Volcanic crater lakes in Western Uganda are a significant natural resource to different societal segments. To the local people, the crater lakes provide a source of livelihood, to the Ugandan government the lakes are a boost to the tourism industry, to researchers, the crater lakes play an important role in studies of comparative limnology and, on a global scale, the Western Uganda volcanic crater lakes are heritage sites to be preserved. Despite their numerous values, the crater lakes continue to experience eutrophication challenges largely due to ecosystem disturbance from human activities around the crater lakes themselves and in their entire catchments, but also due to some natural causes.

The rationale for this study was to evaluate eutrophication in the crater lakes through water quality assessment of six freshwater crater lakes. The evaluation is based on the common trophic state variables; chlorophyll-a (Chl-a), total nitrogen (N), total phosphorus (P), and Secchi depth (SD). Other related physicochemical variables were measured both on site and through laboratory experiments. Eutrophication is associated with phytoplankton growth, especially blue-green algae, and as a result, cyanobacterial compositions were also examined.

Trophic state evaluation of the crater lakes was based on the trophic level index (TLI) approach, using the New Zealand TLI system as a base example. The use of the New Zealand TLI system was primarily aimed at testing whether TLI systems developed from other regions can be used to monitor Ugandan crater lakes and, secondly to gain an insight of the systematic similarities and differences between Ugandan crater lakes and New Zealand lakes.

Linear relationships between log-transformed chlorophyll-a and other TLI variables suggested that in the Ugandan crater lakes, total nitrogen and Secchi depth are good proxies for chlorophyll-a whereas total phosphorus is less, suggesting that crater lakes may be nitrogen limited. When the variables were converted into TLI sub-indices, using the New Zealand system, linear regressions of TLc (Chl-a) against TLs (Secchi depth) showed the same relationship between chlorophyll-a and Secchi depth in the crater lakes as in New Zealand model. However, TLn consistently under-predicted TLc with the New Zealand model suggesting that Ugandan crater lakes produce chlorophyll-a for less nitrogen. TLc and TLp (TP) showed a weak relationship. Generally, the New Zealand TLI system characterises Ugandan crater lakes from a mesotrophic to hypertrophic state, but, the results are not accurate due to lack of similarity among calculated parametric sub-indices. The inaccuracy is attributed to differences in the systematic functioning between Ugandan crater lakes and New Zealand lakes noted above such as predominant nitrogen limitation, high chlorophyll-a synthesis, and a low Chl-a: TN ratio in the crater lakes.

To address the mismatch in the parametric sub-indices, the New Zealand TLI model has been used to formulate a TLI system that is specific to Ugandan crater lakes which increased the similarity in computed TLI sub-indices. The new TLI system improved on the accuracy trophic state estimations and showed that crater lakes range from mesotrophic to hypertrophic state.

Crater lakes with a trophic state above mesotrophic are characterised by extensive catchment modification through agriculture and human settlement. Modified catchments generate nutrients from agricultural activities, such as fertiliser use and farm waste, in addition to household and institutional faecal treatment systems, such as unlined pit latrines and septic tanks. All nutrients generated have the potential to be transported into crater lakes through various pathways. To address the challenge of eutrophication in Ugandan crater lakes, a suitable monitoring programme, such as trophic level index system, should be adopted as the foundation for effective crater lake management. This will also provide a baseline to develop effective nutrient control strategies for craters lakes catchments.