Spatiotemporal dynamics and species-habitat relationships in syngnathid assemblages from a coastal lagoon under eutrophication stress

Syngnathids are a vulnerable fish group strongly associated with vegetated habitats in transitional waters, yet their population biology and habitat preferences remain poorly understood, particularly in environments subjected to eutrophic conditions. This study examines the spatiotemporal variation and habitat selection of syngnathid populations in the shallow areas of the Mar Menor, a hypersaline coastal lagoon undergoing severe eutrophication, which has led to mass macrophyte mortalities. Seasonal surveys conducted in 2018-2019 revealed a relatively simplified syngnathid assemblage as compared to other large European transitional systems, but highly relevant within the lagoon context. Syngnathus abaster was the dominant species, while Syngnathus typhle and Hippocampus guttulatus were less abundant. Juveniles were particularly prevalent in the vegetated bottoms of shallow areas, reinforcing their function as growing and foraging grounds. Habitat models identified species- and size-specific preferences, with S. abaster and S. typhle closely linked to vegetation metrics, while adult classes showed more complex habitat relationships. In contrast, H. guttulatus exhibited a stronger dependence on water quality predictors. Seascape features also influenced syngnathid distributions. Our findings highlight the key role of seagrass and mixed seagrass-macroalgae meadows in supporting syngnathid populations, particularly under a scenario of habitat degradation. Long-term monitoring is essential to track syngnathid responses to environmental shifts and to inform effective conservation strategies in transitional systems facing increasing anthropogenic pressures.