Effect of land use on water quality of tropical headwater streams: comparison across rainforest, cropland, pastureland, and oil palm plantation

Oil palm plantations in Guatemala's northern lowlands have been expanding rapidly over the past two decades, driven by large-scale conversion of tropical forests and croplands. This extensive land use change to oil palm has placed enormous pressure on the region's freshwater ecosystems. The extent to which various land uses impact the quality of tropical stream water remains poorly understood. In this study, we evaluated the effect of different land uses on aquatic macroinvertebrates and stream water quality in the northern lowlands of Guatemala. We assessed 19 sampling sites, including at least four replicated headwater streams for each of the four dominant land uses: tropical forest, cropland, pastureland, and oil palm plantation. In each stream, we measured and analyzed physicochemical water quality parameters in both rainy and dry seasons and sampled benthic macroinvertebrates during the dry season. Subsequently, we used the resulting data to calculate two water quality indices and assess the ecological status of the sampling sites: the Water Quality Index (WQIsimp), based on physicochemical parameters, and the Biological Monitoring Working Party for Costa Rica (BMWP-CR), based on the aquatic macroinvertebrate community. Water quality scores differed among sampling sites, with sites draining forested catchments exhibiting the highest scores and those draining pastureland and oil palm plantations exhibiting the lowest. In contrast, the WQIsimp results did not differ significantly among land uses and were largely influenced by seasonal variations. Overall, water quality scores declined as the proportion of tropical forest within the watershed decreased. Oil palm plantation sites were characterized by the lowest benthic macroinvertebrates species richness and diversity, alongside higher abundance of contamination-tolerant families. These findings reveal significant negative impacts of oil palm plantations on water quality and aquatic macroinvertebrate communities, particularly when compared with forested areas and basic grains croplands. Therefore, safeguarding primary forests and basic grains cropland from further conversion to oil palm is crucial to preserving biodiversity, maintaining ecosystem functions and services, and securing food provision.