Contrasting size responses of loricate ciliates to long-term warming: agglutinated forms decrease while hyaline forms increase

Although relationships between temperature and size are widely discussed in aquatic protists, empirical evidence remains absent for loricate protists, and systematic variations in thermal responses among ecological groups remain unexplored. Using tintinnids as a model system for marine plankton, we present the first comprehensive analysis of temperature-size relationships in loricate protists based on a 72-year dataset (1947–2019) from Jiaozhou Bay. Analysis of 702 microscopic measurements from five tintinnid species revealed fundamentally divergent thermal responses across lorica types. Agglutinated forms demonstrated significant inverse temperature-size relationships (p < 0.001), whereas hyaline forms exhibited positive thermal correlations (p < 0.001). Generalized Additive Models (GAMs) quantified these contrasting responses: entirely agglutinated species experienced 7.61% LOD reduction and 22.05% biomass decline; partially agglutinated species showed intermediate responses with 3.21% LOD decrease and 13.80% biomass reduction (R² = 0.97); conversely, hyaline species increased 3.94% in LOD and 29.98% in biomass. Projections under future warming scenarios (+ 4 °C) coupled with reduced Chl a (-0.1 µg/L) predict asymmetric community restructuring: 14.04% biomass decline for agglutinated forms contrasting with 39.10% increase for hyaline forms. These findings provide critical baseline data for understanding thermal impacts on tintinnids and broader implications for microzooplankton dynamics in marine pelagic ecosystems under global warming.