Preliminary assessment of river ecosystem services in the volcanic area of Mount Merapi, Indonesia

River ecosystem services (RES) are vulnerable to landscape changes mainly by volcanic eruptions. Therefore, this study aims to assess RES in the volcanic area which was affected by the major and minor eruptions of Mount Merapi, Indonesia. The RES referred to the regulating and supporting services of the Krasak River in Jogjakarta. The research involved collecting water and biodiversity samples from two distinct Merapi’s hazard zones (KRB I and KRB II) along the river. Parameters related to regulating services, such as particulate, organic, and nutrient purification, biological control, as well as supporting services like primary productivity, were quantified. We conducted an analysis to understand how landscape conditions interacted with these parameters and employed the t-test to assess differences in RES between the two KRBs. Our findings revealed that the Krasak River exhibited a range of values, including 2.40–5.95 mg/l for Biological Oxygen Demand (BOD), 0.61–3.41 mg/l for nitrate, 0.02–0.11 mg/l for phosphate, 160–60,000 MPN/100 ml for coliform, and 156.3–937 µg/l for chlorophyll-A. These values demonstrated the river’s capacity to perform both regulation and support services. However, certain segments showed variations in ecosystem services, possibly due to the presence of autochthonous matter from aquatic organisms and decomposing organic matters. This showed that volcanic eruption and landscape are closely linked with the water quality and aquatic biodiversity, which affect the ecosystem services.