From indicator-centered assessment to ecosystem-centered interpretation: Toward a conceptual framework for river bioassessment

Despite their widespread application in river bioassessment, biological indicators are still largely interpreted within an indicator-centered paradigm, with individual biological assemblages commonly evaluated in isolation. Although phytoplankton, zooplankton, and benthic macroinvertebrate communities each provide valuable yet complementary ecological information, differences in their responses have often been regarded as inconsistencies rather than as natural expressions of the multidimensional organization of river ecosystems. As a result, the central challenge in contemporary river bioassessment is no longer identifying the most sensitive or universally applicable biological indicator, but developing an ecological framework capable of integrating complementary biological evidence into a coherent interpretation of ecosystem condition. This review addresses that conceptual challenge by shifting the focus from comparing individual biological indicators to developing an ecosystem-centered framework for interpreting complementary biological evidence. Through a critical synthesis of current knowledge on phytoplankton, zooplankton, and benthic macroinvertebrate communities, we demonstrate that these three biological assemblages collectively represent complementary dimensions of river ecosystem organization and functioning. Within the proposed framework, phytoplankton are interpreted as indicators of rapid pelagic responses and primary production, zooplankton as mediators of trophic connectivity and energy transfer, and benthic macroinvertebrates as indicators of long-term habitat integrity and ecosystem memory. Collectively, these assemblages capture production, energy transfer, and ecological processing across fast, intermediate, and slow temporal scales, providing a multidimensional perspective on river ecosystem condition. Our synthesis further demonstrates that variation in biological indicator performance primarily reflects differences in ecological context, environmental drivers, spatial and temporal scales, and assessment objectives rather than contradictions among the indicators themselves. Accordingly, the principal contribution of this review is not the identification of a superior biological indicator, but the conceptual redefinition of river bioassessment from indicator-centered assessment toward ecosystem-centered ecological interpretation. By reframing biological indicators as functionally complementary components of an integrated ecological system rather than competing assessment tools, the proposed framework provides a stronger scientific foundation for biological assessment, supports ecosystem-based monitoring, conservation, restoration, and adaptive river management, and offers a transferable conceptual basis for future research. Ultimately, future advances in river bioassessment will depend less on discovering additional biological indicators than on improving the ecological interpretation of the biological evidence already available.