The consequence of the mass occurrence of
The results of previous studies indicate that the reproduction period of
Most studies on reproductive activity of
Considering the above information, the aim of this study was to indicate the abiotic parameters of water and bottom sediments, which significantly affect the reproduction of
It was hypothesized that, unlike in deep lakes, the reproduction period of this species is more dependent on the temperature of water above the bottom than the water oxygenation.
The research was carried out in 9 lakes. Almost all of them (except Lake Głuchowskie) are located in north-eastern Poland (Fig. 1) in the Iława Lake District, which is the western part of the Masurian Lake District. Unlike other lake districts in Poland, small and shallow lakes are predominant here (surface area < 100 ha, maximum depth < 5 m). Polymictic lakes constitute as much as 64% of all lakes in this region (Marszelewski 2005). The lakes under study (except Lake Głuchowskie) are located in a relatively short distance from the town of Iława. Whereas Lake Głuchowskie is situated about 30 km north of Toruń, near the village of Bielczyny. The studied lakes are shallow, polymictic and small (except Lake Jeziorak and Lake Karaś), and are characterized by a high trophic level (Table 1).
Basic characteristics of the studied lakes – Lake Jeziorak is the longest lake in Poland (length: 27.4 km, surface: 3230 ha). The study in this lake was carried out in a shallow bay part (its morphometry is provided in Table 1) located in the oldest southern part of the lake. F – forest, A – agricultural, M – marsh, T – affected by tourism E.c. – the abundance of elodeids refers to their density in the central part of the lake, assessed arbitrarily on a scale from 1 to 10, where 1 denotes the complete absence of plants, and 10 means that the macrophytes overgrow the entire water volume
Parameter
Macrophyte-dominated lakes
Phytoplankton-dominated lakes
shallower
deeper
Zielone
Karaś
Piotrkowskie
Głuchowskie
Ząbrowo
Jeziorak
Stęgwica
Kolmowo
Silm
Surface (ha)
20.2
423.3
54.4
24.1
12.7
80.0
39.7
43.4
58.8
Maximum length (m)
1375
4100
1450
740
960
2800
1880
1050
1625
Maximum width (m)
240
2250
850
475
215
1000
290
550
800
Maximum depth (m)
2.4
2.8
2.5
4.5
1.6
2.0
1.4
5.7
3.8
Mean depth (m)
1.3
0.6
1.7
2.2
1.1
1.3
1.0
3.1
2.0
Number of sampling locations
6
2
2
2
2
7
7
4
3
Depth at sampling locations (m)
1.5–2.2
1.6 ; 2.0
1.5 ; 2.0
3.0 ; 4.0
1.2 ; 1.5
1.0–1.5
1.2–1.4
2.0–3.0
3.5
Catchment
F
A/M
F
A
F/A
F/T
F
A
F
Wind exposure
low
high
medium
high
low
medium
medium
high
medium
Dominating elodeid species
E.c.
Ch.sp.
Ch.sp.
C.d.
C.d.
Elodeid abundance
5
7
8
8
9
1
1
1
1
The studied lakes were divided into macrophyte-dominated (5) and phytoplankton-dominated (4) lakes (Table 1). The first group included lakes where, due to the high water transparency, submerged plants cover the lake bottom also in central parts. In addition to the depth, the lakes also vary in their exposure to wind, abundance and the taxonomic composition of submerged macrophytes. Almost all of them (except Lake Głuchowskie) are surrounded by forests. The lakes dominated by phytoplankton, on the other hand, included those where, due to the low water transparency, the presence of macrophytes was limited to a very narrow littoral zone. These lakes were divided into two groups: shallower and deeper lakes. The first group included lakes where the ratio of euphotic depth to the depth of a lake was higher than 1, so light reached the bottom of a lake, whereas in the deeper phytoplankton-dominated lakes the above ratio was less than 1, so it was dark at the bottom.
Samples were collected once a month from January to December 2012. Water transparency was measured using a Secchi disc. We used a luxometer Slandi LX204 to assess the light conditions. We also calculated the vertical attenuation coefficient of light under water (E) (Scheffer 1998: 22, Eq. 2). Furthermore, to estimate the amount of light reaching the bottom, we multiplied the E value by the lake depth (D). This index (E × D) is related to the shade level at the lake bottom (the higher ED value, the less light reaches the bottom) (Scheffer 1998). Moreover, we calculated the euphotic depth (Scheffer 1998: 25, Eq. 7), i.e. the depth beyond which the light level falls below 1% of the surface irradiation and is considered too low to maintain positive net photosynthesis of algae.
We measured temperature, conductivity, pH and oxygen concentration of the near-bottom water layer with a core sampler and a MultiLine P4 (WTW) Universal Pocket Sized Meter.
The bottom sediments (0–5 cm top layer) were collected with the same core sampler. Water content was measured by oven-drying sediments to a constant weight at 104°C, and organic matter content (OC) was determined after igniting dried sediments at 550°C for 2 h. The latter parameter was expressed in two ways: (1) as a percentage of dry mass of the sediments and (2) in milligrams of dry mass per unit of fresh sediment volume (10 cm3). The former value is a standard method of presenting the OC of bottom sediments. However, in our opinion, the latter figure is a better predictor of the amount of organic matter at the bottom, potentially available to animals. Sediment oxygen demand (SOD) was estimated by adding 300 ml of 100% oxygen saturated tap water to a special dish containing 20 cm3 of fresh sediments. The oxygen uptake was measured after 1 h, at 20°C.
Samples of Oligochaeta in the macrophyte-dominated lakes were collected using a core sampler (catching area 40 cm2, 7–10 replicate samples). In the phytoplankton-dominated lakes, we used an Ekman-Birge grab (catching area 225 cm2, 2–4 replicate samples). The samples were rinsed using a 0.2 mm sieve and preserved in 4% formaldehyde.
An important aspect of the study was to indicate the relationship between the size of an individual and its ability to reproduce. Given that Oligochaeta – due to the pressure of predators – very often do not have the hind part of the body (Wiśniewski 1979), the size of individuals was determined based on the width of the 8th body segment. According to Jenderedjian & Unanian (1987), the diameter of the 8th segment (the widest part of the body) is the most commonly used and most practical parameter that characterizes the body size of Oligochaeta. The body width was measured using the IPS-512 Image Analyzer and then the individuals were divided into the following size classes:
class
width of the 8th segment (mm)
I
< 0.20
II
0.21–0.30
III
0.31–0.40
IV
0.41–0.50
V
0.51–0.60
VI
0.61–0.70
VII
0.71–0.80
VIII
0.81–0.90
IX
0.91–1.00
X
1.01–1.10
XI
1.11–1.20
XII
1.21–1.30
Individuals at different stages of the reproductive system development (mainly those with the developing clitellum) were considered as capable of reproduction. The proportion of such individuals was not calculated as usual in the whole population, but only among those individuals whose size (in this case, starting from class V) indicated the ability to reproduce. According to the authors of the presented paper, this approach allows for a more precise assessment of the reproductive activity of the species under study, because it does not take into account any individuals that are still too young to reproduce, and whose proportion in the total population may vary.
To indicate which of the measured abiotic parameters of the water and bottom sediments are correlated with the development of the reproductive system of
Small oligochaetes classified into class II, III and IV were not considered in this paper, as no individuals with a developed reproductive system were found among them. Over 2700 individuals of
Number of examined individuals of M – macrophyte-dominated, Ph-D – deeper phytoplankton-dominated, Ph-S – shallower phytoplankton-dominated
Lakes
Macrophyte-dominated
Ph-D
Ph-S
Sum
sum
Class
Zielone
Ząbrowo
Karaś
Piotrkowskie
Głuchowskie
Kolmowo
Silm
Stęgwica
Jeziorak
M
Ph-D
Ph-S
V
163
18
31
33
5
68
11
270
95
694
250
79
365
VI
204
22
15
20
12
100
54
327
91
845
273
154
418
VII
161
17
15
19
18
57
34
230
69
620
230
91
299
VIII
109
20
13
1
6
37
18
92
37
333
149
55
129
IX
26
13
12
4
2
16
12
43
22
150
57
28
65
X
13
10
9
1
5
8
9
6
61
33
13
15
XI
2
2
1
2
5
2
1
15
5
7
3
XII
1
1
1
Sum
678
102
96
78
43
286
142
973
321
2719
997
428
1294
The percentage of individuals with a developed reproductive system among the individuals capable of reproduction (starting from class V) varied in particular lakes and ranged from 14 to 72% (Fig. 2). It should be noted that the highest value was found in Lake Głuchowskie, where the number of examined individuals was the smallest one – only 43. On the other hand, in lakes with the highest number of studied individuals (Lake Stęgwica – 973 and Lake Zielone – 678), the proportion of sexually mature individuals was relatively small, 15 and 17%, respectively. There was no correlation between the lake type and the proportion of individuals with a developed reproductive system.
Single individuals with a developed reproductive system were observed already in class V (Table 3). Lake Jeziorak is distinguished by the fact that the reproductive organs were found in as many as 12% of individuals included in this class. Even though a higher value (20%) was determined in Lake Głuchowskie, only 5 individuals were in this class, thus only 1 specimen was capable of reproduction. It should be noted, however, that the highest percentage of sexually mature individuals in the distinguished classes was determined in the latter lake.
Percentage of sexually mature individuals of M – macrophyte-dominated, Ph-D – deeper phytoplankton-dominated, Ph-S – shallower phytoplankton-dominated
Lakes
Macrophyte-dominated
Ph-D
Ph-S
Average
Average
Class
Zielone
Ząbrowo
Karaś
Piotrkowskie
Głuchowskie
Kolmowo
Silm
Stęgwica
Jeziorak
M
Ph-D
Ph-S
V
0
0
0
0
20
1
0
1
12
2
< 1
1
4
VI
3
18
0
0
67
10
24
7
19
10
7
15
10
VII
20
47
40
26
78
25
15
17
35
24
28
21
21
VIII
42
70
54
100
100
32
44
36
73
46
50
36
47
IX
62
85
83
100
100
75
92
84
82
80
75
82
83
X
85
100
100
100
80
100
67
100
90
94
92
80
XI
100
100
100
50
80
100
100
87
100
71
100
XII
100
100
100
Generally, the percentage of reproducing
The percentage of individuals with a developed reproductive system varied in particular months (Fig. 3). The results do not allow to indicate a period in which the percentage of sexually mature individuals would be only relatively high. There were also no regularities related to the distinguished types of lakes. Noteworthy is the lack of individuals reproducing in the summer months. On the other hand, only individuals with a developed reproductive system were recorded in the period from March to May and in November and December.
Physical and chemical parameters of water and bottom sediments (annual average values) of the studied lakes are presented in Table 4. Water transparency was higher in the macrophyte-dominated lakes (1.7–2.1 m) than in the phytoplankton-dominated ones (0.7–1.4 m). Water temperature ranged from 11.2 to 14.0°C, oxygen concentration was relatively good in all lakes (6.4–10.8 mg O2 dm-3), pH values ranged from 7.2 to 8.7 and conductivity from 199 to 577 μS cm-1, except Lake Zielone, where it was clearly lower (100 μS cm-1).
Mean values of abiotic parameters of water and bottom sediments of the studied lakes percentage of dry mass of sediments in milligrams of dry mass per unit of fresh sediment volume (10 cm3) sediment oxygen demand (mg O2 per 20 cm3 of fresh sediments per 1 h))
Parameter
Macrophyte-dominated lakes
Phytoplankton-dominated lake
shallower
deeper
Zielone
Karaś
Piotrkowskie
Głuchowskie
Ząbrowo
Jeziorak∗
Stęgwica
Kolmowo
Silm
Secchi depth (m)
2.2
2.1
2.1
2.0
1.7
0.8
0.7
1.0
1.4
Temperature (oC)
13.5
13.4
14.0
12.6
12.3
12.1
11.2
12.3
13.4
Oxygen concentration (mg dm-3)
8.0
7.8
9.3
6.4
10.0
10.8
6.9
7.2
7.2
pH
7.5
7.9
8.0
8.6
8.3
8.7
7.2
8.5
8.0
Conductivity (μS cm-1)
100
496
253
577
428
309
199
428
425
E × D
2.4
2.7
2.2
5.5
2.7
4.4
2.9
8.3
7.6
Water content (%)
97.3
94.6
96.6
93.3
95.8
96.1
95.5
94.7
97.4
Organic matter (%)
78.4
39.2
64.9
32.6
49.8
51.7
54.8
34.6
58.6
Organic matter (mg)
170
217
226
242
215
136
252
189
159
SOD
2.6
3.3
3.2
6.2
5.1
4.7
4.3
8.2
3.4
E × D values were higher in the deeper phytoplankton-dominated lakes (7.6 and 8.3) compared to the other lakes (2.2–5.5). Water content in the bottom sediments was relatively high, ranging from 93.3 to 97.4%. The percentage of organic matter content per unit of dry mass of the bottom sediments (OC-%) varied considerably among the studied lakes, ranging from 32.6 to 78.4%. Organic matter content (in mg) per 10 cm3 of the fresh bottom sediments ranged from 136 to 252 mg. SOD values varied considerably from 2.6 to 8.2 mg O2 per 20 cm3 of fresh sediments per 1 h.
The highest values of the Pearson correlation coefficient between the percentage of individuals with a developed reproductive system and the measured parameters were found in the case of conductivity (r = 0.69;
Values of Pearson’s linear correlation coefficient (r) and the significance level (p) between the percentage of sexually mature individuals of E × D – index related to the light conditions at the lake bottom; WC – water content; OC (%) – organic matter content expressed as a percentage of dry mass of sediments; OC (mg) – organic matter content in milligrams of dry mass per unit of fresh sediment volume (10 cm3); SOD – sediment oxygen demand
Statistics
Near-bottom water
E × D
Bottom sediments
Temperature
Oxygen concentration
pH
Conductivity
WC
OC (%)
OC (mg)
SOD
r
−0.22
0.08
0.30
−0.21
−0.10
0.22
0.204
0.648
0.002
< 0.000
0.080
0.226
0.012
0.568
0.204
Research on abiotic parameters that significantly affect the reproduction of
The percentage of sexually mature individuals of
The percentage of sexually mature individuals of
A very important aspect of this study was to identify the abiotic parameters of water and bottom sediments having an important impact on the sexual maturity of
In this study, no correlation between the water temperature and the percentage of individuals with a developed reproductive system was found. These results may be somewhat surprising, since the thermal conditions are considered as one of the most important environmental factors affecting the reproductive activity of aquatic worms. The researchers generally agree that the reproduction of
Low oxygen concentration is another important environmental factor affecting the reproductive activity of
Both in the shallow and deep lakes under study, we found no individuals of