above. Therefore, we suspect a methodological problem associated with seasonal or interannual variation of parameters.

 

Interannual variability in the Gulf of Riga: 1993-1995

Each of the steady-state nutrient budgets described above exhibit imbalances, for which we suspect two principle causes, both related to problems with the steady-state assumption:

Water Budget

Estimates of annual fluxes of water for the Gulf during the period 1993-95 are shown in table 1.  Here, in contrast to LOICZ terminology, Vin represents annual inflow from the Baltic, and Vout represents annual outflow to the Baltic, estimated using inverse modelling ( Håkansson et al., 1998).  VQ, the riverine flows for these years are from Stålnacke (1996).  Estimates of precipitation are from the Estonian Hydrometeoro- logical Institute, Parnu meteorological station (Tarmo Kõuts, personal communication).  Estimates of VE are averages for the period 1981-94 (Omstedt et al., 1997), but in any case are relatively small terms in the overall water budget.  As is clear from the table, there is significant interannual variation in some of the flows.   However, departures of watbal (net inflow) from zero, which indicate deviations in volume from steady state, or annual rates of change of water volume, are relatively small (6% of the average total system volume in 1995; less deviation in the previous two years).

Table 1.  Annual water fluxes of the Gulf of Riga for the period 1993-1995

Water fluxes (km3 yr-1)

1993

1994

1995

Vin

95.5

153.8

131.4

Vout

-141.2

-196.4

-144.9

VQ

31.5

39.8

36.2

VP

11.9

13.6

13.7

VE

-9.4

-9.4

-9.4

WatBal

-11.7

1.4

26.9

Nutrient budgets

The following tables show estimates of annual fluxes of nutrients and their annual  rates of change of nutrient mass in the Gulf of Riga.  Burial fluxes and point source estimates for each year are assumed to be the same as calculated in the steady-state calculation above. The change in mass is estimated in the simplest way as a difference between total amounts, which, in turn, are calulated as the product of average annual concentration and the average (steady-state) water volume. Other, more elaborated schemes, show nearly identical results. In contrast to the LOICZ steady-state convention in which the total change in the mass of nutrients in the system is assumed to be zero, budgets for systems with significant annual variation should take this measured change into account.  So, for example, instead of writing S[sources-sinks] +S[inputs-outputs] =0, we write S[sources-sinks]+S[inputs-outputs] =DM, where DM is the measured interannual change in mass of nutrient for the year.  If DM is large compared to the net flux of nutrients, the estimate of net production or net denitrification may be affected.

Nitrogen

Terms in the total N budget for the years 1993-95  are shown below (table 2).  Riverine nutrient transport for these years is taken from Laznik et al. (1997), annual nutrient input from point sources for 1990 is from HELCOM (1993). The nitrogen contribution from the atmosphere was estimated with avearage (1986/91) concentrations from (Granat 1998) and precipitation from Pärnu station described above. Nutrient concentrations in the inflowing waters are assumed to be the same as in surface waters (0-30 m layer) of the Eastern Gotland Basin, and in the outflowing waters, the same as in the Gulf of Riga, and are extracted from the Baltic Environmental Database (BED) with the SwingStations tool for 1993, 1994, and 1995 and averaged as in the steady-state analysis above. The 'net exchange with Baltic' row in table 2 represents the annual difference between total N flux into the Gulf from Baltic, and out from the Gulf to the Baltic.  For these years, the net flux was negative (i.e. a net loss to the Gulf), and is thus considered an 'export'.  The 'export' associated with burial (the same steady-state estimate as used in the steady-state analysis above) is only about 10% of the loss to the Baltic.  Overall, the annual "source" of total N (excess of estimated imports over exports) for the Gulf of Riga for each of these years is between 5-6 billion moles N, which is between 34-36% of the average mass of total nitrogen in the Gulf (figure 2).  This is in contrast to the annual net change in the water budget, which fluctuates between positive and negative values over the same period.  If the steady-state denitrification estimate of 4.2 x 109 moles N yr-1 is correct, most of the imbalance between imports and exports is lost via this mechanism. The direct estimate of annual change of the nitrogen pool for these years ranges from around +1.2 x 109 moles N yr-1 to -2.2 x 109 moles N yr-1.   Change in the DIN pool ranges from -0.9 x 109 moles N yr-1 to -0.03 x 109 moles N yr-1.  Averaging these values gives estimates of an annual decrease of about 0.5 x 109 moles N yr-1in the nitrogen pool, consistent with the decreasing trend in nitrate concentration over the period 1990-95 reported by (Yurkovskis and Mazmachs, 1996), equivalent to 0.5 to 0.85 x 109 moles N yr-1.

Table 2.  Annual fluxes of a) total N, and, b) DIN, in the Gulf of Riga for the period 1993-1995

Total N budget

1993

1994

1995

Imports

million moles

Riverine

7664.1

8399.7

6482.1

Atmospheric deposition

869.2

1146.4

1122.4

Point sources

381.5

381.5

381.5

Exports

Net exchange with Baltic

-3143.0

-4340.0

-1963.5

Burial

-407.3

-407.3

-407.3

Net flux

5365

5180

5615

Net Change between years

1172

-2178

 

 

DIN budget

1993

1994

1995

Imports

million moles

Riverine

4297.6

4395.4

3767.7

Atmospheric depositionn

869.2

1146.4

1122.4

Point sources

190.7

190.7

190.7

Exports

Net Exchange with Baltic

-1139.0

-877.1

-647.5

Net flux

4218

4855

4433

Net Change between years

-948

-33

 

Phosphorus

Terms in the total P budget for the years 1993-95  are shown in table 3Each of the terms are calculated as in the previous nitrogen budget section, using phosphorus data from the same sources. The 'net exchange with Baltic' row again represents the annual difference between total P flux into the Gulf from Baltic and from the Gulf to the Baltic. As with total nitrogen, the net flux from the Baltic was negative (representing an export of P to the Baltic) for each year.  However, unlike the total N budget, the net flux in total P is negative (excess of estimated exports over imports) for each of these years, by an amount roughly equal in magnitude to the burial term. Overall, the magnitude of the annual net flux of P and DIP for this period is very similar to that of the long-term average budget.  Net change in the total P pool averages around 7.5 x 106 moles P yr-1for 1993-94 and 1994-95, consistent with Yurkovskis and Mazmachs (1996) increasing trend estimate of total P concentration over the period 1975-1995, equivalent to 13 x 106 moles P yr-1.   However, in contrast to generally observed increases in the total P pool, the estimated change in the DIP pool for 1993-94 is strongly negative, indicating a large sink for DIP for that period. 

Following a slightly modified version of the LOICZ guidelines, the annual change in DIP can be subtracted from  the net flux to estimate the net ecosystem production of the Gulf in each year.  This procedure yields an estimate of around 390 mmol C m-2 yr-1 for the year 1993-94 and around 120 mmol C m-2 yr-1 for 1994-95 (assuming a molar  Redfield ratio of 106:1).  While this exercise reveals significant interannual variability in the system (enough to explain the discrepancy in the steady-state P budget),  the average of these net production estimates is about 10% higher than the steady state estimate.  Applying a similar modification to LOICZ procedure used to estimate net denitrification yields values of 4.2 x 109 moles N yr-1for 1993-94 and 4.6 x 109 moles N yr-1for 1994-95, very similar to the steady-state estimate.  These values, together with the interannual variability seen in the N budgets, account for the differences between observed inflows and outflows of N to the system.

Table 3.  Annual fluxes of a) total P, and, b) DIP,  in the Gulf of Riga for the period 1993-1995

Total P budget

1993

1994

1995

Imports

million moles

Riverine

57.0

73.2

57.1

Atmospheric deposition

10.0

11.4

11.5

Point sources

22.1

22.1

22.1

Exports

Net exchange with Baltic

-93.4

-119.0

-81.7

Burial

-35.4

-35.4

-35.4

Net flux

-39.8

-47.8

-26.5

Net change between years

10.2

4.7

 

DIP budget

1993

1994

1995

Imports

million moles

Riverine

40.4

53.3

41.7

Atmospheric deposition

10.0

11.4

11.5

Point sources

11.0

11.0

11.0

Exports

Net Exchange with Baltic

-65.7

-47.0

-31.2

Net flux

-4.2

28.7

33.0

Net change between years -63.9 9.1

Silica

Estimates of riverine sources of Si to the Gulf for the period 1993-95 are close to their long-term average estimates.  The observed trend in Si concentrations has been sharply downward during the period 1985-95 (Yurkovskis and Mazmachs 1996), resulting in reduced concentrations of Si in the waters of the Gulf compared with those of the Baltic proper, and a corresponding net flux of Si from the Baltic to the Gulf, consistent with the estimate in the steady-state analysis.  These estimates of a net influx of Si from the Baltic are in contrast to those of Mägi and Lips (1998), who estimated a net outflux of around 400 million moles Si yr-1 for between November 1993-94.  However, these authors also estimated Si input of approximately 2.5 times this value from 'unknown inputs'.  While uncertainties remain in the Si budget, in particular for the burial term for which we have no annual estimates, the interannual variation in the Si budget  explains a substantial part of the discrepancy between net flux and net change in the Si pool.  The differences between 'net flux' estimates and 'net change' estimates averaged between 1993-94 and 1994-95 are almost exactly explained by the net production associated with DIP uptake in the LOICZ calculation, assuming a Redfield ratio of 16:1 Si:P (molar) and a diatom-dominated phytoplankton community. Somewhat problematically, however, the values for each of the periods 1993-94 and 1994-95 are out of phase with the corresponding productivity estimates.  We must assume that internal cycling processes beyond the scope of this analysis are responsible for these effects.

Table 4.  Annual fluxes of dissolved Si in the Gulf of Riga for the period 1993-1995

Si budget

1993

1994

1995

Imports

million moles

Riverine

2226.8

2665.6

2126.2

Net Exchange with Baltic

+121.4

+40.7

+544.7

Exports

Burial

-1796.4

-1796.4

-1796.4

Net flux

552

910

874

Net change between years 320 -223

 

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