Benthic Biodiversity in Subarctic Stream Networks: Headwaters as Upholders of Variation Through Space and Time

Headwaters often contribute disproportionately to network‐scale benthic biodiversity as differences in community composition between separate headwater streams (β‐diversity) can be high although local (α) diversity may be low. Most research on stream β‐diversity has adopted a spatial perspective, whereas temporal β‐diversity remains largely uncharacterised, but may be highest in headwaters, then decreasing with distance downstream. We studied the spatial α and β‐diversity of benthic invertebrate communities in five subarctic stream catchments in a single sampling year, with four mainstem and four headwater sites each and temporal β‐diversity over five years in one stream catchment. Beta‐diversity was measured as Bray–Curtis dissimilarity, including spatial β‐diversity among sites and temporal β‐diversity among years at the same site. We found that α‐diversity increased with distance downstream, and this pattern persisted across five years. Headwater communities supported consistently higher spatial β‐diversity than mainstem communities. Temporal β‐diversity also tended to be higher for headwater than mainstem communities. Our results show that local environmental conditions in subarctic streams are inherently linked with stream network position, providing evidence for the key influence of location on spatial and temporal variation in stream metacommunities. Our concurrent consideration of both spatial and temporal β‐diversity demonstrates that the contribution of headwaters to network‐wide spatial β‐diversity persists year after year, highlighting the need for network‐scale strategies and actions that ensure their protection.