Population dynamics and secondary production of a dominant molluscan primary consumer, Sulcospira hainanensis (gastropoda : pachychilidae),in Hong Kong streams and their implications for benthic productivity: y Yeung Alex Chee Yu.

Abstract

The lack of any measurements of the secondary production of the prosobranch snail Sulcospira hainanensis (Bröt, 1872) (Pachychilidae), an abundant primary consumer in Hong Kong streams, represents a major gap in our knowledge of the magnitude of benthic productivity in these systems where S. hainanensis constitutes a substantial proportion of community biomass. This study investigated the population dynamics and production of S. hainanensis between February 2011 and February 2012 in 4 Hong Kong streams (2 unshaded and 2 shaded). Annual production by amphibiotic insects was also quantified by emergence traps to evaluate the importance of in-stream production by fully aquatic animals versus that leaving streams in the form of adult aquatic insects. In addition, snail grazing effects on algae and the potential for consequent competitive interactions with benthic insects was compared under different shading conditions and between wet and dry seasons.

Sulcospira hainanensis was ovoviviparous and had balanced sex ratios. Brood size ranged from 52 to 1189, and 3 recruitment episodes were observed in both populations in the one-year study period. Hatchlings reached sexual maturity in 3 – 5 years whereas adults could live for 6 – 12 years. Mean population densities varied from 108.4 to 206.1 individuals m^(-2), while mean biomass was 1003.0 to 4430.2 mg ash-free dry mass (AFDM) m^(-2) and was generally higher in the dry season. Snail production was estimated using the size-frequency, instantaneous-growth (field- and computer-based) and increment-summation (IS) methods, with the IS estimates (1612.8 – 6123.9 mg AFDM m^(-2) 〖year〗^(-1)) considered to most accurately represent production by S. hainanensis. Annual turnover ratios were 1.36 – 2.24 〖year〗^(-1), and production was higher in unshaded streams where growth was more rapid, reflecting higher availability of algal food.

The contribution of S. hainanensis to production by benthic animals as previously estimated in one study site was relatively low (15%) compared with its contribution to total standing biomass (26%). Annual insect emergence varied from 167.5 – 780.2 mg AFDM m-2 year-1, and constituted ~13% of total benthic production in one site, and this preliminary finding suggests that the water-to-land energy flux attributable to emerging insects along tropical Hong Kong streams is rather minor.

The competition effects of S. hainanensis were studied during the dry season of 2012 and the wet season of 2011 by means of snail inclusion-exclusion manipulations. Snails exerted strong effects on algae and insects during the dry season, but did not significantly affect the structure of benthic assemblages. Algae in shaded streams were more severely depleted by snails, though the reduction in snail densities did not lead to increases in insect abundance or biomass. The effects of snails were not detectable during the wet season, when spate-induced disturbances were more frequent and intense. This generally agreed with the harsh-benign hypothesis, which predicts a reduction in the significance of biotic interactions under more disturbed conditions. Therefore, competition between S. hainanensis and insects was important only at base-flow conditions during the dry season.