A decadal change in agricultural land use reduces macroinvertebrate diversity in a headwater stream. A case study in Sri Lanka

Headwater streams are the starting points of river network systems, and they harbor unique biological communities. Many headwaters are under pressure due to anthropogenic impacts such as land use changes, but research on long-term community response to such impacts is scarce. The present study was designed to compare the impacts of land use changes on macroinvertebrates in a decadal period, considering the Waturawa stream, Sri Lanka as a model system. The diversity of macroinvertebrates was assessed in a reach (similar to 600 m) of this headwater stream, at three sampling sites. In this study, each site was visited once a month for three months in 2021. The physicochemical properties of water and the benthic macroinvertebrate diversity were assessed and compared with similar data collected in 2011. Land use changes in the area were also evaluated using Landsat satellite images. During these ten years, approximately 16 % of forested lands were converted to agricultural lands. About 60 % of macroinvertebrate families have disappeared, and 10 % of new families have emerged by 2021 compared to 2011. Present findings revealed that the increment of agricultural lands and deforestation had changed the nutrients and other material loading into this stream. Therefore, a strong negative correlation exists between water phosphate and macroinvertebrate diversity and Total Suspended Solid (TSS) and macroinvertebrate diversity. These observations allow us to conclude that the land use changes in a decade have contributed to the depletion of macro-benthic diversity in the Waturawa stream.