Succession and secondary production of gastropods in the Ebro Delta ricefields

Abstract

We analyze the dynamics of the gastropod community living in the Ebro Delta ricefields in relation to the cultivation cycle. We found six bassommatophoran species, three of which form more than 97% of total: Physella acuta (Draparnaud), Planorbis planorbis (L.), and Lymnaea ovata (L.).

Colonization of ricefields by freshwater pulmonates started in June, with populations increasing until the harvest in October, when the bulk of gastropods showed a decline. Populations recovered quickly and, in some cases reached their yearly maximum of abundance in Autumn. P. acuta predominates over all other species except in May and June, when P. planorbis takes over. L. ovata is always the least abundant of the three main species. The analysis of size classes suggests that there are two breeding seasons for P. acuta and P. planorbis, the first at the end of Summer, with a total replacement of the population, and the second in October–November, without replacement. Conversely, L. ovata shows a wide array of size classes throughout the rice cultivation cycle. This suggests that this species breeds earlier and outside the ricefields.

The variations in population numbers of each species in relation to environmental changes induced by rice cultivation suggest that P. planorbis better exploits the algal cover or other food available at the ricefield bottoms, while P. acuta forages mainly on epiphytic vegetation and adventitious plants.

Gastropod secondary production and turnover ratios in the Ebro Delta fall within the range of natural freshwater systems, but are situated in the zone of higher values.