ICES CM 2006/F:22
Daily feeding patterns and feeding rates of copepod Limnocalanus macrurus in the
Bothnian Sea (Baltic Sea)
The large calanoid copepod Limnocalanus macrurus inhabit deep big lakes and brackish
water bodies. The copepod is living in the Baltic Sea deepest parts. Because of its large
body size the species is important fish prey and play significant role in Baltic Sea
ecosystem. The study to ascertain Limnocalanus macrurus feeding rates – gut clearance
rate coefficient, ingestion rate - and daily vertical migrations related to reproduction and
feeding intensity was performed in October 2005 in the Bothnian Sea. Vertical migration
behaviour of Limnocalanus macrurus adults in the autumn could be attributed to the
reproduction and also feeding pattern. Sampling was done in 125 m deep Bothnian Sea
station after every 3 hours during day and night above, inside and below the thermocline
layer. It was clarified that adult copepods avoid migrating above the thermocline layer,
although the temperature boundary was at profound depth – about 55-75 m. To obtain
knowledge on Limnocalanus macrurus feeding ecology the aspects of feeding rates were
examined.
Keywords: Limnocalanus macrurus, migration, feeding.
Contact author:
Gunta Aišpure: Institute of Aquatic Ecology, University of Latvia, Daugavgrivas 8, LV1048, Latvia [tel: +371 7602301, fax: +371 7601995, e-mail: [email protected]].
INTRODUCTION
The Bothnian Sea is a brackish-water body in Baltic Sea situated in the western part of
Gulf of Bothnia with the maximum depth of 230m and average – 68m (HELCOM 1996).
Copepod Limnocalanus macrurus preferring to live in deep and cold water lakes (Roff,
Carter 1972; Watson 1974; Evans 1990) and brackish water basins (Holmquist 1970; Hernroth,
Ackefors 1979; Vuorinen, Ranta 1987) is comparatively more abundant in this Baltic Sea
part. Feeding is important factor influencing Limnocalanus macrurus population
development, but the feeding ecology of Limnocalanus macrurus is still poor known.
This research was performed to specify Limnocalanus macrurus feeding rates and
feeding intensity in relation to daily vertical migrations.
MATERIAL AND METHODS
The research was carried out from vessel Lotty (Umea Marine Sciences Centre, Sweden)
at Bothnian Sea in October 2005. Sampling was done during night and day in open sea
station with depth of 120m (Figure 1). Before sampling, temperature and salinity profiles
were measured from the top till bottom of water column.
Fig. 1: Sampling area – Gulf of Bothnia with sampling station in the Bothnian Sea.
Field work
Copepods for estimation of feeding rates were collected at daytime. Three vertical haul
through the entire water column was made and obtained samples were immediately
transferred to a filtered seawater. Afterwards 3 sub-samples were filtered on pieces of
plankton net (100 micron mesh size) every 10 minutes 8 times, then net pieces were
double wrapped, folded in aluminum foil and frozen immediately.
Sampling for finding Limnocalanus macrurus daily feeding intensity changes related to
vertical migration were performed at 4 different water layers: 0-20m, 20-40m, 40-60m
and 60-120m. Samples were collected every 2 hours 8 times during one night and day.
After sampling zooplankton was filtered on the net pieces, wrapped double in aluminum
foil and frozen immediately.
Sampling for daily vertical migration changes was performed every 3 hours during
daytime and nighttime at three water layers above, within and under the thermocline (055m, 55-75m and 75-120m), all the samples fixed with formalin.
Laboratory work
All the collected frozen samples were left at –200C till next processing. In the laboratory
20 animals of Limnocalanus macrurus 5th copepodite and adult development stages were
sorted under stereomicroscope and extracted into 5 ml of 90% acetone. Obtained extracts
were stored in darkness at +50C. Chlorophyll a content was obtained using gut
fluorescence method (Harris et al. 2000). Acetone extracts were analyzed with
fluorometer and chlorophyll a values calculated using following formula (Aminot, Ray
2001):
Chl a (mg/m3) = (F - intercept) / slope ,
where F is fluorometric reading of sample; intercept and slope - values from calibration
curve of pure chlorophyll a.
Gut clearance rate coefficient (k) is derived from a model of exponential decrease in gut
fluorescence over time, assuming that a constant percentage of the gut content is
evacuated per unit time (Kiørboe at al. 1985 cited by Harris et al. 2000):
Gt = Go * exp ( -k * t )
where Gt is gut content at time t, and G0 is the initial gut content (Harris et al. 2000).
Ingestion rate (I) is proportional to gut clearance rate but is expressed in different units.
To calculate ingestion rate below mentioned formula were used:
I=K*G
where G is gut contents (amount of chlorophyll a per animal) and K the gut clearance rate
constant (min –1) (Mauchline 1998).
The numbers of Limnocalanus macrurus were obtained by counting adults under the
binocular microscope.
RESULTS AND DISCUSSION
Temperature and salinity
Temperature, salinity and density at the station were determined before sampling (Figure
2). The temperature ranged between 20C and 110C and thermocline occurred at 55-75m
depth. The sea-surface temperature in the Gulf of Bothnia changes without any clear
trend probably due to the large influence of river discharge and vertical mixing
(HELCOM 1996). Most possible the profound thermocline layer in the sampling station
was formed because of windy weather at the sampling time. Salinity varied between 4
and 6,3 ‰, with halocline at approx. 10m depth, and gradually increased towards the
bottom. Consequently the increase of water density was stated from the top till bottom,
but sharp increase was at two depths – around 10m (salinity) and about 60m
(temperature).
Fig. 2: Vertical changes of hydrological parameters in the Bothnian Sea sampling station
in October 5 2005 (A): temperature (0C) and (B): salinity (‰) and density (sigma-t).
Daily feeding patterns
The feeding intensity of Limnocalanus macrurus differed at various depths. The mean
values at day and night of each water layer were at top 0-10m layer 23,31 mg/m3, at 1050m layer 8,65 mg/m3, at 50-90m layer 16,78 mg/m3 and at deepest 90-120m layer 6,80
mg/m3. So the top layer was most effective for feeding, but the second highest feeding
intensity was reached at thermocline layer. Thus the copepods can feed sufficiently also
at the deeper water layers. Because of great depth of thermocline, there couldn’t be any
light from surface and certainly no photosynthesizing algae. Most possible, that
Limnocalanus macrurus at the greater depths were feeding on yielding algae. The sinking
speed of phytoplankton decreases at the layer of sharp temperature change because of
greater water density (Fig 2B).
The maximum chlorophyll a concentration in the guts of L. macrurus – 30,21 mg/m3 –
were found at 0-10m top layer at daytime, but the lowest– 3,46 mg/m3 – at deepest 90120m layer in midnight (Fig.3.). At night and during sunrise the maximums of feeding
intensity with chlorophyll a value respectively 24,51 mg/m3 and 23,74 mg/m3 were
observed in the thermocline layer. The lowest feeding intensity was at deepest 90-120m
layer with chlorophyll a values at midnight only 3,46 mg/m3 and at sunrise 3,88 mg/m3.
In the midnight the numbers of Limnocalanus macrurus adults increased in the
thermocline layer because of vertical migration (Fig.4). But at rest of the night main part
of the population were located in the deepest part of water column – 90-120m.
Chlorophyll a c oncentration (mg/m3)
35
30
25
20
15
10
5
0
21:00 0:00
3:00
6:00
9:00 12:00 15:00 18:00
time (h)
0-10m
50-90m
10-50m
90-110m
Percentage of Limnocalanus
macrurus individual
Fig. 3. Daily chlorophyll a value changes in guts of Limnocalanus macrurus copepodites
(CV) and adults in October 2005 in the Bothnian Sea.
100%
80%
60%
40%
20%
0%
22:00 1:00
75-120m
4:00 7:00 10:00 13:00 16:00 19:00
time (h)
55-75m
0-55
Fig. 4. Limnocalanus macrurus adult percentage abundance changes in 0-55m, 55-75m
and 75-120m water layer in October 2005 in the Bothnian Sea open part.
At various times of day and night Limnocalanus macrurus feeding intensity was the
lowest at 10-50m layer between thermocline and top layer. Apparently this layer was not
preferred by Limnocalanus macrurus for feeding, but used mostly for vertical migrations.
Feeding rates
Exponential model of Limnocalanus macrurus gut clearance rate coefficient (Fig.5A)
shows that chlorophyll a in the copepod guts changes with coefficient 1,121 μg chl a/
ind. Ingestion rate (Fig. 5B) is proportional to the gut clearance rate coefficient and if
adjusted with exponential model, is constant – 12,564 μg chl a /ind*min. The
significance for the both models is relatively high - R2 =0,9082, n=8, p=0.01. Dagg and
Grill (1980) measured the ingestion rate of copepod Centropages tipicus in New York
Bight - 20-40 μg dry wt / ind. Peters and Downing (1984) summarized many literature
data and calculated that the mean feeding rate of marine copepods is 40 μg wet wt / ind *
d, but different copepod species ingestion rates in literature often are described as value
that changes to an asymptotic level according to the concentration of food (Marin et al.
1986; Mauchline 1998; Merrell and Stoecker 1998; Paffenhöfer 1998; Peters and
Downing 1984; Price and Paffenhöfer 1986). It is found that ingestion rate of large
copepod Paracalanus sp. in North Atlantic Ocean can reach nearly 200 μg C / μg C * d
according to food concentration level (Paffenhöfer 1998). Assuming that 1 μg of
chlorophyll is equal to 40 μg C (Verity et al. 1993 cited by Paffenhöfer 1998), ingestion
rate value of Limnocalanus macrurus will be high. The differences could be explained by
additional environmental factors like temperature. It is known that ingestion rate
increases with temperature (White and Roman 1992) and in some copepods with food
concentration to an asymptotic level, so there are seasonal changes in ingestion rate
(Kleppel 1992). As autumn is the reproduction period for Limnocalanus macrurus, the
feeding could be more intense in order to secure a higher quality offspring.
Gut clearance rate
(μg chl a / ind )
24
A (Gut clearance rate)
20
16
12
8
0.1195x
y = 11.209e
4
2
R = 0.9082
0
20
40
chl a
Ingestion rate
( μg chl a/
ind*min.)
250
50
time (min.)
60
70
Expon. (gut clearance rate)
B (Ingestion rate)
200
150
100
y = 125.64e 0.1195x
R2 = 0.9082
50
0
20
Chl a
40
50
tim e (m in.)
60
70
Expon. (Ingestion rate)
Fig. 5: Limnocalanus macrurus copepodite feeding rates in October 2005 in the Bothnian
Sea open part (A): exponential model of chlorophyll a changes in Limnocalanus
macrurus guts in time with gut clearance rate coefficient 11.209; (B): Limnocalanus
macrurus copepodite ingestion rate exponential model where ingestion rate 125.64 μg chl
a/ind*min.
CONCLUSIONS
1)
The top 0-10m layer is most effective for Limnocalanus macrurus feeding where
chlorophyll a concentration in the gut extracts reached 30,21 mg/m3.
2) Because of possible greater density of sinking algae in the thermocline layer
Limnocalanus macrurus feeding intensity reached second greater value - 24,51
mg/m3.
3) The ingestion rate is proportional gut clearance rate coefficient and value of
Limnocalanus macrurus ingestion rate constant is high – 12,564 μg chl a /ind*min.
The feeding rates could be influenced by additional environmental factors like
temperature or food concentration.
ACKNOWLEDGEMENTS
This research was supported by grant from Umea Marine Sciences Centre (Sweden) and
by Europe Social Foundation (ESF) project “Assistance to doctoral students and new
scientists in the University of Latvia”.
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