Trophic basis of production for a mayfly in a North Island, New Zealand, forest stream: contributions of benthic versus hyporheic habitats and implications for restoration

摘要

The leptophlebiid mayfly Acanthophlebia cruentata (Hudson) is restricted to the North Island and some associated offshore islands of northern New Zealand where it commonly occurs in benthic and hyporheic habitats of forested streams. We investigated: (1) life history; (2) secondary production in benthic and hyporheic habitats; and (3) major energy sources contributing to nutrition and production of this species in a pristine forest stream. Most nymphal size classes were present throughout the year, and emergence extended over several months, peaking from February to April. Despite apparently having extended emergence and recruitment periods, Acanthophlebia exhibited a predominantly univoltine life history. Annual benthic production (calculated by the size-frequency method) was 0.318 g dry mass (DM) m~(-2) year~(-1), compared to 4.601 g DM m~(-2) year~(-1) in high-density benthic habitats at the tails of pools, and 34.476 g m~(-3) year~(-1) for colonisation baskets set at 15-45 cm deep in the substratum. On a habitat weighted basis averaged out over the entire sampling reach, it was estimated that 76% of annual production occurred in hyporheic habitats >10 cm below the streambed surface. Gut contents were dominated by fine particulate matter (FPM) ≤75 μm and larger inorganic material on all dates in individuals from both benthic and hyporheic habitats. Fungi were relatively abundant in guts of benthic animals collected on some dates, whereas spores and pollen were relatively common food items in both habitats on occasions. Analysis of the trophic basis of production, based on gut contents and assumed assimilation and net production efficiencies, indicated that benthic secondary production was supported largely by fungi (48% of production) and FPM (37%), whereas FPM supported a higher level of hyporheic production (52%) than fungi (27%). Although stable carbon isotope values suggested dependence on epilithon, the enriched δ~(15)N values for this food source implicated the involvement of a microbial loop whereby a substantial proportion of Acanthophlebia nutrition appeared to be derived from heterotrophs growing in FPM and epilithon that had assimilated dissolved organic carbon. This study has highlighted the significant role that hyporheic habitats can play in the ecology of Acanthophlebia populations in pristine native forest streams. Re-establishment of hyporheic function would appear to be an important component of stream restoration work to enable the successful recolonisation of Acanthophlebia populations at sites where they historically occurred.