Trends in nutrient and oxygen conditions within the kattegat: Effects of local nutrient supply

Estuarine,

Coastal

and Shelf

Science (1988) 26,559-579

Trends in Nutrient and Oxygen Conditions Within the Kattegat: Effects of Local Nutrient Supply

Lars Andersson Department Gothenburg, Received

and Lars Rydberg

of Oceanography, Sweden 21 April

Keywords:

University of Gothenburg,

1987 and in revised

nutrients;

form

17 December

Box

4038,

S-400

40

1987

oxygen content; primary production;

Baltic Sea; Kattegat

The Kattegat forms the outer part of the Baltic estuary. It is characterized by a stable two-layer stratification maintained by approximately equal supplies of low saline water from the Baltic and high saline oceanic water from the Skagerrak. The nutrient supply to these waters increased rapidly during the past decades and oxygen deficits have been reported from different parts of the estuary. In this paper, we have calculated trends in nutrient and oxygen concentrations within the surface and deep waters of the Kattegat and adjacent waters. This has been done with available data for the past decades, with reference to nutrient supply and phytoplankton production. Oxygen concentrations within the deep water decreased from 4.58 to 4,08mll-’ between 1971 and 1982, indicating a 50°, increase in oxygen consumption. Concentrations of Tot-N, Tot-P and inorganic nitrogen increased simultaneously, both in the surface water during the winter and in the deepwater during the summer. Changes in Tot-N and Tot-P were dominated by the Baltic water, while local supply to the Kattegat dominated the changes in inorganic nitrogen. Increases in Tot-N and Tot-P suggest a successively increasing biomass. The importance of local nutrient supply to the Kattegat was studied by comparing expected nutrient concentrations within the surface water (due to exchange with adjacent waters) with actually observed concentrations.

Introduction Low oxygen concentrations and subsequent benthic mortality have been observed in the Kattegat (Figure 1) during the last decade. The phenomenon dominates in the shallow south-eastern part, but is occasionally found elsewhere (Miljiistyrelsen, 1984; Rosenberg, 1985). Similar problems were seen in the German Bight, where it was recently shown that both nutrient concentrations and phytoplankton carbon increased dramatically during the past decades (Berg & Radach, 1985). Studies from separate hydrographic stations in the Kattegat have also shown considerable changes (see e.g. Svansson, 1984; AErtebjerg, 1986). In this paper, we make use of all available hydrographic data from the Kattegat and adjacent waters during the past decades, to evaluate trends in nutrient and oxygen concentrations. 559 0272-7714/88/050559+21

$03.00/O

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programs

The Kattegat is a shallow sea with a stable two-layer stratification, formed by an outflow of low saline water from the Baltic and an inflow of high saline Skagerrak surface water in the north [compare Figure 2(a)]. The surface area, with borders as indicated in Figure 1, is 17 700 km’, the volume 375 km3 and the mean depth 20 m. A hypsographic curve is shown

Nutrient

and oxygen

561

trends

(a) The

Skagerrak

The

Kottegat

The

Belt

Sea

S, Figure 2. (a) Length section through the Kattegat. Sources of water and nutrients are indicated. Main fluxes are shown by thicker lines. (b) Local nutrient supply to the Kattegat (and the Belt Sea) during winter. Baltic water (Ca, S,) mixes with Kattegat deep water (C,, S,) to form Kattegat surface water (S,). Without nutrient sources or sinks, the concentration within the Kattegat attains the value C,‘: due to local supply, the observed value is C, > C,‘; the difference, C, = C, - C,’ is an estimate of the local supply, illustrated by a hypothetical C-S curve.

in Figure 3. The mean depth of the halocline, which is present throughout the year, is 15 m and the volume of the upper layer is approximately 225 km3. Surface water salinities vary between 15 and 30X, due to strongly variable exchange with the Baltic. The deep-water salinity is between 32 and 34%. While the deep-water salinity is nearly constant in the north-south direction, there is a strong gradient in the upper layer, indicating unidirectional, upward wind entrainment. The residence time for the surface water is l-2 months, based on approximately equal supplies of Skagerrak and Baltic water (Stigebrandt, 1983; Rydberg, 1983). The deep water has a residence time varying between 1 and 4 months (Rydberg & Sundberg, 1984). In the south-east Kattegat, where the water depth exceeds the halocline depth by only a few meters, a combination of hampered water exchange and large nutrient supply has caused the most extensive oxygen deficiency. Phytoplankton production is low during winter, and a pool of nutrients builds up from November to February. This pool consists of contributions from deep water, Baltic water, atmosphere, rivers and waste water [see Figure 2(a)]. The deep-water supply dominates

562

L. Andersson

& L. Rydberg

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the inflowing Skagerrak water and the south-east Kattegat

Nutrient

and oxygen trends

563

(see Rydberg & Sundberg, 1984), but all the sources are important. The spring bloom occurs in early March, and empties the pool within lessthan two weeks. Mean inorganic nitrogen and phosphorus concentrations before the bloom are approximately 8 mmol m - 3 and 0.7 mmol me3, respectively. The mean annual primary production for the years 1975-77 was 105 gC mm2(AErtebjerg et al., 1981). Edler (pers. comm., 1987) measureda higher annual production in the south-east Kattegat: between 100 and 160 gC m- * for the years 1981-85. Primary production seemsto be nitrogen limited (AErtebjerg et al., 1981; Rydberg & Sundberg, 1984; Graneli et al., 1986). Hydrographic measurements from frequent ship cruises, comprising salinity, temperature, oxygen and nutrients, have been performed within the Kattegat since the 196Os,mainly by the Fishery Board of Sweden and the Danish Agency for Environmental Protection. The results have been stored at the ICES data center in Copenhagen (Baltic Data Base). Using the Baltic Data Base we have analysed trends in nutrient and oxygen concentrations in the Kattegat and within adjacent waters, differentiating between surface water and deep water and between summer (April-September) and winter (January-February) conditions. An attempt is also made to couple the local nutrient supply to changes in nutrient and phytoplankton dynamics. This wasdone by calculating the expected nutrient concentrations within the Kattegat based on observed salinities and nutrient concentrations within the adjacent waters. The difference between observed and calculated values within the Kattegat gives (during the winter) an estimate of the local supply‘ [see Figure 2(b)]. General data treatment The Baltic Data Baseincludes data from the beginning of the century up to 1982. For this study, we chose nutrient data (NO;, NH:, NO;, PO:-, Tot-N and Tot-P) from 197l-82 and oxygen and salinity data from 1960-82. In all, there were approximately 6000 data on salinity and oxygen, 5000 on phosphorus, and 2000 on nitrogen parameters. Most information originates from the main stations shown in Figure 1, although other stations have frequently been used. To simplify further work, we used monthly mean values within the Kattegat for each month and for each parameter (S, DIN=C(NO; +NO; +NH:), DIP = PO:-, Tot-N, Tot-P and oxygen), dividing between surface water (S < 30%) and deep-water (S>30%0). Each water sample was thereby given equal weight. By this procedure, no account was taken of differences in the amount of data from one month to another, or that results from centrally positioned stations like Anholt might have a higher weight than results from stations near the borders. To get a picture of seasonalvariations, the monthly meanswere used to calculate mean bimonthly concentrations of DIN and DIP and oxygen within the surface water and the deep water of the Kattegat, respectively [see Figures 4 and 5(a-d)]. To prepare a discussion on trends, we also added to these figures the corresponding concentrations from the surrounding waters including the south-east Kattegat, calculated in the sameway and from the samedata baseas above. An area north-east of Las6 was picked as being representative of the inflowing Skagerrak water (S > 30%), assumingthat the deep-water enters east of Las6 (seeSvansson, 1984), and an area in the south-west Baltic (Arkona basin) as representative of the Baltic water (S < 10%). The reasonfor choosing this area, and not the Belt Seawhich is located between the Baltic and the Kattegat, isthat the Arkona basin is an

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Nutrient and oxygen trends

area with horizontal homogeneity and stable conditions. The conditions within the Belt Seaare much more complex, and there are no well defined water masses.As the deep water in the Kattegat only very rarely enters the Belt Sea, the water and salt budget discussed below holds true. However, when the local supply of nutrients to the Kattegat is discussed,the supply to the Belt Sea and the ijresund is included. The lowest oxygen concentrations occur during late summer (Figure 4). Inorganic nutrient concentrations within the Kattegat deep water [Figure 5(b)] are also low during this period, which may seem surprising. However, this indicates that the inflowing Skagerrak water, which originates from nutrient depleted surface waters, effect on the concentrations (Rydberg & Sundberg, 1984).

has a dominant

The most apparent trends within the deep water can be expected during periods of low oxygen concentrations and high decomposition rates. In the surface water, on the other hand, we expect (for nutrients) clear trends during the winter, when internal processes such asprimary producrion, are negligible. Thus, with reference to Figures 4 and 5 (a-d), the following periods were chosen: Oxygen, deep water: August-October (low concentrations); DIN and DIP, surface water: January-February (low uptake); DIN and DIP, deep water: April-September (large decomposition).

Monthly regression.

mean values for these periods were used to calculate trends using linear A correlation

coefficient,

r=

r, was determined

for each parameters;

z (Xi -T) cyi -5) J[Z(x, -x,2] [qy, -y)‘]

where y = 6, + blx, is the linear equation, 6, and b, the coefficients to be determined. The relation between r and the significance level is given by the t-test; t=rx-----

.,/N-2 \z7

The results are shown

in Figure

6-8, summarized

in Table

1 and are commented

on below.

Results Trends

Oxygen trends for the Kattegat deep water and for the inflowing Skagerrak water are shown in Figure 6(a,b). There is a significant decreaseof almost 0.05 ml 1- ’ year - ’ in the Kattegat deep water. Svansson (1984) made a trend analysis from Fladen (see Figure 1) and found a decreaseof 0.013 ml l- ’ year- ’ , basedon data from all seasons.A comparison indicates that the decreaseoccurs mostly in the summer period, which agrees with the

Figure 5. Bimonthly mean DIN (DIP) concentrations (pm) in the years 1971-82. (a): Kattegat surface water (S < 30%0, -); south-eastern Kattegat surface water (S<3OX, -.-.-); southern Baltic surface water (SC 10X, ,); Skagerrak water (S>3OG, - - -). (b): Kattegat deep water (S>30%+ -); south-eastern Kattegat deep water (S > 30’& - - -); Skagerrak water (S > 30YW, ---). (c): Kattegat surface water (S < 30%0, -); south-eastern Kattegat surface water (SC 30%0, - - -I; southern Baltic surface water (SC loo/00, .); Skagerrak water (S>3Ou/oo, ---). jd): south-eastern Kattegat deep water (S> 30%,, Kattegat deep water (S > 303/00,-); -. -. -); Skagerrak water (S> 30%,---).

566

L. Anderson

& L. Rydberg

TABLE 1. Comparison of nutrient .S < 10%0), the Kattegat (surface Skagerrak (S < 300/w)

and oxygen trends within the Baltic (surface water, water, SC 305~; deep water, S> 30%0), and the

January-February

&l)

C (1982)

iiC/h‘t

19.2 17.3 21.6 21.7

0.036 0.056 0,078 0,065

(19cs2)

bC/cst

30.4 23.0 24.1

0.19 0.11 0.12

14,4b 9,7b 11.3* 13.1b

0.0032 0.0012 0.0025

0.54 0.63 0.51 0.82

0.73 0.80 0.73 1.23

0.0014 0.0013 0.0016 0.0031

3.46

5.38

0,016

4.02

9.70

0.043

0.39

0.39

0

0.72

0.69

Tot-N

Kattegat (SW) Skagerrak Baltic (SW) Kattegat (DW)

5,3b 8.7h 7.7D

Tot-P

Kattegat (SW) Skagerrak Baltic (SW) Kattegat (DW)

0.83 0.85 0.62

1.25 1.01 0.99

DIN

Kattegat (SW) Skagerrak (DW) Baltic (SW) Kattegat (DW)

4.7 6.8 3.6

9.4 9.0 4.6

Kattegat (SW) Skagerrak Baltic (SW) Kattegat (DW)

0.61 0.68 0.44

DIP

April-September

0.73 0.65 0.54

0.036 (0.017) (0.007) (0.0009) ( - 0.0002) (0.0007)

(&l)

( -0~0003)

August-October OXY

Kattegat Skagerrak

(DW)

4.58 5.39

4.08 5.33

- 0.0038 ( - 0~0005)

January-December Tot-N

Kattegat (SW) Skagerrak Baltic (SW)

8.6b 9.0b 10.4b

Tot-P

Kattegat (SW) Skagerrak Baltic (SW)

0.61 0.69 0.54

22.0 18.7 22.2 0.86 0.85 0.80

0.10 0.07 0.09 0.0019 0~0012 0.0020

SW, surface water; DW, deep water. “The concentrations, C (mm01 m-‘), for the first and last years and for the trends SC/& month’) were determined from the regression line. Numbers in (mm01 m-3 parentheses are based on too few data or are less significant (compare figures). ‘Concentrations underestimated due to uncertainties in analysis method. The corresponding trends are overestimated.

conclusions of AErtebjerg et al. (1981), who studied oxygen conditions in the south-west Baltic. Figure 6(b) shows that oxygen concentrations of the inflowing Skagerrak water were almost constant during this period; thus the decrease is a local effect. Trends in winter nutrient concentrations within the surface water are shown in Figure 7(a-d). Despite few measurements due to ice and bad weather conditions, it is obvious that DIN [Figure 7(a)] increased by approximately lOOa,, between 1971 and 1982, which corresponds to an increase of 0.43 mmol rn- 3 year ‘. The very large increase in Tot-N

Nutrient

and oxygen

567

trends

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Yeor Figure 6. Monthly oxygen concentrations for the period August-October during the years 1965-82. The trend is based on linear regression. The correlation coefficient, r, the significance level, F(t), and the rate of change, Xi& (in ml l- ’ year - ‘), are indicated. (a) Kattegat deep water (S> 30%). (b) Skagerrak water (S > 30%).

[Figure 7(b)] is probably overestimated, however. Samplesbefore 1975were analysed in a slightly different way than for later years, giving lower values. Tot-P [Figure 7(d)] increased by 0.038 mmol rnw3 year- ’ , while the changesin DIP [Figure 7(c)] were insignificant. Results from Fladen (Svansson, 1984), based on data from all seasons,agree qualitatively with the present study, showing rapidly increasing Tot-N and Tot-P concentrations of 0.4 and 0.02 mmol m3 year- i, respectively, but weak changes in DIP. While there were still too few data on DIN when Svanssonmade his analyses,a couple of years later AErtebjerg (1986) calculated a winter trend at Fladen and found an increaseof 0.37 mm01mm3year-‘, which is similar to that found in the present work. Deep-water nitrogen concentrations [Figure 8(a,b)] increased rapidly: DIN increased by 0.5 mmol rn-’ year-‘, corresponding to a 1500,, increase during 1971-82. A similar

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Nutrient

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increase, 0.37 mm01 m-3 year- ’ was found by AErtebjerg (1986) at Fladen. The increase in Tot-P [Figure 8(d)] was 0.037 mmol mm3 year- ‘, which is surprisingly large as there was no significant change in DIP [Figure 8(c)]. Nutrient trends within the inflowing Skagerrak water were calculated for comparison with the deep water trends [Figure 9(a-d)]. Although there are few data on the inflowing water, it is obvious that the increase in both DIN and Tot-P within the Kattegat deep water cannot be explained by inflowing Skagerrak water. However, the trends in Tot-N and DIP are similar in both areas. Budget

calculations

In combination with the trend study, it might be of interest to evaluate how the local nutrient supply (both land based and atmospheric) of the Kattegat and the Belt Sea influences their nutrient concentrations. This is easily done for the winter period (January-February), when the internal nutrient sources/sinks are small. The Kattegat surface water consistsof a mixture of Kattegat deep water and Baltic water asindicated in Figure 2(a). Salinity conservation gives: WNV,S,)

= QlS, +

Qw‘& - (PI + Q&,

(1)

where QR is the supply of Baltic water and Q, the supply of deep water to the surface water. The fresh-water supply of the Kattegat is small (350 m3 s-i; Svansson, 1975), and negligible for the salinity budget. S,, S, and S, denote the salinity of Kattegat deep water, Baltic water and Kattegat surface water, respectively, while V, is the volume of the Kattegat surface water. The seasonalchange in salt content, 6/6t(V,S,) is an order of magnitude smaller than the other terms (seeRydberg & Sundberg, 1984), which supports a steady-state assumption. The proportions of Q, and Qa forming the Kattegat surface water can then be determined according to:

Q,/Qa = P = (S, - S,)/(S, - Sd

ia

where P is a mixing parameter. Let us further assumethat the nutrient concentration within the Kattegat surface water, C,, can be written as C2 = (C, + C,), where C, is due to local nutrient supply to the Kattegat and the Belt Sea, and C’ is causedby supply from the Baltic and the deep water. Assuming immediate adjustmeLt of Kattegat surface water concentrations to changing concentrations of the surrounding waters [compare Figure 2(b)] we obtain the following equation for Ci: SISt(V2CZ)

= Q,C,

+ Q,C,

- (Q1 + Q&C;

= 0

(3)

which, when combined with equation (2), gives: PC, + c, = (P + l)C,

(4)

Equation (4) can be solved for C; if the salinities and nutrient concentrations of the surrounding waters, including the salinity within the Kattegat surface water, are known. The ‘surplus concentration’, C, = C, - C,‘, caused by local supply can then be calculated

Figure 7. (a-d) Monthly DIN, Tot-N, DIP and Tot-P concentrations within the Kattegat surface water (S-C 30”,,) for the period January-February during the years 1971-82. The trend is based on linear regression. The correlation coefficient, r, the significance level, F(t), and the rate of change, SC/St (in pM year-‘), are indicated.

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Nutrient

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571

from the known surface water concentrations within the Kattegat (C,). The preceding section is illustrated graphically in Figure 2(b). The mean values of C,, C, and C, and the corresponding salinities for each year between 1971and 1982, calculated from the original monthly means, are shown in Table 2. The concentrations within the Kattegat (observed, C,; calculated, C2)), including C, are shown in Table 3. For our present purpose, only DIN and DIP are required, although it was found valuable to determine Tot-N and Tot-P aswell for discussion. As seenfrom Table 2, there are few data available, and mean values are finally basedon observations from the years 1974 and 1978-82. The differences in DIN (DIP) concentrations between Skagerrak water and Kattegat deep water are small. However, there are differences [compare Figure 5(b) and (d)], and becausethere were so few data from the Kattegat deep-water, we used data from the Skagerrak water, i.e. C, and S, instead of C, and S,, to calculate C,’ (seeTable 3). A first estimate of C, = C, - C,’ was thus obtained based on the Skagerrak water. To get an estimate basedon Kattegat deep water (C,), an alternative mean value of C, was determined by reducing the first value with by the factor P*(??, --co), where the bars indicate mean values basedon data from all years. As seen from Table 3, the difference in C, between the methods is of minor importance. Table 3 shows that the mean winter concentrations of DIN and DIP are 8.38 and 0.71 mm01m-3, respectively, of which 1.97 and 0.08 mm01mm3are due to local nutrient supply. Thus, slightly lessthan 25% of the winter DIN pool and 120,bof the DIP pool consists of local supply. The proportions of deep water and Baltic water supply can be determined from the expression: f’c =

(QtW/(Q&,>

= f’(C,IC,)

(5)

The mean values given in Tables 2 and 3 show that 470/b(49oi, for DIP) of the Kattegat winter nutrient pool is derived from the deep water (x Skagerrak water) and 27:); (39O,, for DIP) from the Baltic, the rest being from land-based and atmospheric supplies.

Discussion One motivation for the present work was to seewhether extensive data treatment of the Baltic Data Base could give some new information concerning trends in nutrient and oxygen concentrations within the Kattegat during the past years. AErtebjerg (1986) and Svansson(1984) had already found increasing concentrations of DIN, Tot-N and Tot-P, using data from Fladen in the centre of the Kattegat. The present report confirms their results. A summary of our results is given in Table 1. Another important aim was to investigate whether the trends within the Kattegat were mainly due to increased local nutrient supply (land basedand atmospheric), or if trends within the surrounding waters were also important. For this reason, we needed to know not only how the local nutrient supply changed during the past years, but also how the nutrient trends developed within the Baltic surface water and within the inflowing

Figure 8. (a-d) Monthly DIN, Tot-N, DIP and Tot-P concentrations within the Kattegat deep water (S>3090) for the period April-September during the years 1971-82. The trend is based on linear regression. The correlation coefficient, r, the significance level, F(r), and the rate of change, E/St (in pM year- I), are indicated.

572

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& L. Rydberg

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Nutrient

and oxygen

trends

573

Skagerrak water. Thus, we analysed data also from the southern Baltic (Arkona basin), following the same procedures as used earlier. The results are shown in Figure 10. Increases in Tot-N and Tot-P were similar to those of the Kattegat, indicating that changes within the Baltic are important for the conditions within the Kattegat. DIN also increased in the Baltic water (as indicated by Nehring, 1981), but to a much lesser extent than in the Kattegat. This anomaly will be discussed below. For DIP (compare Fonselius, 1980) the trends within the Kattegat and the Baltic were generally too weak during this period to discuss any change in influence due to the Baltic water. In accordance with the above-mentioned aims, an attempt was made to calculate the yearly supply of Tot-N and Tot-P to the Belt Sea and the Kattegat for the years 1972-82 using data from Fleischer et at. (1982), Miljijstyrelsen (1984), Edler (1984), and AErtebjerg (1986). In contrast to earlier investigators, we tried to estimate the reduction of supply due to sewage treatment. We also assumed that the enclosed Danish fjord, Limfjorden, only partly (200,,) contributes to the Kattegat supply. The results, shown in Figure 11, reveal a considerable increase in the supply of Tot-N, while the supply of Tot-P decreased marginally. Very roughly, one may assume that X)-60*,, of the Tot-N is inorganic, while the corresponding percentage for DIP is probably somewhat lower. The specifications of the supply are still uncertain, however, and parallel information often deviates. Spatial distribution of the supply of Tot-N is shown in Figure 12, (from Rydberg, 1983). The summary of trends given in Table 1 show that DIN increased more rapidly in the Kattegat than elsewhere. Only a minor part of the surface-water increase could be attributed fo the supply of Baltic water. The local nitrogen supplies to the Kattegat and the Belt Sea, however, were almost doubled between 1972 and 1982 (Figure 11). In the budget section, we showed that the local supply of DIN was large enough to raise the concentration by 2.0 mmol mm3 during winter. This calculation was based on mean values for the later part of the period 1971-82. Thus, the increase of the winter concentrations due to increased load may be on the order of 50U,, of these values, or z I mmol rnm3. However, compared to the observed increase from 4.7 to 9.4 mmol me3 (Table I), it is obvious that increased deep-water concentrations must be the main reason for the surface-water increase. There were too few data on winter concentrations within the Kattegat deep water to obtain an estimate of the trend, but the strong increase during the summer period, from 4~0to9~7mmolm3 (Table l), is at least an indication of increasing concentrations during the winter. Comparing the summer DIN concentrations of the inflowing Skagerrak water (C,,) with those of the Kattegat (C,) indicated a dramatic, but possibly overestimated, increase of the difference AC, = C, - C, from 0.42 to 4.32 mmol mm3 between 1971 and 1982 (Table 1). The corresponding difference for DIP, on the other hand, remained practically unchanged at approximately 0.35 mmol m- 3. It is difficult to find a reason for the change in DIN other than that it is a result of increasing net supply of organic nitrogen to the deep-water due to increased nitrogen supply to a nitrogen-limited ecosystem (see e.g. Graneli et al., 1986).

Figure 9. Skagerrak The trend level, F(r),

(a-d) Monthly DIN, Tot-N, DIP and Tot-P concentrations within the water (S>30”,,) for the period April-September during the years 1971-82. is based on linear regression. The correlation coefficient, r, the significance and the rate of change, tin pM year I), are indicated.

so

33.92 34.85 33.24 33.83 33.37 33.60 33.38 32.45 34.04 33.65 32.94 34.21

33.62

Year

1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

Mean”

“Mean

7.88

5.98 7.52 9.34 9.29 8.58 7.27 7.48 8.73

-

DIN

0.69

0.65 0.76 0.61 0.54 0.76 0.56 0.79 0.71 0.71 0.76 0.71 0.69

DIP

values for the whole

17.7

9.3 13.0 17.0 17.1 20.3 19.1 23.2 17.8 17.7

-

Tot-N

(S, > 30)

period

8.54

8.23 8.65 8.74 8.24 8.39 9.17 8.27 8-65 8.59 8.92 8.58 8.01

S,

18.6

18.6 19.0 23.7 21.0 19.7

7.3 15.3 9.5 -

Tot-N

(S, i 10)

1974 and 1978-82.

4.25

4.49 5.10 4.44 3.54 4.56

-

2.52 4.94 3.91 3.38

DIN

are based on the years

0.96

0.82 0.91 0.99 0.82 0.89 0.84 0.92 1.00 1.03 0.97 1.09 0.82

Tot-P

Baltic

0.51

0.39 0.55 0.41 0.44 0.60 0.57 0.44 0.47 0.56

DIP

(January-February) mean concentrations of salinity and nutrients deep water (S, > 300/w, C,) and the Baltic water (S, < lo’?&, Cs)

2. Winter

Skagerrak

Kattegat

TABLE

0.84

o-54 0.69 0.55 0.67 0.82 1.00 0.85 0.77 0.79 l-03 0.76 0.99

Tot-P

(mmol

m

33.31

33.41 34.02 33.25 33.50 32.61 32.87 33.96 33.24 33.14 33.22 32.74 33.83

S,

3, within

(8.48

8.02 6.55 7.43 5.48 9.27 9.26 8.47 9.90

DIN

18.0

-

18.1 20.0 22.9

-

11.0

8.4

0.74)

0.76 0.79 0.63 0.60 0.82 0.61 0.76 0.73 0.82 0.79

DIP

(Se > 30%,

(S, > 30)

water

Tot-N

Kattegat

the Skagerrak

1.04

0.98 1.05 1.07 0.82 1.01 0.89 0.91 1.02 1.26 0.73 1.35 1.08

Tot-P

C,),

the

22-22

Mean” Meanb Mean’

8.38

7.38 5.16 8.92 12.55 10.38

-

5.26 7.13 5.93

DIN

22.5

20.5 22.0 26.3 34.6 20.3

-

0.71

0.67 0.46 0.72 0.89 0.86

0.53 0.69 0.60 0.63 0.90 0.64

-

10.6 11.7

DIP

1.14

0.82 0.95 1.01 0.88 1.01 0.96 0.92 1.09 1.15 1.39 1.38

Tot-P

mean concenrrarions

Tot-N

Measured Kattegat C2

(January-February)

6.15

6.79 7.08 5.83 5.67 6.65

4.89 -

DIN

of salinity

18.2

19.4 19.1 23.5 19.3 18.2

9.4

-

Tot-N

Expected Kattegat c2

and nurrienrs

0.61

0.65 0.65 0.60 0.60 0.63

0.53 0.64 0.52 0.50 -

DIP

(mmol

m

0.90

0.88 0.93 1 .oo 0.94 0.91

0.69 0.78 0.80 0.76 0.87 0.90

Tot-P

2.24 1.97

059 -1.92 3.09 6.88 3.73

1.04 -

-

DIN

(C,).

the Kattegat

deep water

-‘) within

C~,observedconcentration;C~,concentrationcalculatedfromsaliniryandnutrienrconservationassumingnoexternal(orinternal)nutrientsupply to rhe surface water; C. = Cz - Ci, increase in concentration due to local nutrient supply (see text). “Mean values are based on the Years 1974 and 1978-82. hMean based on C, implies that Ci was determined from Skagerrak water (C,) instead of Kattegar Mean based on C,, ~~ R implies thar we have corrected for the deviation from the original equation.

R

21,71 19.60 22.63 23.06 25.65 24.61 20.13 23.15 20.96 21.73 21.14

1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982

based on C, based on C, ~

s

Year

TABLE 3. Winter

4.4 4.3

1.1 2,92.8 15.3 2.1

2.3

-

Tot-N

Local supply c.

surface

0.10 0.08

0.00 0.05 0.08 0.13 0.02 0.19 0.12 0.29 0.23

DIP

wafer

0.23 0.20

0.13 0.17 0.21 0.12 0.14 0.06 0.04 0.16 0.15 0.47 0.47

Tot-P

576

L. Andersson

5 a

& L. Ryiberg

6-

A A

z

o

r=O.31

rt

4k

n

-

A

A

5 +4

t

f(

t 1 >o-995

A 0.2

“I

0.75

g:o.ooE

< F( I ) CO.90

Cd)

0-q

01 1971

ig=0.030

11

f(Ii>O-9975

11

11 75

11

” 80

“1 5

Figure 10. (a-d). Monthly DIN (Tot-N, DIP and Tot-P) concentrations within the Baltic surface water (SC loo,,) for the period January-February during the years 197182. The trend is based on linear regression. The correlation coefficient, r, the significance level, F(t), and the rate of change, K/St (in pM year- I), are indicated.

Nutrient

and oxygen

trends

Figure 11. Local supply deposition) to the Kattegat mainly based on AErtebjerg

577

Year of Tot-N and Tot-P (rivers, waste water, atmospheric and the Belt Sea during the years 1972432. The Figure is (1986), with minor deviations (see text).

The trends in Tot-N and Tot-P within the southern Baltic were large enough to dominate the changes within the Kattegat (Table 1) even if there were also considerable increases within the Skagerrak waters. In fact, if the trends within the southern Baltic are representative for the Baltic proper, which they probably are, they imply that the nitrogen (phosphorus) content of the Baltic proper increased by =0.1-0.2 (0.01) Mt year-‘, compared to a yearly supply of x l-2 (O-08) Mt (Larsson et al., 1985). Thus, * lO-15O,, of the yearly supply may enter a rapidly increasing pool of organic nutrients. To our knowledge, this effect of eutrophication has not been widely discussed,but it seemsto correlate with a strong increasein phytoplankton carbon reported by Berg and Radach (1985). Oxygen concentrations within the Kattegat decreasedconsiderably during the period (Figure 6(a,b), Table 1). The decreasewithin the deepwater was 0.05 ml 1-l year ‘, but lessthan 0.0 1 ml 1- i year - i in the inflowing Skagerrak water. The difference in oxygen concentrations between the Skagerrak and the Kattegat water, Co - C, , was 0.8 1 ml l- ’ (1971) compared to 1.23 ml l- ’ (1982). Assuming that the oxygen consumption is R = Q, (C-C,), this increase implies a 500/h increased consumption, provided that Q, is unchanged. Concluding

remarks

It was suggested that phytoplankton carbon may have increased considerably due to increased nutrient supply. In this connection, one may also inquire how primary production is affected. Two issues are of certain interest: the coupling between winter nutrient pool and spring bloom production, and the coupling between upward transport of nutrients from the deep-water during summer and daily summer production. The winter nitrogen pool increased by lOO’$, between 1971 and 1982 while the deepwater DIN concentrations during summer increased from 4 to 10 mmol rnM3 (seeTable l), with a corresponding increase in the upward nutrient flux. Thus, if there is any correlation at all between primary production and ‘new’ production, these changeswould be expected to exert considerable effects on the daily primary production. Measurements of production have been made at Anholt in the centre of Kattegat since the 1950s(see AErtebjerg, 1986). The daily summer production seemsalmost unchanged. However, the

578

L. Andersson

& L. Rydbern

depasbn The The The

Kattegat 21 Belt Sea 41 Sound 5

0 I

25

50 km I

.

Figure 12. Local sources for Tot-N within ( x IO3 tons) for the period 1977-81 (from based on data from Miljdstyrelsen (1984). was 50000 tons Tot-N (300 tons Tot-P), Rydberg.

.

the Kattegat and the Belt Sea. Mean values Rydberg, 1983). Values within brackets are The atmospheric supply used for Figure 11 i.e. somewhat lower than those given by

techniques used have been altered during this time and there are large gaps in the data set. The conclusion must be that it is not possible, for the moment, to decide whether the cursive within the Kattegat has been influenced by increasing nutrient supply or not. A more complete observation series from the Great Belt, on the other hand (AErtebjerg, 1986), indicates a considerably increased primary production during the 1970s. This work will be continued by updating measurements from the last couple of years, to make new trend analyses based on the period 1975-86. The past few years are better covered than the early 197Os, and we may have a chance to obtain appropriate trends for Tot-N also. However, it appears at present that the nitrogen supply has been fairly constant during the 1980s (AErtebjerg, 1986); this probably implies that the trends within

Nutrient

and

oxygen

579

trends

the water are also broken. In itself this is a good sign, and we can look forward to better data to study the coupling between nutrient supply, nutrient concentrations and primary production. References AErtebjerg, G. 1986 Arsager till og effekter av eutrofiering i Kattegat och Belthavet. 22 nord. symp. om vattenforskning, Laugarvatn, Island, August 1986. Nordforsk, Helsingfors, Finland. AErtebjerg, G., Jacobsen, T. Gargas, E. & Buch, E. 1981 The Belt Project. Evaluation of the Physical, Chemical and Biological Measurements. The National Agency ofEnvironmental Protection, Copenhagen, Denmark. Berg, J. & Radach, G. 1985 Trends in nutrient and phytoplankton concentrations at Helgoland Reede (German Bight) since 1962. ICES, C.M. 1985/L:,?/ Sess. R. ICES, Copenhagen, Denmark. Edler, L. 1984 Vasterhavet. In Gddningav havsomrdden kring Sverige (SNV PM 1808) (Rosenberg, R., ed.). The Swedish National Agency for Environmental Protection, Box 1302, 171 25 Solna, Sweden. Fleischer, S., Rydberg, L. & Stibe, L. 1982 Seasonal nutrient transport to the Laholm bay (in Swedish with English abstract). Vatten 38,51-60. Fonselius, S. H. 1980 On the long time variations in phosphorus in the Baltic surface waters. Meddelande frdn Havsjiskelaboratoriet. No 262. Institute of Marine Research, Lysekil, Sweden. Graneli, E., Graneli, W. & Rydberg, L. 1986 A comparison between nutrient limitation at the ecosystem level and at the community level. Ophelia 26, 181-194. Larsson, I-J., Elmgren, R. & Wulff, F. 1985 Eutrophication and the Baltic Sea: causes and consequences. Ambio 14,9-14. Miljostyrelsen 1984 Iltsvind och fiskdtid i 1981. Omfang og orsaker. Miljiistyrelsen, Strandgade 29, 1401 Kobenhavn, Denmark. Nehring, D. 1981 Hydrographisch-chemische Untersuchungen in der Ostsee von 1969-78. Geodiitiche und Geophysikalische Veriflentlichungen, Reihe IV, Heft 35. Berlin. Rosenberg, R. 1985 Eutrophication-the future marine coastal nuisance? Marine Pollution Bulletin 16, 227-231. Rydberg, L. 1983 Vastkustens hydrografi och nirsalttransporter. Trender och klimatberoende i &.tersjon och vasterhavet. Riid serie 6. Oceanografiska institutionen, Goteborgs universitet, Box 4038, S-400 40, Gothenburg, Sweden. Rydberg, L. & Sundberg, J. 1984 On the supply of nutrients to the Kattegat. Report No. 44. Oceanografiska institutionen, Goteborgs universitet, Box 4038, S-400 40, Gothenburg, Sweden. Stigebrandt, A. 1983 A model for the exchange ofwater and salt between the Baltic and the Skagerrak. Jourrral of Physical Oceanography 13,41 l-427. Svansson, A. 1975 Physical and chemical Oceanography of the Skagerrak and the Kattegat. Institute of Marine Research Report No 1. Institute of Marine Research, Lysekil, Sweden. Svansson, A. 1984 Hydrographic features of the Kattegat. Rapports et Pro&s-Verbaux des Rdunions. Conseil Permanent

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