O P H E L IA , 22(2) : 2 3 7 -2 5 1 (December 1983)

A N N U A L VARIATIONS IN VERTICAL DISTRIBUTION AND DENSITY OF B A T H Y P O R E I A PILOSA LINDSTRÖM

A N D B A T H Y P O R E I A SARSI WATKIN AT JULEBÆK (N O R TH -SEA LA N D , DENMARK)

W i l l y N i c o l a i s e n & E b b e K a n n e w o r f fM arine Biological Laboratory, D K -3000 Helsingsr, Denmark

A BSTRA CT

The annual variations in vertical distribution and density of Bathyporeia pilosa and B. sarsi were investigated on a locality in the near tideless inner Danish waters. The main trends in distribution could be explained by variations Ín water movement, temperature, desiccation and oxygen content o f the interstitial water. Other factors, including permanent water cover and rather low and changing salinities seem ed to be o f minor importance.

Predation w as probably the major source of mortality. Mortality mainly took place in the reproductive period.

IN T R O D U C T IO N

Several investigations have shown that Bathyporeia pilosa and Bathyporeia sarsi and, frequently , o ther members of the genus commonly occur singly or together in the in tertidal zone o f exposed sandy beaches of western Europe (Salvat 1967, Ladle 1975, Khayrallah & Jones 1980a, and other papers quoted therein). It is fu rther know n that the rise and fall of the tide exerts a great direct and indirect influence on their zonation on the shore (Salvat 1967).

B. pilosa and B. sarsi are often found in shallow sandy beaches in inner D anish w aters, and B , pilosa even extends into the Balticproper (Persson 1982). D ue to the essentially tideless nature of these waters, the two species are forced to live in sedim ents w hich nearly always are covered by water; hence living conditions differ very m uch from those of an intertidal zone.

D espite the com m on occurrence of the tw o species in inner Danish waters there are no previous accounts of their vertical zonation and the factors that control it. The present paper reports on the annual variations in vertical zonation and density o f B . pilosa and B. sarsi in the Julebsek beach on the coast of North- Sealand and the controlling factors.

238 W. N I C O L A I S E N &: E. K A N N E W O R F F

M A TER IA LS A N D M E T H O D S

O n the north-facing Julebazk beach, situated about 4 km west of Helsingor, a transect was established and sampled at close intervals for m ore than a year. The transect was nearly at right angles to the shore line. D epending on w eather conditions, between 16 and 20 stations were m easured o u t a t 5 m intervals from a given fixpoint on the shore and m arked w ith tonkin rods (Fig. 2). The w ater level relative to the fixpoint was then m easured and the w ater depth a t each station recorded. From these data the beach profile relative to the fixpoint was determined.

A t each station five bottom samples were taken at distances less than a metre from the tonkin rod. The bottom sampler was a hand-opera ted core sampler covering an area o f 80 cm2 (see Kanneworff & N icolaisen 1983). The animals were transported alive to the laboratory and killed by form aldehyde immediately before examination.

Primarily, identification was based on W atkin (1939a). W e found, however, tha t the two species more easily could be distinguished by the colour o f the contents of the hepatopancreatic caecae which was clearly visible through the cuticle of both living and newly killed animals. In B. pilosa the colour was brownish whereas it was bright green in B. sarsi. In addition, B. pilosa has a red pigmentation on the abdomen which is not present in B. sarsi.

On each sampling occasion the w ater tem perature was m easured a t the inner and outer parts of the profile.

At one occasion grain size analyses were carried ou t by m eans of standard dry sieving techniques.

PH Y SIC A L A N D C H E M IC A L F A C T O R S

Salinity

The surface salinity of the 0 re su n d is a function of the w ater interchange between the N orth Sea and the Baltic. As a result o f the freshw ater run-o ff to the Baltic the surface current is norm ally out-going in the straits of D enm ark. In the Oresund, the depth of the out-going surface layer is norm ally between 6 and 10 m and the salinity is in the range 8-12 %o. Below the hom ogeneous surface layer the w ater normally moves in the opposite direction. The salinity increases regularly w ith depth to reach a m axim um of m ore than 30 %o below 20 m depth. However, the direction of the surface current is often reversed m ainly due to the action of westerly winds. Incomming saline w ater from the K attegat then increases the surface salinity, often to more than 20 %o.

The surface salinity of the 0 re su n d was recorded daily from the Lappegrund light vessel a t a position 2 km off the Julebæk locality. T he lowest salinity

VERTICAL D IS T R IB U T IO N A N D DENSITY OF BAT HYP OREI A 2 3 9

recorded w as ab o u t 8 %o which occur during short periods throughout the year. This is the salin ity o f the Baltic w ater when it enters the 0resund. Generally, each low salin ity period lasted a few days but it occasionally lasted a couple of weeks. These periods alternated w ith periods with higher and more varying salinities. In the w in ter the surface salinity frequently was above 20 %o (max. abou t 25 % o). In the m onths M ay-O ctober it was seldom more than 15 %0.

Temperature

O ur tem pera tu re m easurem ents on the inside and the outside of the sand bar are show n in Fig. 1, together w ith the surface temperatures measured at the L appegrund light vessel.

From A pril to Septem ber, the tem perature on the inside of the sand bar was always above the tem perature at the Lappegrund, often by as much as 10°C (average ab o u t 5 °C ). From September through October it was fairly close to the Lappegrund tem pera tu re whereas it often was below by as much as 3-4°C (close to freezing) in the w inter months.

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20

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1968 1969

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F ig . 1. A nnual v a ria tio n s in tem perature m easured inside of (o) and outside of ( • ) the sand-bar a t Julebsek. T he fully d raw n curve shows the surface tem peratures a t the Lappegrund light-vesse .

240 W. N I C O L A I S E N öe E. K A N N E W O R F F

From early spring through the summer the tem peratures m easured on the outside o f the sand bar were generally also above the Lappegrund tem peratures, bu t the deviations were m uch smaller (average about 1°C). In the au tum n they w ere close to the Lappegrund tem peratures whereas they were close to or above the Lappegrund tem peratures in w inter (by as much as 2 -3 °C).

Hence, from early spring through the summer the tem perature on the inside o f the sand bar was above the tem perature on the outside o f the bar, the average difference being about 4°C. In winter this trend was reversed, as the tem perature on the inside o f the bar was below the tem perature on the outside o f the b a r by 3-4°C on the average.

W ave exposure

As the Julebaek locality faces north it is sheltered from the often strong w esterly winds which prevails in the area. Especially during autum n and w inter strong winds from the north-west and north, however, can cause pronounced alterations o f the beach profile.

Water level

In the inner Danish w ater the tidal amplitude, which is only abou t 15 cm, is superimposed on larger irregular variations due to winds, air pressure variations and variable run-o ff from the Baltic.

The w ater level off Hornbask (10 km north of Julebæk) was m onitored from 1891 to 1925 by the D anish M eteorological Institute. From these observations the following inform ation is obtained.

The m ean m onthly w ater level is lowest in M arch-M ay (about 10 cm below the m ean annual level). From then on, the mean m onthly w ater level increases steadily, and the highest level is reached in July-September (about 10 cm above m ean annual level). The mean m onthly w ater level then decreases un til M arch the following year.

The short-term variations in w ater level may, as m entioned above, be m uch larger. The highest and lowest w ater levels recorded were abou t + 1 7 5 cm and —125 cm, respectively. They occur in w inter time and are o f sho rt du ra tion (less than 2 hr). The short-term variations are much lower and of shorter du ra tio n in the m onths June-September than in the rest of the year. This is especially true fo r the low w ater levels. In the sum mer period low waters o f 40 cm seldom last m ore than an hour o r less and never more than a few hours. O utside the summer period low w aters o f 40 cm often last a day and may occasionally last a week. H igh waters generally last longer than low waters, bu t they are o f less consequence to sediment-living animals.

VERTICAL D IS T R IB U T IO N A N D DENSITY OP BATHYPOREIA

A n n u a l variations in beach profile and sediment factors

According to Ingle (1966) there is a lack of a standardized terminology associated with the beach env ironm ent. O ur use of terms is broadly similar to that of Ingle, but topographic differences necessitate some modifications. The concavity immediately seaw ard o f the shoreline we term the trough or the runnel if the width is only a few m etres. T he bar is the remainder of the profile seaward of the trough. The b reak er zone is situated on the seaward face of the bar. The area between the b reak er zone and the bar top we term the swash zone.

Fig. 2 show s the changes o f the profile throughout the period of study.Between 7 and 30 O cto b er 1967, the changes of the profile were relatively

minor; consisting m ain ly o f a sm oothening-out of the bar. On 13 December the shape o f the p rofile had changed considerably, the accretion being caused by a strong w ind from the no rth (7-8 Beaufort), associated with high water (90 cm above norm al). F rom D ecem ber through February the following year great changes con tinued to take place shorew ard of station XIV and especially on the bar. Seaward o f s ta tio n XIV, however, changes were insignificant. Through the spring, sum m er and au tum n the changes in relief were small compared to the winter. D ue to the slow shorew ard movement of the bar the trough gradually became n arrow er until it was reduced to a runnel in the autumn. On most sampling days a fte r D ecem ber 1967 the highest part of the bar was above the w ater level. T his w as because we chose calm days when possible. However, the bar top m ust have been frequently covered by water as living animals nearly always were present. T hroughout most of 1968, the air-exposed top was situated immediately seaw ard o f the trough. The height of this part of the bar diminished gradually through 1968 until it was nearly leveled out in late autumn. Generally, the breaker zone w as located around station XIV. The exact position varied w ith the w eather conditions.

The transects from 1969, 1970 and 1973 show that the profile changed much after 1968.

From April th rough O ctober, the sediment of the trough was oxygen-deficient as shown by the presence o f black sulphide layers a few mm below the surface. Judged from the absence o f black sulphide layers the sediment of the higher parts of the b a r w as oxidized to several cm depth. At the deep end of the profile the redox cond itions w ere interm ediate between those of the trough and thoseof the higher p a rts o f the bar.

O n one occasion we determ ined the grain size distribution at all stations sampled. In the trough the median grain size was about 170 pm. On the top o the bar the m edian grain size was about 185 /xm. Seaward of the bar top the m edian grain size decreased regularly with water depth and was about 125 pm a t station XVI. T he sedim ent was well sorted, especially on the bar top.

242 W. N I C O L A I S E N & E. K A N N E W O R F F

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A N N U A L C H A N G E S IN D IS T R IB U T IO N A N D A B U N D A N C E O F A N IM A L S

B a th y p o r e ia p ilo s a (Fig.2)

In early October 1967, B. pilosa were found on the top and the seaw ard side of the bar, whereas none were found in the trough. In late O ctober they appeared in the trough, but apart from that the distribution had rem ained nearly unchanged. In December, following a period o f heavy accretion, the top of the bar was exposed to the air and here only a few anim als were found. O n the seaward face of the bar the highest densities now occurred in deeper w ater, viz. at stations XIV to XVI. In the trough the density had increased strongly. From January to 13 M arch, the pattern of distribution changed little. O n the top of the bar only few animals were found. The highest densities were found at stations XVI to XVII. The densities in the trough w ere com parable to those found in December. On 28 M arch the pattern had changed considerably. The density had increased significantly on the top of the bar, and the highest densities occurred closer to the shore (stations XIII to XIV) than on the previous three sampling occasions. The density in the trough was com parable to th a t of a fortnight earlier. The distribution found on 10 April resembled th a t found on 28 March. On 23 April and 8 M ay no anim als were found in the trough. The distributions on the top and seaw ard face of the bar resem bled those found on 28 M arch and 10 April. On 22 M ay newly hatched juveniles had appeared. The distribution pattern resembled that found on 23 April and 8 M ay. The relatively high densities found on the exposed top of the bar are notew orthy . The distribution patterns from 7 June to 26 November remained m uch the same. However, on the exposed top of the bar few animals were found. N o anim als were found in the runnel. The large m ajority of animals were found from immediately seaward of the exposed top to the breaker zone. O n 18 D ecem ber the distribution had shifted seaward and the densities on the highest parts of the bar had decreased significantly. On 28 M arch 1969 it was found th a t the seaw ard move of the population had continued after 18 December.

When comparing the distribution patterns from around the beginning of April in 1968, 1969, and 1970 and 1973 it appears th a t the shorew ard move began earlier in 1968 and 1973 than in 1969 and 1970.

The total number of individuals of B. pilosa collected on a given sam pling day (shown in Fig. 3) is taken as a measure of the true population size.

The slowly decreasing trend in total num bers between O ctober 1967 and early May 1968 was not apparan t in the data from O ctober 1968 to M arch 1969* but this may be due to the uncertainties in sampling. It is safe to conclude* however* that outside the period M ay to September, the m ortality was very small. From M ay to September the m ortality was high as, despite a large pro-

VERTICAL D IS T R IB U T IO N A N D DENSITY OF B AT HYP OREI A 245

T o ta l n u m b er found

1 0 0 0

5 0 0

B, p ilo s ao

B. sars i

...I.

1969

Fig. 3. The total numbers o f Bathyporeia pilosa and B, sarsi collected on each sampling occasion.

duction of juveniles in th a t period, the to tal numbers found in October-Novem- ber 1968 w ere com parable to those found in March-April 1968.

From the to ta l num bers found around the beginning of April in 1968, 1969, 1970 and 1973, it appears th a t the population size may change from year to year. It seems to have been lowest in April 1970, intermediate in 1968 and 1969 and highest in 1973.

There w ere no clear trends for animals belonging to different size groups, developm ent stages o r sex to be differently distributed along the sampling profile.

The varia tions in density of B. pilosa did not in any obvious way correlate w ith the m ino r varia tions in topography of the bar.

B a th y p o r e ia sa rs i (Fig. 2)

In general, the annual changes in distribution pattern and population size follow ed the sam e trends as for B. pilosa. There were, however, some important differences.

As for B . pilosa , the B . sarsi population shifted seawards from January to early M arch. B. sarsi disappeared completely from the though in the course of the w inter. The shorew ard move of B . sarsi apparently began some weeks later than th a t o f B. p ilo sa . The m ain difference in distribution pattern between the tw o species th ro u g h o u t the rem ainder of 1968 was that B. sarsi was comparatively ra rer th an B . pilosa on the higher parts of the bar, being almost totally absen t on the air-exposed top.

A com parison o f the distribution patterns from around the beginning of April in 1968, 1969, 1970 and 1973 shows, as for B. pilosa, that the shoreward move in spring o f B. sarsi seems to have begun earlier in 1968 and 1973 than in 1969 and 1970.

246 W. N I C O L A I S E N &c E. K A N N E W O R F F

Due to the lower numbers present the relative varia tions in to ta l numbers of B. sarsi due to the sampling uncertainties are much larger th an for B. pilosa, and tend to obscure the annual trends in population size changes (Fig. 3). It seems, however, that as a consequence o í a partly unsuccessful breeding season, the total numbers of B. sarsi after September 1968 were low er than those preceding M ay 1968. It is also evident th a t throughout the m ain period o f investigation the population size of B. sarsi was lower than tha t of B. pilosa.

Around the beginning o f April the population size o f B. sarsi w as a t about the same level in 1968, 1969 and 1970, whereas it was considerably higher in 1973.

As was found for B. pilosa, there were no clear trends for anim als belonging to different size groups, development stages o r sex to be differently distributed along the sampling profile.

As with B. pilosa, the density variations o f B. sarsi did n o t correlate w ith the minor variations in topography o f the bar.

Summary o f the main trends in distribution and abundance o f B a th y p o r e ia p ilo s a and B. s a r s i

In general, the distribution of the two species overlapped th ro u g h o u t the period of investigation.

From early spring to autum n (1967 and 1968) both species w ere mainly found on the highest parts o f the bar, i.e. between the top and the breaker zone, and they were absent from the trough (runnel). O n the exposed parts of the bar, only B. pilosa was found in significant numbers.

B. pilosa was commonly found in the trough from late O ctober to mid April. B. sarsi was found in the trough from December to February. In the w inter both species were present in low num bers on the top of the bar. The m ain part o f the populations of both species had been displaced tow ards the deep end o f the transect in the early part of the w inter and rem ained there until the shoreward move started around 1st April (B. sarsi some weeks later than B. pilosa).

Following the appearance of newly hatched juveniles in the m iddle of May 1968, the population sizes started to increase, w ith tha t o f B. pilosa being much more pronounced than tha t of B. sarsi. In 1968 the popu lation size o f B. pilosa was about the same in early spring and late autum n. D ue to a less successful breeding season the population size of B. sarsi was low er in late autum n than in early spring. The m ortality o f both species was high in the breeding period and low outside this period.

In 1967 and 1968 the population size of B. pilosa was constantly higher than that of B. sarsi.

The sampling results from 1969, 1970 and 1973 indicate th a t there was some yeár-to-year variation in the absolute and relative population sizes o f the two species.

VERTICAL D IS T R IB U T IO N A N D DENSITY OF BAT HY P ORE I A 2 4 7

Size, reproductive stage o r sex did no t significantly influence the distribution o f the two species along the profile.

The d istribu tion o f the tw o species on the bar did not correlate with the m inor varaia tions in topography of the bar.

D IS C U S S IO N

The m ain trends in d istribution o f B. pilosa and B. sarsi along the Julebæk transect can be explained by the interplay of the following factors: Water m ovem ent, tem pera tu re, desiccation and oxygen content of the interstitial water.

The w ater inpu t to the interstices of sediments has been shown to be propelled by sw ash energy and the velocity fields associated with waves (Riedl 1971, Riedl, H uang & M achan 1972). The effect of the wave pump increases with wave hight, po rosity o f sediment and decreasing water depth. The effect is strongest und er breaking waves. The depth of the redox discontinuity layer, and hence the thickness o f the oxidized surface sediment, correlates with the water m ovem ent a t the sedim ent surface. Relocation of the sediment increases with degree o f w ate r m ovem ent while the deposition of light organic particles from the w ater colum n decreases. M icrobial activity in sediments increases with tem pera tu re (Fenchel 1969). The strong water movement between the breaker zone and the bar top a t Julebæ k causes high water input into the sediment and a low organic conten t. Consequently, the oxygen content of the interstitial water is high, even at sum m er tem peratures. Relocation of sediment is high in this area, and dra inage o f interstitial w ater occurs regularly at the highest parts. The low w ater m ovem ent in the trough causes low water input to the sediment and a ra the r high organ ic content. Therefore, at higher temperatures the sediment is reduced im m ediately below the surface. In winter, the sediment is oxidized to several cm d ep th due to low temperatures and a somewhat larger water movem ent. R elocation o f sedim ent is relatively low in the trough. The stations seaw ard o f the b reaker zone are in these respects intermediate between those on the higher p a rts o f the bar and those in the trough.

B. pilosa and B. sarsi are very well adapted to the changing conditions in the sedim ents o f exposed beaches. They have no need for a burrow with connection to the sedim ent surface as they obtain their supply of oxygen from the interstitia l w ate r and the ir food from sand grains which they efficiently clean for adhering organ ic m atter. A t higher temperatures they dig fast enough to keep pace w ith an eventual erosion or accretion of the sediment (Nicolaisen 8c K annew orff 1969). However, we have frequently noted that the animals become very sluggish w hen the tem perature is close to 0 °C. In winter, this may cause m any anim als to leave the bottom by accident when erosion takes place.

As the dem and for oxygen of Bathyporeia increases strongly with temperature (Fish & Preece 1970b , and ow n unpublished results) the oxygen content of the

248 W. N I C O L A I S E N 6c E. K A N N E W O R F F

interstitial water is especially critical in summer, As B. pilosa always w as present on the air-exposed parts of the Julebæk profile in sum m er the species m ust be tolerant to w ater drainage from the sediment. This agrees w ith the experimental findings of Preece (1971b). The absence of B. sarsi from these parts o f the bar is m ost likely due to a lesser tolerance to this factor, although this has not beentested experimentally.

Field investigations perform ed by Khayrallah & Jones (1980a) showed that B. pilosa rarely occurs in sediments w ith a m edian particle d iam eter outside the range of 150-220 jjum or with a silt and clay content greater than 2 % . Animal density was not correlated with median particle diameter o r silt and clay content within these ranges. Judging from these results, the small variations in grain size composition on the main p a rt of the Julebæk profile are unlikely to have influenced the distribution of B. pilosa and B. sarsi significantly. In this context it is noteworthy that in w inter high densities were found a t the deepest stations where the median particle diam eter was lowest.

B . pilosa is very tolerant to low salinities. Thus, it is found in the Bay of Finland at salinities of about 4 .5 % (Segerstrâle 1943) and the experimental studies of Preece (1970) also showed a large tolerance to low salinities. To our knowledge, B. sarsi has no t been reported from the Baltic p roper. Furtherm ore, the surface salinities of the northern 0 re su n d seems to be close to the lower limit of the salinity tolerance of this species (unpublished results). It therefore seems possible that the limited breeding success of B. sarsi in sum m er a t Julebæk may, at least in part, be due to low salinities in com bination w ith high tem peratures. It seems likely, however, tha t the main source of m orta lity is predation, mainly by juvenile flounder and plaice which are num erous a t Julebæ k from July to October. At Barsebäck on the Swedish coast of the 0 re su n d we have frequently found that 0-group flounders in July have their stom achs completely filled with B. pilosa, which is the only Bathyporeia species present at this locality. Later on they change diet. This agrees w ith the findings o f Sasdy (1972, quoted in Persson 1982). It is notew orthy tha t a t Julebæ k m ortality mainly takes place in the reproductive period.

The nocturnal swimming activities of Bathyporeia are partly associated with reproductive activity (Watkin 1939b, Colman ScSegrove 1955b, Fincham 1970a,b, Preece 1971a), but they also allow the animals to seek and locate m ore favourable living conditions. The response of B. pilosa to variations in current speed have been investigated by Khayrallah & Jones (1980a). They found that with increasing current speed active swimming decreased and passive drifting increased. At current speeds above 2.5 cm/sec most of the animals drifted passively. Reduced swimming activity of B. pilosa at low tem peratures w as dem onstrated by Preece (1971a).

The information on the behaviour of Bathyporeia in the w ater column is im portant for understanding the distribution on the beach. W hen a bore is

VERTICAL D IS T R IB U T IO N A N D D ENSITY OF B A T H Y P O R E I A

form ed after collapse o f a breaker there is a net onshore movement of water across the su rf zone (Ingle 1966). Part o f this w ater surplus moves seaward tow ard the b reak er zone as a bottom return-current. The movements of stained sand grains show ed th a t the bottom w ater tended to move toward a breaker zone from b o th sides. Passive drifting will, therefore, tend to concentrate the anim als in the su rf and breaker zone. W hen the temperature is close to 0°C digging-in can only be accomplished in the relatively calm water of the trough and a t the deep end o f the profile. The som ewhat higher temperatures at the deep end o f the p rofile increases the animals* ability to dig in.

Based on the above inform ation we interprete the distribution patterns of B. pilosa and B. sarsi a t Julebæ k as follows:

T he increase in tem perature around the beginning of April leads to an increase in swim m ing activity and digging ability of the animals, and both species are m oved by the nea r-bo ttom currents tow ard the breaker and surf zones where the tem peratures is higher. This move contributes significantly to an important early rise in ra te o f various life processes (e.g. m aturation of gonads, feeding and grow th). D uring sum m er the tem perature and oxygen conditions limit the anim als to the b reaker and surf zones. The m ortality during summer is mainly due to p reda tion , although the tem perature and salinity conditions together w ith desiccation on the top o f the bar may be partly responsible in the case of B. sarsi. In w in te r the anim als disappear from the high parts of the bar, due to the com bined effects o f low tem perature and increased relocation of sediment by increased w ater m ovem ent. The animals are carried into the water column and, due to their sluggishness at low temperatures, they are only able to rebury in the calm er areas o f the trough and, mainly, the stations at the seaward end of the profile. M orta lity is low at this time of year due to the absence of predators and to low tem peratures.

O n the nearly tideless Julebæk beach the distributions of B. pilosa and B. sarsi are very sim ilar, the m ain difference being the absence of B. sarsi from the air-exposed p a rts of the bar. The populations of both species are essentially lim ited to the stations which are permanently covered by water. This is in m arked con trast to the distribution of the two species on beaches of Western E urope w ith p ronounced tides, where they occur in the middle and upper parts o f the tidal zone (Fish &c Preece 1970a, Colman &c Segrove 1955a, Ladle 1975, V ader 1965, H olm e 1949, W atkin 1942, Salvat 1967). When the two species occur together in significant densities (Salvat 1967, Vader 1965) the distribution o f B. pilosa extends to a higher tidal level than that of B. sarsi. This supports our assum ption that B. pilosa is more tolerant to environmental stress than B. sarsi.

T he d istribu tions o f the two species on tidal beaches have been correlated w ith the varia tion in several sediment properties, including those used in the above discussion (Salvat 1967, Khayrallah & Jones 1980a, Khayrallah Sc Jones 1 980b ), b u t the m ajo r controlling factors may very well differ with locality.

250 W. N I C O L A I S E N Sc E. K A N N E W O R F F

Seasonal changes in distribution of Bathyporeia have been described earlier. A t La Vigne (southern French Atlantic coast) Salvat (1967) found a seaward displacement in summer of the upper lim it o f d istribution o f B. pilosa, and assumed it to be caused by high tem peratures. A seaw ard displacem ent in sum m er o f B. pilosa was also found by Fish & Preece (1970a) a t the coast of Wales. Persson (1982), w orking at the Swedish coast o f the Baltic, found a seaward move in autum n of B. pilosa and assumed it to be caused by an active search for better feeding conditions. A t the coast o f N orthum berland Ladle (1975) found tha t the population o f B. sarsi was displaced seaw ards in April- M ay. The lack o f a common trend in annual d istribution changes indicates that the controlling factors operate differently at the various localities.

The maximum densities of B. pilosa at Julebæ k are w ith in the range of m aximum densities previously reported from the tidal zone o f w estern Europe (Salvat 1967). From the Baltic proper densities as high as 19 000 B. pilosa per m3 have been reported (Persson 1982). The highest densities o f B. sarsi at Julebæk (4 April 1973) are, to ou r knowledge, the highest densities reported for this species. Thus, on the basis of population densities there is noth ing to suggest that the low salinities and the near absence of tides at Ju lebæ k are unfavourable to the two species. This invalidates the assum ption of Salvat (1967) that B. pilosa is exclusively an intertidal species.

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