Influence of Humic Substances on Phytoplankton Primary Production and Community Structure: A Review

In recent decades, many freshwater and coastal marine ecosystems have experienced increased inputs of chromophoric dissolved organic matter (CDOM) of terrestrial origin, primarily humic substances (HSs). This phenomenon, commonly referred to in English-language literature as brownification, involves the darkening of water bodies due to elevated concentrations of colored organic compounds. The influx of HSs into aquatic systems alters the optical, chemical, and trophic characteristics of the water column. This interdisciplinary review synthesizes current scientific knowledge on the effects of HSs on phytoplankton primary production, trophic structure, and community composition under conditions of climate change. Key mechanisms are examined, including reduced light penetration, enhanced thermal stratification, nutrient transport, and the involvement of HSs in redox processes. Evidence suggests that moderate humification can stimulate productivity through microbial degradation and the photodecomposition of allochthonous organic matter, releasing additional nutrients. However, at high HS concentrations, photosynthetic activity declines, phytoplankton shift toward mixotrophy, and biodiversity decreases. Altered light spectra favor groups with accessory pigments and vertical mobility, such as cryptophytes, dinoflagellates, and cyanobacteria, potentially reducing ecosystem trophic efficiency. Ecosystem responses to humification vary with geographic, climatic, and morphometric characteristics of water bodies. Special attention is given to boreal and Arctic regions, where permafrost thaw and increased precipitation intensify HS runoff. Key research directions are identified, including thresholds of abrupt change, the role of mixotrophy in carbon cycling, and the development of region-specific adaptation strategies. This review underscores the importance of an integrated approach to assessing the ecological consequences of aquatic humification.