LIMNOLOGICA - CORE

ELSEVIER Limnologica 34,199-212 (2004)

http://www.elsevier.de/limno LIMNOLOGICA

Distribution, abundance and life history of Mysis relicta (LOVEN) in the Feldberg Lake District, Germany

Julia Scharf*, Rainer Koschel

Leibniz-lnstitute of Freshwater Ecology and Inland Fisheries, Department of Limnology of Stratified Lakes, Stechlin-Neuglobsow, Germany

Received: August 22, 2003 • Accepted: January 30, 2004

Abstract

Distribution, abundance and life history characteristics of Mysis relicta were studied in the Feldberg Lake District (Lake Breiter Luzin, Lake Schmaler Luzin, Lake Zansen) located in northeastern Germany. Between July 2001 and November 2002 mysids were collected by vertical net hauls. In order to determine the impact of the current trophic conditions on the distribution of mysids in these lakes, oxygen concentration, total phosphorus, chlorophyll a and water transparency were also measured. All investigated lakes are mesotrophic at present. Lake Breiter Luzin exhibited great seasonal and spatial variations in mysid abundance. Density of adults and juveniles had a mean of 44.9 _+ 57.1 and 68.7 + 99.6 m -2, respectively. Highest abundance of adults was 110.4 + 76.5 m -2 in summer, lowest abundances of 2.0 + 4.0 m -2 occurred in spring. For juveniles, highest density of 218.4 + 174.6 m -2 was detected in summer and lowest of 0.8 + 1.8 m -2 in winter. No mysids were caught in any of the daytime hauls, but they were widely distributed throughout the water column at night. Size frequency distribution of mysids suggested that reproduction occurred year-round, the most consistent influx of juveniles occurred in early summer and a smaller second cohort in autumn. Highest mysid abundance was 189.2 + 318.6 adults and 127.0 _+ 66.3 juveniles m -2 in Lake Schmaler Luzin, and 59.6 ___ 5.6 adults and 79.4 + 11.2 juveniles m -2 in Lake Zansen. There were great spatial differences in abundance in both lakes.

Key words: Mysis relicta - glacial relicts - abundance - distribution - Feldberg Lake Dis- trict - trophic status

Introduction

Mysis relicta (Crustacea: Mysidacea) is a benthic-pelagic invertebrate that inhabits off-shore hypolimnetic waters and is adapted to cold temperatures. High abundance of mysids is usually found in deep oligotrophic and mesotrophic lakes but lower numbers also occur in eutrophic waters (HORPPILA et al. 2003). They undergo

extensive diurnal vertical migration (BEETON 1960; BEE- TON & BOWERS 1982) and remain near the bottom during the day, whereas at night, they migrate upwards through the water column to the thermocline. Light is the main regulating factor of this vertical migration because the animals are adapted to darkness and their eyes are easily damaged by strong light levels (LINDSTROM 2000). Thus, in periods of bright moonlight the nocturnal ascent of

*Corresponding author: Julia Scharf, Leibniz-lnstitute of Freshwater Ecology and Inland Fisheries, Department of Limnology of Stratified Lakes, Alte FischerhOtte 2, D-16775 Stechlin-Neuglobsow, Germany; Phone: +49 033082 69970, Fax: +49 033082 69917, e-mail: [email protected]

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200 J. Scharf & R. Koschel

mysids is inhibited and they remain in deeper strata than usual (JANSSEN & BRANDT 1980; BOWERS & GROSSNICK- LE 1978). Temperature affects the upper extent of migration. BEETON (1960) reported that mysids do not migrate through a pronounced thermocline into warmer surface waters, even though they can tolerate higher temperatures for a short time. DEGm~EVE & REYNOLDS (1975) detected a temperature around 22 °C as the upper lethal limit for mysids acclimated to 5 °C.

Mysis, an omnivore, feeds on detritus, benthic inver- tebrates, phytoplankton and zooplankton (JOHANNSSON et al. 2001; VIHERLUOTO et al. 2000; BOWERS & GROSS- NICKLE 1978; LASENBY 8z LANGFORD 1973) and provides a major food resource for planktivorous fish (LASENBY et al. 1986; MORGAN et al. 1978). Therefore, Mysis relicta plays a key role in energy transfer between plankton and fish production as well as between benthic and pelagic habitats.

Life cycles of Mysis relicta vary among populations, lasting one to four years from egg to reproducing adult (ADARE & LASENBY 1994; JOHANNSSON 1992; BEETON & GANNON 1991; MOEN & LANGELAND 1989; BERRILL ~; LASENBY 1983; MORGAN 1980; MORGAN • BEETON 1978). Breeding mostly starts in autumn and young are released from early spring to summer (N~sJE et al. 2003; JOHANNSSON 1992) but reproduction can also occur several times in one year (POTHOVEN et al. 2000; MORGAN & BEETON 1978).

Mysis relicta has generally been treated as a prime ex- ample of a "glacial relict" faunal element. It is widely distributed in fresh- and brackish waters in the northern regions of America and Eurasia which were glaciated in the past. In Germany, historical studies showed that this

species could be found in five lakes, all located in the Mecklenburg Lake District (THm~,~MAYN 1925, 1928). Three of these lakes are situated in the Feldberg Lake District. In the 1980's, this German mysid stock declined dramatically due to anthropogenic eutrophication processes (WATERSTRAAT 1988), thus higher quantities could only be detected in Lake Breiter Luzin. Some remediation measures were started in the 1980's to stop the wastewater discharge into the Feldberg Lake District and in 1996/97 lake restoration was arranged in the Basin Carwitz of Lake Schmaler Luzin. This restoration changed the trophic condition of Lake Schmaler Luzin, total phosphorus content in the water column of Basin Carwitz was reduced by more than 33% from 1995 to 1997 (KoscHEL et al. 2001).

The main goal of this paper is to give a review of the present distribution of Mysis relicta in the Feldberg Lake District (Lake Breiter Luzin, Lake Schmaler Luzin, Lake Zansen) in relation to the trophic condition of the lakes. More specifically, the study is centred on the question whether the remediation efforts in the Feldberg lakes led to a recovery of the mysid population by im- proving the physicochemical parameters of their habitat. Furthermore, abundance and some aspects of life history characteristics of the mysid population in Lake Breiter Luzin are examined to compare it with mysid stocks oc- curring in lakes of other regions.

Study site

The Feldberg Lake District in Mecklenburg-Vorpom- mern, northeastern Germany (53°20'N, 13°28 %) was

L

- 5 8 - - 56 0 500 1000 m

N Fig. 1. Contour map of Lake Breiter Luzin with the 5 sampling locations. Digital maps provided by the Ministry of the Environment Mecklenburg-Vorpommern (see: Methods).

Limnologica (2004) 34, 199-212

formed during the last glaciation by melting of "dead" ice blocs and erosion of melt water channels (SCHMDT 1997). Several lakes are situated in this region, partially

Lake Sehmaler Luzin

o Sampling points depth [rn] ! ] - 8 - o

-16 - -8 -24 - -16 - 3 0 - -24

m l ~ - 3 2 - -30

N / /l

500 1000 m

Fig. 2. Contour map of Lake Schmaler Luzin, Central Basin (north) and Basin Carwitz (south) with the 7 sampling stations. Digital maps provided by the Ministry of the Environment Mecklenburg-Vorpom- mern (see: Methods).

Mysis relicta in the Feldberg Lake District, Germany 201

connected with each other by channels. The present study focused on three lakes in this area. Lake Breiter Luzin (Fig. 1) is one of the largest lakes in this system (surface area 345 ha, volume 77 mill. m 3) and is one of the deepest lakes in the Southbaltic Lake District of Germany with a maximum depth of 58.3 m and a mean depth of 22.3 m. Lake Schmaler Luzin (Fig. 2) is long (6 km) and narrow (surface area 144.9 ha, volume 20.9 mill. m 3, maximum depth 33.5 m, mean depth 14.5 m) and is connected to Lake Breiter Luzin by a small channel. The third lake, Lake Zansen has a surface area of 138.2 ha, a volume of 23.1 mill. m 3, a mean depth of 16.7 m and its maximum depth is 42.2 m (Fig. 3).

Methods

Between July 2001 and November 2002 Mysis relicta were collected monthly in the water column of Lake Breiter Luzin, but not during the time of ice cover in De- cember and January. Samples were taken at new moon (+ 3 nights) around midnight at five fixed stations. Four of them were 25 m deep and the fifth station was located at the deepest point of the lake (58.5 m, Fig. 1). Addi- tional samples were taken at the neighbouring Lake Schmaler Luzin in October and November 2001, as well as in September 2002 at seven fixed stations in areas from 18 m to 34 m depth (Fig. 2). Lake Zansen was sampled in September 2002 at five 20 m deep stations (Fig. 3), likewise at new moon (+3 nights) around midnight. For collection a plankton net (diameter 0.4 m, length 1.4 m, mesh size 150Hm) was lowered within 1.5 to 2 m of the bottom, allowed to rest for 1 minute and then raised vertically to the surface at 0.5 to 0.8 m sec -~, with two replicates at each station.

Lake ZanSen N

D Sampling points ;pth [m]

o ~ _ ~ -16- -8

-24 - -i 6 -32- -24 -4o- -32 -42- -40

o 500 lOOO m I I . . . . i

I '111

Fig. 3. Contour map of Lake Zansen and the 5 sampling points. Digital maps provided by the Ministry of the Environment Mecklenburg-Vor- pommern (see: Methods).

Limnologica (2004) 34, 199-212


In April, July, and October 2002, vertical net hauls were taken at four locations in Lake Breiter Luzin (depth 25-38 m) with depth intervals from 0-10 m, 10-25 m and 25-35 m around noon and midnight at new moon (_+ 3 nights). One sample was collected per depth interval at each location. For this purpose the net was equipped with a closing mechanism.

All samples were preserved in 4% formalin-sugar so- lution for size and gender analysis. Males were identified by the presence of thickened and bifurcated fourth pleopod and females by the presence of a marsupium. All individuals without either of these characteristics were classified as juveniles. Body length was measured from the tip of the rostrum to the base of the telson to the nearest millimetre.

Sampling for physicochemical parameters was con- ducted once a month at the deepest location of Lake Brei- ter Luzin, in the Central Basin and Basin Carwitz of Lake Schmaler Luzin and at two sites in Lake Zansen. Be- tween May and September sampling took place bi-week- ly in Lake Breiter Luzin. Total phosphorus (TP), chlorophyll a (Chl a), oxygen concentration (02) and water transparency are considered in detail here. One-litre water samples for the determination of TP and Chl a were taken with a Limnos-Water Sampler (LIMNOS, Turku - Finland) in 0, 2.5 and 5 m depth and were pooled to an integrated sample of the whole euphoric zone. TP was measured with a Perstop flow injection analysis system TECATOR FIASTAR 5010/5030. The analyses followed the Tecator Application Notes (ASN) 60-03/83

(detection limit 0.005 mg 1-1; now ASN 5602). For TP measurements, the unfiltered water samples were auto- claved for 30 min (1.5-2.0 bar) after persulfate (K2S208) addition. Samples for Chl a analysis were filtered through membrane filters (ME 28 Schleicher & Schuell, 1,2 ffm, 2 filters per sample) and stored at -18 °C for a maximum of 3 month. Filters were homogenized (Ultra- Turrax T 25, 2 min), extracted in acetone (12 to 24 h in darkness, followed by centrifugation) and analysed spec- tro-photometrically with a LAMBDA 2 Perkin-Elmer spectrophotometer (detection limit approximately 1 fig 1-1) for Chl a. Calculation of Chl a concentration was per- formed according to STR~CKLAND & PARSONS (1968). 02 and water temperature were measured in situ (WTW OX 196, WTW Weilheim) at 1-5 m intervals. Water transparency was determined in the morning by Secchi disc (Hydro-Bios Kiel, white, 0.2 m diameter).

For statistical analyses, the program SPSS 9.0.1 for windows (© SPSS. Inc.) was used. Univariate analysis of variance (two-way ANOVA) was applied to test the effect of the two variates depth layer and month on mysid abundance. The Scheff6 Post hoc-Test was added to compare the abundance at different months. Also the effects of the two variates date and sampling station on the abundance of adults and juveniles were analysed by two-way ANOVA. The Scheff6 Post hoc-Test was used to compare abundance at different sampling stations and dates. Using one-way ANOVA and the Duncan Post hoc- Test, the differences in abundance during the sampling period were examined.

400

375

350

325

300

,--, 275

E 250

5 225 [ • A d u l t s .z.

I =,, 200 [] Juveni les .~ 175 C

150

125

7101 8/01 9/01 10/0111/01 2/02 3/02 4/02 5/02 6102 7102 8/02 9/02 1010211/02

Fig. 4. Seasonal changes in mysid abundance of adults and juveniles in Lake Breiter Luzin between July 2001 and November 2002. Mean over all sampling sites _+ standard deviation, n = 10.

Limnologica (2004) 34,199-212

Results

240 220 200

180

E 160 6 140 z "~ 120 >,

100 C 80

60

Lake Breiter kuzin

Mysid abundance showed great seasonal and spatial variation in the water column. During the sampling period mean density was 44.9 _+ 57.1 m -2 and 68.7 + 99.6 m -2 for adults and juveniles, respectively. Highest densities were 218.4 + 174.6 juveniles and 110.4 + 76.5 adults m -2 in summer and autumn. Lowest densities of 0.8 + 1.8


and 2.0 _+ 4.0 for juveniles and adults m -2 occurred in winter and spring (Fig. 4). The mean monthly density of juveniles was highest in July both in 2001 and 2002. After this peak, juvenile abundance declined but slightly increased again in November 2001 and October 2002. Low numbers were found in winter and early spring. In May abundance of juveniles started to increase again. Mean monthly abundance of adults showed an annual maximum in August 2001 and October 2002. Lower peaks were detected in November 2001 and August 2002. During winter and early spring, the number of adults collected was low, with densities starting to increase from May, similar to the juveniles. Differences in densities between July/August 2001 and the following winter/spring were significant for juveniles and adults (ANOVA, F = 4.7, p < 0.001 and F = 2.3, p < 0.05, Dun- can Post hoc-Test p < 0.05).

40

20 b

1 2 3

Digital lake maps were provided by the Ministry of the Environment Mecklenburg-Vorpommern (Um- weltministerium M-V, Abteilung Integrierter Umwelt- schutz und Nachhaltige Entwicklung - Seenreferat - , 2001/2002) and used to indicate the sampling sites in Fig. 1-3.

4 5

Sampling points

I N Adults

[] Juveniles

Fig. 5. Mysid abundance at different sampling stations in Lake Breit- er Luzin between July 2001 and November 2002. Stations no. 1-4 at 25 m, no. 5 at 58.5 m. Mean _+ standard deviation, n = 30.

m

C , m

,,C

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10-25

25-35

0 20 40 I I I

60 80 100 120

Density [No. m "2]

i i

t

I

140 I

160

I

180

[] April

[] July

• October

Fig. 6. Mean nocturnal mysid abundance in different depth intervals in April, July and October 2002 in Lake Breiter Luzin. Mean + standard deviation, n = 4.

Limnologica (2004) 34,199-212

2001

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n

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20 August

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2002

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o , , , ,~_,-t-~, , ,~ ,~ ,~ , - - , ,,,,,,,,, , 2 4 6 8 10 t 2 14 16 18 2 0 451o3o ~~km July

o . . . . . . . . ,,,,,, , ~ , ~ , ~ , m ,,,~, , 2 4 6 8 10 12 14 16 18 20

453o ~ August

o . . . . . . . ,~,~, ,m-~,~, ~, , , , ~ , 2 4 6 8 10 t 2 t 4 16 18 20

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September

,m.&:..~m,;: . . . . .

2 4 6 8 10 12 14 16 t 8 20

October

.-. , , . . . . . . 2 4 6 8 10 12 t 4 16 18 20

45 November 30

2 4 6 8 10 12 14 16 18 20

Body l e n g t h [ m m ]

Fig. 7. Size-frequency distribution of Mysis relicta in Lake Breiter Luzin between July 2001 and November 2002, sampled at night in the entire water column at all stations.

60

,~ 10 E " " 15

" 20

25

I J I I I I I

0 2 4 6 8 10 12 14

Oxygen concentra t ion [rag I "1]

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16


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x 14. May

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Water temperature [°C]

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Water temperature [~C]

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.= 20

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0

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15

20

25

30

35

40

45

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Water temperature ['13]

Fig. 8. Oxygen concentrations and water temperature of Lake Breiter Luzin [A], Lake Schmaler Luzin (Basin Carwitz) [B], and Lake Zansen (Middle) [C] at selected sampling times in 2002.

Mysid densities of juveniles and adults were significantly different among sampling stations (univariate analysis of variance F = 28.6, p < 0.001 and F = 19.9, p < 0.001). Thus abundance at sampling station 5, the

deepest point of the lake, was significantly lower than at the other stations (Scheff6 Post hoc-Test, p < 0.05). Moreover, the abundance of adults at the deep station was low throughout the sampling period (Fig. 5). At the

Limnologica (2004) 34,199-212


Table 1. Selected trophic criteria and trophic status according to LAWA (1998) and OECD (1982) of Lake Breiter Luzin (n = 30) and Lake Schmaler Luzin (n = 19)in 2001/2002 and of Lake Zansen (n = 8)in 2002 (means +_ standard deviation of the euphotic zone). Original data of Lake Zansen are provided by the Ministry of the Environment Mecklenburg-Vorpommern.

8reiter Luzin Schmaler Luzin, Schmaler Luzin, Zansen Basin Carwitz Central Basin

Secchi depth [m] 3.0 ___ 1.1 5.4 ___ 1.3 6.3 +_+_ 1.7 4.4 + 1.2 Total phosphorus [tJg 1-1 ] 22.9_+9.9 20.8_+7.5 22.7+7.7 33.0-+15.0 Chlorophyll a[pg k 1] 8.2 _ 3.4 3.7 -+ 1.5 3.3 _-+ 2.0 4.1 _ 2.8 Actual trophic status mesotrophic mesotrophic mesotrophic mesotrophic

same time, mysid densities were also affected by the sampling date (juveniles: F = 41.8, p < 0.001; adults: F = 9.4, p < 0.001). And there were also interactions among both variates, sampling station and date (juveniles: F -- 7.9, p < 0.001; adults: F = 3.1, p < 0.001).

No mysids were caught in any of the daytime net hauls in the water column, whereas at night, they were widely distributed at the different depth intervals (Fig. 6). In October and July, mysids occurred in the whole water column. Highest densities were found in the upper (0-10 m) and middle depth layer (10-25 m) in July and in the middle layer in October. In April, only a few individuals were caught at the 10-25 m depth. However, distribution between the three depth intervals was not sta- tistically significant but there were significant differences in the distribution among months (univariate analysis of variance, F = 2.6, p > 0.05 and F = 12.2, p < 0.001). The seasonal variation in abundance was mostly due to the low numbers of mysids in April. Den- sity differences between April/July and April/October were significant (Scheff6 Post hoc-Test, p < 0.001 and p < 0.01), whereas differences in July/October were not significant (p > 0.05). Also no interactions between the two categories of months and depth layers could be detected (F = 1.8, p > 0.05).

Size-frequency distributions of mysids changed throughout the season, but cohort development was not as clear in 2001 as it was in 2002 (Fig. 7). In February 2002, only a few individuals of the previous year's cohort existed, they were 11 to 16 mm long. Three cohorts co-existed in March, when young-of-the-year of about 3 mm in length entered the population. The two previous year's cohorts were 8 mm and approximately 12 to 17 mm long by this time. These three cohorts could be followed through the succeeding months, in which the largest sized individuals disappeared from the population by August. In November, young-of-the-year from the spring cohort were 6-10 mm long. In 2001, these cohorts were less easily distinguished, but the main peak of young-of-the-year was discernible throughout the year, reaching approximately the same body length in autumn as in 2002. Recruitment of young to the population

could be observed in almost all months, with highest abundance from May to June. From September to November, another separate cohort of newly hatched individuals appeared in both years.

Lake Breiter Luzin is stratified during the summer and 02 concentrations showed a minimum in the metalimnion from June/July to October (Fig. 8). In October 2002 metalimnic 02 concentrations declined to 4.3 mg 1-1. But in October 2001, there were nearly anoxic conditions at depths between 11 and 12 m (KOSCHEL, unpublished data). During thermal stratification 02 concentrations declined also in the hypolimnion. At the end of summer, there were less than 2 mg 1-2 02 detected below 40 m. Below 45 m, anoxic conditions predomi- nated in both years. Water temperature dropped from 20.4 °C at the surface to 5.8 °C in 15 m depth in July 2002 (Fig. 8). Average hypolimnetic temperature during summer stratification was 4.8 °C. Mean water transparency during the study period was 3.0 m and ranged from 1.25 m in June 2001 to 5.4 m depth in May 2002 (Table 1). The average TP concentration in the euphotic zone was 22.9 ktg 1-1, with a minimum of 10 ktg 1-2 in August 2002 and a maximum value of 45 ktg 1-1 in March 2001 (Table 1). Chl a concentration ranged from 4 to 15/~g l-land averaged 8.2 ktg 1-1 (Table 1). With these selected trophic criteria Lake Breiter Luzin can be classified as mesotrophic, according to LAWA (1998) and OECD (1982).

Lake Schmaler Luzin

Mysids in Lake Schmaler Luzin showed high quantities in the water column, especially in September (Fig. 9). High densities occurred in Basin Carwitz, in the southern parts of the lake, at sampling stations no. 4-7. In the Central Basin, the northern parts of the lake, at sampling stations no. 1-3, low abundance was determined (Fig. 10). However, there were significant differences in adult abundance between the stations (ANOVA, F = 6.6, p < 0.01) and the sampling dates (F = 3.6, p < 0.05) as well as in the interactions of both categories (F = 3.9, p < 0.05). The variations between sampling stations

Limnologica (2004) 34,199-212

350

300

~,~ 25O E 6 200 z

150 e , ,

a 100

50

9/01 10/01 9/02


• Adults

[] Juveniles

Fig. 9. Mysid abundance of adults and juveniles at the sampling dates in Lake Schmaler Luzin. Mean of sampling sites _+ standard deviation, n = 14.

55O

50O 450

~'~ 400 E 350 5 z 300

250 200

c~ 150

100

50 i

2

J I I I

3 4 5 6 7

Sampling points

• Adults

[] Juveniles

Fig. 10. Mysid abundance at different sampling stations in Lake Schmaler Luzin in September 2001/2002 and October 2001. Sampling points 1-3 were located in the northern Central Basin and stations 4-7 in the southern Basin Carwitz. Mean _+ standard deviation, n = 6.

•'• 7O

76 60

5o

"~ 40

3o

2O

100

90

8O

70

6O

5O 40 30

2O

10

1

i i i

2 3 4

Sampling points

5

m Adults

Juveniles

Fig. 11. Mysid abundance at different sampling stations in Lake Zansen, September 2002. Stations no. 1-3 were located in the southern basin, stations no. 4-5 in the northeastern part of the lake. Mean _+ standard deviation, n = 2.

Limnologica (2004) 34,199-212


were only due to significant differences in mysid densities between station number 7 and all others (Scheff6 Post hoc-Test p < 0.05). Referring to juveniles, there were also differences in abundance between sampling stations (F = 12.2, p < 0.001) and sampling dates (F = 13.6, p < 0.001) and there were interactions between both variates as well (F = 4.3, p < 0.05). But the differences between the sampling stations were not as clear as it was for the adult densities. There were significant differences between stations 1-3 and 7 as well as between number 3 and 5 (Scheff6 Post hoc-Test, p < 0.05).

Highest densities in the Basin Carwitz were 189.2 + 318.6 adults and 127.0 +_ 66.3 juveniles m -2. Thus, the Mysis relicta population in this part of the lake is equiva- lent to the one found in Lake Breiter Luzin.

Compared to Lake Breiter Luzin, Lake Schmaler Luzin did not show any 02 minimum in the metalimnion. Nevertheless, there was also a decrease in hypolimnetic 02 concentration during the summer (Fig. 8). In October 2001 and 2002, there was less than 2 mg 1-1 oxygen below 15 m in the Basin Carwitz. The deepest parts of Basin Carwitz, below 30 m depth, were anoxic at the end of summer stratification. In the Cen- tral Basin, the upper boundary of the anoxic zone was located slightly higher, at about 25 m depth. Water temperature declined from 21.8 °C at the surface to 5.3 °C at 15 m depth in July 2002, whereas below 15 m, mean temperature was 4.9 °C during summer stratification (Fig. 8). Mean water transparency was 5.4 m and 6.3 m in Basin Carwitz and the Central Basin, respectively (Table 1). Maximum value in the Central Basin was 9.8 m in November 2001, minimum 3.6 m in July 2001. In Basin Carwitz, water transparency ranged from a maximum of 7.8 m in September 2001, to a minimum of 3.5 m in June 2001. Mean TP in the euphotic zone was 20.8 ktg 1-1 in Basin Carwitz and ranged from 11 ,ug 1-1 in June 2001 to 36 ,ug 1-1 in April 2001. In the Central Basin TP concentration of the euphotic zone was some- what higher. It ranged from 12 ktg 1-1 in June/August 2001 to 36 /~g 1-1 in March 2002 and averaged 22.7/~g 1-1 (Table 1). Mean Chl a content was 3.7 ,ug l -I and ranged from 2 to 8 ,ug h 1 in Basin Carwitz. In the Central Basin Chl a values were slightly lower, with a mean of 3.3 pg 1-1, a minimum of 2 ktg 1-1, and a maximum of 10 ktg 1-1 (Table 1). Lake Schmaler Luzin is a mesotrophic system, according to LAWA (1998) and OECD (1982).

Lake Zansen

In September 2002, during a singular sampling occa- sion, the occurrence of Mysis relicta in Lake Zansen could be detected. In the southern basin (sampling station 1-3), highest abundances were 59.6 + 5.5 adults and 79.4 _+ 11.2 juveniles m -2. In the north-eastern parts (sta-

tions 4-5), mysid densities were lower and no adults could be observed (Fig. 11).

Hypolimnetic 02 concentration decreased during summer stagnation in Lake Zansen. In October 2002, there was less than 2 mg 1-1 0 2 below 23 m depth in the central part of the lake (Fig. 8). But there was no anoxic area during the summer. In the northern part of Lake Zansen, 02 concentrations were slightly lower than in the central part. 02 values were less than 2 mg 1-1 at 13 m depth. Water temperature in July 2002 declined from 20.6 °C at the surface to 5.7 °C at 15 m and mean hy- polimnic temperature during summer stratification was 5.2 °C (Fig. 8). Mean water transparency was 4.4 m and ranged from 2.3 to 6.1 m depth (Table 1). TP concentration in the euphotic zone ranged from 20 to 70 ktg 1-1 and averaged 33 ktg 1-1 (Table 1). Mean Chl a was 4.1 pg 1-1 with a minimum value of 1.3/~g 1-1 and a maximum of 11.2 ~g 1-1 (Table 1). According to LAWA (1998) and OECD (1982), the trophic status of Lake Zansen is classified as mesotrophic.

Discussion

The present study determined the occurrence of mysids in all three lakes with high abundance in Lake Breiter Luzin and in the southern parts of Lake Schmaler Luzin and lower densities in Lake Zansen. Thus the current re- search showed a wider distribution of Mysis relicta in the Feldberg Lake District than investigations suggested in the 1980's. WATERSTRAAT (1988) detected larger quantities of mysids in Lake Breiter Luzin only, in Lake Zansen just a few individuals were caught and in Lake Schmaler Luzin no mysids occurred at that time. There- fore, the recent distribution of mysids is more comparable to historical records at the beginning of the twentieth century than to those of the eighties. In the 1920's, high quantities of mysids occurred in Lake Breiter Luzin as well as in the southern part of Lake Schmaler Luzin (TmENEMANN 1925, 1928). In other parts of Lake Schmaler Luzin and in Lake Zansen, only low numbers of mysids existed. There was no exact information on densities in the early studies. Therefore, a comparison of the population size is impossible.

Anthropogenic eutrophication of the lakes in the 1980's was the main reason for the decline of the mysid population (WATERSTRAAa" 1988). Loss of oxygen, especially in the hypolinmion, deteriorated the living conditions of mysids. From 1924 to 1981 02 decreased in the hypolimnion of Lake Breiter Luzin during summer stratification and there was HRS formation at the bottom since 1976 (RICHTER 1982). The same occurred in Lake Schmaler Luzin, where loss of oxygen was reported in the hypolimnion from 1962 to 1981 during summer stratification. No oxygen could be detected below 25 m

Limnologica (2004) 34,199-212

depth and H2S appeared up to 25 m in 1980. In the late summer of 1995, one year before lake restoration started, the majority of the hypolimnion was lacking oxygen (KOSCHEL et al. 2001). Since 1962, a decrease in hypolimnetic 02 content during summer stratification was seen in Lake Zansen and from 1970 to 1972, H2S appeared up to 36 m depth. In 1980 there was also a lack of oxygen in the metalimnion (RrCHTER 1983).

HORPPILA et al. (2003) suggested that unfavorable physicochemical conditions may indirectly cause a re- duction in mysid population by forcing them into habitats where they are vulnerable to fish predation. Refer- ring to the Feldberg Lake District that means the low oxygen content in the hypolimnion, caused by eutrophication, forced the mysids to areas of shallower depth, where higher water temperature and more intensive fish predation reduced the population.

In the last 20 years, water quality in the lakes of the Feldberg Lake District improved markedly: total phosphorus loading in Lake Breiter Luzin decreased from about 0.4 g m -2 a -~ in the 1980's to 0.07 g m -2 a -~ during 2001-2002 (KosCHEL, unpublished data). Lake Schma- ler Luzin also recovered as a result of a lake restoration program with deep water aeration and artificial calcite precipitation as the principal measures (DITTRICH & KOSCHEL 2002). Mean annual total phosphorus in the euphotic layer decreased from 30 ,ug 1 -t in 1995, before treatment (KosCHEL, unpublished data), to 21 pg 1 -I in the present study. Only the deepest parts of the lake (30-34 m) were free of oxygen in autumn 2002 (Fig. 8). Thus the population of mysids could disperse again. The mysid avoidance of the northern parts of Lake Schmaler Luzin may be caused by lower habitat quality, e.g. lower oxygen concentrations, but further investigations are


necessary for any concrete interpretation. Trophic conditions also improved in Lake Zansen. At the end of summer stagnation low oxygen values could only be measured below 35 m depth (Fig. 8). By 2002 oxygen deple- tion was not evident any more. The avoidance of the north-eastern parts of Lake Zansen by mysids can be ex- plained by the inflow from neighbouring Lake Wootzen which is eutrophic.

Mysis relicta, widely distributed in the northern hemi- sphere, can be divided into four species groups on the basis of electrophoretic analysis (V~aNOLA et al. 1994). The North American populations are identified as an in- dependent species (IV), distinct from three European groups. Species group III is known from a subarctic lake, species lI lives in Ireland, southwestern Scandi- navia and Karelia, whereas species I is distributed in Finland, Poland, and Germany. The German population in Lake Breiter Luzin is comparable to those of other lakes as determined by nighttime vertical net hauls. These lakes have varying physicochemical parameters depending on their trophic status (Table 2).

In Lake Michigan, mean densities at a nearshore station (40-45 m depth) ranged from 41 to 168 individuals m -2 over three years. Means of an offshore station (110 m) in the same sampling period were 210 to 373 m -2 (POTHOVEN et al. 2000). Maximum densities occurred in spring, early summer and autumn and were always associated with high catches of juveniles. LEHMAN et al. (1990) reported mysid numbers of 110 m -2 on average, ranging from 25 to 645 m -2 at a 100 m station in Lake Michigan. At different stations, between 30 and 50 m depth in the same lake in 1976, mysid abundance ranged from a low of 54 m -2 in December to a high of 1019 m -2 in September and averaged 466 m -2 (GROSS-

Table 2. Physicochemical parameters in various types of lakes inhabited by Mysis relicta. Data of temperature (T), total phosphorous (TP) and chlorophyll a (Chl a) are presented as epilimnetic means or mean range of several sampling stations.

Lake Year, T TP Chl a Secchi depth Type Reference sampling time [°C] [tJg I-q [pg I -~] [m]

Michigan 1994-I995, April-October

Ontario 1995-1997, May-October

Ontario 1996, April-May

Ontario 1990, May

Ontario 1981-1986, July-October

Hiidenvesi 1999, May-October

11.4-13.1 4.7-4.9 - - oligotrophic

9-21 9-12 1-2 5-7 mesotrophic

2.9 8.7 1.5 - mesotrophic

7.1 12.9 4.5 - mesotrophic

15-21 - - - mesotrophic

10-20 49 - 0.9 eutrophic

CARRICK et al. (2001)

HALL et al. (2003); GRAY et al. (1994)

MILLARD et al. (2004)

MILLARD et al. (2004)

JOHANNSSON & O%ORMAN (1991)

HORPPILA et al. (2003)

Limnologica (2004) 34, 199-212


NICKLE & MOROAN 1979). Abundance in Lake Ontario at a 125 m station was lowest in spring but increased in June and July due to the recruitment of young to the population (JOHANNSSON 1992). Mean abundance over a 7- month period for different years ranged from 217 m -2 to 602 m -2. KJELLBERG et al. (1991) observed mean density of about 200 individuals m -2 with maximum values in spring in Lake Mjcsa.

JOHANNSSON (1992, 1995) recognized that mysid densities varied in Lake Ontari o . Spatial patterns varied sea- sonally but in all seasons mysid densities increased as the depth of the sampling station increased from 20 m to 100 m. Even though samples in this study were not collected over a depth gradient, it is clear that mysid abundance in Lake Breiter Luzin did not increase with greater depth. Throughout the sampling period, densities were always lower at the lake's deepest point as compared to the 25 m stations (Fig. 5). Lower oxygen concentrations in some months may have been the reason for the avoidance of these regions. Below 40 m water depth, oxygen concentrations from August to December were of < 1 mg 1 -~. SHERMAN et al. (1987) reported mysid occurrence only at oxygen concentrations of > 1 mg 1 -~ while THIENEMANN (1925) discovered mysids just at oxygen values > 1.4 mg 1-1.

As shown by LEHMAN et al. (1990) for a population in Lake Michigan, diurnal vertical migration in Lake Breit- er Luzin caused the most conspicuous variation of mysids in time and space. Daytime vertical tows at all stations captured no animals, whereas at night mysids were distributed throughout the water column. But this vertical migration may be limited by increasing temperatures above the thermocline during summer and autumn. Mysids usually avoid warmer surface waters (BEETON 1960). DEGRAEVE & REYNOLDS (1975) reported rapidly increasing mortality of Mysis at temperatures above 13 °C. This could explain the low abundance in the upper layer (0-10 m depth) in October 2002 at Lake Breiter Luzin (Fig. 6). The temperature in this layer was above 13 °C at sampling time. However, mysid accumu- lation at the thermocline during the night to avoid warm epilimnetic water, as reported by BEETON & BOWERS (1982) and BEETON (1960), could not be observed at Lake Breiter Luzin. Nevertheless, sampling in smaller depth intervals would have been necessary to detect vertical distribution more accurately. The distribution of mysids in the Feldberg Lake District was not restricted by higher temperatures in deeper water layers. Hypolim- netic temperature was about 4 °C to 6 °C throughout the year (Fig. 8).

As in Lake Michigan, some reproduction occurred year-round in Lake Breiter Luzin. However, the size-frequency distribution suggested high influxes of juveniles to the total population in spring and in some cases in late summer or in late winter (POTHOVEN et al. 2000). Also

MORGAN & BEETON (1978) detected newly released juveniles in spring, summer, and early winter in Lake Michigan, indicating that some females produce a second brood within one year. A second brood may be the reason for the newly hatched individuals in autumn as seen in the size-frequency distribution of the present study (Fig. 7). This again could be responsible for the slight increase of the population in November 2001 and October 2002 in Lake Breiter Luzin (Fig. 4). In Lake Ontario breeding of female mysids started in October and most of the young were released between February and June (JOHANNSSON 1992). Size-frequency distribution in Lake Ontario showed at least two cohorts in April, when young-of-the-year were 4.5 mm in length and the yearlings were 11-14 mm long. One could fol- low the young-of-the-year cohort until end of October, when they reached approximately 10 mm in length.

Quantitative sampling of mobile and unevenly distributed organisms such as mysids may be problemati- cal. However, by comparing published data, it can be as- sumed that the estimated densities in Lake Breiter Luzin reflect the true abundance. SHEA & MAKAREWICZ (1989) showed that mysid abundance was accurately estimated by night-time vertical net hauls. They took benthic sled tows at night at the same time as they towed vertical net hauls and found that only 2-5% of the population re- mained at the bottom at two stations in Lake Ontario at 100 m and 35 m depth. Also GROSSN~CKLE & MORGAN (1979) detected that vertical net hauls at night are more accurate than day sled tows because the latter are only effective at intermediate depths in Lake Michigan. How- ever, during winter, the population of mysids at all sizes was low and could not account for the abundance of adults observed in early summer because the young, born in the spring, could not become adults in that time. Some animals must have been on the lake bottom and they may have a different spatial distribution during winter. In Lake Ontario, there was a great spatial and seasonal heterogeneity in densities and translocation of mysids. Strong hypolimnetic currents, as suggested by JOHANNSSON (1995), were a means of dispersal. Catch rates and size structure can also be affected by varying movements of some size groups (MORGAN & T~IRELKELD 1982). It is apparent that population dynamics of Mysis relicta vary between years, different lakes, and depend on several factors, suggesting that multi- year studies are necessary to adequately characterize mysid populations.

Acknowledgements We are grateful to J. Mathes from the Ministry of the Environ- ment Mecklenburg-Vorpommern providing digital maps of the lakes and unpublished data on Lake Zansen. M. Thomas

Limnologica (2004) 34, 199-212

helped processing the digital lake maps. We also appreciate J. Dalchow, R. Degebrodt, R. J~ihrling, H. Koschel, M. Lentz, U. Mallok, T. Mildebrath, R. Rossberg and M. Sachtleben for assistance in sample collection and sample treatment. Linde Adams is acknowledged for correcting the English. This study was funded by the Federal Ministry for the Environment, Na- ture Conservation and Nuclear Safety (BMU), represented by the Federal Nature Conservation Agency (BfN), and the Min- istry of the Environment Mecklenburg-Vorpommern.

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