Zooplankton taxa occurrences and composition associated with a cage aquaculture farm on Lake Victoria, Uganda

Study Extent

An annual quarterly monitoring sampling is ongoing since 2011 and the current data presents datasets from 2011 to June 2020 for Napoleon Gulf, northern Lake Victoria. Three transects; downstream, within, and upstream cages, with accompanying control point sites were established. However, the sampling design has so far been distorted due to changes in cage setup and expansion of cages within the Napoleon Gulf.

Sampling

Zooplankton samples were collected with a conical plankton net (Nansen type; mesh size 60 µm; mouth diameter 0.25 m), towed vertically through the water column, as described by Mwebaza-Ndawula (1994). Each sample was washed with tap water in the laboratory over a 53 µm sieve to remove the preservative and then diluted to a suitable volume, depending on the concentration of organisms in each sample. Sub-samples of 2, 2, 5 and 10 mL were taken with a wide bore automatic pipette from a well agitated sample. The sub-sample series were performed to consider the more abundant organisms in 2, 2 mL series, and the rarer organisms in 2, 2, 5, 10 mL series. Each sub-sample was put into a counting chamber and examined under inverted microscope (Hund, Wetzlar, Germany) at X100 magnification for taxonomic determination, and X40 for counting and organism body measurements.

Quality Control

Some zooplankton were able to be identified to species level using published taxonomic keys (Sars 1895; Pennak 1953; Brooks 1957; Rutner-Kolisko 1974; Koste 1978; Boxshall & Braide 1991; Korinek 1999). Taxonomic names were cross-checked using the World Register of Marine Species (WoRMS). Volume densities of organisms were calculated from the counts data, with reference to the sample net mouth diameter and water column depth at each sampling site (Mwebaza-Ndawula, 1998).

Method steps

1. Collection of the zooplankton In the field, a conical plankton net (Nansen type; mesh size 60 µm; mouth diameter 0.25 m), towed vertically through the water column to have an integrated sample. Three hauls were taken per site and were combined to make a composite sample.
2. Preserving the samples The composite sample was preserved with sugar-formalin mixture. The sugar was to stop the ballooning of cladocerans for easy identification.
3. Identification of zooplankton taxa In the laboratory, samples were washed using a sieve of 53 µm to remove the fixatives. Organisms were identified to the smallest taxonomic level possible using taxonomic keys (Sars, 1895, Pennak, 1953, Brooks, 1957, Rutner-Kolisko, 1974, Koste, 1978, Boxshall and Braide, 1991, Korinek, 1999). Density of organisms were calculated from the counts data, with reference to the sample net mouth diameter and water column depth at each sampling site (Mwebaza-Ndawula, 1998)

Freshwater zooplankton retained in a plankton (Nansen) net of 60µm mesh size

1. Afrocyclops
   rank: genus
2. Ascomorpha
   rank: genus
3. Asplanchna
   rank: genus
4. Bosmina longirostris
   rank: species
5. Brachionus angularis
   rank: species
6. Brachionus bidentata
   rank: species
7. Brachionus budapestinensis
   rank: species
8. Brachionus calyciflorus
   rank: species
9. Brachionus caudatus
   rank: species
10. Brachionus dimidiatus
    rank: species
11. Brachionus falcatus
    rank: species
12. Brachionus forficula
    rank: species
13. Brachionus patulus
    rank: species
14. Brachionus plicatilis
    rank: species
15. Calanoidae
    rank: family
16. Cephalodella
    rank: genus
17. Ceriodaphnia cornuta
    rank: species
18. Ceriodaphnia dubia
    rank: species
19. Chydorus
    rank: genus
20. Cyclopoidae
    rank: family
21. Daphnia longispina
    rank: species
22. Daphnia lumholtzi
    rank: species
23. Diaphanosoma excisum
    rank: species
24. Euclanis
    rank: genus
25. Filinia longiseta
    rank: species
26. Filinia opoliensis
    rank: species
27. Hexanauplia
    rank: class
28. Hexathra
    rank: genus
29. Keratella cochlearis
    rank: species
30. Keratella tropica
    rank: species
31. Lecane bulla
    rank: species
32. Lecane luna
    rank: species
33. Macrothrix
    rank: genus
34. Mesocyclops
    rank: genus
35. Moina micrura
    rank: species
36. Polyarthra vulgaris
    rank: species
37. Synchaeta
    rank: genus
38. Synchaeta pectinata
    rank: species
39. Thermocyclops decipiens
    rank: species
40. Thermocyclops emini
    rank: species
41. Thermocyclops incisus
    rank: species
42. Thermocyclops neglectus
    rank: species
43. Thermocyclops oblongatus
    rank: species
44. Thermodiaptomus galeboides
    rank: species
45. Trichocerca
    rank: genus
46. Trichocerca cylindrica
    rank: species
47. Tropocyclops confinnis
    rank: species
48. Tropocyclops tenellus
    rank: species

The study area was in Napoleon Gulf, Northern part of Lake Victoria (Ugandan portion). Intensive fish cage farming is on the increase in this portion of the lake.