Phytoplankton as indicator of ecological status: morpho-functional insights into seasonal and spatial variability for monitoring urban shallow freshwater blue spaces

Abstract

Freshwater blue spaces (FBS) are natural or artificial surface waters accessible to humans, either proximally or distally. They are highly susceptible to nutrient enrichment and eutrophication, which alter aquatic communities. As a recently recognised biosphere-engineer, phytoplankton is a key target in urban monitoring programs Thus, we aim to evaluate the effectiveness of phytoplankton morpho-functionalclassification schemes to better assess environmental changes in urban FBS. We applied two morphofunctional approaches across 15 urban waterbodies in different seasons (cold and warm) and regime types (clear water, phytoplankton-turbid, or free-floating-plant dominated). We also examined the relationship between within-lake spatial heterogeneity (presence/absence of macrophytes) and trophic status with phytoplankton structure. Results showed that phytoplankton assemblages varied significantly due to seasonality, regime type, trophic status, and within-lake heterogeneity. Key ecological groups, such as bloom-forming and potentially toxic Cyanobacteria, indicated phytoplankton-turbid and hypertrophic waters (IndVal: > 50%). Given their relative simplicity compared to taxonomic approaches and their sensitivity indetecting degraded conditions across diverse FBS, morpho-functional methods are valuable tools for environmental monitoring, complementing traditional assessments based on physical and chemical parameters. We also emphasise the importance of considering seasonal and within-lake variability when designing sampling strategies to assess the ecological status of urban FBS.