Hydrobiologia 337: 85-92, 1996. 85 @ 1996 Kluwer Academic Publishers. Printed in Belgium. Succession and secondary production of gastropods in the Ebro Delta ricefields Jacob González-Solís & Xavier Ruiz Dept. Biologia Animal (Vertebrats), Facultat de Biologia de la Universitat de Barcelona, Av. Diagonal 645, f l Barcelona 08028, Spain J~ Received 12 July1995; in revised fonn21May1996; accepted 4 June 1996 , ~ Key words: agroecosystem, breeding cycles, pulmonates, availability r-" Abstract We analyze fue dynamics of fue gastropod cornmunity living in fue Ebro Delta ricefields in relation to fue cultivation cycle. We found six bassornmatophoran species, three of which form more than 97% of total: Physella acuta (Draparnaud), Planorbis planorbis (L.), and Lymnaea avara (L.). Colonization of ricefields by freshwater pulmonates started in June, with populations increasing until fue harvest in October, when fue bulk of gastropods showed a decline. Populations recovered quickly and, in some cases reached their yearly maximum of abundance in Autumn. P. acuta predominates ayer all other species except in May and June, when P. planorbis takes overo L. avara is always fue least abundant of fue three main species. The analysis of size classessuggeststhat fueTe are two breeding seasons for P. acuta and P.planorbis, fue first at fue end of Surnmer, with a total replacement afilie population, and fue second in October-November, without replacement. Conversely, L. avara shows a wide array of size classesthroughout fue rice cultivation cycle. This suggeststhat this species breeds earlier and outside fue ricefields. The variations in population numbers of each species in relation to environmental changes induced by rice cultivation suggest that P. planorbis better exploits fue algal cover or other food available at fue ricefield bottoms, while P. acuta forages mainly on epiphytic vegetation and adventitious plants. Gastropod secondary production and turnover ratios in fue Ebro Delta fall within fue range of natural freshwater systems, but are situated in fue zone of higher values. r'" Introduction Ricefields are extreme examples of variable aquat- ic habitats, owing to pesticide treatrnents, adjustrnents Freshwater gastropods make up a large proportion of in water level and other agricultural operations (Forés g benthic animal biomass in variable aquatic habitats & Comín, 1986). Freshwater gastropod communities ;, (McMahon, 1983; Bosnia et al., 1990). This is because may playa keystone role in fue transfer of matterand 1.. .' semelparity, short life spans, earlyage at maturity, high energy in fuese agroecosystems. Also ricefields are ¡i growth rates, and high reproductive capacities maxi- oftensituated in ancient wetland areas of alluvial plains '" mize fue production of offspring in variableenviron- or deltas, where they benefit from periodic natural ments(McMahon, 1983).They may therebyprovide flooding (Forés& Comín, 1992).By the progressive an important source of food for a variety of predators loss of wetlandhabitats in Europeduring the present (Bosniaet al., 1990; Crowl & Covich, 1990; Wollheim century(Finlayson et al., 1992), ricefieldsare now fue & Lovvom, 1995). Moreover, their role in aquatic only majar flooded areasavailablefor invertebrates, ecosystems seems crucial to fue growth and mainte- fish, andwaterbirds (Fasola & Ruiz, 1996). Dance of macrophytes (Underwood & Thomas,1990; Ricefields provide excellent large-scale experimen- Underwood,1991). tal field situations, which can be used to compare fue ecology of their communitieswith those of perma-