Aquatic insects in a forested and an urban stream: The importance of riparian forest, microhabitats and climate influence

Aims This study evaluated the environmental conditions, abundance, and diversity of aquatic insects in streams in a secondary forest and an urban area in Bragança, Pará, Brazil. Methods From October to December 2023, data on aquatic insect taxa (diversity and abundance), environmental variables (electrical conductivity, pH, dissolved oxygen, total dissolved solids, and oxidation-reduction potential), and disturbance levels, assessed using the Habitat Integrity Index (HII). Results A total of 423 individuals were collected, 69 of them collected in the secondary forest streams and 354 in the urban streams, distributed across six orders: Ephemeroptera, Trichoptera, Odonata, Diptera, Coleoptera and Hemiptera. The urban stream harbored 22 insect families, while 12 families were recorded in the secondary forest stream. Abundance and richness declined over the dry months in both streams. Berger-Parker dominance index increased over time, while the Margalef diversity index decreased, indicating shifts in insect assemblage structure. Both streams were classified as ‘‘Disturbed’’ (HII < 0.70). Electrical conductivity, total dissolved solids, phosphate and dissolved oxygen were associated with variations in the insect assemblage composition and abundance. Gomphidae was strongly associated with phosphate concentrations, Corduliidae with total dissolved solids and electrical conductivity, and Coenagrionidae and Caenidae responded positively to dissolved oxygen. The urban stream experienced complete flow loss during the dry season, likely associated with severe drought conditions, compounded by regional forest loss and urbanization processes. Conclusions Higher abundance in the urban streams likely reflects anthropogenic disturbances that reduced sensitive taxa and favored the dominance of tolerant ones. Higher number of aquatic insect families in urban streams may be related to higher microhabitat diversity, which likely supported the survival of species with different ecophysiological requirements.