Fifty years of primary production measurements in the ... - CiteSeerX

Journal of Sea Research 56 (2006) 1 – 16 www.elsevier.com/locate/seares

Fifty years of primary production measurements in the Baltic entrance region, trends and variability in relation to land-based input of nutrients Lars Rydberg a,⁎, Gunni Ærtebjerg b , Lars Edler c a

Department of Oceanography, University of Gothenburg, PO Box 460, SE 405 30 Gothenburg, Sweden b National Environmental Research Institute, PO Box 358, DK 4000 Roskilde, Denmark c Swedish Meteorological and Hydrological Institute, B. 31, Nya Varvet SE 42671 V Frölunda, Sweden Received 12 December 2005; accepted 30 March 2006 Available online 27 April 2006

Abstract Inter-annual variations and long-term trends in phytoplankton primary production (PP) within the Baltic entrance region (the Kattegat and the Belt Sea) are presented and discussed. The study employs the core of Danish monitoring data, with measurements at 6–8 different sites from the past 20–50 years. Temporal development of the annual PP is compared with changes and variations in the land-based nutrient inputs and to other, independent, Swedish and Danish PP data. Spatial and seasonal variations based on annual and monthly PP, respectively, are evaluated. There are large variations on all scales; annual PP ranges from 50 to 500 g C m− 2, with maximum values in some Danish fjords and minimum values in open waters during the 1950s. The Kattegat and the Sound have a lower mean production (135–165 g C m− 2; 1981–2000) than the Great and Little Belts (185–220 g C m− 2). Compared to 1950s and 1960s, the daily PP has changed from being almost constant between March and October, to having two more distinct maxima, one in March and one between July and September. It is obvious that annual mean production has increased considerably since the 1950s, but also that this increase took place before 1980. For data after 1980, we find a co-variation between annual nutrient loads and regional mean PP; years of low total nitrogen (TN) and total phosphorus (TP) input to the region coincide with low PP and vice versa. Waste-water treatment and measures in agriculture have reduced the land-based input of TN by about 1/3 and input of TP by about 2/ 3 since the 1980s, enough to cause a substantial decrease in the surface water nutrient concentrations. A simultaneous, but weak downward trend in the regional mean PP can be seen up to 1997, after which the trend is broken. Higher production from 1998 is most likely an effect of changes in the method used for determination of PP. © 2006 Elsevier B.V. All rights reserved. Keywords: Primary production; Belt Sea; Kattegat; Nutrient loads

1. Introduction We investigate inter-annual and seasonal variations and long-term trends in the phytoplankton primary ⁎ Corresponding author. Tel.: +46 31 7732856. E-mail address: [email protected] (L. Rydberg). 1385-1101/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2006.03.009

production (PP) within the Kattegat and the Belt Sea. The region, shown in Fig. 1A, comprises the shallow entrance area of the Baltic estuary, which is subject to large variations in salinity and vertical stratification on timescales from days to weeks. Measurements of primary production from these waters date back to the 1950s, when Steemann Nielsen (1964) started bi-

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L. Rydberg et al. / Journal of Sea Research 56 (2006) 1–16

monthly sampling at a couple of lightships: Anholt Nord, near to Anholt E, and Läsö Rende in the Kattegat and Halsskov Rev in the Great Belt (see Fig. 1A). After the lightships were withdrawn in the mid1970s, the Danish-EPA (now NERI) and other parties, including Swedish authorities initiated cruise measurements at several different sites within the area. This study comprises about 5000 measurements of daily

PP, mainly from NERI (3000) and the Danish counties, carried out from 1953 to 2002. The data have been used by NERI and other parties to evaluate variability and development of the PP in relation to various other parameters such as nutrient concentrations, temperatures, visibility, Chl-a and plankton communities (e.g. Anon, 1998, 2002a). Our contribution is an update of the figures and findings reported

Fig. 1. A. The entrance to the Baltic Sea indicating depths, borders between the areas and stations mentioned in the text. B. A cross-section obtained 3–5 April 1974 showing the salinity distribution from the Kattegat through the Great Belt up to Darss Sill.


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Fig. 1 (continued).

in the earlier works. We also include recent PP data and carry out a specific comparison with the landbased nutrient inputs. Monitoring of land-based nutrient inputs, from rivers and point sources, was initiated gradually from the 1970s although entire coverage was not reached until the end of the 1980s (Kronvang et al., 1993). In this study, we employ data on land-based, monthly loads of total nitrogen (TN) and total phosphorus (TP) for the period from 1975 to 2001. Atmospheric inputs and nutrient supply from adjacent waters are employed for discussion. Recently it has become increasingly obvious that measures to counteract eutrophication (waste water treatment and reduced input of nutrients to agriculture) have had impact. The input of nutrients to the area is decreasing substantially and nutrient concentrations in the surface water clearly respond (Carstensen et al., 2005). 1.1. Primary production and nutrient inputs Primary production in the region features a seasonal variation with low production rates from November to February. The productive season is initiated by a spring bloom, usually in March or early April, which empties the surface water nutrient pool within a few weeks. The summer production is relatively constant, if averaged over several years, but we may find a period of low production just after the spring-bloom and also a second peak in between July and September. Estimates of annual production rates, from different sites and different years, vary from about 50 to 500 g C m− 2 (or 150 to 1000 mg C m− 2 d− 1, if calculated as mean

daily summer production, from May to September; e.g. Anon, 1998). The lowest rates refer to early lightship data from the Kattegat (Steemann Nielsen, 1964), whereas maxima are found in some Danish fjords (Anon, 1998). By comparing these lightship data with data from more recent projects in the Kattegat (1984–93; Heilmann et al., 1994), Richardson and Heilmann (1995) suggested a long-term increase in PP: from less than 100 to about 200 g C m− 2 y− 1. Following Nixon (1992), and assuming that nitrogen is the most limiting nutrient, Richardson and Heilmann (1995) argued that the cause of this change was an increase in the total nitrogen (TN) supply to the region. However, we think that other and more comprehensive PP data from the region, particularly the aforementioned series from NERI and the Danish counties ought to be taken into account in a study of regional development. In addition, studying inter-annual variations of the PP in relation to the nutrient supply may give new insight. Recently, we have also seen efforts to model how the various modes of nutrient supply to the area (land-based, atmospheric and exchange with adjacent waters) affect the production rates (Rasmussen and Gustafsson, 2003; Rasmussen et al., 2003). These authors, and likewise Carstensen et al. (2003) commonly emphasise the importance of the land-based nitrogen supply as a primus motor for the primary production. The landbased load of TN (TP) to the Kattegat and the Belt Sea dominates the annual nitrogen budget at an average of 100 000 (4000) tons (1989–2001; Ærtebjerg et al., 2003), while the atmospheric load of TN is about 45 000 tons and almost negligible for TP (328 tons y− 1;

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Table 1 Stations and approximate number of PP data included in the Danish monitoring programs, the NERI database (the number of years with data is indicated within parenthesis if fewer than 10) (3 for 2000-02) 1950–59 Little Belt South (LBS) The Sound (Ven) Great Belt (Halsskov Rev) Kattegat W (Ålborg Bight) Kattegat SW (Gniben) Little Belt North (LBN) Kattegat E (Anholt E)

1960–69

59 (5)

100 (5) 100 (5)

80 (6)

180

Carstensen et al., 2005). The magnitude of the deepwater nutrient input in the northern Kattegat is about 6000 (1000) tons/month during the productive summer season (Andersson and Rydberg, 1993), i.e. of the same order as the atmospheric input for nitrogen, but the major input for phosphorus. On an annual basis, almost all TN added to the region is retained there (e.g. Andersson and Rydberg, 1988; Andersson, 1996). The magnitude of the land-based inputs of TN and TP indicates that there are reasons to expect a response between variations in nutrient input and PP, at least on longer time scales (annual and upwards) and for the region as a whole (see Section 1.2). 1.2. PP variability and its causes A closer look at PP data from the region, such as the project data of Richardson and Christoffersen (1991) and those of Heilmann et al. (1994) reveals large temporal and spatial variability. This scatter also shows up in the annual and monthly production rates from individual stations, as monthly data are based on one or, at best, a couple of observations per month (Table 1). A major cause of this strong variability is, we assume, the rapidly shifting hydrographic conditions within the region. Fig. 1b shows a typical synoptic salinity section, from the outer Kattegat to the southwestern Baltic. However, the conditions are highly disturbed by large-scale wind and air pressure variations, and the waters of the entrance region are moving back and forth (Stigebrandt, 1983; Andersson, 2002). This barotropic motion contributes to large temporal salinity variations. It dominates over the two-layer estuarine forced circulation, just as the tides do in most river estuaries, and brings about strong but variable vertical mixing, particularly in the Belt Sea. The residence time for the waters in the region is 1–4 months (e.g. Rydberg, 1987) in relation to the surrounding Skagerrak (34 psu) and Baltic waters (8 psu). However, because of the strong gradients (Fig.

1970–79

1980–89

1990–99

2000–02

Total

50 (5)

180 60 175 (7) 90 40 85 60

>400 100 >500 >200 60 210 50 (7)

>70 50 (2) 150 >70 50 50 (2)

>600 230 >1000 500 150 360 400

60 (5)

15 (3) 30 (2)

1B), changes from low-saline Baltic waters to highsaline Skagerrak waters may occur almost instantly. Occasionally deep water may reach the surface, and during periods of strong in- or outflow, a thorough vertical mixing may take place. Such strong hydrographical variability jeopardises the observations of PP. However, the residence times indicate the lower limit for which regional mean PP estimates may be established, given that the frequency of observations is high enough. Whereas the rapidly shifting hydrography is just an indicator of variations in the PP, the real causes are related to the major differences in plankton flora (e.g. Anon, 2002c; AlgAware at www.smhi.se) and nutrient concentrations (Andersson and Rydberg, 1988; Ærtebjerg et al., 2003) between Baltic and the Skagerrak waters. The high-salinity Skagerrak waters, entering the Kattegat from the north, have high nutrient concentrations most of the year, adding a relatively large nutrient load to the Kattegat (see above). By contrast, the surface water of the Baltic Sea, which enters at the southern end, has lower nutrient concentrations (Anon, 2002c). Therefore, outflowing Baltic water normally reduces nutrient levels in the entrance region. Differences in methodology (e.g. in situ or deck incubation and the algoritms for calculating PP from light, Chl-a and biomass data) and sampling are other complicating factors that have to be taken into account, particularly in comparing data from different sources. For example, more recent studies (Richardson and Christoffersen, 1991; Björnsen et al., 1993) have shown that substantial PP takes place in or below the halocline, indicating that observations based on surface water sampling only may have underestimated the PP. 1.3. Approach to data in this study There are obvious restrictions to our possibilities to establish reliable trend and variability analyses. On the other hand, there are few marine areas, if any, which are


more intensively studied in terms of PP than the Baltic entrance region. Measurements of primary production in the area were initiated at an early stage. During the 1980s and 1990s, several stations were visited more or less monthly. Records indicating seasonal and long-term variations up to the early 1990s are shown in Fig. 2. The NERI data alone comprise more than 3000 measurements of PP (Table 1). For the long-term trends in PP, we will focus on decade-wise variations based on annual or seasonal means, starting from the 1950s. For inter-annual variations, we focus on regional means for the period 1981–2002 and compare those with the land-based nutrient loads. Studies of the spatial variability in PP are based on the most comprehensive set of data from 1989 to 1997. As already indicated, research on PP has delivered a large set of project data (e.g. Richardson and Christoffersen, 1991; Heilmann et al., 1994; Carstensen et al., 2003). The Swedish monitoring programs also involved some more comprehensive measurements of primary production within the region. These data are used for comparisons and discussion. Section 2 describes the data sets used, including how sampling and calculations of PP were carried out. Section 3 shows the results of analysis of PP data and nutrient loads. Section 4 compares nutrient loads and PP and discusses the results in the light of other PP data.

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2. Data 2.1. Primary production. NERI data Lightship observations of phytoplankton primary production were initiated by Steemann Nielsen in the early 1950s, and continued for about 20 years with several interruptions. The measurements were carried out twice per month all year round. Sampling was done at 4 depths: 2, 7, 15 and 22 m. In situ 14C incubations were done for half a day. The methods used for calculations of daily (and monthly mean) primary production are described in Steemann Nielsen (1964). The lightship series used here include monthly primary production from Anholt E in the Kattegat (1954–60 and 1963–70), from Halsskov Rev in the Great Belt (1953–57 and 1966–70) and from Ålborg Bight in the western Kattegat (1966–70). However, data from Läsö Rende (1963–65 and 1975–77) are also available at NERI, but this series was less continuous than the others, and therefore not included in this study. Frequency and number of raw data from the lightships are shown in Table 1. When the lightships were withdrawn, the PP observations continued, employing incubator measurements on board cruising research vessels. These observations were introduced with the Danish Belt Project in 1975 (Ærtebjerg et al., 1981). Here, the sampling depths were chosen at 75, 25 and 2% of the

Fig. 2. Monthly PP data from lightship and later cruise measurements in the open Danish waters of Kattegat and the Belt Sea (from Anon., 1994).

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surface irradiance level, whereas incubation was done for 2 h, following procedures described in BMB (1976) and Anon. (1988). The cruise data from 1975 and onwards cover a great many stations from the Arkona Basin in the Baltic (Fig. 1A) to the northern Kattegat, some of which have been running at a relatively high frequency, particularly since 1981. Frequently sampled stations (or pools of closely neighbouring stations, here referred to as areas) within the region of interest include Kattegat W (Ålborg Bight), Kattegat SW (Gniben), Kattegat E (Anholt E), The Sound (Ven), Little Belt South (LBS), Little Belt North (LBN) and Central Great Belt (Halsskov Rev; Fig. 1A). Data from these and other stations have been continuously evaluated in annual reports from NERI (Anon, 1994, 1995, 1998, 2002a). Table 1 indicates the approximate frequency of

measurements at the various stations. From 1981 to 1988 the typical frequency was 0.5–1 observation per month. From 1989 to 1997, the frequency was increased to 1–2 observations per month. After 1997, the number of stations was reduced, but the frequency was increased at those that were left, namely LBS, Central Great Belt, LBN, the Sound and Ålborg Bight. The series from the Central Great Belt is the single most comprehensive, featuring more than 1000 PP data and a weekly resolution since 1990. It also includes several replicates, which where not accounted for in the numbers given in Table 1. Kattegat SW was also excluded because the data were too sparse, particularly before 1980. Thus, only six stations were included in Figs. 3–5. Three sets of figures with PP data from Kattegat W (Ålborg Bight), Kattegat

Fig. 3. Summer primary production, the arithmetic mean PP from May to September, calculated for years with observations from at least 4 out of 5 months.


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Fig. 4. Decade-wise mean production, obtained from integration of the monthly climatology (see Fig. 5).

E (Anholt E), The Sound (Ven), Little Belt South (LBS), Little Belt North (LBN) and Central Great Belt (Halsskov Rev) were evaluated: A. Summer production, which is the arithmetic mean PP from May to September, calculated for years with measurements from at least 4 out of 5 months (Fig. 3; mg C m− 2 d− 1). Because there are more data from the summer months than from the rest of the year, this quantity gives a better resolution than the annual production. The relationship between summer production and annual production (4/3 in mg C m− 2 d− 1) is estimated in Appendix A. B. Decade-wise mean production, which is an average obtained from integration of the monthly climatology (see below), thus indicating changes in the long-term annual mean production (Fig. 4; g C m− 2 y− 1).

C. Decade-wise monthly mean (climatology) production, which is calculated on the basis of the monthly PP within each decade, from the 1950s and onwards (Fig. 5; mg C m− 2 d− 1). The climatology production indicates seasonal variations and long-term trends or changes. At least 3– 4 y of monthly data have been used for each decadal cycle. Annual (or yearly) production is the arithmetic mean value of the monthly mean PP, calculated for years with data from at least 11 mo (g C m− 2 y− 1). Single missing months are interpolated from adjacent months. 2.2. Nutrient supply The land-based nutrient supply consists of inputs from rivers and so-called point sources, i.e. nutrient

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Fig. 5. Decade-wise monthly mean (climatology) production, calculated on the basis of the monthly PP within each decade.

loads in industrial and municipal waste water. In addition, there are airborne inputs to the sea surface, mainly as wet and dry deposition of nitrogen. In Sweden, nutrient loads from major rivers and point sources have been monitored with a monthly resolution since the 1970s. In Denmark, where runoff is dominated by small streams, a full covering was not reached until 1989. However, measurements in the major streams show no general changes in nutrient concentrations from the late 1970s to the early 1990s, indicating that reanalysis based on the loads through the major streams should give satisfactory results for the total riverine loads also before 1989 (Kronvang et al., 1993). Thus, for the period from 1975 to 1988, Danish nutrient loads have been estimated from the correlation between monthly runoff and nutrient load in the early 1990s. German loads are commented on below. Loads from point sources are estimated from a combination of measurements (in larger sewage plants) and bulk estimates (for smaller sources: typically 4 kg N and

1 kg P per person per year). Airborne loads have been measured at single points since the 1970s. At first there were only few observations and much uncertainty concerning the accuracy with which the atmospheric load of nitrogen could be estimated over water. Today, the situation has improved considerably (Anon, 2002b). Although still not quite satisfactory, atmospheric loads are now available for the whole entrance region. Load data are collected by countries and counties, but not in an entirely consistent way. We used data collected by NERI. Their data base contains monthly land-based loads of TN and TP from 1975 to 1996 for the Kattegat and the Sound (borders indicated in Fig. 1A). Loads from the river Göta Älv and the city of Gothenburg entering in the northernmost part of the Kattegat have been omitted because these loads are mainly exported to the Skagerrak and their effects on the region of interest limited. Monthly loads to the Belt Sea were available for Denmark, but not for Germany, except for 2001–2002. However, estimates of annual loads are available for all


countries (county-wise), which means that land-based annual loads could be obtained both for the Kattegat and the Belt Sea for 1975–2001. Atmospheric deposition data from NERI were available for 1989–1997. Later an improved deposition model was employed (Anon, 2002b). It suggests that the 1989–1997 data are somewhat underestimated. 3. Results 3.1. Primary production in General Several aforementioned reports are based on the NERI PP data and include comparisons between Danish inshore and open water stations in the Kattegat and the Belt Sea. In general, the inshore waters feature clearly higher PP with annual maxima of up to 500 g C m− 2 compared to 100–200 g C m− 2 in the open waters (Anon, 1997). The reports also indicate a long-term increase of the PP, as shown in Fig. 2 (Anon, 1994). Fig. 2 compares lightship data from the 1950s and 1960s with cruise data from the 1980s. There is an increase in mean daily PP from between 200–400 mg C m− 2 d− 1 to between 400–800 mg C m− 2 d− 1. It is also obvious that the spring bloom production and the late summer production have become more outstanding in the series from the 1980s. Spatial variability within the open waters of the region, based on NERI cruise data from 1971–79 (Ærtebjerg et al., 1981) is indicated in Table 2. Although the temporal resolution is not quite satisfactory, there are obvious differences in the annual PP, where the maximum values (195 g C m− 2 y− 1) are found in the Great and Little Belts and the minima (90 g C m− 2 y− 1) in the open Kattegat waters (Gniben, Anholt E). At Halsskov Rev in the Great Belt, the mean summer PP (May and September) amounts 886 mg C m− 2 d− 1. Below we investigate whether more recent data available at NERI and a re-evaluation can add to our knowledge, and also to what extent nutrient supply and Table 2 Annual primary production in Danish waters 1971–79 (from Ærtebjerg et al., 1981) g C m− 2 y− 1 NW Kattegat (2 stations) S Kattegat (2 stations) The Great Belt incl. Mecklenburger Bight (5 stations) The Sound (4 stations) Fehmarn Belt and Arkona Deep (4 stations)

125 90 195 100 115

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variations in PP are correlated. We will consider seasonal and spatial variability in the PP, but also inter-annual variability and long-term trends. 3.2. Variability and long-term trends in primary production The daily mean summer production from the six main areas (Kattegat W and E, Little Belt N and S, The Sound and the Great Belt) is shown in Fig. 3. Each bar represents an average of about 10 PP observations (cf Table 1). However, the results are scattered and far from all years can be accounted for, because of too few observations (data from less than 4 mo). The longest series (those from Kattegat W, Kattegat E and the Great Belt) show that values from 1980 and onwards are clearly higher than those of the 1950s and 1960s. There is also a decreasing trend in all Belt Sea data, starting from the early1980s and continuing up to1997, after which all series show a higher production. It is also obvious that the Belt Sea stations feature a higher summer production (600–1000 mg C m− 2 d− 1) than the open sea stations Kattegat E and Gniben (300– 500 mg C m− 2 d− 1), whereas The Sound and Kattegat W feature intermediate levels (500–600 mg C m− 2 d− 1). The same conclusions can be drawn from Fig. 2 and Table 2. Remarkable is the high spring bloom production at Läsö Rende, which reaches 1400 mg C m− 2d− 1, compared to between 800–1000 mg C m− 2 d− 1 at the other stations. The spring bloom is also delayed compared to all other stations in the region, taking place in April, more simultaneously with spring blooms in the North Sea and Skagerrak (Lindahl et al., 1998). Co-variation between different areas is not easily seen from Fig. 3, although two years of extremely low runoff (1996 and 1997: Carstensen et al., 2005) coincide with low production at all Belt Sea stations including the Sound and the SE Kattegat. The year 1999, with a very high runoff, also shows summer PP clearly above the average. The decade-wise mean production, shown in Fig. 4, is based on data from all months, i.e. on annual PP. Like Fig. 3, but more clearly, Fig. 4 also indicates the general increase from the 1950s to the 1970s and a decrease from the 1980s to the 1990s. All stations have a higher production after 2000. On the decadal time scale, we can also see a consistent co-variation between most stations, including those in the Kattegat, which was not obvious in comparisons of summer production from individual years (Fig. 3). As indicated in Fig. 2, enhanced spring bloom production in March-April and a second maximum during late summer are features that have become more

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pronounced in data after 1980. This change was also pointed out by Richardson and Heilmann (1995). In Fig. 5, the seasonal variations are depicted as decadewise monthly mean (climatology) production. Fig. 5 corroborates this change. The open waters of the Kattegat feature a lower summer production than the Belt Sea, whereas the spring bloom production is similar. The Sound has the lowest spring bloom production of all areas. It is probably due to a greater impact of Baltic water, which has winter nutrient concentrations well below those of the Belt SeaKattegat region (Ærtebjerg et al., 1981) including a later onset of the spring bloom. As shown in Fig. 4, decadal mean PP values show more consistent co-variation between different stations, than do the summer PP data of Fig. 3. We assume that this difference appears because of too few observations (some of these latter averages contain just four observations whereas others may contain 10–20), and data from individual stations are not generally repre-

sentative of the mean PP. Even series with a very high resolution (days-weeks) show large variations, indicated for example in the data of Richardson and Christoffersen (1991). In order to reduce the large spatial and temporal variability, and for comparison with nutrient inputs, a regional average PP was calculated using annual PP data from the three most frequently visited stations: Kattegat W (Ålborg B), the Great Belt (Halsskov Rev) and Little Belt S. This was done for the years 1981–2002, the period with the highest frequency of observations (Table 1). Data from these stations also show distinct similarities both on annual (Fig. 3) and decadal (Fig. 4) timescales. The series obtained is based on an average of about 70 data per year, half of which are from the Great Belt station. Thus, there are more data from the western part of the region. This is further discussed, together with the land-based nutrient inputs, in Section 3.3. Inter-annual variations are large: from less than 150 to about 300 g C m− 2 y− 1. The downward trend observed in the regional mean PP from the 1980s is

Fig. 6. Annual land-based supply of (top) total nitrogen (TN) and (bottom) total phosphorus (TP) to the Belt Sea and the Kattegat.


Fig. 7. Atmospheric load of nitrogen to the Kattegat and the Sound, monthly observations.

broken in 1998. The most likely cause of the increase in PP thereafter is a change in methodology. The new method for calculation of PP was introduced by NERI in 1998. This and other variations in PP will be discussed in Section 4 after data on nutrient inputs have been evaluated.

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therefore the effects on total inputs are also restricted. Fig. 6 shows a decrease of the TP load to the region from an average of 7000 tons y− 1 (1980–89) to less than 3000 tons y− 1 (1993–2001). For nitrogen (Fig. 6) there is a decrease from 135 000 tons y− 1 (1980–89) to less than 100 000 tons y− 1 (1993–2001). It corresponds to lowered inputs of the order of 2/3 and 1/3, respectively. Regarding the magnitude of other nutrient sources (see Section 1.1), it is obvious that such strong reductions will affect nutrient concentrations but most likely also primary production. Monthly atmospheric loads of TN to the Kattegat are shown in Fig. 7, covering the period from 1989 to 1996. Maximum loads occur during summer, but the variations are large, mainly mirroring the differences in precipitation. Although the yearly atmospheric loads of nitrogen are smaller than the land-based (approx. 50% on average; Section 1.1.), the difference in seasonal patterns, where the land-based supply is lower during summer, makes the atmospheric input of nitrogen relatively more important. Atmospheric loads of phosphorus are small compared to

3.3. Land-based and atmospheric nutrient supply Temporal development of the land-based supply of TN and TP to the region is seen in Fig. 6, which shows the yearly input from 1975 to 2001. Large inter-annual variations in the loads (most pronounced for TN) depend almost exclusively on the variations in freshwater runoff. In other words, the co-variation between Kattegat and Belt Sea data, obvious in both parts of Fig. 6, is mainly a result of regionally similar rainfall and runoff (Carstensen et al., 2005). However, compared to the 1980s, the 1990s feature lower nutrient inputs. The reduced inputs follow comprehensive measures in sewage plants and agriculture (Anon, 2002a; Carstensen et al., 2005), and the decadal change is only to a minor extent affected by rainfall. The most dramatic change is seen in the TP loads around 1990 (Fig. 6), when phosphorus reduction was introduced for the city of Copenhagen and other Danish cities. In Denmark, the TN load has decreased by 14% due to reduced waste water input and 20% due to decreased TN loads from agriculture. Input of TP was reduced by 75% (Carstensen et al., 2005). Also in Sweden and in Germany, several countermeasures were taken during the period. However, the loads from Germany and Sweden are comparatively small (e.g. Andersson and Rydberg, 1988; Carstensen et al., 2005), and

Fig. 8. Monthly loads of (top) TN and (bottom) TP to the Kattegat (incl. the Sound, excl. River Göta älv).

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other sources and not shown here. The load of TP to the Belt Sea was estimated at 328 tons y− 1 by Carstensen et al. (2005). For comparison, Fig. 8 shows monthly landbased loads of TN and TP to the Kattegat. During summer, the land-based load of TN is almost as low as the atmospheric load. 4. Discussion 4.1. Trends in primary production. Changes in methodology We have reviewed NERI's data base on primary production in the Baltic entrance region. Series from 1953–2002 are included in the study. In all, we used about 3000 PP data from 6–8 stations, investigating the decade-wise seasonal variability (Figs. 2 and 5), the mean summer production (Fig. 3) and the decade-wise mean PP (Fig. 4). By using the three most frequently visited stations, Little Belt S, Great Belt and Kattegat W, comprising totally about 1500 data since 1980 (see Table 1), we calculated a regional mean annual PP for the years 1981–2002 (Fig. 9), assumed representative for the western Kattegat and the Belt Sea. From the longest PP series (Figs. 3 and 4), we recognised a substantial increase in the annual PP, with an approximate doubling from less than 100 g C m− 2 y− 1 (1950s–1960s) to nearly 200 g C m− 2 y− 1 (1980s– 1990s). The size of this increase, however, may be questioned for several reasons. Methods have changed (Section 2.1) and there are fewer data before1980 (Table 1) than thereafter. The Great Belt is the only area where the number of data before 1980 is satisfactory (Fig. 4; Table 2). A cautious interpretation of the NERI data suggests that the average increase in PP from the 1950s and 1960s up to the 1980s (see below) is 50–100%. Data

Fig. 9. Yearly mean PP, based on data from the Great Belt, Little Belt S and Kattegat W, and land-based supply of TN to the Kattegat and the Belt Sea.

Table 3 PP data along a longitudinal section in the eastern Kattegat from Anholt to SE Skagerrak (data from Heilmann et al., 1994) Time for cross-section

PP

PP(SD)

March 1992 March 1992 April 1984 May 1987 May 1988 May 1990 May 1991 May 1992 May 1993 Sep 1992 Oct 1990

146 1306 195 986 475 689 615 898 599 444 879

75 641 83 553 81 385 346 248 247 135 375

Each number represents approximately 10 measurements. PP and SD (standard deviations) are given in mg C m− 2 d− 1.

from the 1990s, in addition, indicate a slight decrease in PP compared to 1980s. Richardson and Heilmann (1995), in comparing the old lightship data from the Kattegat with their own observations during1984–93 suggested a doubling (or more) of the PP. Their observations, which comprised data from 11 expeditions in the area east of Anholt and Läsö northward to the southern Skagerrak, gave a yearly mean production of 190 g C m− 2 y− 1 (Table 3; Heilmann et al., 1994). The NERI station Kattegat E, on the other hand, shows an annual mean PP between 110 and 160 g C m− 2 y− 1 only (Fig. 4) for the same years. Although the areas of observations were not exactly similar, difference in methodology is the most likely cause of this relatively large difference in PP. Richardson and Heilmann (1995) were using a new technique for determination of PP, which involved the use of continuous fluorescence measurements (see also Richardson and Christoffersen, 1991). As commented by the authors, high fluorescence, recorded in the halocline area and in deeper waters, raises the level of (gross) production considerably (by up to 30% compared to traditional methods, used by NERI). The different methodology explains partly why Richardson and Heilmann (1995) find the increase in PP from earlier periods so large. From 1998 onwards, a similar methodology was employed by NERI. This probably explains why NERI PP data from 1998 and onwards are markedly enhanced, compared to data from the preceding periods. However, the effect of this new methodology may differ between areas, depending, for example on the total depth. In waters shallower than the mean depth of the halocline (15 m) the change in PP is probably less pronounced. Comparing summer PP at Kattegat W (which is


shallow) with that of the Great Belt and Little Belt S also indicates a larger increase at the latter stations (Fig. 3). The difference is even more pronounced when the fjord stations are compared with those of the Belt Sea (as in the NERI report Anon, 2004). The Belt Sea stations are deeper and clearly have a larger increase. The NERI report addresses a possible effect of the change in method, but emphasises other factors such as land-based nutrient supply, wind mixing and high temperatures as causes of increased PP. Assuming that 30% of the observed PP from 1998– 2002 is due to the change in methodology, we indicate in Fig. 9 what is probably a more realistic development of the PP. In fact, it looks as if the difference could be even larger. In a study performed in open waters near Gilleleje (Fig. 1), Richardson and Christoffersen (1991) reported an annual production of 290 g C m− 2 (based on almost weekly PP observations during the whole of 1989). NERI data generally indicate a relatively low production for 1989, with a regional average of about 160 g C m− 2 y− 1. Even though the Gilleleje area may be subject to strong upwelling, the large difference confirms the impression that the new method results in higher values of PP. 4.2. Spatial variability in primary production Lower production and a less apparent increase in PP in the open parts of the Kattegat than elsewhere in the region were pointed out at an early stage by Steemann Nielsen (1975) and later also by Ærtebjerg (1986). At Kattegat E (Anholt E), the annual PP averages 135 g C m− 2 for the period 1981–2000. In the Great and Little Belts, annual mean PP levels are 185–220 g C m− 2 (Fig. 4) and at Kattegat W (Ålborg B) and SW (Gniben) PP is 150 g C m− 2. The Sound averages 140 g C m− 2. These average data include the years of ‘enhanced production’ (1998–99) caused by the change in methodology. A 30% difference means that the values given above would have been approx. 3% lower, if the old method had been used. Carstensen et al. (2003) used a different set of PP observations from the Kattegat, carried out through the Danish National Aquatic Monitoring and Assessment Program (DNAMAP). The program included more than 1000 measurements from 1989 to 1997. A few data coincide with the NERI data, but most data are fully independent. Table 4 shows mean values from four open sea stations, with an annual range in PP from 96 (Anholt/Kattegat E) to 109 (Kullen) g C m− 2 y− 1. Five coastal stations with depths of about 10 m average 170 g C m− 2 y− 1. The data confirm the impression that

13

Table 4 Annual mean PP during 1989–97 (from Carstensen et al., 2003) STATION Aalbaek Hals Barre Gudenå Kattegat W (Aalborg Bight) Gilleleje NE Kattegat SW (Gniben) Kullen Kattegat E (Anholt E)

Depth (m)

Nos of data

PP (g C m− 2 y− 1)

10 10 8 13–15

150 140 170 70

172 167 171 100

10–15 25

80 66

173 106

25–30 >30

53 65

109 96

Raw data from DNAMAP (http//mads.dmu.dk).

the open waters have a lower production (see also Carstensen et al., 2003). The difference is even more pronounced if a comparison is made with fjord stations (e.g. Anon, 1998). Higher PP in coastal regions is likely to be caused by higher nutrient availability, due to either land-based inputs and/or faster recirculation of nutrients (more efficient vertical mixing) in shallower waters. Carstensen et al. (2003) also discussed the relatively low PP values compared to those of Heilmann et al. (1994). However, we find the low levels consistent with the NERI data: during the years from 1989 to 1997, the average PP (according to Fig. 9, based on data from the high range Belt Sea stations and Kattegat W) is also well below the long-term mean PP, or about 175 g C m− 2 y− 1. Thus, the DNAMAP data seem fully comparable with NERI data. Other series of independent PP data in the region are the Swedish measurements from the Sound and the Laholm Bay in SE Kattegat. The Swedish series are comprehensive, with 12–24 observations per year, thus allowing for calculations of annual production, as shown in Table 5. However, they are carried out for restricted periods of time. For the period from 1985 to 2000, annual PP in the Sound averages 130 g C m−2 y−1, which is marginally lower than the Danish average (140 g C m− 2y− 1). However, comparison of data for individual years indicates a poor correlation (not shown). On the other hand, the Sound is subject to very strong hydrographic variability, and moreover the methodology also differs, both in terms of sampling and in calculation methods. Data from Laholm Bay in SE Kattegat originate from two periods during the 1980s and 1990s (see Table 5). The yearly mean PP from those measurements is 165 g C m− 2 y− 1, or 25% above the mean in Kattegat E. It makes sense, because the stations are situated

14


Table 5 Annual primary production, g C m− 2, in the Laholm Bay (SE Kattegat) and the Sound

different methods used by NERI (before and after 1998) can be compared.

Year

The Sound

4.3. Correlation between PP and nutrient loads

1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Mean

70 148 79 152 60 76 136

113 140 118 117 180 136 130 120 170 150 135 140 57 100 118 132 148 122

Laholm Bay St 19

Laholm Bay St HV

167 166 168 104 114

157 201

190 245 134

144

185

Data are from SMHI (unpublished).

near two of the largest rivers entering the Kattegat (Rydberg et al., 1990). Data from Laholm Bay also show large seasonal variations with pronounced spring and autumn blooms. To summarise, the results obtained by a more thorough investigation of PP data from the region do not generally differ from results obtained earlier by using limited parts of the NERI data (Anon, 1994, 1995, 1997, 1998) or by using separate investigations (e.g. Richardson and Heilmann, 1995; Carstensen et al., 2003). We can see a consistency in the NERI data after 1980, where the most frequently sampled stations show similar long-term trends and inter-annual variations. Data from NERI and DNAMAP (1989–97) seem to coincide well, although we have not compared station by station. Even Swedish PP data from the Sound and the SE Kattegat seem to produce consistent long-term averages. However, it seems important to formulate a mathematical relationship such that the results from the

Investigating the water nutrient concentrations of the entrance region for the years 1989–2002, Carstensen et al. (2005) found an obvious decrease in the phosphorus concentrations (for both TP and DIP – i.e. phosphate). The decrease was most pronounced in coastal and bottom waters, but substantial in surface and open waters as well. Also for the nitrogen concentrations (TN and DIN) there was a decrease, although less obvious. On the other hand, the interannual variability in nitrogen concentrations was larger and more clearly related to the variations in nitrogen loads. Both effects are consistent with the variations in the land-based nutrient loads, where the phosphate load features a larger long-term decrease but has less obvious inter-annual variations (Fig. 6). Thus, the conclusion of Carstensen et al. (2005) that both nitrogen and phosphorus concentrations are affected by variations in nutrient loads seems highly consistent. Now, to what extent is there a correlation between the nutrient input and the primary production? In Fig. 9 we have plotted the annual land-based load of TN and TP together with the regional mean, annual PP for the period 1981–2002. Obviously, both series show a high correlation with PP data, particularly for inter-annual variations, but less apparent concerning long-term changes. This may lead us to think that nitrogen inputs affect the annual PP more than phosphorus inputs. But, as pointed out by Carstensen et al. (2005) in their discussion, both phosphorus and nitrogen are probably important. Their effects may differ considerably with season and position. Both data sets use regional data, although there are large differences in both PP and concentration levels within the region. Questions about limiting nutrients may not be solved on a regional scale, but have to be approached more locally, in time and space. The large variations in PP levels within the region are far from understood. The effect of the Baltic Sea is certainly large in the southern parts of the region and more so in the Sound than in the Belts. The waters of the Sound may change from Kattegat to Baltic water within a few days, whereas the Great and Little Belts may develop their own characteristic waters, more sensitive to local inputs. Mixing conditions are different, the salinities as well, and also the landbased input of nutrient varies. Some of these effects are incorporated in the modelling by Rasmussen and Gustafsson (2003).


The Kattegat, on the other hand, is likely to be more influenced by the Skagerrak circulation and nutrient inputs from the North Sea. Surface water from the North Sea enters the Kattegat as deep water and contributes considerably to the Kattegat’s nutrient budgets (Andersson and Rydberg, 1993). We expected that years of high inflow of very nutrient-rich water from the continental rivers would influence the production, as seems to happen in the eastern Skagerrak (Lindahl et al., 1998). Such events are clearly seen in nutrient data from the Kattegat (e.g. in 1989, 1994, 1995 and 1999), but seem not to result in markedly changed PP in the Kattegat and the Belt Sea. 5. Conclusions To conclude, this re-evaluation of NERI's PP data from the Kattegat and the Belt Sea has shown that the large scatter can be reduced if mean values in space and time are created. This is probably self-evident, but averaging the annual PP over decades (Fig. 4) brings about clear spatial differences between areas but also a consistent temporal variability. The spatial average of annual PP for the period 1981–2000, using the most frequently visited stations (with a visible inter-annual co-

15

variation), also showed a clear co-variation between PP and the land-based nutrient loads on an inter-annual timescale. The co-variation is more pronounced for nitrogen than for phosphorus (because of larger interannual variability). The long-term decrease in landbased nutrient loads (over the past 15 years) is accompanied by a moderate decrease in primary production, of the order of 10%, presuming that higher PP from 1998 onwards is due to a change in methodology. This decrease is likely to be the result of decreased land-based nutrient loads. The very large decrease in the land-based input of phosphorus to the area (from 7000 to less than 3000 tons y− 1) showed a clear response in terms of lower phosphorus concentrations (Carstensen et al., 2005), but we are not able to judge to what extent the decrease in PP is an effect of lower phosphorus or lower nitrogen input. Nitrogen reduction was relatively less and the effects on the nutrient concentrations also much smaller. This reevaluation indicates that more detailed investigations of the relationships between production and nutrient supply are needed to understand the relative importance of nitrogen and phosphorus. Such investigations must include vertical mixing and regeneration of nutrients at the bottom as factors of major influence on the rate of PP.

Appendix A The correlation between yearly PP and summer PP was discussed in Section 2. The figure below shows the correlation, based on NERI data from two stations in the Belt Sea area, the Great Belt (⁎) and Little Belt S (·). All years of observations (see Figs. 3 and 4) are included. The figure indicates a high correlation. The ratio, evaluated in mg C m− 2 d− 1 is 4/3.

16


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