**5. Conclusions**

comprehensive inventory of watershed-scale ecosystem services knowledge across major US watersheds. More specifically, our analysis summarizes the scientific literature since 2000 within the context of the number of studies investigating the biophysical supply and social demand for ecosystem services. We found a temporal trend in the number of publications similar to that found from international studies following the global development trend in this research area [3, 22]. Our results emphasize the urgent need to implement interdisciplinary frameworks that take into account the interdependent social, economic, and biophysical dynamics of shared water resources and the need for using integrative approaches to capture

Overall, our results showed that the number of studies investigating regulating and provisioning services was higher relative to those investigating cultural services. This finding is consistent with similar studies across the globe, where research on the supply and demand of ecosystem services has focused mainly on provisioning and regulating services [24, 25]. In the Mediterranean region, for example, [21] showed that provisioning services attracted much more scientific attention, which is also consistent with most of the findings related to the assessment of ecosystem services in European landscapes [13, 23]. Furthermore, using the CICES classification, we found that from a total of 308 ecosystem services studied across all US watersheds, regulating services (e.g., filtration, sequestration, storage and accumulation by ecosystems, habitat maintenance, and chemical conditions of freshwaters) were most commonly studied, while cultural services (e.g., educational and cultural heritage) were the least studied. As recently highlighted by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), there is an urgent need for global efforts by governments, academia, and society to promote knowledge of earth's biodiversity and ecosystems, with the aim of informing sustainable policy and management of natural resources [26, 27]. One of the key components of the IPBES approach is the notion of nature's contributions to people, which recognizes the critical role that culture plays in defining all links between humans and ecosystems. We therefore argue that there is also a need to recognize the important role that cultural services and values can play in water resources management in the USA and the need to operationalize the role of indigenous and local knowledge in under-

Different disciplines have traditionally assessed ecosystem services separately [18, 24], which has led to the conclusion that ecosystem services values are multidimensional, and thus their evaluation must be conducted from the ecological, social, and economic perspective [23, 28, 29]. Although we found a small percentage of studies that used this multidisciplinary approach in their assessments, our results showed that most of the studies conducted across US watersheds implemented a biophysical approach, which points out the gap of integrating different approaches into ecosystem service research [30, 31]. We believe that this gap is due to the absence of a shared theoretical framework, as we found that over 78% of all studies in the USA did not use a standard ecosystem services framework. In a recent article, [32] concluded that integrated valuation of ecosystem service supply and demand still faces challenges in understanding the tradeoffs among ecosystem services. With regard to ecosystem service demand, it is necessary to use systematic methods for different stakeholders (beneficiaries, impairers, and managers) because of their different

different value domains [18, 23].

90 Ecosystem Services and Global Ecology

standing watershed's contribution to people [26, 28].

Overall, we found that the use of the supply and demand framework of ecosystem services for watershed-scale studies in the USA has been extremely limited. The majority of the watershed case studies were found in the eastern half of US, with very few in the Southwest. Studies implementing biophysical assessment of the ecosystem services supply were located in the Southeast and Midwest, while studies investigating the social demand of ecosystem services were concentrated along the east coast of the USA. In addition to identifying the gaps in our knowledge of watershed-scale ecosystem services across the USA, we call attention to the scale issue in ecosystem services research, which describes the mismatch between the scale at which ecosystem services are provided and the scale at which those services are used, valued, or managed [16]. Future studies should not only address multiple spatial and temporal scales; they should also assess different stakeholder scales, from the individual to the community to the municipality to the state, and beyond.

Understanding and quantifying tradeoffs between ecosystem services, considering their ecological, cultural, and economic value, is a key challenge for water resources management and planning in the USA [36] and beyond [37]. Our study demonstrates the knowledge gap across US watersheds in terms of integrating biophysical, sociocultural, and economic dimensions to assess the biophysical supply and social demand for services, which is key for increasing public awareness of the importance of river systems in maintaining human well-being [3, 38]. Moving forward, we would like to see more comprehensive ecosystem service studies at watershed scales using integrative (yet standard) approaches to assess tradeoffs at multiple spatiotemporal and stakeholder scales.
