Trophic state of dry zone reservoirs utilized for culture-based fisheries development in Sri Lanka

The Trophic State Index (TSI) can be used to classify lakes, rivers, and reservoirs into different trophic categories as well as to estimate the degree of eutrophication and the corresponding potential fish yield of a water body.  Relations between Carlson’s TSI models and culture-based fisheries (CBF) in non-perennial reservoirs are well documented, but their applicability in larger, perennial reservoirs remains less comprehensive.  In the present study, 48 reservoirs of varying sizes were examined to investigate the relationships between TSI values derived from Secchi disk depth (SDD) and chlorophyll-a content (Chl-a). Perennial reservoirs varying in size were selected from northern districts: Anuradhapura, Kurunegala, Puttalam and southern districts: Hambanthota and Monaragala. From March 2018 to August 2019, conductivity, pH, water temperature, dissolved oxygen, Secchi disk depth (SDD), chlorophyll-a (Chl-a), and sediment organic matter content (SOM) were measured monthly at five randomly selected sites within each reservoir.   Principal Component Analysis (PCA) based on ln(x+1) transformed limnological data, ordinated northern and southern reservoirs separately into two distinct clusters. The first three Principal Components explained 98.9% of the cumulative variance. Conductivity, pH and Chl-a were found to be more important in explaining the variance. When calculated, TSI(SDD) values in each reservoir were well above the TSI(Chl-a) due to the turbidity caused by non-algal particles or colour. When compared with the reservoir size, there were no correlations (p>0.05) between the two TSI types. Log natural converted SDD and Chl-a values showed no significant correlation (p > 0.05) with ln-transformed SOM values across the reservoirs.  In conclusion, Chl-a values are more suitable for assessing the trophic state of reservoirs in general since non-algal particulates or colour dominate light attenuation. Further research is needed to find simplified tools such as SDD to relate to the trophic status, thereby improving fisheries management in reservoirs of Sri Lanka.