Agricultural Land-Use Legacy, The Invasive Alga Didymosphenia geminata and Invertebrate Communities in Upland Streams with Natural Flow Regimes

The integrity of freshwater ecosystems worldwide is under threat from agriculture and invasive species. Past agricultural activity can have persistent effects on aquatic diversity even decades after restoration, and the spread of invasive species is increasingly difficult to prevent due to globalisation. In the South Island of New Zealand, the invasive diatom Didymosphenia geminata (Didymo) causes nuisance blooms in streams. The impact of Didymo on stream invertebrate communities in upland streams with natural flow regimes remains poorly understood. We investigated the relationships between legacy effects of agriculture, Didymo and benthic invertebrate communities at 55 stream sites in Mahu Whenua, a 530 km2 conservation area comprising four former New Zealand high-country farms. The farms were destocked of sheep 4–9 years before stream sampling started. Kick-netting was used to collect macroinvertebrates from 7–23 streams within each farm to provide a land-use legacy gradient. Moreover, samples from 16 sites with clearly visible Didymo mats covering most of the stream bed (indicating high biomass and a dominant role in the biofilm) were compared with 39 sites without such Didymo mats. Total invertebrate taxon richness and EPT richness (taxon richness of larval mayflies, stoneflies and caddisflies) were lower in the stream catchments destocked most recently. When Didymo was present, relative EPT abundance was lower than when Didymo was absent, and Deleatidium mayflies decreased whereas midges and oligochaetes increased. These results highlight the need to look at past land-use practices when restoring high-country streams after agricultural impacts. They also show that Didymo can have negative effects on invertebrate communities in upland streams with natural flow regimes, a stream type previously overlooked in studies on this invasive diatom.