Temporal and Spatial Variability of Zoobenthic Communities in Archipelago Waters of the Northern Baltic Sea-Consequences of Eutrophication?

~ Int. Revueges. Hydrobiol. 1 76 1 1991 1 3 i 433449 L ~~

ERIK BONSDORFF, KATRI AARNIO & EVA SANDBERG

Department of Biology and Huso Biological Station Abo Akademi University, Abo, Finland

Temporal and Spatial Variability of Zoobenthic Communities in Archipelago Waters of the Northern Baltic Sea-Consequences of

Eutrophication?

key words; zoobenthos, spatial variability, temporal changes, eutrophication, Macoma balthica

Abstract

A large-scale spatial (52 sampling stations in two areas; one previously affected by numerous sources of enrichment, and one pristine area used as a standard reference for coastal monitoring) and temporal ( 1972-1 989) analysis of hydrography and benthic macrofauna in the archipelago waters of Aland, northern Baltic Sea, revealed significant changes of the ecosystem. The nutrient levels exceeded the criteria for eutrophicated waters at all stations, and the benthic macrofauna showed significant alterations in all parameters analyzed. In the area previously (1972-73) disturbed, reduced complexity in terms of diversity and evenness (i.e. functional responses) were recorded, whereas the changes in the previously undisturbed area are classified as structural. The main conclusions are that (a) no truly “natural” coastal reference areas can be found in the northem Baltic, and (b) the importance of the local effluents will add to the large-scale effects of eutrophication.

I . Introduction

The natural balance of the Baltic Sea has long been studied from many points of view (JANSSON 1980, LEPPAKOSKI 1980, VOIPIO 1981, ELMGREN 1984), as this semi-enclosed water-body is unique in many features, such as its low salinity, high relative proportion of limnic (river-) inflow, and marked seasonality. The monitoring schemes cover most aspects from hydrography and chemistry through most biotic levels, including zoobenthos (for a review, see LEPPAKOSKI & BONSDORFF 1989). The main efforts, however, have been on either the open sea (for instance within the framework of HELCOM; ANDERSIN et al. 1978, Baltic Sea Environment Commission-HELCOM 1986), or in polluted areas closer to the coasts. Only recently have coastal monitoring programmes gained in importance, as it has become clear that the relatively shallow areas (above the halocline) are highly productive and also the first areas to be affected by the indirect effects of the dif- fuse loads we call eutrophication (CEDERWALL & ELMGREN 1980, LARSON et al. 1985, ELMGREN 1989).

The present investigation aims at illustrating the use of soft-bottom zoobenthos in eva- luating the spatial distribution and effects of the long-term changes in the coastal ecosystem of the northern Baltic Sea. The sampling strategy used was to revisit a large number of sampling stations previously investigated to record any overall changes in time and space (CEDERWALL & ELMGREN 1980, PEARSON et al. 1985). Thus two areas in the Aland archipelago, northern Baltic Sea (Fig. 1) were chosen, as an extensive baseline study of hydrography and zoobenthos had been conducted there in 1972-73 (HELMINEN 1975). One area was already in the early 1970’ies considered organically enriched, whereas the

434 E. BONSDORFF et al.

60'15"

. ' Tr V . -

26"E

Figure 1 . The investigated areas in the h a n d archipelago, northern Baltic Sea. (A) reference area (NW Aland; stations V-Z exemplifying a transect through the area), and ( B ) F&jsundet-Lumparn (stations L 6-L 19 representing a transect through the area; station L11 is the site of the year-to-

year long-term analysis).

other is still used as a natural reference area for coastal monitoring in both Finland and Sweden. Furthermore, the zoobenthos of the Aland archipelago has been extensively investigated during the past decades, which allows us to correct for possible regular long-term fluctuations (HELMINEN 1975, WESTERBERC 1978, BONSDORFF et al. 1986, LEPPAKOSKI et al. 1986, BONSDORFF 1988).

2. Study Areas

The investigated areas (Fig. 1) are located in the archipelago of the &and islands in the central northern Baltic Sea (60" N, 20° E), on the border between the Baltic proper and the Bothnian Sea. The reference area (NW Aland; A in Fig. 1) is principally unaffected by industrial or municipal effluents, and only marginally influenced by runoff from land. In this area 24 stations (1-26 m depth) were sampled in May-June 1989, covering a

Variability of Zoobenthos in Archipelago Waters 435

range of habitats from the innermost sheltered bays with very soft sediments to the outermost archipelago zone bordering the open sea. This area is compared to Fiirjsundet-Lum- parn (€3 in Fig. I ) , situated in the eastern part of Aland, ranging from sheltered fjord-like bays to open areas with a maritime character (SANDBERG et af. 1989). In this area, classified as eutrophicated already in 1975, 28 sampling stations (2-36 m depth) were visited in May-June 1989. These areas have previously been described by HELMINEN (1975), LEPPAKOSKI et al. (1986), and SANDBERG et al. (1990).

3. Material and Methods

3.1. Hydrography and sediment properties

At each station, the basic hydrographic parameters (transparency, t OC, S %o, pH, oxygen saturation, and the main nutrients Tot-N and Tot-P) were analyzed from the surface (0.5 m) and the bottom near water (0.5 m above the sediment surface). The quality of the sediment was described, and the organic content measured as loss on ignition (3 h at 500 “C) after drying for 24 h at 60 OC (DYBERN et al. 1976).

3.2. Benthic macrofauna

In order to allow direct comparisons with the studies conducted in the early 1970’ies, the methods used for sampling and sorting the benthic fauna followed those used by HELMINEN (1975). At each sampling station 5 replicate grab samples were taken with an Ekman-Birge type hand-ope- rated box corer (17 x 17 cm). All samples were sieved on a 1.0 mm mesh size screen, and the resi- due was sorted alive in the laboraty immediately after sampling. For sorting each sample was pored on a white tray and the animals picked without the use of a microscope. Occasional checks of the remains were done under a dissecting microscope, as was all determinations to genera and species. All species were counted separately from each sample, and their wet weight was measured to the nearest 0.1 mg. For the bivalve Macoma hafthica the length-frequency distribution (1.0 mm clas- ses) was measured to get an impression of the age-distribution within each population. All faunal sampling was conducted in May-June 1989.

3.3. Numerical treatment of the data

In order to analyze and graphically visualize the spatial (within and between areas) and temporal (1972-73 to 1989, within and between areas) distribution of the zoobenthos, and to detect possible significant differences and changes, all data was treated using the following computer programmes: StatViewTM SE + Graphics for the Macintosh (Abacus Concepts, Inc.) and Cricket GraphTM v. 1.3 (Cricket Software, Inc.). To test for overall changes or differences a one-way ANOVA was used, coupled with Student’s t-test for individual comparisons of means. The Shannon-Wiener diversity (H’; H’>O) and it’s evenness component ( J ; 0 6 J S 1) were calculated using the programme Mac- Divind for Macintosh (courtesy of Mr. HANS G. HANSSON, T j h o Marine Biology Laboratory, Sweden). The number of replicates and stations sampled individually provided a sufficient basis for the numerical treatment at the levels of precision chosen (WESTERBERG 1978, DOWNING 1989).

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4. Results

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4.1. Hydrography and sediment properties

The physical and chemical conditions in the bottom near water and the sediment in the reference area (Fig. 1) are shown for a transect ranging from the inner to the outer parts of the archipelago (Fig. 2). The organic content of the sediment (1.18-10.0 %; mean value: 6.86%) showed no significant trend in this area, and all values fall within the

’ 0 Oxygen (%I TOI-P WI) TOI-N (pgiyio

V T A A l - B S 0 P X Z

Figure 2. Physical and chemical parameters in the bottom near water and the organic content of the sediment in NW Aland (area A;

stations V-Z).


natural range of the archipelago areas. The transparency (given as Secchi-depth) increased towards the open sea, with a total range of 1 to 4.5 m (mean value: 2.5 m). This gradient was aIso reflected in the basic hydrographical parameters, with a slight reduction in pH-values towards the sea, and simultaneously increasing salinity (2.80 to 5.86 %O S ; Fig. 2). Oxygen saturation in the bottom water was high (mean value 93.48 %). The nutrient content of the bottom water was surprisingly stabile throughout the transect, with Tot-N ranging from 289.45 to 547.03 pg/l (mean value: 454.21 pd), and Tot-P from 24.64 to 53.65 pg/l (mean value: 36.27 pg/l). The relatively high phosphorus content at all stations falls clearly above the limits for eutrophicated waters as classified by the Finnish National Board of Waters (1985).

For the area known to be affected by various effluents already in the early 1970’ies, the hydrographical and chemical conditions followed somewhat similar trends (Fig. 3). The organic content of the sediment ranged from 5.27 to 9.17 % (mean value: 9.32 %), with no significant differences within the area, but with higher average-values than in the reference area (Table 1 ; p < 0.1). The transparency of the water increased outwards, and

Table 1. Comparison of the physical and chemical parameters of the bottom near water, the organic content (%) of the sediment, and number of species (S , abundance ( A ;

Farjsundet-Lumparn 1989. All values are means of all investigated stations for each area. *=p<O.I, **=p<O.O5 and ***=pcO.Ol, n.s.=nosignificant difference.

ind/m’) and biomass ( B ; ind/m2) in the zoobenthic community in NW d land 1989 and in

NW-hand

Depth (m) 12.7 n.s. Transparency (m) 2.5 n s .

7.93 PH Oxygen (%) 93.48

36.27 ** T0t-P Tot-N (lLg/l) 454.2 I n.s Organic content (%) 6.86 n.s No. of Species (S) 6.5 * * Total abundance (A) 1954 n.s Total biomass (B) 119.4 * *

* * * * *

Farjsundet-Lumpam

10.8 2.5 7.6 I

80.54 62. I9

494.7 1 9.32 3.8

1599 67.3

ranged from 1.5 to 5.9 m (mean value: 2.5 m; no significant difference between areas), and the basic hydrographical and chemical features showed similar trends as for the reference area (Fig. 3), with significantly lower pH-values (Table 1; p c 0.01), indicating an increased oxygen consumtion in the sediments. Average oxygen saturation in the bottom near water was significantly lower (Table 1 ; p c 0.05) than in the reference area (mean value: 80.54 %), indicating a higher organic load, as reflected in the organic content of the sediment. The content of basic nutrients in the bottom water showed a decreas- ing trend from the inner bays to the open areas (Tot-N: 321.32 to 498.60 pg/l; mean value: 494.71 pg/l, and tot-P: 30.76 to 92.62 pg/l, mean value: 62.19 pg/l). Although the nitrogen-values were higher in F&jsundet-Lumpam than in N W Aland, no statistically significant difference was recorded, due to the large variability between stations within each area. For phosphorus, however, the values in the reference area were significantly lower than in Lumparn-Fajsundet (Table 1; p < 0.05).

29 Int. Revue ges. Hydrobiol. 76 (1991) 3


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Figure 3. Physical and chemical parameters in the bottom near water and the organic content of the sediment in Farjsundet-Lumparn

(area B; stations L 6-L 19).

4.2. Benthic macrofauna

The basic community data (number of species, total abundance and total biomass) for both investigated areas is presented in Figs. 4 and 5. In this presentation a direct comparison to the situation in 1973 is given. In both areas the overall complexity of the community has decreased in form of a significantly reduced number of species present (NW-

Variability of Zoobenthos in Archipelago Waters

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Figure 4. Number of species (S), total abundance (A) and total biomass ( B ) in NW Aland in 1989 and 1973 (sta-

tions V-Z).

Aland 1989: 2: 6.5 spp, 1973: R: 7.9 spp; p=0.025, and Farjsundet-Lumpam: 1989: 1: 3.8 spp, 1973: R: 5.1 spp; p=O.O19). The difference between areas in 1989 was also significant, with a more complex'community in NW Aland (Table 1, p < 0.05).

The total abundance of the zoobenthos had increased significantly in NW Aland (1954 ind/mz 1989 vs. 1195 ind/mz 1973, p = 0.037; Fig. 4), whereas the increase in Fkjsundet- Lumpam, although important, was not statistically significant (1559 ind/m* 1989 vs. 899 ind/rnz 1973, p = 0.066, Fig. 5). The pattern is similar in both areas with an increase in total abundance, and no significant differences were recorded between areas in 1973 or 1989 (Table 1.).

Total community biomass increased significantly between 1973 and 1989 in the refer- 29.


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Figure 5. Number of species (S), total abundance (A ) and total biomass ( B ) in Farjsundet-Lumparn in 1989 and 1973 (stations L 6-L 19).

ence area in NW h a n d (1 19.4 g wwt/m2 1989 vs. 55.3 g wwt/m2 1973; p = 0.002). mainly due to the overall increase in abundance and the large proportion of bivalves (Maco- ma balthicaj present in the system. In Farjsundet-Lumpam a slight, but not significant, increase was observed (67.3 g wwt/m2 1989 vs. 43.8 g wwt/m2 1973; p=O.162). In the previously eutrophicated area, the biomass in 1989 remained on a significantly lower level than in the previously unaffected reference area (Table 1 ; p < 0.03, which indicates that the latter at present is in a transistory phase (sensu PEARSON & ROSENBERG 1978). The spatial distribution of the biomass-increase within areas shows significantly increasing trends towards the open coast (Figs. 4 and 3, which indicates that the changing environmental factors causing these large-scale changes at community level are imported

Variability of Zoobenthos in Archipelago Waters 44 1

to the local ecosystem. Thus they are part of an even more extensive process possibly affecting the entire coastal ecosystem of the northern Baltic Sea (CEDERWALL & ELMGREN 1980, ELMGREN 1989).

The pattern in both areas is similar for all measured parameters, with a clear trend towards an impoverished community with few species, high abundance and high biomass (Fig. 6) . This trends is further underlines by the structural changes in the species compo-

l o 1 t t

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Figure 6. Comparison of n. s. number of species, abundance and biomass of all stations in NW h i n d 1973/ 1989 and in Farjsundet- Lumparn 1973/1989. n s . = no significant difference, * =p<O.I , * * =p<0.05

L 73 L 89 ‘and*** =p<O:OI

sition that have taken place between the sampling years. In the reference area in NW Aland a shift in the relative importance of species is evident with a dominance of stress- tolerant species (sensu LEPPAKOSKI 1975), such as chironomid larvae and Macoma


balthica, whereas the dominating amphipod in the system (Pontoporeia afinis) has been pushed towards the open coast (Fig. 7). In the locally polluted area, Farjsundet-Lumparn,

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Figure 7. Relative dominance of species in the zoobenthic community in NW .hand, a) 1973, b) 1989.

this trend is even more obvious, in that the transect from the inner parts towards the open areas is divided in zones dominated by chironomids, M . balrhica and in the outermost parts, P. affinis (Fig. 8).

The structural changes described above also manifest themselves in terms of functional responses. Thus the diversity and evenness (i.e. the relationships between the occur- rence of a species and its numerical importance) in NW h a n d have largely evened out over the entire transect, with the exception of the innermost parts of the system, where the community has been impoverished in all parameters (Fig. 9). Thus no significant changes were recorded in diversity (mean-H’ 1.499 in 1989 vs. 1.678 in 1973; p = 0.564) and evenness (mean4 0.559 in 1989 vs. 0.594 in 1973; p=0.707). This functional response to the changes in the community are even more pronounced in F&jsundet-


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L6 L7 L8 L12 L18 L19 1989

Figure 8. Relative dominance of species of the zoobenthic community in Farjsundet-Lumpam, a) 1973, b) 1989.

Lumpam, where the overall changes in diversity (mean-H 1.029 in 1989 vs. 1.696 in 1973; p = 0.01 1) and evenness (mean4 0.455 in 1989 vs. 0.703 in 1973; p = 0.029) are statistically significant (Fig. lo). For both functional parameters measured the temporal trends have been similar in both areas investigated, and no significant differences between areas could be noted in 1989 (H’: p = 0.168, and J : p = 0.423).

At population-level, the impact of eutrophication can be illustrated with the relative size-distribution of the bivalve Macoma balthica (Fig. 11). At two localities close to each other in the polluted area, the population at the outer station (St. L 18) displayed a cha- racteristic age distribution with mainly juvenile and early adult stages (2-9mm; 1-3 years), whereas the inner station (St. L 12) showed a largely opposite distribution. At this site the population was senile with few young animals, and the main size interval1 ranged from 10 to 19mm (4-6 years old mussels). In this case the recruitment is poor in the inner parts of the gradient, and with time the population can be expected to disappear completely (cf. Fig. 8). Thus the zone totally dominated by chironomid larvae will extend

444 E. BONSDORFF ef al.

V T A S O P X Z

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!c l , o

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Figure 9. The diversity (H') and evenness (4 of the zoobenthic

V T A S 0 P X 2 community inNWAland, a11973, Station b) 1989.

even further, indicating the gradual migration of the entre community as the importance of stress increases with time (LEPPAKOSKI 1975). In NW h a n d the Macoma-population at all localities found still displayed a size-distribution indicative of active recruitment in form of annual settling of juveniles.

On a more detailed temporal scale this type of development can be illustrated for one sampling site (St. L 11 in Farjsundet-Lumparn), where the gradual increase in organic load of the water has been followed since 1972 (Fig. 12). In this case the beginning of the deterioration of the community is seen already in the mid 1970-ies, with a slight period of recovery in 1985-88. Even if the data in all community parameters analyzed show large interannual variability, the trend is evident, and this example illustrates the gradual shift in community characteristics from an overshoot in early eutrophication towards a total destruction.

5. Discussion

The differences between areas recorded in hydrographical, chemical and biological parameters, partly support the interpretation that local sources play an important role in the structuring of the aquatic ecosystem. Thus changes over time at one site (Fig. 12), although significant, may not reflect anything else than a very local process. At population level (i.e. Macoma balthica in this case) local changes in the size-structure may reflect adaptive responses to truly altered conditions (MOLSA er al. 1986, BONSEORFF & WENNE 1989).


L6 L7 LB L12 L18 L19

L6 L7 LB L12 L18 L19 Station

Figure 10. The diversity (14') and evenness (J) of the zoobenthic community in F&jsundet- Lumpam, a) 1973, b) 1989.

15 * L

E al ;3 10 g c al 'P m - $ 5

0- . - . - . . . . . . . . . . . 1 2 3 4 5 6 7 8 91011121314151617181920212223

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Figure 1 1 . The relative size-distribution of Macoma balthica at two localities (L 12 and L 18) in Farjsundet-Lumpam in 1989.


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Figure 12. Long-term (1972-89) changes in the zoobenthic community (species, abundance and biomass) at station L 1 1 in F&jsundet-Lumpam.

The alternative strategy is to sample many stations (in this case a total of 52 stations) over a large area with long time-intervals between samplings. In order to correctely quan- tify changes, the sampling regime should involve both adequate number of samples and stations (DOWNING 1989). Any overall changes recorded will then reflect processes on a much larger scale (CEDERWALL & ELMGREN 1980, PEARSON et al. 1985), although the detailed processes may remain largely unknown. LEPPAKOSKI (1975) and PEARSON & ROSENBERG (1978) gave the principal framework to analyze changes in benthic communities along either temporal or spatial gradients in connection with organic enrichment or


pollution, but both concepts still aim at interpreting mainly local changes. LEPPAKOSKI & BONSDORFF (1989), in an attempt to separate the effects of pollutants from natural environmental gradients, clearly demonstrated the need of large-scale analysis. This type of overall comparisons have been undertaken for the entire Baltic Sea (ELMGREN 1989), but the level of uncertainty in such an analysis remains high, as only few comparative studies have yet been undertaken (e.g. CEDERWALL & ELMGREN 1980, JARVEKULG & SERE 1985, KARJALA & LASSIG 1985, ANIERSIN 1986).

In the present analysis, the area analyzed is large and homogenous enough to allow rigid statistical analysis and an interpretation at both local and regional level. Thus in this case the drastic local changes in time cannot be explained only by an increased local influence of pollution and/or eutrophication, as the changes recorded cover the entire set of stations throughout the environmental gradients studied. As the background levels of nutrients are high, and the general changes in the zoobenthic communities agree with the predictive models suggested by PEARSON & ROSENBERG (1978) and PEARSON ef al. (1985), our conclusion is that the changes recorded reflect a true alteration of the entire coastal ecosystem of the northern Baltic Sea. This, however, indicates that the vector causing the ecosystem to change is imported from elsewhere. In extension, this has to be interpreted as a warning signal that the entire Baltic ecosystem is undergoing drastic changes caused by man, as similar trends are recorded in many areas of the Baltic above the halocline (LARSSON er al. 1985, ELMGREN 1989), and the opposite is taking place below it (ANDERSIN ef al. 1978, LEPPAKOSKI 1980).

6. Summary

Macrozoobenthos was used in a large-scale survey (52 sampling stations) to estimate the present status of the archipelago area of the h a n d islands (northern Baltic Sea). Transects sampled in 1989 were compared to data from 1972-73 (regional analysis), and it was shown that, in spite of point-sources of pollution or eutrophication significantly affecting the biota at the local level, the major change has occurred at the regional level. Thus all primary community parameters registered (number of species, total abundance, total biomass) showed significant changes in both major areas covered, and in a previously affected area these changes manifested themselves also at the functional level in terms of altered diversity and evenness. It is concluded that the ultimate reason for these extensive changes are to be found in the overall changes in the hydro-chemistry of the Baltic Sea, which can no longer be limited to any specific region. This implies that the entire Baltic ecosystem may well be undergoing changes that can no longer be easily altered, and the concept of reference areas may have to be altered, and new baselines set for future research.

7. Acknowledgements

We thank Huso biological station (Abo Akademi University) for placing excellent working faci- lities at our disposal, and for letting us use unpublished background data. The Academy of Finland, the Provincial Government of Aland, and the Maj and Tor Nessling Foundation are gratefully ack- nowledged for financial support.

448 E. BONSWRFF et al.

8. References

ANDERSIN, A.-B., 1986: The question of eutrophication in the Baltic Sea-results from a long-term study of the macrozoobenthos in the Gulf of Bothnia.-Publ. Water Res. Inst. Helsinki, 68:

ANDERSIN, A.-B., J. LASSIG, L. PARKKONEN and H. SANDLER, 1978: The decline of macrofauna in deeper parts of the Baltic proper and the Gulf of Finland.-Kieler Meeresforsch., Sonderh. 4:

Baltic Sea Environment Protection Commission-HELCOM, 1986: Baltic Sea Environment Proc.

BONSDORFF, E., 1988: Zoobenthos and problems with monitoring: an example from the &and area.-Kieler Meeresforsch., Sonderh. 6: 85-98.

BONSDORFF, E., E. LEPPAKOSKI and C.-S. OSTERMAN, 1986: Patterns in postimpact successions of zoobenthos following physical and chemical disturbance in the northem Baltic Sea.-Publications of the Water Research Institute, National Board of Waters, Finland, nr. 68, pp. 117-121.

BONSDORFF, E., and R. WENNE, 1988: A comparison of condition indices of Macoma balrhica (L.) from the northem and southern Baltic Sea.-Neth. J. Sea Research 23: 45-55.

CEDERWALL, H., and R. ELMGREN, 1980: Biomass increase of benthic macrofauna demonstrates eutrophication of the Baltic Sea.-Ophelia, Suppl. 1: 287-304.

DOWNING, J. A., 1989: Precision of the mean and the design of benthos sampling programmes: cau- tion revised.-Mar. Biol. 103: 23 1-234.

DYBERN, B. I., H. ACKEFORS and R. ELMCREN (eds.), 1976: Recommendations on methods for marine biological studies in the Baltic Sea.-Baltic Marine Biologists Publ. 1: 1-98.

ELMCREN, R., 1984: Trophic dynamics in the enclosed, brackish Baltic Sea.-Rapp. P.-v. RCun. Cons. int. Explor. Mer. 183: 152-169.

ELMCREN, R., ,1989: Man’s impact on the ecosystem of the Baltic Sea: energy flows today and at the turn of the century.-Ambio 18: 326-332.

Finnish National Board of Waters, 1985 : Saaristomeren kalankasvatustoimintaa koskeva vesien- suojelusuunnite1ma.-Rep. No. 374.

HELMINEN, 0.. 1975: Bottenfaunan i den Mandska skarg%rden.-Medd. Huso biol. stat. 17: 43-7 1. JANSSON, B.-O., 1980: Natural systems of the Baltic Sea.-Ambio 9: 128-136. JARVEKULG, A., and A. SEIRE, 1985: Long-term changes in the bottom fauna of Tallin Bay and their

KARJALA, L., and J. LASSIG, 1985: Studies on the benthic macrofauna in the Tvarminne area, Gulf

LARSSON, U., R. ELMCREN and F. WULFF, 1985: Eutrophication and the Baltic Sea: causes and

LEPPAKOSKI, E., 1975: Assessment of degree of pollution on the basis of macrozoobenthos in

LEPPAKOSKI, E., 1980: Man’s impact on the Baltic ecosystem.-Ambio 9: 174-181. LEPPAKOSKI, E., E. M. BLOMQVIST and E. BONSDORFF, 1986: The &and archipelago as a reference

for coastal monitoring of the Northern Baltic Sea.-Baltic Sea Environment Proc. 19: 521-530. LEPPAKOSKI, E., and E. BONSDORFF, 1989: Ecosystem variability and gradients. Examples from the

Baltic Sea as a background for hazard assessment.-In: L. LANDNER (ed.), Chemicals in the aquatic environment. Advanced hazard assessment. Springer-Verlag, Berlin, Heidelberg, pp. 6-58.

MOLSA, H., S. HAKKILA and M. PIJHAKKA, 1986: Reproductive success of Macoma balfhica in relation to environmental stability.-Ophelia, Suppl. 4: 167-177.

PEARSON, T. H., A. B. JOSEFSSON and R. ROSENBERG, 1985: Petersen’s benthic stations revisited. I. Is the Kattegatt becoming eutrophic?.-J. Exp. Mar. Biol. Ecol. 92: 157-206.

PEARSON, T. H., and R. ROSENBERG, 1978: Macrobenthic succession in relation to organic enrichment and pollution of the marine environment.-Oceanogr. mar. Biol., Ann. Rev. 16: 229-3 11.

SANDBERG, E., K. AARNIO and E. BONSWRFF, 1989: Bottenfaunans utveckling i nordvhtra &and och i Lumparn-omrAdet-en jamforelse av situationen 1972-73 och 1989.-Rapport till Alands landskapsstyrelse, Nr. 71,44 pp.

SANDBERG, E., K. AARNIO and E. BONSDORFF, 1991: Long-term changes in the archipelago waters of hand-a comparison of the bottomfauna in 1972-73 and 1989.-Memoranda SOC. Fauna Flora Fennica (in print) (In Swedish with English abstract).

1 02- 1 06.

23-52.

19: 1-553.

causes.-Hydrobiol. Res. 15: 148-154.

of Finland, 1964-1967 and 1973-1976.-Hydrobiol. Res. 15: 169-181.

consequences.-Ambio 14: 9-14.

marine and brackish-water environments.-Acta Acad. Aboensis, Ser. B, 35 ( 2 ) : 1-96.


VOIPIO, A. (ed.), 198 1 : The Baltic Sea.-Elsevier Scientific Publishing Company, Amsterdam,

WESTERBERG, J., 1978: Benthic community structure in the Aland archipelago (N Baltic) represent-

ERIK BONSDORFF, KATRI AARNIO & EVA SANDBERG Department of Biology and Huso biological station Abo Akademi University SF-20500 Abo, Finland

418 pp.

ed by samples of different sizes.-Kieler Meeresforsch. Sonderh. 4: 53-60.

Documents

Temporal and Spatial Variability of Zoobenthic Communities in Archipelago Waters of the Northern Baltic Sea-Consequences of Eutrophication?