The ecology of five species of heptageniid mayflies was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two-year period (1977-79). Quantitative benthic samples were taken from different microhabitats in the stream on 47 occasions and, in combination with light-trap catches of adult mayflies, were used as a basis for investigating life-history patterns and estimating secondary production. Electrogena sp. (49.8 individuals 0.5 m−2; 60% of total heptageniid densities) and Cinygmina sp. (15.7 individuals 0.5 m−2) were the most abundant heptageniids in TPKFS, followed by Epeorus sp. (9.1 individuals 0.5 m−2). Iron sp. and Paegniodes cupulatus, which were present in relatively low densities, made up < 10% of total heptageniid abundance. In terms of mean biomass (1977-79), Electrogena sp. and Cinygmina sp. were equally important (10.3 vs. 10.5 mg ash-free dry weight (AFDW 0.5 m−2), while Epeorus sp. was ranked third (6.8 mg AFDW 0.5 m−2). There was, however, considerable inter-year variation in heptageniid standing stocks: mean densities and biomass of Cinygmina sp. in 1977-78 were (respectively) eight and nine times greater than in 1978-79, while Iron sp. and P. cupulatus exhibited the converse trend, with densities more than twice as high and biomass more than three times greater in 1978-79. Heptageniid abundance in TPKFS seemed to follow an annual pattern of wet-season decrease and dry-season increase, but this tendency was more apparent in some species (e.g. Iron sp.) than others (e.g. Cinygmina sp.). The life histories of TPKFS heptageniids were unclear. All species showed rather stable larval size-frequency distributions throughout the study period, which was indicative of asynchronous growth and continuous recruitment. Adult flight periods did not exhibit strong seasonality. Calculations of secondary production were based upon the conservative assumption that each species was univoltine, but multivoltine life histories may have been possible for these tropical mayflies.
Total heptageniid annual production over the period 1977-79 was 212.0 mg AFDW 0.5 m−2, and was slightly higher during 1977-78 than in 1978-79 (225.0 vs. 202.1 mg AFDW 0.5 m−2). Inter-year variations in production reflected variations in annual mean biomass: production of Cinygmina sp. was 7.9 times greater in 1977-78 than in 1978-79, but production of Iron sp. was 2.7 times higher during 1978-79, and that of P. cupulatus was 4.6 times greater. As a consequence of these inter-year variations, species-specific production decreased in the order Cinygmina sp. > Electrogena sp. > Epeorus sp. > Iron sp. > P. cupulatus in 1977-78, but the sequence was Electrogena > Iron sp. > Epeorus sp. > P. cupulatus > Cinygmina sp. in 1978-79. For the study period as a whole (1977-79), the order was Electrogena sp. > Cinygmina sp. > Epeorus sp. > Iron sp. > P. cupulatus. Production:biomass (P:B) ratios varied from 4.8 (Cinygmina sp., 1977-78) to 11.1 (Iron sp., 1977-78), and P:B ratios (1977-79) decreased in the order Iron sp. > Epeorus sp. > Electrogena sp. > P. cupulatus > Cinygmina sp. Production calculations and P:B ratios are highly sensitive to the conservative assumptions made about heptageniid voltinism in TPKFS, and therefore may be significant underestimates.
Epeorus sp., Iron sp., and P. cupulatus were most abundant in midstream microhabitats but Cinygmina sp. was more numerous close to the stream banks. The microdistribution pattern of Electrogena sp. was less distinct, but larvae seemed to favour midstream microhabitats. Multivariate statistics revealed that the microdistribution of all heptageniids (except Electrogena sp.) was influenced significantly by sediment grain-size characteristics rather than by algae or detritus, but the proportion of variation (4-14%) in the abundance of each heptageniid species accounted for by sediment characteristics was low.