Traits Thesaurus: Toward semantic harmonisation of FAIR trait-based data on aquatic organisms

Trait-based approaches have become increasingly essential in biodiversity and ecological research, particularly for monitoring and understanding community and ecosystem responses to biodiversity loss and habitat changes. Organismal functional traits are key indicators that link community structure and organisation to large-scale ecosystem processes. Despite the growing application of trait-based approaches across all ecological domains, the absence of standardised trait definitions, units, and terminologies has led to substantial heterogeneity, hindering data integration, synthesis, and reuse across studies, taxa, and spatial scales. This lack of semantic harmonisation represents a major lacuna in biodiversity knowledge, impeding cross-study and cross-systems trait comparison. Hence, we present the Traits Thesaurus, a controlled vocabulary with over 1500 trait concepts developed to standardise trait data and metadata for aquatic organisms. The Traits Thesaurus expands and integrates five existing Thesauri (Phytoplankton, Macroalgae, Zooplankton, Macrozoobenthos and Fish Traits Thesauri) into a unified framework. Beyond expanding and consolidating existing vocabularies, it also introduces newly developed term collections, including standardised units of measurement associated with measurable traits and additional conceptual domains. The Traits Thesaurus provides stable URIs, clear definitions, preferred and alternative labels, and mappings to external controlled vocabularies. The Traits Thesaurus is aligned with FAIR and Linked Open Data principles, international standards and enables full interoperability, data annotation and integration across disciplines and existing biodiversity information systems. With this initiative, we aim to foster a global, community-driven consensus on harmonised trait concepts, enabling effective large-scale data mobilisation, interoperability, and reuse of trait information to advance trait-based aquatic ecology and biodiversity research.