Growth and secondary production of mayflies was studied in three Hong Kong streams during 1995-1997. Mayfly growth was measured in the laboratory and in situ at Tai Po Kau Forest stream (TPKFS), and in an open and shaded site of the Lam Tsuen River (LTRO and LTRS respectively). Five mayfly species were studied: Cinygmina sp. and Electrogena sp. (Heptageniidae), Procloeon sp. and Baetis sp. (Baetidae) and Choroterpes spp. (Leptophlebiidae). Secondary production of each species was calculated over a two-year period at all three sites, and in Shing Mun River (SMR). The trophic base of mayfly production in all stream sites was determined by measuring the isotopic composition of mayflies and potential food sources during the dry and wet seasons of 1996-97.

Eggs of Procloeon sp., Cloeon sp. and Choroterpes spp. developed parthenogenetically in the laboratory, with hatching success of 71, 73 and 13% respectively. Baetis sp. and Procloeon sp. with the fastest growth rate, completed nine to 11 generations a year; Choroterpes spp. achieved six to nine generations, while Cinygmina sp. and Electrogena sp. completed at least four generations. Growth rates and development times were signifficantly affected by food and temperature. Mayflies grew faster and emerged sooner when provided with abundant food, both in the laboratory and in the field. Temperature had a positive effect on the growth rate of most species, and growth rate was mass-specific. Growth models including initial dry weight and temperature explained a significant amount of the total variation in field growth rates, and were used to calculate secondary production by the instantaneous growth method.

Secondary production estimates varied temporally and spatially. Total annual mayfly production (all five species combined) declined in the order SMR>TPKFS>LTRO>LTRS. Heptageniids contributed over 50% of the total mayfly production in most stream sites, although Baetis sp. represented 34% of the total production at LTRO. Greatest inter-year variation was observed in LTRO and LTRS, where production decreased by over 70% during 1996-97, while production at TPKFS remained relatively stable. Inter-year differences in total mayfly production reflected declines in the abundance of dominant species. Annual turnover ratios of all mayflies were among the most rapid recorded in the literature. Maximum P/B ratios for Cinygmina sp. and Electrogena sp. were 36.2 and 33.7; values were 80.9 adn 76.8 for Baetis sp. and Procloeon sp., and 46.6 for Choroterpes spp. Secondary production calculated by the size-frequency method was approximately 35% higher than estimates obtained with the instantaneous growth method.

The isotopic carbon and nitrogen composition of allochthonous food sources showed little temporal variation at all stream sites. Periphyton was generally more DELTA13 carbon-enriched during the dry season. Isotopic signatures of mayflies during the dry season revealed that Cinygmina sp., Electrogena sp., Baetis sp. and Choroterpes spp. fed mainly on periphyton and filamentous algae in all stream sites except LTRS. However, the assimilation of allochthonous sources increased during the wet season. Procloeon sp. at TPKFS and SMR were highly DELTA13 carbon-enriched and utilized an unknown food source - possibly cyanobacteria.