HELGOLJ~NDER MI'ERESI.JNTERSUCHUNGEN E lelgol,~nder b,'leeresunters. 5 I, 399-416 (1997)

Long-term changes in the benthic communities of the Pomeranian Bay (Southern Baltic Sea)

J. K u b e ~* , F. G o s s e l c k 2, M . P o w i l l e i t ~ & J. W a r z o c h a 3

~Inst i tute of Balt ic Sea Research , S e e s t r a B e 15, D - I 8 I 19 Rostock , G e r m a n y

2Ins t i tu te of A p p l i e d Ecology, L i n d e n w e g 2, D - 18184 N e u Broders tor f , G e r m a n y

r Fisher ies Ins t i tu t e Gdyn ia , ul. Kol la to ja 1, PL - 81-332 Gd~.'nia, Po land

ABSTRACT. Long-term changes in the macrofauna of the Pomeranian Bay were studied by com- paring survey data from the 1950s, 1980s, and 1990s. The studv area has undergone significant eutrophication during the period of investigation. Biomass of filter-feeding blwdves increased sig- nifi~antly. Spatial distribution patterns of several species have changed. Strong decreases in species richness were caused by oxygen depletion at stations deeper than 15 m. Saduria entomon, .Monoporeia u/finis, and Ponloporeiu femorata vanished entirely between t981 and 1993. Although a causal relationship between simultaneous increases of nutrient levels and macroben- !hic biomass cannot be veri.~ied, eutrophicat!on is proposed to be the ma~or process affectip,~ changes in macrofauna assemblages. In addition, changes in hydrography dnd climate increased frequency and seventy of oxygen depletion events in the Pomeranian Bay since the mid 1980s.

I N T R O D U C T I O N

Seve ra l a t t e m p t s h a v e b e e n m a d e to exp la in l o n g - t e r m c h a n g e s in the macro - z o o b e n t h o s of the Baltic Sea. S e v e r e b iomass r educ t ions or e v e n m a c r o f a u n a d e a t h

w e r e of ten o b s e r v e d b e l o w the ha loc l ine (I-leip, 1995). A g e n e r a l c h a n g e from b iva lve - d o m i n a t e d towards p o l y c h a e t e - d o m i n a t e d c o m m u n i t i e s was d o c u m e n t e d in the d e e p e r

par is of the s o u t h e r n Baltic (Anders in et al., 1978; H E L C O M , 1990). T h e o x y g e n con-

cen t ra t ion of the d e e p e s t wa te r l eve l had s ta r ted to d e c r e a s e by the end of the 19th cen tu ry and fell to a lmos t neg l i g ib l e c (mcen t ra t ions in all d e e p e r bas ins in the ear ly

1990s. This anoxia has b e e n a t t r ibu ted to bo th m a n - m a d e eu t roph i ca t i on and c l imat ic c h a n g e s (e.g. Fonsel ius , 1972; G a r g a s et al., 1978; Ger lach , 1994).

M a n y au thors h a v e also d e s c r i b e d a s ign i f i can t inc rease in m a c r o f a u n a b iomass a b o v e the ha loc l ine as a c o n s e q u e n c e of h i g h e r food supp l i e s (Pearson & R o s e n b e r g ,

1978; C e d e r w a l l & Ehngren , 1980; Brey, 1986). H o w e v e r , s ince 1980 o x y g e n d e p l e t i o n

has also caused s t rong f luctuat ions in the ben th i c b iomass of severa l sha l low bays dur- ing per iods of s t rong s trat i f icat ion (Gosse lck & Georg i , 1984; Weige! t & Rumohr, 1986).

It is s u p p o s e d that the o b s e r v e d r e m a r k a b l e c h a n g e s in c o m m u n i t y compos i t i on w e r e

c a u s e d by an inc rease in o rgan ic inpu ts to the s e d i m e n t d u e to e u t r o p h i c a t i o n a n d a c h a n g e of h y d r o g r a p h i c condi t ions (Weigel t , 199l ; Prena, 1994).

�9 Addressee for all correspondence.

[) Biologlsche Anstalt Helgoland, Hamburg

400 J. Kube, K Gosse/ck, 3,1. Powillelt & .l. Warzocha

This paper describes long-term changes in the macrozoobenthic communi t ies of the Pomeranian Bay (southern Baltic Seal by compdring the 1950s with the I980s and 1990s. The Pomeranian Bay is a shallow transition zone between the Oder Estuary and the deeper Arkona and Bornholm Basins (Fig. 1). Due to its topography and hydrography, recent macrofauna communit ies are influenced by both an increase in organic riverine loads and severe oxygen depletions in the deeper basins (Powilleit et al., 1995; Kube et al., 1996). The aim of our study is to discuss the long-term changes in the macrozooben- thos in relation to fluctuations and long-term trends in the environmenta l conditions.

MATERIAL AN[) METHODS

We compared original data sets from studies carried out from 1955 to 1958 (Lowe, 1963), [rom I980 to 1982 (Gosselck, 1985; Warzocha, 1995) and in 1993 (Powilleit et a l , 1995]. In all studies a heavy, 0.1 m 2 Van Veen grab, a sieve with l-ram mesh size, and 4% Formalin for storage were used. All investigators collected three samples at each station and date. Unfortunately, we do not know how much grabs used dur ing different periods of investigation differed regarding the penetrat ion depth and the strength of shock wave.

Fig. 1. Map of the area under investigation

Long-term changes in the benthic communities of the Pomeranian Bay 401

Table l. Number of stations sampled in different habitat types of the Pomeranian Bdy during three investigation periods

Depth Area Number of stations

Lowe (1963) C3osselck (1985) Powilleit et al. Warzocha (1995) (1995)

5-I0 m Oder Bank 7 2 3 10-15 m near shore 5 5 8 10-20 m off shore 23 7 17 20 m Sassnitz Deep 1 I - 2 20-30 m Arkona Basin 10 7 5 20-30 m Bornholm Basin 2 2

The data sets compared here are based on 58 stations sampled by L6we (1963), 23 stations samp!ed by Gosselck (1985) and Warzocha (199.'5) and 35 stations sampled by

Powilleit et al. (1995). Although all three station grids covered the main habitat types of the study area, sampling sites diltered be tween investigation periods (Table 1, Fig. 5).

The border between Germany and Poland runs through the middle ol the study area and,

therelore, till investigations were restricted either to the German or Polish part of the bay.

Samples were mainly collected outside of the reproduction time in Apri l /May and October /November . About 20% of the samples were taken in July/August . However,

the influence of high spat densities on species abundances is almost negligible, because most of the juveniles pass through the meshes of the l-ram sieve (Powilleit et al., 1995).

The Pomeranian Bay harbours about 40 different macrofauna species (Kube et al.,

1996) but abundances of only 14 species could be used for a cluster analysis. The patchiiy distributed biue mussel, lMytiM~ edulis, and all associated crustaceans were

excluded from the cluster analysis because they cannot be sampled precisely with only three replicate grabs. Due to the patchy distribution of Mytilu.s-clunlps, relative 95%- confidence limits were greater than 100% of the mean values (Powilleit et al., 1995). We also had to exclude all small polychaetes and the mud snail HydrobJa ulvae from the

cluster analysis because these species were not counted by L6we (1963) and Warzocha (1995). Furthermore, three infaunal species (Hediste diversicolor, ~Marenzelleria viridis, ,Mya arenaria), burying deeper than 5 cm, were excluded from the cluster analysis.

Howe.ver, the species that could be used for the cluster analysis are representa t ives

of all major habitat types of the study area. i.e. the deeper parts along the slopes of the adjacent Arkon,J and Bornholm Basins, the exposed shallow sandy central parts, and the shel tered river mouth (Table 2).

A b u n d a n c e data were also used to plot species distribution maps. The maps were computed by kriging (Cressie, 1991).

Biomass comparisons were restricted to the bivalves ,~1. edulis, NIacoma balthica, and NI. arenaria. These species account for more than 90% of the total zoobenthic biomass (Kube et al., 1996). They are character ized by d longevity of more than ten years and a low P/B ratio (Kube, 1996b).

Bivalve biomass data at LOwe (1963) and Powilleit et al. (1995) were recalculated by allometric shell length-ash free dry weight (AFDW) relationships for samples taken

402 J. Kube, F. Gosselck, M. Powilleit & J. Warzocha

O

�9

a~

e~

q~

%

u~

Cq

C4

t ~

CO

~ & ~ ~ ~ = = . . . . ~ S ~ ~ ~ o

F~

Long-term changes in the benthic communities ot the Pomeranian Bay 403

in April 1993 (Kube, 1996b). Biomass data of Gosselck (1985) and Warzocha (1995)

were excluded from the analysis, because they had not measured the shell length of

bivalves. As already mentioned, the patchily distributed, mobile Mytilus-clumps cannot be

sampled precisely with less than 10 grab subsamples. The data set of L6we (1963) pro-

vided enough paral lel grab samples per cluster group to est imate biomass means. Biomass data from Powilleit et al. (1995) were val ida ted by addit ional d redge samples

and video observat ions (Kube, 1996b). Biomass values of NI. arenaria obta ined from Van Veen grab samples represent

only about 70% the standing stock (Powilleit et al., 1995).

LONG-TERM VARIABILITY OF ENVIRONMENTAL FACTORS

O c e a n o g r a p h y

The water column in the Pomeranian Bay is usually well mixed down to a depth of

about 15 m by wind forcing, As a consequence , fresh water supplied by river runoff affects the entire water column (Trzosinska & Cyberska, 1992). There is a strong nega-

Fig. 2. Intensity index of inflows of highly saline water into the Baltic Sea (annual maximum) between 1946 and I993 (redrawn from Franck & Matth~ius, 1992, see MatthSus & Franck, 1992 for calculation of the index). Asterisks indicate benthos investigations. Values above the asterisks give the total number of major inflow events for the ten year periods preceding each benthos

investigation

404 J. Kube, F. Gosselck, M. Powilleit & J. Warzocha

tive relationship be tween runoff data of the Oder River and annual salinity means

(1950-1992, r = -0.7, p < 0.05, n = 41). Annual salinity means showed no trend within the last 40 years. Values fluctuated

be tween 6-8','~ near the mouth of the Oder and 7-97~ on its banks INehrinq, 1990;

Cyberski, 1992). A strong vertical salinity gradient occurs at the steep northern edges of the bay.

The 10~, halocline is known to fluctuate there be tween 15-35 m depth near the Arkona Basin and 40-60 m depth ne.ar the Bornholm Basin. Changes in halocline depth depend

mainly on the f requency and intensity of the inflow of highly saline water from the Kat-

tegat into the Baltic Sea (Nehring et al., 1994; Matthaus & Lass, 1995). Occasionally,

upwel l ing- l ike events are responsible for an uprising of the halocline. As a result, deep

water from the Arkona Basin propagates into the bay via the Sassnitz Deep. Such

upwel l ing events somet imes cause strong stratification in the western part of the

Pomeranian Bay at depths be tween 10 and 15 m (Lass, pers. comm.). The oxygen of the highly saline deep water, inflowing from the North Sea during

the winter, is deple ted during periods of stagnation. Hence, the oxygen concentrat ion

of the deep water below the halocline depends on both, on biological activity and on

the f requency and intensity of major inflows. Until the 1970s, major inflows were

observed more or less regularly (Fig. 2). Seven teen major inflow events were observed within a period of ten years preceding the invest igat ions of L6we (1963), and e leven

inflows occurred within the ten years before the studies carried out by Gosselck (1985)

and Warzocha (1995). A long lasting s tagnat ion period since 1983 resulted in ex t reme

decreases of oxygen concentrat ions in the Bornholm Basin and, during summer, even in the Arkona Basin (Franck & Matth~ius, 1992; Nehr ing et al., 1994). This stagnation

period was interrupted in January 1993 (Matth~ius & Lass, 1995).

T e m p e r a t u r e

Fluctuations of annual means of water tempera ture in the Pomeranian Bay are

strongly related to the severity of the preceding winter season (Cyberska, 1992). Although the overall trend of the severeness index of winter seasons is negat ive

(1946-1993, r = -0.21, p >0.05, n = 49), means of severeness indices of a period of ten

winter seasons before the beg inn ing of the invest igation did not differed (Fig. 3). Differ- ences occurred in the course of the severeness index over the three ten year periods.

Whereas the invest igat ions of L6we (1963), Gosselck (1985), and Warzocha (1995~ fol-

lowed a period of cold winters p receded by a period of mild winters, opposi te condi-

tions preda ted the invest igat ion of Powilleit et al. (1995).

F o o d s u p p l y

The nitrogen input of the Oder River increased from 10000-20000 t �9 a -1 in the 1960s to 70000-80000 t �9 a Z in the 1990s (Cyberska et al., 1992, 1993; Lampe, 1993). Surface

chlorophyll a concentrations have doubled in the whole study area be tween 1970 and

1990 (Scbulz & Kaiser, 1986; Nakonieczny et al., 1991; Renk, 1992). The increase in phyto-

plankton biomass resulted in a decreasing water transparency and a heavy loss of macro-

phytobenthos in the adjacent Greifswalder Bodden (Messner & Von Oertzen, 1991}.

Long-term changes in the benthic commumties of the Pomeranian Bay 405

Fi(j. 3. Index ot the severity of the winter seasons between 1946/47 cind 1!)94/95 (sum of minus degrees Iddily meansl per winter season, data from the Meteorological Station Warnem/inde, F.R.G.). Thin horizontal line shows the long-term average. Thick horizontal lines show the average

for the ten-year periods preceding ectch benthos investigation

Furthermore, an increasing accumulat ion of organic matter was observed in sedi-

ments of the shel tered Greifswalder Bodden and in the Sassnitz Deep (Lampe, 1993; Leipe et al., 1995; Nausch, pets. comm.). However, no accumulat ion of organic matter

was found for sandy sediments of the Pomeranian Bay above the 15 m isobath (Neu-

mann & Bublitz, 1969; Koine, 1995).

RESULTS

L o n g - t e r m c h a n g e s of t h e m a c r o z o o b e n t h o s a s s e m b l a g e s

Eight groups of stations were separa ted by cluster analysis at a 45% similarity

threshold and were computed to spatial distribution maps for all three invest igat ion periods (Figs 4 and 5). The first group combines stations with a high species richness in

the southern Arkona Basin. The second group covers shallow coastal locations near the

Oder Mouth and near the Greifswalder Bodden. Group three is separa ted into two main sub-groups 3a and 3b, represent ing exposed sandy stations on the shallow Oder

Bank and its deepe r surroundings, respectively. The fourth group contains deeper sites

in the southeast of the bay, which are character ised by low densities of characterist ic

406 J. Kube, F. Gosselck, M. Powilleit & J. Warzocha

Fig. 4 I)endrocjram oi the 116 stations using group-avera(je clustering trom Bray-Curtis simildrities on square root-transtormed abunddnces (presence > 1%). The eicjht groups of stations separated at a 45"',, threshold are indicated (symbols and numbers}. Additiondlly, the third station group was

divided into the sub-groups 3a and 3b

species of the sand bottom community (Bathyporeia pilosa, Cerastoderrna lamarcki). Group six represents stations in the Sassnitz Deep with a high species richness. Group

seven includes stations in the deeper northern part at the slope to the Bornholm Basin.

This station group lacks several species typical for the western Baltic Sea. Groups five and eight show a heavily reduced species richness (Table 2).

The spatial distribution maps (Fig. 5} show changes in the areal extension of cluster

groups from one investigation period to the next, especial ly in the western half of the

bay. Cluster group one and six, represent ing diverse macro launa assemblages in the southern Arkona Basin and in the Sassnitz Deep, almost d i sappeared b e t w e e n 1957

and 1993. The Oder Bank cluster group (3a) covered the largest part of the Pomeranian

Bay in the 1950s. Since 1981 its extension has been restricted to the shal low central parts of the Oder Bank. The deeper sandy areas are now part of the cluster group 3b.

Today, a wide range is also covered by cluster group four, as compared with the 1950s

when it was restricted to deepe r parts in the West near the Isle of Rugia in the 1950s.

The cluster groups five and eight represent stations, more than 15 m deep, sampled in

1981 and 199:3 in the Sassnitz Deep and along the former Oder Riw.~r bed. These assem- blages were absent during the invest igation period of L6we (1963}.

C h a n g e s in i n d i v i d u a l s p e c i e s

Distribution pattern of several species under invest igat ion have changed be tw een

1958 and 1993. The crustaceans Monoporeia affinis and Pontoporeia fernorata vanished

entirely since 1981 (Fig. 6). Only a few single individuals of the isopod Saduria

14~0 1500

Fig. 5. Maps showing the spatial pattern of different cluster groups during the investigation periods of L6we (1963), Gosselck (1985), Warzocha (1995), and Powilleit et al. (1995). Symbols are

the same as in Fig. 4. See text for further explanations

408 J. Kube, ]:. Gosse lck , M. Powil le i t & J. Warzocha

M o n o p o r e i a aff inis

I ' ~ , : ' : i : : ' ~ o.~~57

P o n t o p o r e i a f e m o r a t a

[ % ~ - - ~ . ~ ; ~i~' " . " 1 I o o o

t ~ " 1 : : . : : ~s~176 L.."( , . ~1oo :

- ~ : . : : : ~,o ~ i

�9 . . . : : : : ~

....

. , ,.

~ �9 e ~

~ , # . . ~ 2 ~ . ~ ~ �9 �9 _.

~ / ) ~ �9

~;~\..

~ I ~ ' ~ ' ~ . ~" �9 �9 . .

2

~ . " ". ~ . <

Fig. 6. Distribution of/Vlonoporeia affinis and Pontoporeia femoratu (incl. �9 m -') during the investi- gation periods of L6we (1963), Gosselck (1985), Warzocha (1995), and Powilleit et al. (1995)

entomon w e r e found s ince 1993. All o t h e r spec ies , that w e r e a b u n d a n t in t he sou the rn par t of the A r k o n a Basin and in the Sassn i tz D e e p b e t w e e n 1955 and 1980 w e r e d iscov-

e r e d to be v e r y scarce in 1993. T h e c o c k l e C. lamurcki and the a m p h i p o d B. pilosa a lmos t d i s a p p e a r e d from the s o u t h w e s t of the s tudy area. T h e i r dens i t i e s r e m a i n e d sta-

ble only on the O d e r Bank (Fig. 7). Dens i t i e s of the a m p h i p o d Corophium volutator h a v e p robab ly i n c r e a s e d n e a r the O d e r Mou th . No c h a n g e s w e r e o b s e r v e d for the iso- pod Cyathura carinata and the a m p h i p o d Leptocheirus pilosus (Fig. 8).

Long-term changes in the benthic communi t ies of the Pomeranian Bay 409

Bathyporeia pilosa

" I ~ " . . ~ : :: I~ooo

4

I ,~ ~" , ~ i ~ ~ ~ooo ! i ' , . :~:.;::!;; )] 100

Cerastoderma lamarcki

i\J \ ."

, ~" : ~ & : �9

- ~ ,!ii~v ' __. ~--~. � 9 . �9 ~ , . - : ~ _ ~

F i g . 7 Distribution of Cerastoderma lamarcki and Bathyporeia pilosa (ind.. m :) durin 9 the inves- tigation periods of Lowe (1963), Gosselck il.985), Warzocha [1995), and Powilleit et al. [1995)

M a c r o z o o b e n t h i c b i o m a s s

C h a n g e s in the biomass distr ibut ion pa t te rn b e t w e e n the 1950s and 1990s are shown for three b iva lve species in Fig. 9. The dis t r ibut ion pat tern of NI. edu l i s and M. arenaria

have not c h a n g e d within the past 35 years. The b iomass distr ibution pa t te rn of M. bal th-

ica in 1993 looked almost like an inve r sed picture of its 1950s distribution. Biomass va lues of hi. bul th ica have dec reased in the sou thwes t and increased north of the O d e r Bank.

410 J. Kube , F. G o s s e l c k , M. Powi l le i t & J. W a r z o c h a

Corophium volutator

....... 1110o0 %,/-'..' "::I~: .;: �9 : . M 500

" ~ . . . . i . . . . . ~ 2 0 0 -~/~/ : ' : " ' " H100

~ ~ ' . ' : i :~: "1~ ~

Cyathura carinata

I/.: ~.:.:. M2oo F ~ ( - . . . . . . . H,oo

. . L ~ , ~ . . . . . �9 Hso

! ~ ~ 1981

.--2 t . .

I ~ ~ , ~ 1993

"'1

/ ' - h . ~ "~ , " ~ g

',

4 ~ T j

Fig. 8. Distribution ot Corophium volutator and Cyathura carinuta lind. �9 m-') during the investiga- tion periods of L0we [1963), Gosselck !1985), Warzocha (1995), and Powilleit et al. (1995)

T h e m e a n b i o m a s s or M. edulis a n d M. arenaria w as a b o u t e i g h t t i m e s h i g h e r in

1993 t h a n in t h e 1950s. T h e i n c r e a s e is s t r o n g l y s ign i f i can t . M e a n b i o m a s s v a l u e s of M.

balthica s h o w e d no d i f f e r e n c e b e t w e e n t he tw o i n v e s t i g a t i o n p e r i o d s (Tab le 3).

To o b t a i n a m o r e d e t a i l e d v iew, q u a n t i t a t i v e c h a n g e s in b i v a l v e b i o m a s s e s w e r e

a l so c a l c u l a t e d s e p a r a t e l y for c l u s t e r g r o u p s 3a, 3b, 4 a n d 6 (Tab le 3). L a r g e d i f fe r -

e n c e s o c c u r r e d b e t w e e n t h e c l u s t e r g r o u p s . W h e r e a s t h e b i o m a s s of M. edulis

s t r o n g l y i n c r e a s e d in t h e s o u t h w e s t , b i o m a s s v a l u e s d i d n o t i n c r e a s e o n t h e O d e r

L o n g - t e r m c h a n g e s in the ben th i c c o m m u n i t i e s of the P o m e r a n i a n Bay 411

Mytilus edulis "~ ~ 1 . �9 g AFDW/m~

<~:...." �9 ,~~-~ i Y/Z~ ~::~iG~,~ 13o

. . . . 4 ,Z'~"~4P ~ & : :: :'i~i:: : u ,

~ ~ 1957 I ~ ~ 1993

Macoma balthica

~ : , : ~ . �9 , . .~..

; \ i N ~" / [ 9

,,.i 4.- ~,~, "-{" Mya arenaria

g AFDW/m 2 I ~ .:: : . "

%,... . . , 0 ow,m. _ _ . , . . ? " �9 .

" ~ :-~'~'~ ~, " ' ' . " iI~.:,?;"X 'J ,~ ...... 5 0

Fig. 9. Biomass distribution of ~Mytilus edulis, ,Macoma balthica and hlya arenuria (g AFDW �9 m-9 during the investigation periods of Lowe (1963) and Powilleit et al. (1995)

B a n k . T h e b i o m a s s of M. arenuria i n c r e a s e d in all c l u s t e r g r o u p s , e x c e p t for d e c r e a s e

in c l u s t e r g r o u p 6. H o w e v e r , in al l c a s e s s a m p l e s i zes w e r e too s m a l l to o b t a i n s ign i f i -

c a n t r e su l t s .

412 J. Kube, F. Gosse l ck , M. Powil le i t & J. W arzo ch a

Table 3. Changes in the biomass values (g AFDW �9 m :) of Mytilus edulis IMe), Macoma balthica (Mb) and Mya urenuria {Ma) in the Pomeranian Bay. Asterisks denote statistical significant

changes (" "" p < 0.001, Mann-Whitney U-test)

Cluster group Species 1955-58 !993 Factor ot (n 1955-58/n 1993) (L6we, 1963) (Powilleit et al., 1995) changes

in the mean _+ se median mean • se median mean

3a Me 3.4 _ 1.9 0.4 1 !.5 • 7.3 5.32 3.4 (13/6) Mb 2.5+0.4 2.2 3.1 __.0.9 2.1 1.2

Ma 5.4 +_ 2.5 1.1 12.6 • 4.2 8.9 2.3

36 Me 0.3 • 0.1 02 0.3 • 0.2 0.1 0.8 116/3) Mb 1.2 • 02 1.1 2.5 • 1.5 2.3 2.0

NId 1.1 • 0.2 1.0 9.7 _+ 4.6 12.7 9.0

4 Me 1.9 • 1.7 0.3 14.5 • 8.1 2.0 7.8 i3/8) Mb 1.4 _+ 0.6 1.8 2.2 • 0.6 1.9 1.{5

NId 0.4 • 0.2 0.4 13.6 _-a- 6.4 5.0 34.6

6 Me 0.8 • 0.4 0.0 2.1 • 12 2.1 2.6 113/3) Mb 3.6 + 0.5 4.(1 6.1 = 2 0 6.9 1.7

Ma 2.4 • 1.0 0.fi 0.5 : 1).2 0.4 0.2

all samples Nle 1.1 - 0.5 0.2 9.7 • 2 9 2.0 8.5" * " {58/33) Mb 2.5 -'- 0.3 2. I 2.6 _+ 0 4 2.3 1.0

Ma 2.4 • 0.7 1.0 19.7 • 5.0 8.6 8.1 " * "

D I S C U S S I O N

R e l i a b i l i t y of d a t a

C o m p a r i n g da ta that w e r e g a t h e r e d by cl i l ferent samplin~.j m e t h o d s is a g e n e r a l

p r o b l e m in e v a l u a t i n g pas t a n d p r e s e n t d i s t r ibu t ion p a t t e r n s in the b e n t h o s (Reise el

al., 1989). Unfor tuna te ly , ionf l - t e rm da ta se r i e s are o f t en l ack ing for sub l i t to rd l m a r i n e

a reas , d u e to the e n o r m o u s cos ts of s h i p b o a r d s a m p l i n g . As a c o n s e q u e n c e , a lmos t all

d o c u m e n t a t i o n s of l o n g - t e r m c h a n g e s in the m a c r o f a u n a ot the Baltic Sea a r e b a s e d on

r e i n v e s t i g a t i o n s of a r eas that h a d b e e n s t u d i e d i n t e n s i v e l y seve ra l d e c a d e s ago {Rosen-

b e r g &M ol l e r , 1979; C e d e r w a l l & E l m g r e n , 1980; Brey, 19861.

For s h a l l o w wa te r s in the w e s t e r n a n d s o u t h e r n Baltic Sea, w e c o n s i d e r it u n i m p o r -

l an t to r e s a m p l e exac t ly the s a m e s i tes in the s a m e s ea s o n , b e c a u s e :

1. Repos i t ion in 9 the r e s e a r c h vesse l on exac t ly t he s a m e s ta t ion tha t h a d b e e n s a m p l e d

d e c a d e s ago is imposs ib le . Our o w n e x a m i n a t i o n s of the p rec i s ion of t he n a v i g a t i o n

s y s t e m w h i c h was u s e d in t he 1950s r e v e a l e d a m a x i m u m er ror of +_2 km for off

s h o r e a r e a s w i thou t vis ible l a n d m a r k s .

2. Of ten , t h e r e are no g r e a t d i f f e r e n c e s in s p e c i e s c o m p o s i t i o n in s a n d y a reas , d u e to a

very low s p e c i e s r i chnes s (Kube, 1992; Kube, 19966; this s tudy).

3. Dens i ty va r i a t ions c a n n o t be r e l a t e d to s e a s o n a l osci l la t ions , p r o v i d e d by r e p r o d u c -

t ive cycles . T h e y are s u p e r - i m p o s e d by s tochas t i c va r i a t ions in h y d r o g r a p h i c cond i -

t ions (Arntz & Rumohr, 1986; Brey, 1986; Kube , 1992).

4. A b r a s i o n of s e d i m e n t a n d h e d l o a d t r a n s p o r t i n d u c e d by local w i n d s o f t en c a u s e

c h a n g e s in l a r g e - s c a l e p a t c h i n e s s w h i c h c a n n o t be c o v e r e d by t a k i n g t h r e e pa ra l l e l

g r a b s a m p l e s at a s ing le s ta t ion (Kube, 1996a).

L o n g - t e r m c h a n g e s in the ben th i c c o m m u n i t i e s of the P o m e r a n i a n Bay 413

5. T h e s a m p l i n g s e a s o n h a s no s t r o n g e l f ec t on e s t i m a t e s of b i o m a s s v a l u e s c a l c u l a t e d

f rom s t a n d a r d i z e d l e n g t h - w e i g h t r e l a t i o n s h i p s of b i v a l v e s wi th a h i g h l o n g e v i t y a n d

low P/B ratio.

H e n c e , it s e e m s to us t h a t the c o v e r a g e of all i m p o r t a n t h a b i t a t t ypes a n d a l a rge

n u m b e r of s a m p l e s is m o r e i m p o r t a n t for a s ta t i s t i ca l e v a l u a t i o n t h a n s p a t i a l a n d sea -

sona l accu racy . T h e to ta l n u m b e r o | s a m p l e s u s e d in this s t u d y w as m u c h h i g h e r t h a n

t he s a m p l e s izes of t h e i n v e s t i g a t i o n s of C.ederwal l & E l m g r e n (1980) a n d Brey (1986).

R e s p o n s e s to e u t r o p h i c a t i o n , a n d f l u c t u a t i o n s in h y d r o g r a p h y a n d c l i m a t e

E u t r o p h i c a t i o n is u sua l ly d e f i n e d as a c o m p l e x of p h e n o m e n a t r i g g e r e d by a n

i n c r e a s e of l i m i t i n g n u t r i e n t s , w h i c h l e a d s to i n c r e a s e s in b e n t h i c a b u n d a n c e , b i o m a s s ,

a n d n u m b e r of s p e c i e s b u t a lso to a n o x i c c o n d i t i o n s in s e d i m e n t s a n d m a s s i v e d ie -of f of

b e n t h i c a n i m a l s ( P e a r s o n & R o s e n b e r g , 1978; Help , 1995). Effects of e u t r o p h i c a t i o n a re

diff icul t to m e a s u r e a n d diff icul t to e x p l a i n in a s t r a i g h t f o r w a r d m a n n e r , b e c a u s e b e n -

thic c o m m u n i h e s u sua l l y r e s p o n d in two ways ; f u n c t i o n a l l y by i n c r e a s i n g t he i r p r o d u c -

tivity, a n d s t r u c t u r a l l y by a d a p t i n g t he i r c o m p o s i t i o n to the h i g h e r food s u p p l y

( B e u k e m a , 1991 ).

In t he Bal t ic Sea , r a t e s of i n c r e a s e in b i v a l v e b i o m a s s a b o v e the h a l o c l i n e v a r i e d

b e t w e e n 5 a n d 4 7 % of the ini t ia l v a l u e s p e r y e a r (Table 4). We o b s e r v e d a n a n n u a l r a t e

at i n c r e a s e at a b o u t 2 0 % ol the v a l u e s e s t i m a t e d in the 1950s w h i c h c o r r e s p o n d s wel l to

the r a t e s of i n c r e a s e in n i t r o g e n inpu t . H o w e v e r , l a rge d i f f e r e n c e s o c c u r r e d b e t w e e n

i n d i v i d u a l spec i e s . T h e b i o m a s s e s of the f i l t e r - f eede r s M. edu l i s a n d M. urenar ia

i n c r e a s e d by a l m o s t 2 5 % annua l ly . No i n c r e a s e w as f o u n d for C. l a m a r c M a n d / v l , bal th-

ica, t he b i o m a s s d o m i n a n t s of the b e n t h i c c o m m u n i t i e s in t h e 1950s. T h e s e f i n d i n g s a re

in a c c o r d a n c e wi th t he r e su l t s r e p o r t e d by Cede . rwa l l & E l n l g l e n (1980), w h o also

d e s c r i b e d t ha t .'M. ba l th ica was r e p l a c e d by M. e d u l i s as the d o m i n a n t spec i e s . T h e

c h a n g e in d o m i n a n c e s s e e m s to b e r e l a t e d to a l a r g e r c o m p l e x of causes ; c o m p e t i t i o n

for t oad a n d space , m e c h a n i c a l d i s t u r b a n c e by d r i f t ing m u s s e l c lumps , a n d shi f t s in the

r edox p o t e n t i a l m s u m m e r d u e to o r g a n i c l oad ing .

Table 4. Long-term increase in the biomdss of molluscs in shallow sublittoral waters of the Baltic Sea. Initial biomass: biomass at the begin of the investigation period, ldter biomass: biomass at the end oi the investigation period (~ data lrom Cederwall & Elmgren, 1980: investigations 1920-23 and 1976/77 around the islands of Oland and Gotland, :~data from Brey, 1!)86; investigations

1961-65 and 1982.83 in Kiel Bay, and ' this stud.v)

Species Initial biomass Later biomass Mean increase Increase rate (g AFDW �9 m-:) (g AFDW �9 m :~) (g AFDW. m-") (% - a : of the

initial biomass)

,'vlytilus edulLs: 0.09 2.3 0.04 46.5 Cerastoderma spp.' 0.03 O. 14 0.002 8.5 &lacomu baithica ~ 0.6 t 1.96 0.025 4.1 iHya arenana: 0.003 0.05 0.001 30.3 all Mollusca z 2.1 12.8 0.54 25.5 Mytilus edulis ~" 0.2 2.0 0.05 25 Macoma balthica ~ 2.1 2.3 0.006 0.3 ,'Vlya arenarla ~ 1.0 8.6 0.22 22

414 J. Kube, F. Gosselck, M. Powilleit & J. Warzocha

Spatial variations in the biomass increase ol M. edul is demonstrate how strong lim- itations by other ecological factors can be. In the Pomeranian Bay, motile Myti lus-

clumps lay at tached to Myaoshells on the sea floor. Depending on the current situation, single clumps can aggregate to patches of several square meters or disperse. Perma- nent erosion and strong bedload transports on the shallow Oder Bank prevent a suc- cessful set t lement of mussel spat and encourage an accumulat ion of musse l -c lumps in the deeper southwest of the bay. As a consequence, distribution patterns of M. edul is

have not changed considerably during the past 35 years. Below 15 m, all stations showed a decrease in species richness in the 1990s. These

changes were caused by an increase in oxygen depletion events in the Arkona Basin since 1983 and upwe.lling of deep water with lowered oxygen saturat ion from the Arkona Basin into the Pomeranian Bay via the Sassnitz Deep. Gosselck (1985) described a rate of increase in macrobenthic biomass of 16'7o - a -~ be tween the 1950s and 1980s. M. balthica dominated macrobenthic biomass in 1980. Its biomass had increased annual ly by 53% of the initial value. First signs of macrofauna death in the Arkona Basin were noticed in 1984 (HELCONI, 1990). A change in the M. bal th ica pop- ulation trom larger to smaller individuals was observed and its biomass decreased. The total n u m b e r of speci~;s decreased from ,10 to 20 be tween 1980 and 1986. The reduction in the numbers of the crustaceans P. fr and Diastylis rathkei was particularly striking.

Changes in the species composition of the macrofauna of the Pomeranian Bay does not result in an increasing proportion of deposit-feeders, as reported previously by other investigators (Help, 1995). Our analyses rather suggest an increase in the portion ot filter-feeders until 1993. A high rate of resuspension of phytoplankton and particu- late organic matter in the shallow parts of the bay might be the cause for these findings. This hypothesis is supported by the fact that no organic matter was accumula ted in the sediments.

Besides eutrophication, some of the observed phenomena seem also to be tr iggered by variations in oceanographic and climatic conditions. Although changes due to eutrophicat ion in the benthic communit ies of the Baltic Sea become more and more obvious, we are still unable to quantify the observed effects.

Events of oxygen depletion were found to increase in the southern Baltic Sea since the 1980s (Weigelt, 1991; Franck & Matthaus, 1992). The large gap of important inflow events since 1983 might have been an important cause for the observed drastic changes in the oxygen regime of the Arkona Basin. Otherwise, the increase in benthic biomass until the 1980s might have accelerated the speed of oxygen deplet ion below the pycnocline dur ing summer stratification.

Bivalves, the biomass dominants , show highly successful recrui tment dur ing sum- mers following a cold winter (Beukema, 1982; Kube, 1996a). This positive effect can establish extraordinari ly high biomasses after a series of severe winters due to the long life span of bivalves and, therefore, accelerates the speed of biomass increase (Beukema, 1989). Indeed, the populat ion structure of M. arenaria differed strongly be tween the 1950s and the 1990s. Whereas smaller size classes prevailed dur ing the late 1950s, the populat ion was domina ted by large specimens in 1993 (Kube, 1996a). This observation agrees with differences in the time course of the winter severeness index dur ing the ten-year periods preceding both investigations.

L o n g - t e r m c h a n g e s in the ben t h i c c o m m u n i t i e s of the P o m e r a n i a n Bay 415

Acknowledgements. This is publication No. 262 of the Institut f/.ir Os t seeforschung Warnemunde , Germany. This s tudy was suppor ted by the Federal Ministry of Research and Technology (BMBF) under grant n u m b e r 03F0105B.

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