Abundance of macroinvertebrates in sites with varying degradation levels in Lake Wamala

Study Extent

The study was conducted between October 2016 and October 2017 in sites with different levels of land degradation. Papyrus was the common vegetation adjacent to water and the different sites of the lake are surrounded by different thicknesses of this vegetation although, in some sites, it has been cleared up to the lake’s shoreline. Seven sampling sites were selected for the study. The sites were categorized into three as follows: (1) Less degraded sites (2) Degraded sites, and (3) Riverine sites, based on the level of adjacent land degradation, the thickness of the shoreline vegetation and location near the river mouth. Within each site, three sampling stations were established at Shoreline, 250 m and 500 m in an inshore-offshore direction.

Sampling

At each site and sampling stations, benthic macroinvertebrates were collected using a Ponar grab (238 cm2 open-jaw area). Three vertical hauls of sediment at each sampling station were taken and then pooled into a composite. Each composite sample was then sieved through a 400µm nitex mesh to concentrate the sample. All the samples were placed in clean, labeled sample bottles and preserved with 5% sugar formalin solution for taxonomic identification and enumeration in the laboratory.
In the laboratory, the formalin-fixed benthic macroinvertebrate samples were rinsed through tap water to remove preservatives before subsampling and sorting of the organisms. All the animals were separated, counted, and identified to the smallest taxonomic level possible under a dissecting microscope following taxonomic keys by Mandahl-Barth (1954), Pennak (1953) and Merritt and Cummins (1997). Numerical abundance (individuals per square meter, Ind.m-2) was then calculated for each taxon.

Quality Control

The samples were immediately processed in the field and treated with formalin to keep the organisms of interest intact. To avoid loss of organisms during sample processing, appropriate mesh sizes during sieving. Appropriate taxonomic keys were used.

Method steps

1. Collection of the macroinvertebrates: In the field, sediment samples were collected using a Ponar grab with an open jaw surface area of 238 cm2. At each site, three sediment samples were obtained. The three samples were mixed and concentrated to form one composite sample for each site.
2. Preserving the samples: The composite sample for each site was separately preserved in 5% formalin to maintain the organisms in good condition prior to analysis in the laboratory.
3. Identification of macroinvertebrates: In the laboratory, formalin was rinsed off from each sample and placed in white flat-bottomed trays. Using pairs of forceps, all benthic macroinvertebrates were sorted from the sediment and the individual taxa identified to the lowest possible taxonomic level using appropriate identification keys and a dissecting binocular microscope at 4x 25 magnification.

1. Ablabesmyia
   rank: genus
2. Baetis
   rank: genus
3. Caenis
   rank: genus
4. Ceratopogonidae
   rank: family
5. Chaoborus
   rank: genus
6. Chironomidae
   rank: family
7. Chironomus
   rank: genus
8. Clinotanypus
   rank: genus
9. Cloeon
   rank: genus
10. Ephemerythus
    rank: genus
11. Hirudinea
    rank: class
12. Libellulidae
    rank: family
13. Libellulidae
    rank: family
14. Oligochaeta
    rank: class
15. Petrophila
    rank: genus
16. Podocopa
    rank: order
17. Povilla adusta
    rank: species
18. Procladius
    rank: genus
19. Tanypus
    rank: genus

The resource covers Lake Wamala. The lake is a UNEP designated environmental change hotspot (https://na.unep.net/atlas/webatlas.php?id=391).