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Open Journal of Marine Science, 2015, 5, 280-289

Published Online July 2015 in SciRes.

http://www.scirp.org/journal/ojms

http://dx.doi.org/10.4236/ojms.2015.53023

Environmental Factors and Seasonal

Variation in Density of Mussel Larvae

(Mytilus

edulis)

in Danish Waters

Hans Ulrik Riisgård

, Kim Lundgreen

, Daniel Pleissner

Marine Biological Research Centre, University of Southern Denmark, Kerteminde, Denmark

Department of Bioengineering, Leibniz-Institute for Agricultural Engineering Potsdam-Bornim e. V., Potsdam,

Germany

Email:

[email protected]

Received 29 May 2015; accepted 13 July 2015; published 16 July 2015

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Abstract

Mussel larval densities may fluctuate considerably on both small spatial and short temporal scales.

So far, only few and scattered data on the occurrence of mussel larvae have been reported from

Danish waters. However, seasonal variation in density of blue mussel (Mytilus

edulis)

larvae as re-

lated to environmental factors (temperature, salinity, phytoplankton biomass) is basic informa-

tion of relevance for future line-mussel farming in Danish waters. Here we report on the density of

mussel larvae in a number of potential farming sites in the inner Danish waters. The mussel larval

density measured in Skive Fjord, an eutrophicated inner branch of Limfjorden, during a period of

20 years, from 1989 to 2009, along with corresponding temperatures and chlorophyll

makes up

the most important series of data reported here. In most years, a pronounced spring density peak

and a subsequently lower autumn peak could be seen in Skive Fjord, but most conspicuous in the

period 1993 to 2002 where the mean maximum spring larval density was 319 ± 260 ind∙l

−1

. Fur-

ther, data on mussel larval densities have been recorded on 4 locations in the Great Belt region:

Kerteminde Bay in 2008 to 2011, and in 2008 at 3 other locations: Musholm Bay, Svendborg Sund,

and Horsens Fjord. The maximum spring densities in the studied waters were observed in Skive

Fjord, typically in May, whereas 10 to 100 times lower peak densities were found at the other lo-

cations studied. The reported observations show that mussel larvae are omnipresent in the stud-

ied areas and it is suggested that the larval density is sufficient for recruitment to future line-

mussel farms.

Keywords

Mussel Larval Density, Seasonal Variation, Environmental Factors,

Mytilus edulis

Corresponding author.

How to cite this paper:

Riisgård, H.U., Lundgreen, K. and Pleissner, D. (2015) Environmental Factors and Seasonal Variation

in Density of Mussel Larvae (Mytilus

edulis)

in Danish Waters.

Open Journal of Marine Science,

280-289.

http://dx.doi.org/10.4236/ojms.2015.53023

L 111 - 2016-17 - Endeligt svar på spørgsmål 126: Spm., om ministeren vil fremsende de nævnte rapporter Miljømuslinger og muslinger som supplerende virkemidler fra 2013 og Combiopdræt 2015 samt artiklerne fra Aquaculture International 2014 og Open Journal, til miljø- fødevareministeren

1. Introduction

H. U. Riisgård

et al.

The common blue mussel,

Mytilus edulis

L. may reproduce at any time of the year, but in the temperate zone

spawning is most frequent during spring and summer

[1].

Temperature seems to be the most important factor

that determines the annual reproductive cycle, and a cold winter tends to synchronize spawning the following

spring

[2]-[4].

The fertilized egg develops within 24 h to a trochophora larva that soon after secrets the first lar-

val shell to become a free swimming D-shaped veliger larva (100 to 120 µm shell length) feeding on small 2 to

4 µm particles

[5]-[9].

This stage lasts between 3 and 5 weeks

[10],

i.e.

time to reach ~300 µm, depending on

environmental factors (temperature, food ration, salinity) until metamorphosis when an extensible foot appears

and the larva becomes a pediveliger. The pediveliger larva seeks out different types of substrates in preparation

for settlement which involves a sequence of swimming and crawling behavior that culminates in attachment

once a suitable substrate is chosen. After settlement, morphological change (metamorphosis) results in a young

post-metamorphic mussel

[5] [9].

It is well known that mussel larval densities may fluctuate considerably on

both relatively small spatial (tens of metres) and short temporal scales (days) depending on the hydrography

[11],

but although information on density and dispersal of mussel larvae is crucial for understanding recruitment to

the adult wild stock and to aquaculture farm-ropes, only few and scattered data have up to now been reported

from Danish waters.

The Danish mussel production mainly comes from fishery on wild stocks of blue mussels in Limfjorden

which is a shallow-water system that connects with the North Sea to the west and Kattegat to the east (Figure

1).

The annual landings of blue mussels were about 100,000 tons in the 1990’s resulting in overfishing and a sub-

sequent reduction of the mussel stock, and in 2006-2008 the mussel fishery declined to about 30,000 tons per

year which led to restrictions and a national policy that aims at developing a sustainable production of cultured

mussels in balance with the extensive fishery of mussels

[12]-[14].

In recent years, the total annual Danish mus-

sel harvest has been around 35,000 tons, with 70% coming from Limfjorden. However, eutrophication and sea-

sonal oxygen depletion in Limfjorden cause high mortality of bottom-living wild mussels during late summer

[15],

and therefore, line-mussel farming has recently been introduced to increase the production of mussels and

to mitigate the habitat disturbance of mussel dredging

[16].

The aim of a recent mussel-research project (MarBioShell, 2008-2012) was to evaluate the potential of the

Great Belt region between the Kattegat and the Baltic Sea (Figure

as a new line-mussel cultivation site to re-

duce the pressure on Limfjorden. Based on studies of actual growth of mussels in net-bags and on farm-ropes,

Riisgård

et al.

[16]

found that the growth rates in Great Belt compare quite well with the growth in Limfjorden

in spite of the lower salinity in Great Belt

[17]-[19],

and further, the deeper water and faster current speeds in

Great Belt are likely to prevent the eutrophication problems encountered in Limfjorden. The wild blue mussels

in the Great Belt have never been commercially exploited, but the MarBioShell studies showed that line-mussels

can grow from settlement in spring to more than 30 mm in shell length in November

[17].

However, to reach the

traditional consumer size of at least 45 mm takes about 18 months because of the winter period with weight loss

and subsequent re-growth during the next season, and therefore, Riisgård

et al.

[17]

suggested a new approach

of line farming of 30 mm “mini-mussels” during one growth season.

On this background, knowledge on seasonal variation in density of mussel larvae in Great Belt has become

important. Limfjorden and Great Belt have different levels of phytoplankton biomass (chlorophyll

a),

salinity

and current regimes, and therefore it has been of interest to compare the seasonal variation in density of mussel

larvae in Limfjorden with the abundance of mussel larvae in Great Belt and nearby waters as done in the present

study, where a 20 years long time series of mussel-larval data from Limfjorden is compared with new observa-

tions made on 4 locations in the Great Belt region. Such knowledge is of basic importance for understanding

time of mussel-larval settlement and amount of new recruitment to future mussel farms.

2. Material and Methods

2.1. Study Sites in Limfjorden and the Great Belt Region

Limfjorden connects the North Sea with the Kattegat, the mean water depth is about 5 m, and dominating wes-

terly winds cause a water exchange between the North Sea in west and Kattegat in the east (Figure

1),

and there

is a permanent horizontal salinity gradient, with 32 to 34 psu in the west and 19 to 25 psu in the east. Limfjorden

281

H. U. Riisgård

et al.

Figure 1.

Map of Denmark showing

Mytilus edulis

larval sampling sites in Limfjor-

den: (1) Skive Fjord, and in the Great Belt region: (2) Musholm Bay, (3) Svendborg

Sund, (4) Kerteminde Bay, (5) Horsens Fjord.

is eutrophic receiving nutrients from the catchment area which is dominated by agriculture

[15].

The water cur-

rent velocity 1 m above the bottom varies typically between <1 to 6 cm∙s

−1

, and the annual mean chlorophyll

concentration in the period 1982 to 2006 in the central northern part of Limfjorden (Løgstør Bredning) was 7.5 ±

12.1 µg∙l

−1

[18].

Data on mussel larval density in Skive Fjord, which is an inner branch of Limfjorden (Figure

1),

during the

period 1989 to 2009 were obtained from the Danish national monitoring program, Danish Nature Agency, Dan-

ish Ministry of Environment. According to the official monitoring guidelines (Miljøcenter Ringkøbing 2008), 20

l of water were sampled by means of a submersible pump (150 l∙min

−1

) resulting in equally sized subsamples

taken through the water column from bottom to surface by hauling the pump up with a speed of 0.5 m∙s

−1

. The

sampled water was filtered (60 µm mesh) to obtain a representative sample of the mussel larvae. Samples were

preserved in basic Lugol’s solution and examined in the laboratory by Orbicon A/S in order to determine the

concentration of mussel veliger larvae (and other zooplankton organisms).

Great Belt is one of the 3 Danish Straits (i.e. Little Belt, Great Belt, and The Sound), which form the transi-

tion between the high saline North Sea and the brackish Baltic Sea (Figure

1).

The water exchange between the

Baltic Sea and the open sea is driven both by the river run-off and by the meteorological conditions. The surface

salinity in the Great Belt is low, less than 20 psu whereas the salinity beneath the permanent halocline at about

15 m depth is high, about 30 to 34 psu. The current speed is usually about 50 cm∙s

−1

, and the annual mean chlo-

rophyll

concentration measured in the period 2000 to 2010 at 1 m depth in the northern Great Belt was 2.8 ±

2.4 µg∙l

−1

[17]-[19].

The present study reports on mussel larval density in the Great Belt region, including Horsens Fjord (Figure

1),

during the period 2008 to 2011. Water samples were collected at the 4 locations in 2008: Musholm Bay,

Svendborg Sund, Kerteminde Bay, and Horsens Fjord, and further in Kerteminde Bay in the period 2009 to

2011 by using a small conical zooplankton net (Ø24 cm, KC Denmark) with a mesh size of 80 µm and a 80 ml

282

H. U. Riisgård

et al.

collection cylinder. The plankton net was dropped to a known depth and then slowly hauled to the surface. The

sample was then transferred to a 100 ml bottle containing Lugol to conserve the mussel larvae. Before counting

in the laboratory using a reversed microscope (Leitz Labovert FS), the sample bottle was turned upside down

several times to ensure an even distribution of the mussel larvae. The total number of counted larvae was ex-

pressed as individuals per liter of filtrated water. The filtrated water volume (V) was calculated from the circular

area of the plankton net (A = 0.045 m

) and the length (L) of the vertical haul through the water column (V =

which was dependent on the depth at the sampling location (2 to 6 m).

2.2. Environmental Data

Data on chlorophyll

salinity and temperature in the Great Belt region and Skive Fjord were obtained from the

Danish national monitoring program, Danish Nature Agency, Danish Ministry of Environment. Temperature and

salinity were measured with a CTD at heights spaced 0.2 m apart, from 0.8 m below the surface down to about

0.3 m above the bottom. Chlorophyll

(chla) was determined from water samples taken from a depth of 1 m

and subsequently analysed at an authorized laboratory according to Danish national standards.

2.3. Statistical Analysis

One-way repeated measures analysis of variance (ANOVA) and Pearson product moment correlation (SigmaPlot,

version 12) were used to evaluate seasonal larval densities and temperature data, respectively. In all cases a sig-

nificance level of

= 0.05 was used.

3. Results

3.1. Larval Densities in Limfjorden and Great Belt Region

3.1.1. Limfjorden

The mussel larval density measured in Skive Fjord during a period of 20 years, from January 1989 to January

2009, along with corresponding temperatures and phytoplankton biomass expressed as chlorophyll

are shown

Figure 2

and

Figure 3.

In most years, a pronounced spring density peak and a subsequently lower autumn

(93)

(200, 120, 130)

T (°C), chl

(µg l

-1

)

-5

200

180

160

140

120

100

(s,787)

(s,511)

(s,283)

L (ind. l

-1

)

Figure 2.

Mytilus edulis.

Temperature (T, ˚C), chlorophyll

(C, µg∙l

−1

) (upper graph) and mussel larval

density (L, ind∙l

−1

) (lower graph) in Skive Fjord in the period 1989 to 1999. Spring (s) and autumn (a) peaks

are indicated. Values exceeding 75 µg chla∙l

−1

and 200 ind∙l

−1

are shown in brackets. Mean ± S.D. salinity in

the period was 24.3 ± 2.0 psu.

283

H. U. Riisgård

et al.

T (°C), chl

(µg l

-1

)

(110)

-5

150

120

(s, 712)

(s, 267)

L (ind. l

-1

)

Figure 3.

Mytilus edulis.

Temperature (T, ˚C), chlorophyll

(C, µg∙l

−1

) (upper graph) and mussel larval

density (L, ind∙l

−1

) (lower graph) in Skive Fjord in the period 2000 to 2009. Spring (s) and autumn (a)

peaks are indicated. Values exceeding 75 µg chla∙l

−1

and 150 ind∙l

−1

are shown in brackets. Mean ±

S.D. salinity in the period was 23.1 ± 2.1 psu.

peak can be seen. This trend, however, seems to be most conspicuous in the period 1993 to 2002 (with mean

maximum larval densities of 319.1 ± 260.0 ind∙l

−1

), and therefore the mean maximum spring and mean autumn

mussel larval densities for this period have been compared to previous (1989 to 1992, with 32.0 ± 30.3 ind∙l

−1

)

and subsequent (2003 to 2009, 56.7 ± 38.6 ind∙l

−1

) periods in

Table 1.

The few scattered data for the period

1989 to 1992 are not adequate for statistical tests, but statistical analysis showed that the spring density of mus-

sel larvae in the period 1993 to 2002 was significantly different (P

0.05) from the subsequent period 2003 to

2009 (F

(1,15)

= 7.528,

= 0.041). However, the difference in autumn larval density in the same two periods (46.2

± 35.6

versus

37.5 ± 35.8 ind∙l

−1

) was not significantly different (F

(1,15)

= 0.00249,

= 0.962). The mean annual

temperatures in Skive Fjord from 1982 to 2009 (Figure

varied between 9.5 to 13˚C, and a statistical test

showed that there has been no significant increase in the mean annual temperatures (P = 0.452).

3.1.2. Great Belt

The densities of mussel larvae recorded in Kerteminde Bay in the period 2008 to 2011 are shown in

Figure 5,

and the date of maximum spring density of 3.4 to 7.9 ind∙l

−1

appears from

Table 2.

The larval density recorded

in Musholm Bay, Svendborg Sund and Horsens Fjord in 2008 are shown in

Figure 6

along with salinity, tem-

perature and chlorophyll

and mean values of the latter in the sampling period are shown in

Table 3.

It is seen

that the maximum spring densities vary between 2.7 (Musholm Bay) and 12.4 larvae∙l

−1

(Svendborg Sund).

There is no statistical difference (P

0.586,

= 1.936 with 3 degrees of freedom) in mussel larval density be-

tween the 4 locations in the Great Belt region. However, due the limited number of measurements of larval den-

sity in Kerteminde Bay, only the period between July and October 2008 could be investigated for all locations.

4. Discussion

A pronounced spring peak in mussel larval density is typical for all locations studied, and the maximum spring

densities are found in Limfjorden (Figure

and

Figure 3, Table 1

and

Table 4)

whereas much lower peak den-

sities are found at the 4 locations in the Great Belt region. (Figure

and

Figure 6, Table 2

and

Table 3).

The

first yearly occurrence of mussel larvae, typically between mid-May and early June, appears to be closely re-

lated to the annual temperature cycle which apparently synchronize the spring spawning which seems to take

place when the water temperature has increased to about 12˚C, as reflected in the subsequent first appearance of

284

H. U. Riisgård

et al.

Temperature (°C)

Figure 4.

Annual mean (±S.D.) temperatures in Skive Fjord in the period 1982 to 2009.

-2

T (°C), S (psu), chl

(µg l

-1

)

L (ind. l

-1

)

Figure 5.

Mytilus edulis.

Salinity (S, psu), temperature (T, ˚C), chla (C, µg∙l

−1

), and mussel larval density (L,

ind∙l

−1

) in Kerteminde Bay in 2008 to 2011.

Table 1.

Mytilus edulis.

Mean ± S.D. maximum larval densities in Skive Fjord in spring and autumn in 3 periods between

1989 and 2009 along with corresponding water temperature, salinity and chlorophyll

Years

1989-1992

1993-2002

2003-2009

1989-1992

1993-2002

2003-2009

Season

Spring

Autumn

Max. density (ind∙l

−1

)

32.0 ± 30.3

319.1 ± 260.0

56.7 ± 38.6

11.0

46.2 ± 35.6

37.5 ± 35.8

Temp. (˚C)

18.4 ± 3.6

13.8 ± 3.9

13.4 ± 3.4

14.3

15.3 ± 1.7

16.9 ± 1.7

Salinity (psu)

23.0 ± 2.6

22.0 ± 1.8

22.8 ± 1.6

25.9

24.7 ± 1.9

24.3 ± 1.5

Chla (µg∙l

−1

)

19.3 ± 2.5

17.9 ± 8.0

7.9 ± 6.5

17.0

31.8 ± 39.4

11.8 ± 11.5

285

H. U. Riisgård

et al.

Figure 6.

Mytilus edulis.

Salinity (S, psu), temperature (T, ˚C), chlorophyll

(C, µg∙l

−1

), and mussel larval

density (L, ind. l

−1

) in 2008 in (a) Musholm Bay; (b) Svendborg Sound; (c) Horsens Fjord.

Table 2.

Mytlilus edulis.

Maximum density of mussel larvae (peak) in Kerteminde Bay along with corresponding tempera-

ture, salinity and chlorophyll

measured at the peak date in the years 2008 to 2011.

Year

2008

2009

2010

2011

Date

17-July

22-June

16-July

18-May

Max. density (ind∙l

−1

)

3.4

4.6

3.9

7.9

Temp. (˚C)

16.8

14.1

20.1

10.9

Salinity (psu)

16.6

20.8

13.6

14.2

Chla (µg∙l

−1

)

1.7

0.9

2.4

1.6

Table 3.

Mytlilus edulis.

Maximum density of mussel larvae (peak) on 3 mussel larval sampling locations in 2008 along with

corresponding temperature, salinity and chlorophyll

measured at the peak date.

Location

Musholm Bay

Svendborg Sund

Horsens Fjord

Date

10-July

17-June

Max. density (ind∙l

−1

)

2.7

12.4

5.7

Temp. (˚C)

18.4

15.3

14.8

Salinity (psu)

12.9

15.8

20.4

Chla (µg∙l

−1

)

0.7

3.2

mussel larvae. After the maximum summer peak, mussel larvae may be found at low densities throughout the

season until a less pronounced secondary and shorter autumn peak may appear, as typically seen in Limfjorden

(Figure

and

Figure 3, Table 4),

but not in the Great Belt region (Figure

and

Figure 6).

In all cases, the

mussel larvae disappear when the autumn temperature has decreased below about 12˚C. This pattern reflects the

annual spawning cycle of mussels in temperate waters as described by Dare

[3].

The significant decrease in veliger larval density in Limfjorden after 2002 is not obvious. Statistical analysis

showed that the annual mean temperatures in Limfjorden (Skive Fjord) did not change significantly in the period

1982 to 2010 (Figure

4),

but the decrease in larval density may perhaps be correlated with overfishing (dredging)

and reduction of the mussel stock in Limfjorden

[12] [14],

or a tendency towards more severely reduced oxygen

concentrations and mass-killing of mussels during summer

[15].

Dolmer & Stenalt

[13]

measured the mussel larval densities from April to September 1999 at 2 locations in

Limfjorden, namely in the northern central part (Løgstør Bredning) and in Skive Fjord. At both locations the

286

Table 4.

Mytlilus edulis.

Maximum mussel larval density (L), water temperature (T), salinity (S) and chlorophyll

concen-

tration (C) on collection dates in Skive Fjord in the period 1989 to 2009. s/a = spring/autumn.

Year

1989

s/a

Date

10-Jul

23-May

1-Jun

7-Sep

4-Jun

23-Aug

16-May

5-Sep

27-Jun

4-Sep

13-May

9-Sep

20-May

15-Sep

11-May

28-Sep

21-Jun

11-Oct

10-May

7-May

17-Sep

8-Apr

9-Sep

26-May

1-Sep

27-Sep

17-May

12-Sep

22-May

25-Sep

7-May

23-Jul

9-Jun

6-Jul

6-May

10-Aug

Max. L (ind∙l

−1

)

193

145

787

511

283

117

712

267

112

100

114

T (˚C)

20.6

14.2

20.4

14.3

15.1

15.0

15.1

16.1

20.0

16.1

9.3

16.2

13.5

14.3

12.5

14.3

15.8

12.5

18.4

10.4

14.3

7.6

18.5

13.1

16.9

13.6

12.0

16.8

12.6

16.6

11.0

17.7

20.2

20.1

11.7

20.5

20.9

23.0

23.8

23.1

4.8

3.7

2.1

3.5

26.2

24.2

25.5

23.8

25.1

11.0

5.9

5.7

7.7

13.0

19.4

23.2

20.4

21.6

21.3

22.9

10.0

15.0

8.1

34.0

19.0

34.0

20.2

25.9

24.6

27.6

21.3

24.9

21.4

23.7

21.7

25.3

23.4

26.8

24.6

25.5

23.2

23.4

20.3

22.0

17.0

15.0

2.0

34.0

49.0

27.0

9.0

17.0

13.0

18.0

130.0

17.0

20.0

22.0

14.0

11.0

25.3

17.0

S (psu)

23.5

C (µg∙l

−1

)

H. U. Riisgård

et al.

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

287

H. U. Riisgård

et al.

densities were below 20 larvae∙l

−1

until mid-April, when the densities begun to increase and subsequently peak

in mid-June with 579 larvae∙l

−1

and in May-June with 167 larvae∙l

−1

in Løgstør Bredning and Skive Fjord, re-

spectively. A small secondary peak was observed in Løgstør Bredning in August-September whereas no secon-

dary autumn peak was observed in Skive Fjord. The observations from Skive Fjord are not completely in agree-

ment with the observations for 1999 shown in

Figure 2,

where a distinct summer peak (with 98 larvae∙l

−1

Table

and an autumn peak (44 larvae∙l

−1

) is seen. This underlines the importance of spatial and temporal variations

which in Limfjorden are mainly caused by a combination of wind- and density-driven water movements

[15].

Larsen

et al.

[20]

investigated the temporal patterns of bivalve larvae in the Isefjord, Denmark, and found that

the total concentration of bivalve larvae peaked in late June and early July, with one plankton sample having a

much higher concentration than the other samples with a density of 188 ind∙l

−1

. A small autumn peak with 7.8

ind∙l

−1

was observed in late August, and between these peaks the density ranged between 1.4 and 6.4 ind∙l

−1

. In

an earlier study in the Isefjord, Jørgensen

[4]

followed a cohort of bivalve larvae, mainly (about 90%)

Mytilus

edulis,

during its residence in the plankton. The initial mean density of 90 µm shell length larvae was 3150

ind∙l

−1

in late May, but this very high density must be considered as “an extreme peak situation in boreal neretic

waters”

[11],

and has not been report ever since. The abundance of mussel veliger larvae has also been studied

in the embayment of Knebel Vig, Denmark, by Fotel

et al.

[11]

who found that the larval density varied from

1.7 to 40.4 ind∙l

−1

in 1994 and 2.7 to 99.0 ind∙l

−1

in 1995. These observations along with the present account on

mussel larvae in Limfjorden and the Great Belt region show that mussel larvae are omnipresent in the inner

Danish waters, although the density may vary conspicuously between areas due to differences in number of

spawning mussels and hydrography. A recent study by Larsen

et al.

[21]

showed that even a low density of 2

larvae∙l

−1

in the vicinity of a suspended mussel-rope in Great Belt (Kerteminde Bay) resulted in 58 and 291 ju-

venile mussels cm

−1

after 16 and 27 d, respectively. Thus, the larval density in most Danish waters seems to be

sufficient for recruitment to many future line-mussel farms.

Acknowledgements

This work formed part of the MarBioShell project supported by the Danish Agency for Science, Technology and

Innovation for the period January 2008 to December 2012. Thanks are due to Sandrine Serre, Coralie Barth-

Jensen, and Caroline Vandt-Madsen for practical assistance, to the Danish Nature Agency, Danish Ministry of

the Environment, for providing zooplankton and hydrographical data.

References

[1]

[2]

[3]

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