Negative oxygen trends in Swedish coastal bottom waters

Volume 21,'Number Z/July 1990 0025-326X,'90 S3.00+0.00 © 199(I Pergamon Press plc

Marine Pollution Bulletin, Volume 21. No. 7, pp. 335-339, 1991).

Printed in Great Britain.

Negative Oxygen Trends in Swedish Coastal Bottom Waters RUTGER ROSENBERG Universily of G6teborg, Marine Research Station at Kristineberg, 450 34 Fiskebiickskil, Sweden

Temporal trends in annual minimum oxygen concentrations in the bottom water of 14 marine coastal areas have been investigated from the early 1950s or 1960s up to 1984. At twelve stations a significant declining trend could be demonstrated, but at three of these that trend could have been caused by increased recordings over time. The likely reason for the decline is suggested to be increased large scale eutrophication.

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Skagerrak 0~/ Excessive input of nutrients to coastal marine waters cause algal blooms, dominance of filamentous macroalgae and decreased transparency. In enclosed and stratified areas such eutrophication can also result in bottom oxygen deficiency with adverse effects on benthic organisms and demersal fish. This has been reported in recent years from the northern Adriatic Sea (Stachowitsch, 1984), Chesapeake Bay (Seliger et al., 1985), the Baltic Sea (Cederwall & Elmgren, 1980) and the Kattegat (Rosenberg, 1985; Rosenberg & Loo, 1988; Pihl, 1989). Eutrophication is not the only factor causing these effects, but a significant contributor. There are few long-term statistical trends in biological and chemical measurements concerning the parameters mentioned above. Temporal change in oxygen concentration could be a useful warning signal indicating change in the marine environment. The standard method for measuring oxygen has remained unchanged over the last century (Winkler, 1888), which facilitates long-term comparisons. Decreasing trends in oxygen concentration have been shown in several basins in the Baltic Sea between 150 and 400 m over the period 1900 to 1968 (Fonselius, 1969). Similar negative trends have been shown for the northern Adriatic Sea bottom water in all seasons except winter between 1911 and 1984 (Jugtid et al., 1987). In this paper trends in bottom water oxygen concentrations will be presented from 14 semi-enclosed fjordic areas on the Swedish Skagerrak coast over the period 1950-1984. Areas Investigated

This investigation comprises stations (Fig. 1) from the open coast to semi-enclosed areas; the latter with

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poor ventilation of the bottom water. The water is stratified more or less all year round, with brackish water at the surface (-25%0), a halocline at 10-20 m, and deeper oceanic water below (32-34')00). Further details are in Svansson (1975). To the north, stations 1-3 and 5 are rather well ventilated at the bottom, whereas station 4 is in a deep basin at about 62 m just inside a sill at about 50 m. Here the water may stagnate. The hydrography of the Gullmarsfjord (Fig. 1) is described by Svansson (1984), and by Lindahl & Hernroth (1983) who also present some fairly recent oxygen recordings (1978-1981) in the deep basin. Stations 6 to 14 are along a gradient of increasing stagnatioh from south towards north and east. The 335

Marine Pollution Bulletin

depth profile is given in Fig. 2. According to Bj6rk (1983) the bottom water is ventilated from the south, with no stagnation up to at least station 12, whereas at station 9 the bottom water is renewed only about once a year. West of station 9 is a sill at 10 m and south-west of station 6 is a sill at 9 m. These reduce bottom water renewal to a less than annual occurrence. According to Ehlin (1971; cited in Bj6rk, 1983) the mean dePth of this fjordic system (Fig. 2) is 13.5 m. The fresh water supply through rivers is 10-15 m 3 s-1; temperature variation at 20 m is 3 to 150C and salinity varies between 28 and 32.5%o. The level of phosphorus has been shown to be clearly elevated in the semi-enclosed parts of the system in the 1960s (S6derstr6m, 1971). The main towns in the area are Uddevalla (46 000 inhabitants), Stenungsund (17 500) and Lysekil (14 900). All have had tertiary sewage treatment plants installed since the early 1970s. The main industries in the area are petrochemical industries in Stenungsund and a refinery near station 2. None of these polluters can be considered to have a significant impact on the oxygen demand close to the sea bed.

Material and Methods The values of oxygen, temperature and salinity used in this report were all obtained from 'Hydrographical data' measured by the National Board of Fisheries in Sweden. Anon. (1970) was used for the period 1950 to 1966 and after that Anon. (1967-1974) and Anon. (1973-1984). The earliest oxygen recordings used here were from 1950. At some stations the measurements began later O.j

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(Table 1). At most stations 3-6 annual measurements were made, but in some years it could be fewer or more. There is, however, no statistical ( F ( t ) > 0 . 0 5 ) increase or decrease in the number of recordings over the time period (given in Table 1) at the stations 1-9 and 13 and 14. At stations 10-12 there was an increasing trend (F(t)<0.05) in number of recordings by time, which could bias the results at these two stations. The annual variations in bottom water oxygen concentrations were high, e.g. at station 11 they were often recorded to be between 1 and 6 ml 1-1. For the statistical treatment, therefore, the lowest value each year at each station was used. Simple linear regression was used and the slope of the line was tested for significance by t-test. At some stations hydrogen sulphide was recorded, and the values given as gmol/l were converted to 'negative oxygen' in ml 1-I by multiplying the gmol-results by the factor - 0 . 0 4 4 (Anon., 1988).

Results The range of all bottom oxygen concentrations at each station is given in Table 1. The variation was great at all stations. Low minimum values ( < 1 ml 1-1) were recorded at the most enclosed stations 4 and 6-11. At stations 6-9 hydrogen sulphide was measured several times. The annual minimum bottom oxygen concentration at each station is plotted in Fig. 3. The regression line is also shown together with the 95% significance interval for the slope of the line. When all three lines show a declining trend with time, the trend is statistically significant. The statistical significance level for the slope of the line is given in Table 1. Thus, the annual minimum concentrations show a significant ( F ( t ) < 0 . 0 5 ) declining trend at all stations except at numbers 2 and 7. It was highly significant (F(t)<0.001) at stations 5, 11, 13, and 14. If the stations with the lowest minimum concentrations (6-9) are pooled together the declining trend was highly significant (F(t) = 0.0008). Bottom temperature and salinity were also examined at station 9. The annual maximum and minimum values

TABLE 1 Stations for bottom oxygen concentration recording, period of measurement and approximate depth. Maximum and minimum values are given, where recordings of hydrogen sulphide are converted to 'negative oxygen'. Number of recordings (n) of annual minimum oxygen concentrations used for the statistical treatment and the significance level (F(t)) are also indicated. Values < 0 . 0 5 indicate significant (s) trends. (Note that the number of oxygen concentration recordings have increased with time at stations 10 to 12, which make the statistical treatment at these three stations uncertain.) Station

Period of recording

Approx. depth (m)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

1962-84 1962-'84 1963-84 1965-84 1951-84 1950-84 1953-84 1951-84 1951-84 1951-84 1952-84 1952-84 1953-84 1954-84

25 25 33 62 55 40 55 55 40 23 23 20-40 24 27

Abyfjord BroOord Maim6 drag Sahk/illefjord Tr6skeln Kolj6flord Borgilafjord Kalv6fjord Havstensfjord Bj6rningarna Dokorsviken Halsanabbe Asker60ord Hake0ord

336

O,-conc. min.-max, 2.3-7.8 1.8-8.2 3.5-7.5 0.1-6.8 2.6-6.9 -2.0-5.8 -1.8-6.2 - 1.6-6.4 -0.8-7.0 0.1-7.2 0.6-7.0 1.2-8.2 3.3-7.6 2.5-7.4

n

22 22 21 17 29 33 27 33 33 32 31 31 31 29

F(t)

0.014 0.084 0.004 0.027 0.0001 0.023 0.085 0.026 0.029 0.001 ').001 0.003 0.0005 0./1036

Significance s s s s s s s s s s s s

Volume 2 1 / N u m b e r 7/July 1990

did not show any significant ( F ( t ) > 0 . 1 ) trends, and nor did temperature and salinity measured at the same time as the annual minimum oxygen concentrations. Discussion

The results show that bottom oxygen concentrations have dropped at 12 out of 14 stations over the last decades. Of these 12 stations the number of oxygen recordings had increased over time at three stations (10-12), which could have influenced the statistics at these stations. If these three stations are omitted, the statistical analysis gave a significant declining oxygen concentration trend at 9 out of 14 stations. The reason why no significant trend could be obtained for station 7 is that oxygen concentrations

8

around zero were already recorded in the 1950s (Fig. 3). The measurements at station 2 did not commence until 1962, thus the short time-series could be one reason for not obtaining significance. Another reason could be that a refinery was established close to station 2 in the early 1970s, and the large tankers stir the bottom water and sediments in that area (Cato et al., 1986). At station 5, at the sill in the entrance to the Gullmarsfjord, a highly significant negative trend in oxygen concentrations was recorded. This is especially noteworthy as there is no local large input of organic material in the area and the Gullmarsfjord is a nature reserve. A significant trend in annual minimum oxygen concentrations over the period 1951-1984 was earlier demonstrated for the deep basin (ca. 110 m) of the Gullmarsfjord (Josef~on, pets. comm.).

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Fig. 3 Temporal trends in annual minimum bottom oxygen concentrations at stations 1 to 14. The regression line is shown together with the 95% significance level for the slope of that line.

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Marine Pollution Bulletin

Examination of the sediments in the three most enclosed fjords, stations 6-8, indicated that oxygen deficit began in these areas in the period 1956-1962. At that period lamination of the sediments appeared for the first time (Wallin & Oster, 1986). Adverse biological effects have been observed in the region described in this paper. Josefson & Rosenberg (1988) examined the benthic fauna in three fjordic areas in 1986 and reported significant reductions in total mean abundance and biomass compared with ten years earlier. They suggested recently increased periods of hypoxia in the bottom water as the most likely reason for the impact. Similarly, the benthic fauna in the Gullmarsfjord was recently reported to have been affected by oxygen deficiency (Josefson & Widbom, 1988). The commercially important shrimp Pandalus borealis was quite abundant in the bottom waters of the three most stagnant fjords (stations 6-8) in 1909, but not in the adjacent Havstenstjord (station 9). In 1910, however, only a few shrimps were captured at stations 6-8 and the author suggested oxygen deficiency as a likely reason for that, but no measurements were made (Bjrrck, 1913). Thus, we cannot rule out the fact that periodic oxygen deficiency could have occurred also about 70 years ago. However, the situation is worse today with hydrogen sulphide and recently declining trends in bottom water oxygen concentration in these three fjords. The vertical depth distribution range of macro-algae was found to be smaller in the 1970s and 1980s when compared to the 1920s in the vicinity of stations 4, 6 and 9 (Srderstrrm, 1970; Michanek, 1972; Svane & Grrndahl, 1988). The causes for this change could perhaps partly be ascribed to methodological differences, as suggested by Svane & Grrndahl (1988), but the main reason is more likely an increased input of nutrients making the waters less transparent due to increased phytoplankton biomass. It seems probable that the main reason for the declining trends in bottom oxygen concentrations is the increased inputs of nutrients to the region over the last decades causing widespread eutrophication problems. As there were no significant temporal trends in temperature and salinity, as demonstrated for station 9, it seems unlikely that the recent lower oxygen concentrations could be caused by increased water temperatures or exchanges with water masses with different salinities. It is known that the input of total nitrogen via the rivers on the central Swedish west coast has increased by 30-40% as a mean over the period 1978-1987 compared to ten years earlier (Anon., 1989). The input of nitrogen to the Saltkfillefjord (north-east of station 4) increased by 29% over that same period. For phosphorus, however, no increases were observed. The inner and central part of the Saltkiillefjord was severely affected by pollution from a sulphite pulp mill up to 1966, when it closed down. The benthic fauna was significantly reduced in those parts, but unaffected in the outer part where the oxygen recordings presented here were obtained (Rosenberg, 1976). The sulphite pulp mill effluent could have reduced the temporal 338

trend in oxygen presented here, but rather not added to it. It has been estimated that the inputs of nitrogen and phosphorus to the Skagerrak and the Kattegat have increased by a factor of about 6 and > 10, respectively, during this century (Elmgren & Rosenberg, 1987). As a consequence a significant increase in winter concentrations of inorganic nitrogen has been observed in the Kattegat surface water during 1971 to 1982. During the same period a significant decrease in bottom oxygen concentrations was recorded from the same area (Andersson & Rydberg, 1988). Over the last decade many reports have documented worsened oxygen concentrations in stratified waters and other effects caused by eutrophication in the marine environment (e.g. Lindahl & Hernroth, 1983; Rosenberg, 1985; Ju~tid et al., 1987; Rosenberg & Loo, 1988; Josefson & Rosenberg, 1988). These results and the declining trends in oxygen concentrations shown in this paper cannot be caused by local inputs, but must be taken as signs of large scale eutrophication. Such deterioration in the coastal marine environment, demonstrated in areas known to be particularly sensitive or where monitoring is particularly careful, must be taken as serious warning signals. Other less sensitive areas are likely to be threatened in the near future unless the input of nutrients to coastal waters is significantly reduced. Technical and financial support was given by Gunnar Eriksson, Birthe Hellman, Lars-Ove Loo, Kerstin Rosenberg and the National Swedish Environment Protection Board. Comments on the manuscript were made by Drs. Alf Josefson, Odd Lindahl and Tom Pearson. I sincerely thank all of you.

Andersson, L. & Rydberg, L. (1988). Trends in nutrient and oxygen conditions within the Kattegat: effects of local nutrient supply. Estt~ar Coast. Shelf. Sci. 26,559-579. Anon. (1967-1974). Hydrographical data. Medd. Havsfiskelab. Lysekil (mimeo.), 38, 41, 80, 82, 83, 84, 85, 93, 104, 112, 116, 132,148, 160, 168. Anon. (1970). Hydrographical observations from the fjords of Bohusl/in during the years 1893-1966. Medd. Havsfiskelab. Lysekil 77. Anon. ( 1973-1984). Hyd rographical data. National Board of Fisheries, Sweden (mimeo.), 1-37. Anon. (1988). Guidelines for the Baltic monitoring programme for the third stage. Part B. Physical and chemical determinants in sea water. In Baltic Sea Environment Proceedings No. 27 B, Helsinki Commission, Helsinki. Anon. (1989). Miljranalys ,~lvsborgs Ifin. La'nsso'relsen, Viinersborg, Sweden (mimeo.). Bjrrck. W. (1913). Bidrag till k~innedomen om nordhafsriikans (Patldalus borealis Kr.) utbredning och biologi i Kattegatt och Skagerack. Svenska lqydrografisk-biologiska Kommissionens Skrifter 4, 1- 11. Bjrrck, G. (1983). Vattenutbyte och skikmingsfrrhS.llanden i fjordarna innanfrr Orust och Tjrrn. Oceaplografiska instittttionen, Unitersit.v of GiSteborg, Sweden. Cato, I., Mattsson, J. & Lindskog, A. (1986). Tungmetaller och petrogena kolv/iten i Brofjordens bottensediment 1984, samt frrg.ndringar efter 1972. University of Gdteborg, dept. of Marine Geology., Report A,i). 3, 1-97. Cederwall, H. & Elmgren. R. (1980). Biomass increase of benthic macrofauna demonstrates eutrophication of the Baltic Sea. Ophelia Suppl. 1,287-304. Elmgren, R. & Rosenberg, R. (1987). Hur mf.r havet? Forskning och Framsteg 7, 52-58. Fonselius, S. H. (1969). Hydrography of the Baltic deep basins 111. Fish. Bd Sweden. Ser. flydrogr., Rep. 23. Josefson, A. B. & Rosenberg, R. (1988). Long-term soft-bottom faunal changes in three shallow fjords, west Sweden. Neth. J. Sea Res. 22, 149-159.

Volume 21/Number 7/July 1990 Josefson, A. B. & Widbom, B. (1988). Differential response of benthic macrofauna and meiofauna to hypoxia in the Gullmar Fjord basin. Mar. Biol. 100, 31-40. Ju~ti6, D., Legovic, T. & Rottini-Sandrini, L. (1987). Trends in oxygen content 1911-1984 and occurrence of benthic mortality in the northern Adriatic Sea. Eustar. Coast. Shelf Sci. 25,435-445. Lindahl, O. & Hernroth, L. (1983). Phyto-zooplankton community in coastal waters of western Sweden--an ecosystem off balance? Mar. Ecol. Prog. Ser. 10, 119-126. Michanek, G. (1972). A review of world seaweed resources. In Proc. 7th Intern. seaweed syrup., Japan (1971), pp. 248-250. Pihl, L. (1989). Effects of oxygen depletion on demersal fish in coastal areas of the south-east Kattegat. In Reproduction, Genetics and Distributions of Marine Organisms (J. S. Ryland & P. A. Tyler, eds), pp. 431-439. Olsen & Olsen, Fredensborg, Denmark. Rosenberg, R. (1985). Eutrophication--the future marine coastal nuisance'? Mar. Pollut. Bull. 16,227-231. Rosenberg, R. (1976). Benthic faunal dynamics during succession following pollution abatement in a Swedish estuary. Oikos 27, 414427. Rosenberg, R. & Loo, L.-O. (1988). Marine eutrophication induced

oxygen deficiency: effects of soft bottom fauna, western Sweden. Ophelia 29, 213-225. Seliger, H. H., Boggs, J. A. & Biggley, W. H. (1985). Catastrophic anoxia in the Chesapeake Bay in 1984. Science 228, 70-73. S6derstrrm, J. (1971). The capacity of coastal waters to use nutrients in Bohusl~in, Sweden. Botanica Marina 14, 39-52. Stachowitsch, M. (1984). Mass mortality in the Gulf of Trieste: the course of community destruction. Mar. Ecol. 5,243-264. Svane, I. & Grrndahl, F. (1988). Epibiosis of Gullmarsfjorden: an underwater stereographical transect analysis in comparison with the investigations of Gislrn in 1926-29. Ophelia 28, 95-110. Svansson, A. (1975). Physical and chemical oceanography of the Skagerrak and the Kattegat. Fish. Bd Sweden. Inst. Mar Res. Rep. ,¥o. 1. Svansson, A. (1984). Hydrography of the Gullmar Fjord. Medd. Havsfiskelab. Lysekil. Nr 297. Wallin, M. &Oster. O. (1986). Sedimentologisk unders6kning av ndgra fjordar norr om Orust. Naturgeografiska instutionen, University of Uppsala. Winkler. L. (1888). The determination of dissolved oxygen in water. Berichte d. Deutch. chem. Gesellsch. 21, 28-43.

Ii Marine Pollution Bulletin, Volume 21, No. 7, pp. 339-342. 1990. Printed in Great Britain.

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Using Seabirds to Monitor Mercury in Marine Environments The Validity of Conversion Ratios for Tissue Comparisons DAVID R. THOMPSON, FIONA M. STEWART and ROBERT W. FURNESS

Applied Ornithology Unit, Department of Zoology, University of Glasgow, Glasgow G12 8QQ, Scotland, UK

The '7 : 3 : 1 rule' for converting mercury concentrations between feather, liver and muscle tissues was evaluated by measuring total and methyl mercury levels in liver, muscle and body feathers of a range of seabird species. Mean mercury concentrations were used to calculate feather:liver (both total and methyl) and feather:muscle ratios, the results obtained being compared with the predicted values of 2.3 and 7.0. Feather: liver ratios were found to approximate to 2.3 when liver methyl mercury concentrations were considered, but elevated inorganic mercury concentrations in the livers of some species resulted in greatly reduced feather:liver ratios for total mercury. Feather:muscle ratios varied from 3.8 to 15.3. Factors likely to affect the value of feather:liver and feather:muscle mercury concentration ratios, such as the predominant form of mercury present in the liver tissue, sampling date relative to the stage of the moult sequence and types of feather used for analysis, are discussed and we emphasize that the 7 : 3 : 1 conversion ratio used by a number of authors should be treated with caution.

Birds, especially top predators, have been widely used as monitors of a range of environments. Birds offer many advantages as monitors, not least of which is the possibility of using feathers to assess environmental levels of heavy metals. A recent review of the role of seabirds as monitors of metals in the marine environment has been provided by Walsh (in press). Methyl mercury is deposited into the growing feather, binding strongly to disulphide linkages (Crewther et al., 1965) and is unaffected by a variety of rigorous treatments (Appelquist et al., 1984). By measuring the mercury concentration of a representative sample of body feathers (Furness et al., 1986), one is able to assess inter-species, geographical and historical mercury level differences in large numbers of live birds. Many studies have demonstrated positive correlations between feather mercury concentrations and those in internal tissues (for example, Furness & Hutton, 1979; Hutton, 1981; Ohlendorf et aL, 1985). Indeed, several authors have claimed that there is a ratio of 7 : 3 : 1 for mercury concentrations (fresh wt) in feathers, liver tissue and muscle tissue, respectively 339