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122 Seiten, Note: Degree Awarded
List of Tables
List of Figures
List of Plates
Chapter 1. Introduction
1.2. Objective and Rationale of the study
1.3. Study area
Chapter 2. Materials and Methods
2.1. Sampling sites
2.2. Sampling Protocols
2.2.1. Physico-chemical factors
2.2.2. Chlorophyll a and Phytoplankton
2.2.4. Crustacean zooplankton
2.2.5. Sampling for diel migration studies
2.3. Sample Analysis
2.3.1. Chlorophyll a and Phytoplankton
2.3.3. Crustacean zooplankton
2.4. Data Analysis
Chapter 3. Result
3.1. Meteorological conditions during the sampling period
3.2. Physico-chemical factors
3.2.2. Dissolved oxygen
3.2.5. Secchi depth
3.3. Chlorophyll a and Phytoplankton
3.3.1. Chlorophyll a
3.5. Crustacean zooplankton
126.96.36.199. Spatial pattern
188.8.131.52. Temporal trend
3.5.3. Size structure
184.108.40.206. Spatial pattern
220.127.116.11. Temporal trend
3.6. Diel migrations
3.6.1. Diel Vertical Migration (DVM)
18.104.22.168. Chlorophyll a
22.214.171.124. Crustacean zooplankton abundance
3.6.2. Diel Horizontal Migration (DHM)
126.96.36.199. Crustacean zooplankton abundance
Chapter 4. Discussion
4.1. Physico-chemical factors
4.1.2. Dissolved oxygen
4.1.5 Secchi depth (Zs)
4.2. Chlorophyll a and Phytoplankton
4.2.1. Chlorophyll a
4.4. Crustacean zooplankton: Abundance and size structure
4.5. Diel migration studies
4.5.1. Diel Vertical Migration (DVM)
4.5.2. Diel Horizontal Migration (DHM)
Chapter 5. Conclusions
Ballincollig reservoir is a small eutrophic lake located west of Cork city and south of Ballincollig town in county Cork, Ireland. The reservoir was studied for six months, from 7th February through 27th July, for the factors that may influence or regulate population dynamics (i.e. abundance and size structure) of the crustacean zooplankton. Various physico-chemical and biological parameters were measured and related to abundance and size structure of the crustacean zooplankton species/groups using the non- parametric Spearman’s correlation test. Friedman’s test was employed for the purpose of data analysis.
Three cladoceran species: B. coregoni Baird, B. longirostris O.F.Muller, D. hyalina Leydig, and a genus Ceriodaphnia Dana as well as cyclopoid copepods were investigated during the study period. Water variables such as subsurface water temperature, dissolved oxygen, conductivity, and pH as well as secchi depth and chlorophyll a concentration did not show statistically significant spatial variation within the reservoir where as they varied significantly over time.
Factors such as fish predation (top-down control), type and availability of phytoplankton (bottom-up control), water temperature and competition for resources among the crustacean zooplankton were found to regulate or influence abundance and size structure of the crustacean zooplankton. Top- down control by fish was important particularly in structuring the spatial dynamics of the crustacean zooplankton population within the reservoir whereas bottom-up control, water temperature and competition regulated mainly the temporal trend of the population dynamics. The cyprinid fish fauna appeared to influence the temporal dynamics of the crustacean zooplankton rather indirectly through enhancement of phytoplankton availability by stirring up the nutrients from the sediments due to their feeding habit.
First and foremost my thanks goes to the Almighty God who has helped me in all aspects from beginning to end. Next I am very grateful to my supervisor, Dr Debbie Chapman, who was very cooperative and helpful to me throughout my research work. The friendly and welcoming approach Dr Chapman had for me in the midst of her busy time was inspiring and unforgettable to me.
I am deeply indebted to Development Cooperation Ireland (DCI) for sponsoring and funding my entire study and research during the last two years. I want to express my sincere gratitude to Allen Whitaker and Luke Harman, both from my department (ZEPS), without whom my field work would have not been possible. Thanks to Nora Buttimer from ZEPS also for her help and cooperation in analyzing the water samples.
Thanks to Cathleen O’Sullivan from the department of Mathematics and Statistics, UCC, for her guidance in data analysis. Finally I would like to thank my family and all my friends for their encouragements, love and prayers.
2.1. Locations of the sampling sites
3.1. Summary of Friedman’s test for the spatial pattern in the average subsurface water temperature (oC), n = 9, during the entire sampling period
3.2. Summary of Friedman’s test for the temporal trend in the average subsurface water temperature (oC), n = 5, during the entire sampling period
3.3. Summary of temperature (oC)-depth (m) profiles measured at site 01(near tower) during the entire sampling period
3.4. Summary of Friedman’s test for the spatial pattern in the average subsurface water dissolved oxygen (mg / L), n = 8, during the entire sampling period
3.5. Summary of Friedman’s test for the temporal trend in the average subsurface water dissolved oxygen (mg / L), n = 5, during the entire sampling period
3.6. Summary of dissolved oxygen (mg / L)-depth (m) profiles measured at site 01(near tower) during the entire sampling period
3.7. Summary of Friedman’s test for the temporal trend in the average subsurface water conductivity (μS / cm), n = 5, during the entire sampling period
3.8. Summary of Friedman’s test for the temporal trend in the average subsurface water pH, n = 5, during the entire sampling period
3.9. Summary of Friedman’s test for the temporal trend in the average secchi depth (m), n = 5, during the entire sampling period
3.10. Summary of Friedman’s test for the temporal trend in the average chlorophyll a (mg / m), n = 5, during the entire sampling period
3.11. Summary of Friedman’s test for the comparison in the average abundance, n = 10, of each crustacean zooplankton species/group, among the sampling sites during the entire sampling period
3.12. Summary of Friedman’s test for the comparison in the average abundance, n = 10, among the crustacean zooplankton species/ groups, at each sampling site during the entire sampling period
3.13 (A to E). Summary of Friedman’s test for the temporal trend in the average abundance, n = 5, of each crustacean zooplankton species/group during the entire sampling period
3.14. Spearman’s correlation test of relationship between the crustacean zooplankton abundance, the water variables and chlorophyll a
3.15. Summary of Friedman’s test for the spatial pattern in the average size of the crustacean zooplankton species/groups during the entire sampling period
3.16 (A to E). Summary of Friedman’s test for the temporal trend in the average size, n = 5, of each crustacean zooplankton species/group during the entire sampling period
3.17. Summary of temperature (oC)-depth (m) profiles measured at sampling site 01 (near tower) for DVM study
3.18. Summary of average abundance (no / L) of the crustacean zooplankton species/groups sampled for DHM study
4.1. Trophic classification scheme for lake waters proposed by the OECD based on the secchi depth (water transparency) (From: McGarrigle et al., 2002)
4.2. Trophic classification scheme for lake waters proposed by the OECD based on the chlorophyll a concentration (From: McGarrigle et al., 2002)
2.1. A sketch of the study area, Ballincollig reservoir, and locations of the sampling sites
3.1. A comparison of subsurface water temperature values (oC) among the sampling sites and a trend in the average subsurface water temperature, for n = 5, during the sampling period
3.2. A comparison of subsurface water dissolved oxygen values (mg / L) among the sampling sites and a trend in the average subsurface water dissolved oxygen, n = 5, during the sampling period
3.3. A comparison of subsurface water conductivity values (μS / cm) among the sampling sites and a trend in the average subsurface water conductivity, n = 5, during the sampling period
3.4. A comparison of subsurface water pH values among the sampling sites and a trend in the average subsurface water pH, n = 5, during the sampling period
3.5. A comparison of secchi depth values among the sampling sites and a trend in the average secchi depth, n = 5, during the sampling period
3.6. A comparison of chlorophyll a (mg / m) values among the sampling sites and a trend in the average chlorophyll a, n = 5, during the sampling period
3.7. A comparison of average abundance (no / l) of the two phytoplankton categories and the average chlorophyll a over the sampling period
3.8. Frequency of occurrence of the prey items (crustacean zooplankton, algae and others) in the diet of different fish specimen
3.9. Percentage composition (%) by number of prey items from fish gut analysis
3.10. Abundance of each of the crustacean zooplankton species/ groups (A to E) identified within the reservoir during the entire sampling period
3.11. A) Average size (μ m) of the B. coregoni Baird measured at the different sampling sites during the sampling period
B) Average size (μm) of the B. longirostris O. F. Muller measured at the different sampling sites during the sampling period
C) Average size (μm) of the Ceriodaphnia Dana measured at the different sampling sites during the sampling period
D) Average size (μm) of the D. hyalina Leydig measured at the different sampling sites during the sampling period
E) Average size (μm) of the Cyclopoid copepods measured at the different sampling sites during the sampling period
3.12. Average chlorophyll a concentrations (mg / m) sampled at different depths of site 01(near tower) for DVM study
3.13. Average abundance (no / L) of the crustacean zooplankton species/group at different depths at site 01on 6th July
3.14. Average abundance (no / L) of the crustacean zooplankton species/groups at different depths at site 01 on 20th July
3.15. Average abundance (no / L) of the crustacean zooplankton species/groups sampled for DHM study on 6th July
3.16. Average abundance (no / L) of the crustacean zooplankton species/groups sampled for DHM study on 20th July
1.1. Different aspects of Ballincollig reservoir
I. a) Abundance (no / L) of B. coregoni Baird during the study period
b) Abundance (no / L) of B. longirostris O. F. Muller during the study period
c) Abundance (no / L) of Ceriodaphnia Dana during the study period
d) Abundance (no / L) of D. hyalina Leydig enumerated during the study period
e) Abundance (no / L) of Cyclopoid copepods enumerated during the study period
II. a) Average (i.e. mean) size (μm) of B. coregoni Baird measured during the study period
b) Average (i.e. mean) size (μm) of B. longirostris O. F. Muller measured during the study period
c) Average (i.e. mean) size (μm) of Ceriodaphnia Dana measured during the study period
d) Average (i.e. mean) size (μm) of D. hyalina Leydig measured during the study period
e) Average (i.e. mean) size (μ m) of Cyclopid copepods measured during the study period
III. a) The 95 % confidence interval for an estimation of a population mean size (μm) of B. coregoni Baird measured during the study period
b) The 95 % confidence interval for an estimation of a population mean size (μm) of B. longirostris O.F.Muller measured during the study period
c) The 95 % confidence interval for an estimation of a population mean size (μm) of Ceriodaphnia Dana measured during the study period
d) The 95 % confidence interval for an estimation of a population mean size (μm) of D. hyalina Leydig measured during the study period
e) The 95 % confidence interval for an estimation of a population
mean size (μm) of cyclopoid copepods measured during the study period
IV. a) Length-Weight measurement of rudd (Rutilus rutilus L.)
b) Length-Weight measurement of the hybrid (of rudd and bream) fish
c) Length-Weight measurement of gudgeon (Gobio gobio L.)
Zooplankton are small animals that live suspended in the water column and that are carried mainly by water currents rather than by their own ability to move. The freshwater zooplankton principally comprise protozoans, rotifers, and crustaceans, and these range in size from small protozoan flagellates less than 2 μm in their longest dimension to large crustaceans of several millimetres long. Crustacea, together with Rotifera, are the dominant groups of zooplankton communities in freshwater lakes in terms of biomass and productivity (Paterson, 2001).
The majority of cladoceran species are microscopic, with most of the body being contained within a bivalved chitinous carapace. Protruding from the carapace is a head bearing a pair of large biramous second antennae that are used to propel the animals through water actively, which gives them their common name, water fleas. The first antennae and the second maxillae are much reduced. The head also bears a single huge compound eye formed by the fusion of ancestral compound eyes from each side of the head. The eye is not stalked but can be rotated in various directions by the associated musculature. The thoracic region bears five to six pairs of appendages, which generate feeding and respiratory currents. Food particles are filtered from the water by fine setae on the thoracic appendages. The abdominal appendages are absent in cladocerans (Pechenik, 1996).
The copepods comprise two major subgroups namely calanoid and cyclopoid. The two subgroups are distinguished in that the former possess long antennae that stretch down virtually all the length of the body and the mature females bear a single egg sac in contrast to the latter that possess short antennae and the mature females bear a pair of egg sacs (Moss, 1988). Locomotion in the planktonic copepods, like in cladocera, is accomplished primarily by the action of biramous second antennae. The adult members of copepods are usually a little larger than cladocerans.
Cladocera and copepods have different life histories and strategies. The cladocera, like rotifers, are parthenogenetic whereby females produce asexually broods of, usually diploid (2N), eggs that hatch into females without fertilization. The eggs are born dorsally in pouches deep in the carapace and soon develop into young that resemble the parent in all essentials, except size. The young are then liberated and grow into females large enough to reproduce. However, at certain times of the year, such as during adverse environmental conditions (e.g. temporary drying up of ponds or shortage of food), the males (haploid, N) make their appearance, and the females acquire the power of producing special eggs (haploid, N) requiring fertilization. The fertilized eggs (diploid, 2N) become thick walled, often termed as “winter or resting” eggs, darker and more opaque than the “ordinary or summer” eggs. The “winter” eggs eventually give rise to further generations of parthenogenetic females when suitable conditions recur. The “winter” eggs are contained within resistant cases known as ephippia (sing. ephippium) although these are not common to all cladoceran females. They are more developed in Daphniidae and Macrothricidae, but occur in various less developed states in Bosminiidae and many Chydoridae (Scourfield and Harding, 1966; Kalff, 2002).
In copepods, however, each generation is sexual, and before the new mature adults are formed, there are eleven successive moults in the life cycle. The first six, after the eggs hatch, are juveniles called nauplii, which look quite different from the adults. The next five stages, the copepodites, do look like the sixth copepodite, the adult (Ibid).
The cladocera include both the filter feeder herbivorous genera, such as Daphnia and Bosmina, and the carnivorous genera (e.g. Leptodora and Polyphemus) that actively grasp their preys, usually the smaller zooplankton. Free-living omnivorous suspension feeding is normal for the planktonic copepods, mainly the calanoids, but a number of the cyclopoid copepods are carnivorous. The size of particles taken, however, could vary depending on the size of the organism (Kalff, 2002).
In freshwater lakes or reservoirs zooplankton population dynamics, such as abundance and size structure, species composition and distribution, etc., can be influenced by changes in the physico-chemical conditions of the water such as water temperature (Wolfinbarger, 1999), bottom-up forces (i.e. availability and suitability of food) (Makarewicz, 1985), top-down forces (i.e. predation), or competition for food. For example, temperature can adversely affect zooplankton populations through biotic effects such as increases in filamentous cyanophytes, which are not easily filterable by crustacean zooplankton, or increases in predators (Threlkeld, 1987 cited in Wolfinbarger, 1999). However, temperature has also been positively correlated with zooplankton birth rates in laboratory experiments (Hebert, 1978).
Predation pressure on zooplankton communities in lake or reservoir systems could be from invertebrates (Yan and Pawson, 1997; Boudreau and Yan, 2003), fish (Christoffersen, et al., 1993; Dettmers and Wahl, 1999; Tatrai et al., 2003; Vasek et al., 2003), or both invertebrates and fish (Almond et al., 1996; Boronat and Miracle, 1997). Therefore, high densities of zooplanktivorous fish and invertebrate predators can affect the general size structure and taxonomic composition of zooplankton communities. Filter-feeding fish, such as silver carp, can also influence the dynamics of zooplankton populations via competition for food (Lu et al., 2002).
Fish can also positively affect the zooplankton community, directly through the release of zooplankton from predation by macroinvertebrate predators (Sorano et al., 1993) or indirectly by enhancing phytoplankton growth through nutrient regeneration (Attayde and Hansson, 2001). Furthermore, populations of small, inconspicuous zooplankton taxa may also benefit from size selective fish predation because they are released from competition with larger zooplankton (Slusarczyk, 1997).
Diel vertical migration (DVM) is an upward during darkness and downward during daylight migration behaviour among cladocerans and copepods mainly in response to predator pressure (Lampert, 1989; Loose, 1993). Most species migrate upward from deeper waters to more surficial strata as darkness approaches and return to deeper strata at dawn. Thus maximum numbers can be found in the surface layers some time between sunset and sunrise. In other cases, twilight migration results in two maxima in surface layers, one at dawn and another at dusk (Wetzel, 2001).
Diel horizontal migration (DHM) of zooplankton is a daytime aggregation of zooplankton in macrophyte beds and a night time movement out into the pelagic zone that usually occurs in shallow lakes containing fish (Kalff, 2002). This is because shallow lakes, in contrast to the deep thermally stratified lakes, cannot provide vertical refuges for crustacean zooplankton to undergo DVM in order to avoid predation pressure (Wright and Shapiro, 1990). Studies conducted by various authors (e.g. Timms and Moss, 1984; Davies, 1985) have suggested that in shallow lakes with sufficient development of the littoral zone, submerged macrophytes provide Daphnia with spatial refuges from fish predation during daytime. However, Carpenter and Lodge (1986) have indicated that because many juvenile fish often use the vegetation as daytime refuges against predatory (piscivorous) fish, the value of macrophyte patches as shelters for zooplankton may diminish. In contrast, Gliwicz and Rykowska (1992) suggested that large- sized zooplankton try to avoid high predation by planktivorous fish by moving far into the littoral zone.
Jeyaraj (2003), in the first limnological study of Ballincollig reservoir, pointed to fish predation as the major factor that may shape the zooplankton community of the reservoir. However, because the study was limited only to a single season (summer) a further more thorough study covering a longer period of time was necessary to look for the factors that may influence the dynamics of the crustacean zooplankton, both spatially and temporally, in the reservoir. Thus the present study was aimed at investigating both the abiotic (i.e. the physico-chemical parameters of water such as temperature, dissolved oxygen, Secchi depth, conductivity and pH) and biotic (i.e. chlorophyll a concentration, phytoplankton species composition and fish predation) factors that may influence the dynamics (i.e. population abundance and size structure) of the crustacean zooplankton. Investigation of diel vertical migration (DVM) and diel horizontal migration (DHM) of the crustacean zooplankton was also another objective of this study.
Zooplankton were chosen for this study because they are important components in aquatic food webs forming a vital connection, in transfer of energy, between primary producers (i.e. phytoplankton) and the higher level consumers such as predatory invertebrates and fish. Therefore detectable changes in abundance or species composition of zooplankton can reflect changes in the aquatic environment that may affect phytoplankton and as an early indication of imminent changes in the food conditions that may affect fish and other consumers (Clark, 1992). Furthermore, because zooplankton species respond quickly to environmental perturbations such as pollution, change in temperature, etc their standing crop and composition can indicate the quality of water mass in which they occur. For example, research conducted from 1976-1978 on zooplankton communities in a large group of lakes of different trophic states in north-eastern Poland indicated that parameters of structure of zooplankton communities such as percentage of cyclopidae in crustacean biomass and the ratio of cyclopidae to cladocera can be used as indices of lake trophy and also enable the calculation of the trophic state index of the lakes (Karabin et al., 1997).
Ballincollig reservoir (Plate 1.1 and Fig 2.1) is located west of Cork city and south of Ballincollig town in the county Cork, Ireland. The reservoir is located at approximately 51 51’ N and 008 35’ W and is more or less elongated in shape, in the direction of the river channel, with a maximum length of 169 m along its north-south axis, whereas the width along the east-west axis has a maximum value of 111 m. The inflow and outflow of the reservoir are located at the south western and north eastern ends respectively. The reservoir has a total surface area and volume of 1650 m (1.65 ha) and 51850 m respectively (O’Flynn, 2004).
It has an artificial outlet tower which in the past was used to take water out of the reservoir around the north western end. It is a shallow reservoir with the maximum depth of about 5 m at the north western end, near the outlet tower. There is also an indication that, apart from the main deepest area near the outlet tower, there is another deep pool towards the south eastern end that reaches about 4 m (O’Flynn, 2004). According to O’Flynn (2004) the upstream, i.e. the southern end, is generally shallower than the downstream, i.e. the northern end, due to increased siltation, probably as a result of natural eutrophication over many years. Jeyaraj (2003) also indicated that the reservoir basin in the wind direction, i.e. the deepest part near the outlet tower, is probably filled with sediment. The depth gradient of the reservoir is much steeper on the western side than on the eastern side.
Abbildung in dieser Leseprobe nicht enthalten
Plate 1.1. Different aspects of Ballincollig reservoir: South eastern end (A), Eastern margin (B), North eastern end-an outlet (C), and North western margin (D).
The bathymetric study of the reservoir conducted by O’Flynn (2004) showed that the reservoir has a residence time of about 32 days. The reservoir has been described as shallow and hypertrophic undergoing some stratification during summer, although this might be temporary due to the strong winds leading to vertical mixing (Jeyaraj, 2003). The reservoir is also characterised by highly overgrown bankside vegetation and a catchment which is mainly pasture for livestock.
Although the reservoir was originally built, sometime in the 1800s by the British army for the supply of drinking water (Jeyaraj, 2003; O’Flynn, 2004), it is now only used as a coarse fishery, mainly as a carp fishery. It is known to support various species of cyprinids, such as carp (Cyprinus carpio L.), tench (Tinca tinca L.), bream (Abramis brama L.), rudd (Scardinius erythrophthalamus L.), roach (R utilus rutilus L.), as well as gudgeon (Gobio gobio L.) and eel (Anguila anguila L.) (Caffrey, 1996). A comparative study conducted by Caffrey (1995) on three fish species namely carp, tench and rudd showed the presence of large size classes of carp, weighing up to 26 lbs and more than 56 cm long, as well as fast growing tench. The same study revealed the suitability of the reservoir for benthivorous fish due to its deep muddy substrate and ample reedy margins for spawning.
Seven sampling sites were selected both along the longitudinal (North- south) and cross-sectional (East-west) transects of the reservoir as in Figure 2.1. The location of each site was recorded using a Global Positioning System (GPS) device (Table 2.1). Sites 01 through 05 were regularly sampled for crustacean zooplankton abundance and size structure. The near tower (01) and western margin (02) sites, respectively, were used also for the diel vertical migration (DVM) and diel horizontal migration (DHM) studies. The centre of the reservoir (07) and midway between the inlet and reedy margin (06) were used only for the purpose of DHM sampling (Table 2.1).
Table 2.1. Locations of the sampling sites.
Abbildung in dieser Leseprobe nicht enthalten
It was assumed that the sampling sites were situated sufficiently far apart to ensure the independence of the parameters measured at each site. However, weather factors such as heavy rain and stormy winds that could interfere with the assumption of independence of the sites were recorded and taken into account during data analysis.
Abbildung in dieser Leseprobe nicht enthalten
Figure 2.1. A sketch of the study area, Ballincollig reservoir, and locations of the sampling sites.
Zooplankton populations invariably are distributed in a patchy manner posing a difficulty both in sampling and data interpretation (Deibel, 2001 and Clesceri et al., 1998). The crustacean zooplankters, especially, move actively and may shoal, both vertically and horizontally, and thus make quantitative sampling more difficult (Moss, 1998). An attempt was made to maintain consistency in the basic procedures employed in sampling and to standardise the procedures employed during the entire sampling period. Meteorological or weather conditions, such as wind strength and direction, rain, storm and runoff, were all recorded during each sampling time because these may potentially influence the physico-chemical characteristics of the water and zooplankton population abundance. The reservoir was sampled for six months from February to July 2005 every three weeks except during the last two months (June and July) when it was sampled every two weeks due to the expectation of greater biological changes resulting from the warm and sunny weather.
The physico-chemical parameters of water namely temperature, dissolved oxygen, conductivity, pH, as well as Secchi depth were measured directly on site using appropriate meters and a Secchi disc. Temperature and dissolved oxygen were measured with a handheld WTW OXI330 long cable oxygen meter that is equipped with both oxygen probe and built in temperature sensor. Temperature and dissolved oxygen depth profiles were also measured at the deepest site, site 01 (near tower). Secchi depth, pH and conductivity were measured using a black-white 20 cm diameter Secchi disc, handheld WTW pH330 meter and handheld LF330 conductivity meter respectively.
Chlorophyll a is the most abundant and important pigment which generally constitutes 2 to 5% of the dry weight of an algal cell (Bronmark and Hansson, 1998). Thus chlorophyll a was sampled and measured to give an approximate indication of total phytoplankton biomass. In addition phytoplankton samples were taken to determine abundance of the different species present. Samples, for chlorophyll a and phytoplankton analyses, were obtained between 0.5 m and 1 m depth of water column using a 1 L Ruttner bottle sampler. The sampler was lowered in the open position to a desired depth and then closed automatically by a messenger that slides down the supporting cord. Chlorophyll a was sampled at all sites throughout the sampling period while phytoplankton was sampled only at three sites on each sampling occasion. The phytoplankton samples were preserved in the field with Lugol’s iodine solution, 0.3 mL to 100 mL sample (Clesceri et al., 1998).
Two fish species (rudd- Rutilus rutilus L. and gudgeon- Gobio gobio L.) and a hybrid of rudd and bream of different size classes were obtained from an angler on 9th August for gut analysis. Carp (Cyprinus carpio L.) were also caught but could not be included in the gut analysis because permission could not be obtained to remove carp from the reservoir.
Water column samples were taken with a simple conical plankton net, 300 mm (diameter) and 250 m (mesh size), for the investigation of zooplankton abundance and size structure. The plankton net was used at most of the sites throughout the sampling period because it enabled a larger number of individuals to be gathered when scarce or where a large biomass was required (Clesceri et al., 1998). A jug was used to collect large volumes of samples at the shallower and reedy sites (sites 03 and 04) because it was not possible to haul the net properly.
Three replicate vertical hauls were obtained, from a known depth, at each of the sampling sites for analysis. The net was lowered into the water column and then hauled upward hand over hand at a steady speed of 0.5 m/sec. During each vertical haul the net was rinsed at least twice from outside with distilled water using a plastic wash bottle, so that the net contents were washed into the collecting jar at the codend. The sample in the jar was then concentrated by filtering through a filter cup of 60 m mesh size.
The concentrated samples were finally preserved with a 5 % formalin solution (final dilution). Similarly at sampling sites 03 (inlet) and 04 (reedy margin) three replicate jug samples of known volume were obtained and filtered through a 60 μ m mesh, followed by preservation in 5 % formalin (Downing and Rigler, 1984; Clesceri et al., 1998; Paterson, 2001). However, while formalin treatment of zooplankton preservation is useful, it has a distinct disadvantage when morphological structures or population dynamics of the species are the subject of the study. This is because 5 % formalin solution takes a relatively long time to kill the animals and distortion of the body structure often occurs. Cladoceran (e.g. Daphnia, Bosmina, etc) carapaces balloon making it difficult to measure the body size correctly for the estimation of size structure of the population. Therefore, in order to overcome this negative effect of formalin preservation, the crustacean zooplankton samples were filtered in a net filtration cup and immediately immersed in a 95 % ethanol to kill the organisms quickly before being transferred to 5 % formalin solution (Downing and Rigler, 1984).
The maximum volume (Vm) of water that could be filtered during a net haul from a defined depth (d) was estimated with the formula, Vm = r d, where r is the radius of the net orifice (Clesceri et al., 1998). This volume is the maximum because clogging of the net mesh by phytoplankton and other particles can restrict filtration. In fine netting, even the fine netting itself can also cause some water to be diverted from the net’s path.
In addition to the regular sampling of the reservoir for abundance and size structure of the crustacean zooplankton, special sampling was carried out to investigate for diel vertical migration (DVM) and diel horizontal migration (DHM) in the population. This was conducted twice in July at a fortnightly interval. Sampling was carried out between 12:00 and 14:00 (day) and 21:00 and 22:30 (night) local time. A Ruttner bottle sampler (1 L) was used to take samples from specific depths.
The deepest site, near the tower (site 01), was sampled for DVM at every 1 m interval for the first 3 m depth of the water column. Two replicate samples of crustacean zooplankton were obtained from each depth and pooled together for analysis for a particular depth. Sampling for DHM was conducted at the west margin (02), midway between the inlet and reedy margin (06) and the pelagic area or centre of the reservoir (07). Similarly two replicate samples of the crustacean zooplankton were obtained from 1 m depth of the water column at all three sites and processed in the same way as the DVM samples.
Concurrently the sites were also sampled for physico-chemical parameters (i.e. temperature and dissolved oxygen) and chlorophyll a to investigate whether there was any relationship between these environmental variables and the crustacean zooplankton diel migration behaviour.
Samples for chlorophyll a were processed immediately on arrival at the laboratory. For each sample 500 mL was filtered through a 4.7 cm Whatman GF / C filter paper. The filter paper was then rolled with a pair of forceps and placed in a centrifuge tube containing 14 mL of 96 % methanol (Vollenweider, 1974). After 24 hours in a dark place the filter paper was squeezed against the rim of the tube using forceps to ensure that all the extract was removed. The filter paper was then discarded and the extract was centrifuged at 3500 rpm for 7 minutes to settle resuspended cells and fine particles of the filter paper. The supernatant was then decanted into a 4 cm path length cell and absorbance was read using a Shimadzu UV mini 1240 UV-VIS Spectrophotometer at 665 nm (Clesceri et al., 1998).
The absorbance values were converted into chlorophyll a concentration, without correcting for the phaeopigments, using a formula, chlorophyll a (mg / m) = (13.9*A (v+1)) / (d*V), where: A is absorbance at 665 nm, v is volume of solvent in mL, d is cell path length in cm and V is volume of sample filtered in mL (Clesceri et al., 1998).
The preserved phytoplankton samples were concentrated and analysed in accordance with the method given by Clesceri et al. (1998). Sedimentation was preferred to filtration and centrifugation, in concentrating the samples, due to its non-selective and non-destructive nature. Subsamples of 5 mL were randomly drawn with a graduated pipette from a well mixed sample and then transferred to two 2.5 mL settling chambers. This was left in a refrigerator at least for 24 hours to ensure sedimentation.
Phytoplankton were identified and counted under an inverted compound microscope using 200x total magnification. Identification was made to a generic level using an identification key produced by Belcher and Swale (1977). Abundance, of all the genera identified, was calculated as field count, which is expressed as the number of units (i.e. unicellular, colonial or filamentous forms) per mL of a sample. Only some random fields in the subsample were counted and the field count was then calculated as: Field count (No / mL) = (C* At)/ (Af *F*V), Where, C = number of organisms counted, At = total area of bottom of settling chamber (mm), Af = area of a field (mm), F = number of fields counted, V= volume of subsample counted (Clesceri et al., 1998).
Immediately on arrival in the laboratory all the fish specimens were dissected for the gut contents after measuring and categorizing them to different size classes. The 29 rudd (Rutilus rutilus L.) specimens were categorized into two size classes: Fourteen individuals of 5-10 cm (total length) and fifteen individuals with a total length 10-15.9 cm. There were only two specimens of the bream and rudd hybrid fish measuring 10 cm and 11.4 cm (total length). All the 13 gudgeon (Gobio gobio L.) specimens constituted one category of 7.9-10.3 cm total length.
A dissection was made and the gut contents were separated and stored in an aqueous solution of 70 % ethanol for further analysis (Bagenal, 1978). The preserved gut contents were examined under both the normal and inverted compound microscopes. Identification was made using the same identification keys as in sections 2.3.1 and 2.3.3. Frequency of occurrence and percentage composition of the prey items (i.e. crustacean zooplankton
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