Nitrogen interchanges generated by biogeochemical processes in a Galician ria

Marine Chemistry, 45 (1994) 167-176 0304-4203/94/$07.00

167

© 1994 - Elsevier Science B.V. All rights reserved

Nitrogen interchanges generated by biogeochemical processes in a Galician ria Ricardo Prego Instituto de lnvestigaciones Marinas ( CSIC), Eduardo Cabello, 6, E-36208 Vigo, Spain (Received January 26, 1993; revision accepted July 14, 1993)

Abstract The biogeochemistry of nitrogen in the ria of Vigo is approached on the basis of a box model which allows for calculation of the physical transport of nitrate, nitrite, ammonium and organic nitrogen, and the fluxes caused by photosynthesis, remineralisation and sedimentation of nitrogen. The ria of Vigo receives between 4 and 22 mols -I from exterior coastal water, a flux far greater than that of freshwater. The balance of organic nitrogen indicates that there is always an exportation to the outer ria, in the range of 0.5 to 5 mol s -~ . The enrichment of the incoming water is produced by photosynthetic activity in spring and summer, and by river and sediment contributions in winter. Inorganic nitrogen is only exported during the rainy season, when phytoplankton activity is low. During the rest of the year, the ria retains 3.5 mol s-1 of the total nitrogen received. As a result of these fluxes, the residence time of nitrogen in the ria varies between 15 and 97 days. In the spring and summer of 1986, photosynthesis of the ria of Vigo consumed 7.4 mol s -1 , 24% of which was derived from the remineralisation in the aphotic zone in the ria. Part of the synthesised PON is sedimented and remineralised on the continental shelf, or is reintroduced into the ria. This plays an important role in fertilising the area, not considered until now in Galician rias. Both remineralisations supply 40% of the inorganic nitrogen to the euphotic layer in the ria.

1. Introduction

The biogeochemistry of nutrient salts provides very useful information with respect to biological processes in the seas. This information is particularly interesting in the case of nitrogen (Cooper, 1937). The various compounds nitrate, nitrite and ammonium - provide a more complete picture of the biological processes than do silicate or phosphate. The nitrogen cycle in general, and the organic nitrogen cycle in particular, in the ria of Vigo were studied in the 1960's by Fraga (1960, 1967) and subsequently by Prego (1988, 1990a,b, 1992a). Inorganic nitrogen, and nutrient salts in general, maintain a low concentration in the photic layer in the ria, where phytoplankton activity is significant. Phosphate is not a SSDI 0304-4203(93)E0021-P

limiting factor and variations in phosphate do not appear to indicate variations in the biomass (Fraga, 1960). The seasonal change in nutrient salts in the ria of Vigo is normal for a coastal area with a temperate climate (Vaccaro, 1965). The nitrogen cycle in this ria has been described in terms of the variation presented by organic nitrogen (Fraga, 1967) and by the nitrates (Mourifio et al., 1984). This cycle is similar to the one given by Aston (1980) for an estuary. The amount of organic matter in the ria of Vigo shows a marked seasonal variation, with two peaks: one in spring, and the other towards the end of summer, which may combine to become one, depending on the climatic conditions (Vives and Fraga, 1961); and two minimums, one in winter and another, less accentuated, in summer. Nitrate concentration,

R. Prego/Marine Chemistry 45 (1994) 167-176

168 57

54

51

8e48

45

42

39

36

I

I 4 2 ° 15

4 2 ° 15.

12

12,

9

9 57

54

51

8"48

45

42

39

36

Fig. 1. The ria of Vigo. Its outer boundaryis marked in the North Mouth (23 m depth) and the South Mouth (59 m) by dashed lines. This is the longestof the rias, 33 km, of the four Galician Rias Bajas (Muros, Arosa, Pontevedraand Vigo), the secondin area, with 156 km2, and the third in capacity,containing3.275 km3 of water. In 1986, the nine stations shown in the box were sampled seven times. on the other hand, is at its maximum in the ria water in winter. This paper quantifies, for the first time in the case of the ria of Vigo, the nitrogen fluxes caused by physical transport and biogeochemical processes, with the intent of reaching a better understanding of the behaviour of nitrogen in the Galician rias. 2. Survey area

The ria of Vigo is located in the northwest of the Iberian Peninsula (Fig. 1). It is the most westerly of the four Rias Bajas, which penetrate into the northwest of the Iberian Peninsula along an approximately S W - N E axis (Fig. 1). The major fresh-water contribution to the ria occurs in the Inlet of San Sim6n (Prego et al., 1990), where the rivers Oitab6n and Redondela flow into it; the flows from the river Lagares and the sewers of Vigo are low. Fresh water is on the

surface and gradually mixes with saltwater as it leaves the ria. Hydrologically, the system behaves like a partially-stratified estuary (Bowden, 1980). Surface transport towards the platform which is produced is compensated by the influx of subsurface water on the bottom. This flux system means that the box model (Prego and Fraga, 1992) can be applied. Its limit is considered to be in the two sections at the northern and southern mouths of the ria (Fig. 1, dashed lines). The layer of water corresponding to the inflow comprises the lower layer of the box, while the outflow belongs to the upper layer. Based on this division and the balance of water and salt, the residual flows in the ria of Vigo have been calculated by Prego and Fraga (1992). The hydrographic characteristics of the ria of Vigo depend on the flow system described. This is very much influenced by the flow of fresh water and the prevailing winds (Prego, 1990a). In this regard, 1986 may be taken as an example of a

R. Prego/Marine Chemistry 45 (1994) 167-176

standard year because rainfall and wind (Prego et al., 1990) were similar to what may be considered a normal year (Saiz et al., 1961; L6pezJurado, 1985).

169 OCEAN atmosphere

Fo r

c upper layer

lower layer

ri

3. Sampling and analysis As regards the spatial factor, sampling in the ria of Vigo was carried out in five stations along the axis of the ria, which were complemented, outside the ria, by a further three, up to a 150 m isobath, and a last one in the north mouth (Fig. 1). As regards the time factor, seven transects were made (Prego et al., 1988) from February to October 1986. At each station, water samples were stored in Niskin oceanographic bottles, following the order of depths of 0, 5 10, 20, 30, 40, 50, 60, 80, 100, 120 and 150 m. Immediately following the sampling nitrite, nitrate, ammonium and organic nitrogen were analysed using a TECHNICON AAII autoanalyser. Nitrite was determined by diazotation (Hansen and Grashoff, 1983); accuracy was ±0.02 /zmolkg -1. Nitrate was determined by reduction to nitrite in a C d - C u column. The NH4C1 buffer was substituted by a mixture of citric acidcitrate (following Mourifio and Fraga, 1985) in order to avoid problems of contamination in the analyses of ammonium and organic nitrogen; accuracy was +0.05 #molkg -l. Ammomium was determined by the reaction in an alkaline medium with hypochlorite (Grasshoff and Johannsen, 1972); accuracy was +0.04 #molkg -~. Organic nitrogen was determined by oxidation to nitrate with peroxydisulphate under UV light of the sample buffered with tetraborate (Prego, unpubl.). Then the method described for nitrate was applied. The oxidation reagent was prepared by dissolving 18.75 gm of K2S208 and 14.5 gm of borax in Milli-Q water, up to 1 1. For the buffer, 1.54 g ofmonohydrated citric acid was dissolved and 33.8 g of dihydrated trisodic citrate in distilled water up to 1 1. The colour reagent is the same used in the analysis of

RIVER

RIA

,

sediment

'°

,o DIN ~

PON + DON

R TP DIN ~

PON ~

Ftw

I DON

Fa

-T--TZs--T--DIN <-~ PON ~

DON

Fig. 2. Box model for the ria of Vigo applied to nitrogen fluxes. Incoming nitrogen fluxes to the lower layer of the ria via the North Mouth and South Mouth (Fi) vary due to the fluxes caused by remineralisation (R) and sedimentation (S) of PON (particulate organic nitrogen). Meanwhile, in the upper layer of the ria, the same occurs as regards the outgoing nitrogen fluxes via the Mouths (Fo)and contributed to freshwater (Ffw)because of photosynthesis (P) and the fall (D) in PON. An interchange between both layers is also produced by the ascending (Fa) and the descending (Fd) nitrogen fluxes.

nitrate. The average production of oxidation was calculated at 87%, analysing compounds which normally comprise more than 50% of the organic nitrogen in sea water. Organic nitrogen concentration was obtained by comparison with a nitrate pattern, after subtracting the content of nitrate, nitrite and ammonium (average production in oxidation of which was found to be 100%), and dividing the resulting value by the oxidation efficiency of a sulphanylamide standard. Accuracy of the method was calculated to be +0.2 #mol kg -l . The results of ria nitrogen analyses were published by Prego et al. (1988).

4. Method and results In the two-layer box model, mentioned above, for the ria of Vigo it was taken that, in each layer, the sums of the physical fluxes (F) and biogeochemical processes (B) should be zero for any nitrogen compound. These nitrogen inputs or outputs are summarised in Fig. 2. The physical fluxes correspond to nitrate, nitrite, ammonium and organic nitrogen influxes into freshwater (Ffw) and salt water (Fi), outflowing (Fo), ascending (Fa), and descending (Fd) between the boxes (Fig. 2). These fluxes were calculated as the product of the flows (Prego and

R. Prego/Marine Chemistry 45 (1994) 167-176

170 North Plouth . . . . . ~" ....

South Nouth NO3

..... ~ ....

NO2

. . . . . ~- ....

NH~,

. . . . . . ....

Norg

NO3

=

•

NO;, •

NH~

A

N org

--

16 14 i

12

'

u;.;s

i

i

i

i

i

i

i

•

c;ion'

10 8

6

:ill,,." /:"-,

4 T Z lower

E

section

8

6

o,. .....

-..

4 2 0

v , ~ u

n u

. t

,.~

,Y,,

z

:E

--

-,-"

Fig. 3. Average concentrations of nitrate, nitrite, ammonium and organic nitrogen (in #mol-Nkg -l) in the north and south sections of the mouth and in the upper and lower layers of the ria of Vigo on the sampling dates.

Fraga, 1992) for the concentrations (Fig. 3). The results are shown in Table 1. The biogeochemical processes in nitrogen in the ria of Vigo are due to biological activity. Particulated organic nitrogen (PON) is photosynthesised (P) in the upper layer, i.e. the outflowing water mass, at the expense of nitrate, nitrite and ammonium. Then a part (D) of PON falls to the lower layer, i.e. inflowing water mass, where it is sedimented (S) or remineralised (R), returning part of the inorganic nitrogen to the sea water (Fig. 3). The boundary between the two layers in the ria of Vigo, with zero residual velocity (Prego and Fraga, 1992), coincides substantially with the compensation depth, marked by 1% luminosity, according to the Secchi disk. As a result,

photosynthesis takes place mainly in the upper layer. The box model uses inorganic nitrogen fluxes to calculate the photosynthesis and cannot differentiate between gross photosynthesis and respiration. The value obtained corresponds to what Codispoti et al. (1986) and Minas et al. (1986) have defined as "net community production". The following equations are based on the above: Upperlayer, inorganic nitrogen • F/w - Fo + Fa -

+ P = O

(1)

Upperlayer, organic nitrogen • F/w-ro+ra-r

-P+D=O

(2)

171

R. Prego/Marine Chemistry 45 (1994) 167-176 Table 1 Nitrogen fluxes for the ria of Vigo considered as a box (see Fig. 2) Flux (mol s -I )

Incoming (Fi)

Outgoing (Fo)

Descending (Fcl)

Ascending (Fa)

Freshwater (Ffw)

28 February NO3 NO2 NH4~

9.51 0.77 2.13

18.25 0.65 3.39

3.57 0.09 1.01

13.54 0.83 4.33

6.15 0.22 1.77

Norgamc

9.21

13.09

3.16

14.49

1.69

4.14 0.48 0.38

3.58 0.34 0.34

1.19 0.08 0.19

5.29 0.60 0.98

0.90 0.03 0.80

4.31

5.94

1.18

5.09

0.60

N0~N0~-

NH~-

3.23 0.21 0.14

0.22 0.03 0.02

0.04 0.01 0.03

3.35 0.23 0.69

0.34 0.02 0.49

Norgamc

2.71

3.33

0.99

4.13

0.38

7.25 0.26 0.10

0.96 0.18 0.14

0.59 0.05 0.03

9.37 0.49 0.25

0.19 0.01 0.45

7.24

9.62

1.25

5.74

0.33

NO3 NO2 NH +

2.15 0.08 1.12

0.10 0.03 0.22

0.06 0.01 0.06

1.77 0.08 1.24

0.09 0.01 0.47

Norgamc

2.23

2.42

2.37

5.09

0.32

10.12 0.36 0.63

2.66 0.19 1.32

2.58 0.25 0.82

16.98 0.96 3.13

0.09 0.01 0.48

6.77

1 l.g9

15.69

16.44

0.30

NO3 NO2 NH +

2.32 0.16 0.24

0.30 0.04 0.14

1.24 0.18 1.30

3.63 0.47 2.81

0.11 0.01 0.53

Norgamc

1.69

2.70

3.65

4.62

0.41

5 March NO3 NO2

NH~Norgamc 26 May

31 May NO3 NO~

NH~Norgamc 7 July

4 September NO3 NO2 NH4~

Norgamc 3 October

The fluxes are the product of the water flows (Prego and Fraga, 1992) and the nitrogen concentrations (see Fig. 3). The flux in fresh water has been calculated from three diferent origins (Alvarez, 1980): non-contaminated water, contaminated water from the river Lagares, and sewage from Vigo.

Lowerlayer, inorganic nitrogen • Fi-Fa+Fd+R=O

(3)

Lowerlayer, organic nitrogen : F~-Fa+Fa-R-D+S=O

(4)

In Eqs. (2) and (4) D, PON fall, has opposite signs because when PON is added to one layer, it must have been withdrawn from another. The same holds true for P in Eqs. (1) and (2), and R in Eqs. (3) and (4); the appearance of organic nitrogen is coupled with the disappearance of inorganic nitrogen, and vice versa.

R. Prego/Marine Chemistry 45 (1994) 167-176

172 Table 2 Biogeochemical nitrogen processes for the ria of Vigo Process (mol s-I ria-I)

Photosynthesis (p)

PON fall (O)

Remineralisation (R)

Sedimentation (S)

28 February 5 March 26 May 31 May 7 July 4 September 3 October

+ 0.12 -2.87 -4.77 -8.81 -3.18 - 13.83 -4.36

+ 0.19 - 1.45 -4.96 -4.01 -3.80 -2.69 -3.04

+ 1.63 + 0.41 +0.61 + 1.83 -0.39 + 6.31 + 1.47

+ 3.94 - 1.44 -3.92 -4.93 -3.70 -2.40 -2.29

Processes were calculated by applying the data from Table 1 to Eqs. (1-4). Photosynthesis and remineralisation are related to inorganic nitrogen. For this reason, a negative value of these processes indicates the disappearance of inorganic nitrogen in a layer, and a positive value its appearance. A negative value of PON fall and sedimentation indicates the output of PON from the upper layer to the lower layer and the lower layer to sediment, respectively.

The biogeochemical processes occurring in the ria of Vigo are the four unknowns of the system of Eqs. (1-4), and are calculated by substituting the fluxes for their values, shown in Table 1. The results are shown in Table 2. 5. Discussion

5.1 Incoming nitrogenflux The ria of Vigo received between 4 and 22 mols -1 (Table 1) from the offshore coastal water, approximately 45% of which corresponded to organic nitrogen and the greater part of the rest to nitrate. The nitrogen contributing to the fresh water was normally much less (Table 1), around 1 mols -1. This value was higher during winter, as in the case of an estuary (Wells and Froelich, 1984) due to a high terrestrial contribution of nitrate. The larger fluxes of incoming nitrogen in the ria from its offshore waters occur during the rainy season and upwelling events. In the first case, the greatest water flow in the ria (Prego et al., 1990) is the main cause. On 28 February (Table 1), 2116 mols -1 originated from sea water and 9.8 m o l s -1 from fresh waters. Both caused a marked gradient of nitrate and organic nitrogen in the ria, and a minimum isoline of nitrate on the continental shelf. In the second case (Table 1, 31 May and 4 September), the

incoming water mass, North Atlantic Central Water (NACW), transported 14.9 and 17.9 mol s -1 into the ria, approximately three times more than in the case of non-upwelling conditions. Various authors (see Prego, 1992b) have already commented on upwelling off Cape Finisterre (Fig. 1) as being the cause of the productive richness of the Galician rias. This assumption is now justified and quantified for the ria of Vigo. The greatest flux is due not only to an acceleration in the ria flow caused by the Finisterre upwelling (Prego and Fraga, 1992), but also to an increase in the nitrogen concentration in the incoming flux. The largest water flows export PON from the ria which returns remineralised. A strengthening effect is produced in the fertilising capacity of the upwelling (Fss in Fig. 4), which, until now, has not been considered in the rias (Ro in Fig. 4). Thus, for 4 September, according to the isopycnes (Prego and Fraga, 1992), the concentration of nitrate in the water moving into the ria should be 8.0 #molkg -1 (Prego et al., 1988), a similar value to that obtained for the standard concentration of 8.3 #M of nitrate, obtained by Fraga et al. (1985) for the NACW. As the concentration of inorganic nitrogen in the incoming water via the southern mouth is 11.4 #molkg -1 (Fig. 3), this implies a 27% enrichment in inorganic nitrogen relative to the total incoming flux (Table 1). On other days the recycled amount is less. It varies

R. Prego/Marine Chemistry 45 (1994) 167-176 OCEAN

RIA

it~gphere

letvnter

Fd

D=4.40-O.48Fa(Ning)

upperle~¢ "~ler t - lover Ro=O24+0+30Fo(Norg)

Fi(Ning)~

RIVER

P =o 06÷OOtFa (Nlng)

(Ning)=0 II÷062R

*-- Fo (Norg)=099+09BP

173

~fw

I

T--

R =-I.64÷~53P

Ira

FSS 3~llim*nt

~ S =0 I0÷~ 90 D

Fig. 4. Flux dependence in the ria of Vigo according to the highest correlation coetticiens ( r > 0 . 9 4 , except: Ro, r = 0.77; D, r = 0.89; S, r = 0.85) between the fluxes of Tables 1 and 2, taken by pairs. Ro is remineralisation outside the ria and Fss is the inorganic nitrogen concentration in the incoming seawater before remineralisation. The other symbols are explained in Fig. 2. Units are mol s -l.

between 14 and 30% of the inorganic nitrogen fluxing into the ria (Table 3). 5.2 Biogeochemical processes

The biogeochemical processes of nitrogen in the ria may be approached using lineal regressions between the calculated fluxes (Tables 1 and 2) which show the highest correlation coefficients (Fig. 4). The upwelling varies with the wind, its intensity changes inter- and intraannually. Therefore it is better to refer to the fluxes as equations rather than quantities (Fig. 4). As expected (Butler et al., 1979) photosynthesis shows the best correlation in spring and Table 3 Percentages, according to their origin, in the contribution of inorganic nitrogen to the upper layer of the ria Freshwater

28 February 37 5 March 24 26 May 17 31 May 6 4 September 3 3 October 13

Incoming water Remineralisation (F, + Ro) inside the ria 56 70 46+25 61+ 14 35 +27 26 + 30

7 6 12 19 35 31

Fss is the percentage in the sub-surface seawater, off the continental platform. Ro is the contribution from remineralisation over the platform, outside the fla.

summer with the ascending flux of nitrate (Fig. 4). When seasonal upwelling off Finisterre (Fraga, 1981) affects the ria of Vigo, photosynthesis of organic nitrogen is high (Table 2). Phytoplankton consumed 8.8 mols -1 of inorganic nitrogen (68 m g - N m - 2 d -]) on 31 May, and 13.8 mols -1 (144 m g - N m - 2 d -1) on 4 September. A relaxation in upwelling, as in the one detected on 7 July and 3 October, or in the hydrographic conditions in the ria on 26 May (common in spring), forces photosynthesis to maintain values of about 4 mol s -1. The situation on 7 July is a prime example of what occurs when there is a decrease in the influx of nitrate (Table 1). Photosynthesis was much (+4×) less than during upwelling (Table 2), and light penetration in the water reached almost 30 m at station 3 (Prego et al., 1988), i.e. photosynthesis also occurred in the incoming water flow since remineralisation shows a negative value (-0.39 mol s-I; Table 2). This is in contrast with what occurs when there is upwelling bloom: light penetration is less (about 10 m; Prego et al., 1988) and photosynthesis only occurs in the outgoing water flow. Remineralisation in the ria of Vigo mainly depends on photosynthesis (Fig. 4). The negative value of the independent term means that, as photosynthesis becomes less in the upper layer of the ria, it gradually becomes more important in the lower layer, as previously commented for 7 July. Remineralisation of organic nitrogen in the ria of Vigo occurs throughout the year. The presence of organic nitrogen in the upper layer of the ria is always accompanied by ammonium and nitrite in the lower layer (Fig. 3). On 28 February, it was due to a remineralisation (1.6 mol s-l; Table 2) of resuspended nitrogen (3.9 m o l s - l ; Table 2); while on other dates it was due to the decomposition process of phytoplankton. Remineralisation follows the same vertical decomposition sequence as described by Codispoti (1983). This sequence is displaced (Prego, 1990b) when the residual flow in the ria increases, and transports the PON to the outer

R. Prego/Marine Chemistry 45 (1994) 167-176

174 Table 4 Residence time of nitrogen in the ria of Vigo Total nitrogen Outgoing flux Residence time (106 mol) (mol s -l) (days) 28 February 5 March 26 May 31 May 7 July 4 September 3 October

46.5 31.6 35.2 42.0 42.2 56.9 46.6

35.38 11.64 7.52 15.83 6.47 18.46 5.57

15.2 31.4 54.2 30.7 75.5 35.7 96.8

The outgoing nitrogen is the outflux of the ria (Table 1) plus the nitrogen sedimentation (Table 2), except on 28 February.

ria: production, as a result of upwelling, is centred on the surface and near the innermost part of the ria; maximum ammonium values occur at 5 m depth in the central part; maximum nitrite values occur at 20 m depth near the ria mouth; and the highest nitrate concentration occurs at 80 m depth on the shelf outside the ria. Remineralisation occurs in the lower layer of the ria of Vigo by the transport of PON from the euphotic zone (Fig. 4). If the different sources of inorganic nitrogen in the ria of Vigo are considered (Table 3) the role of the remineralisation of nitrogen inside the ria increases from winter to the end of summer, which rises to 32% of the total supplied to the ria. Consequently the net photosynthesis in spring and summer is between 30 and 60% due to remineralised nitrogen, inside and outside the ria. Without this regeneration, new production in the upper layer would be approximately only 60% of the total. Remineralisation never becomes complete. The minimum concentrations obtained for organic nitrogen are around 5 mol kg -1, values similar to the ones obtained at sea by other authors (see Williams, 1975; Malta and Yanada, 1990). Sedimentation of PON in the ria of Vigo turned out to be a direct result of falling matter, in accordance with Fig. 4. This varied between 10 and 30 m g - N m -2 d -1. The sedimentation values in Table 2 are always less than the fall, nearing 55% of photosynthesis. There was even a negative value, i.e. resuspension, on 28

February (25 mg-Nm-2d-1). This must be normal at certain times in winter: the high river flow received (168 m3s-1; Prego and Fraga, 1992) means that there is a re-release of nitrogen. Almost 4 mol s -1 pass from the sediment to the column of water (Table 2), as occurs in the canals of the Danish Wadden Sea (Henriksen et al., 1984), where nitrogen derived from the sediment is finally exported to the coastal zone. This phenomenon is important enough to bear an influence on the upper layer of the ria (Table 3, fall= 0.2 mols-l).

5.3 Outgoing nitrogen flux In winter, the balance of nitrogen fluxes in the ria of Vigo (Table 1) is favourable for exportation to the ocean. In February, 13.8 mol-s -~ more flux out than come in, and in March, this value is 2.7 mols -1 (Table 1). The quantity depends, in the first place, on the river contribution and, secondly, on the resuspension of sediment. During the rainy season the ria of Vigo enriches the offshore waters with (mostly inorganic) nitrogen (Table I). As a result, the residence time of nitrogen is short in winter, i.e. 15 days (Table 4). During the rest of the year, the contrary occurs, as in other coastal areas with small fluvial contributions (Rydberg and Sundberg, 1985). The ria retains 2.2-7.0 mols -1 of inorganic nitrogen. In spring and summer, when the ria behaves like a nitrogen trap, it retains 2-5 mols -1 (+20 mg-Nm-2 d -1) of PON in the sediment, and exports between 0.2 and 5.1 mol s -1 of organic nitrogen (Table 1). Therefore the ria is enriched by nitrogen throughout the year (Fig. 4), although during upwelling events the residence time of nitrogen is approximately one half of that for non-upwelling events (Table 4). The difference between the incoming and outgoing fluxes (Table 1) indicates that there is a continuous exportation of organic nitrogen to the outer ria. To this can be added the further exportation of inorganic nitrogen during winter, mainly due to the nitrate which is derived from

R. Prego/Marine Chemistry 45 (1994) 167-176 r i v e r c o n t r i b u t i o n s . A c c o r d i n g to t h e c o r r e l a tions, t h e i n o r g a n i c n i t r o g e n e x p o r t a p p e a r s t o be d e p e n d e n t o n r e m i n e r a l i s a t i o n in s p r i n g a n d s u m m e r (Fig. 4). A b o u t 0.11 m o l s -1 o f the i n c o m i n g flux is n o t u s e d a n d a p p r o x i m a t e l y 6 2 % o f r e m i n e r a l i s e d flux leaves t h e r i a (Fig. 4). E x p o r t a t i o n o f o r g a n i c n i t r o g e n is a f u n c t i o n o f t h e p h o t o s y n t h e s i s , as s h o w n in Fig. 4, i.e. although the biogeochemical processes of nitrogen in t h e ria a r e c o m p l e x , t h e ria, as a n e x p o r t ing a g e n t o f o r g a n i c n i t r o g e n , s h o w s a s i m p l e d e p e n d e n c e o n t h e influx o f n i t r o g e n , w h i c h is p r o b a b l y a r e s u l t o f the l i m i t i n g c h a r a c t e r i s t i c s of nitrate.

Acknowledgements I gratefully acknowledge the valuable comm e n t s p r o v i d e d b y t w o a n o n y m o u s referees. I w o u l d like t o e x p r e s s m y t h a n k s f o r t h e c o o p e r a t i o n g i v e n b y t h e scientific m e m b e r s o f t h e c a m paign "Galicia IX", and the crew of the R/V Garcia del Cid. T h a n k s a r e a l s o d u e t o I a n Emmett for revision of the English and Maria A n g e l e s G a r c i a f o r p r e p a r a t i o n o f t h e figures.

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