Impacts of Limnoperna fortunei on Zooplankton Florencia Rojas Molina, Susana B. José de Paggi and Juan César Paggi
Abstract In addition to phytoplankton, adult Limnoperna fortunei feed actively on animal prey, consuming over 150 different organisms, including Protista, Rotifera, Nematoda, Cladocera, Copepoda, Ostracoda, and L. fortunei larvae. Animals are present in the stomachs of 96 % of the individuals analyzed representing, on average, 67 % of ingested biomass. Rotifers are the most frequent animal prey, followed by cladocerans and copepod nauplii. Comparison between the abundances of prey in plankton samples and the diet of L. fortunei indicates that all animals are positively selected, with the highest selectivity for the rotifers and small cladocerans. Selectivity is positively associated with prey size and negatively with avoidance capabilities. This selective grazing pressure is probably responsible for the fact that rotifer densities have dropped in several water bodies associated with the Middle Paraná River after L. fortunei colonized the area, while cladocerans and copepods remain at pre-invasion levels. In addition, grazing pressure probably accounts for significant post-invasion decreases in zooplankton density differences during low- and high-water periods. Densities of L. fortunei veligers normally exceed those of rotifer + crustacean zooplankton for 8–9 months of the year, underscoring their potential significance as competitors of other zooplankton for food, and as food for various animals. Keywords Limnoperna fortunei · Golden mussel · Ecological impact · Zooplankton grazing · Grazing selectivity F. Rojas Molina () · S. B. José de Paggi Instituto Nacional de Limnología (INALI, Consejo Nacional de Investigaciones Científicas y Técnicas-Universidad Nacional del Litoral), Facultad de Bioquímica y Ciencias Biológicas (Universidad Nacional del Litoral), Ciudad Universitaria, 3000 Santa Fe, Argentina e-mail:
[email protected] S. B. José de Paggi e-mail:
[email protected] J. C. Paggi Instituto Nacional de Limnología (INALI, Consejo Nacional de Investigaciones Científicas y Técnicas-Universidad Nacional del Litoral), Ciudad Universitaria, 3000 Santa Fe, Argentina e-mail:
[email protected] © Springer International Publishing Switzerland 2015 D. Boltovskoy (ed.), Limnoperna fortunei, Invading Nature - Springer Series in Invasion Ecology 10, DOI 10.1007/978-3-319-13494-9_10
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Introduction Benthic–pelagic coupling mechanisms may involve strong effects of suspension feeding bivalves on the plankton community, dramatically affecting planktonic abundance and structure by selective feeding (Wong and Levinton 2006). In order to understand the effects of mussel grazing on the plankton, it is important to determine both the amount and the type of organisms retained, ingested, and rejected (Cohen et al. 1984; Caraco et al. 1997; Pace et al. 1998; Rojas Molina and José de Paggi 2008). Plankton that is withdrawn from the water column and digested by the mussels or trapped in their pseudofeces may constitute a significant loss of biomass from the pelagial (Horgan and Mills 1997). Although phytoplankton is usually the major source of nutrition for bivalves (Cohen et al. 1984; Bastviken et al. 1998; Prins et al. 1998), several studies have shown that they can also feed on zooplankton (MacIsaac et al. 1995; Wong et al. 2003; Wong and Twining 2003; Wong and Levinton 2005).
Animals in the Diet of Limnoperna fortunei: Field Data and Laboratory Experiments Although most data on the feeding of Limnoperna fortunei are centered on the phytoplankton (see Chapter “Nutrient Recycling, Phytoplankton Grazing, and Associated Impacts of Limnoperna fortunei” in this volume), the zooplankton represents a significant fraction of the diet of the mussel. In a survey carried out in several floodplain environments of the Middle Paraná River, Rojas Molina et al. (2010) recorded a total of 156 taxa in the stomach contents of the golden mussel, covering a wide spectrum of sizes ranging from 4 to slightly over 1000 µm in length. Algae included 81 taxa (Cyanobacteria, Chlorophyceae, Xanthophyta, Bacillariophyceae, Euglenophyta, and Dinophyta), whereas animals were represented by 46 species of Rotifera, 17 of Cladocera, 4 of Copepoda, several kinds of Protista, Ostracoda, and Nematoda, as well as larvae of L. fortunei (Table 1). Despite the fact that animals were represented by much lower numbers of individuals (18 ind./stomach) than phytoplankton (1825 ind./stomach; Fig. 1), they were present in 96 % of the individuals analyzed. Rotifers, especially Keratella spp. and Lecane spp., were the most frequent animal prey items accounting for 81 % of all animals, with a mean abundance of 15 ind./stomach (maximum: 233 ind./stomach). Cladocera, mainly represented by Chydoridae and Bosminidae, were less frequent (present in 64 % of the stomachs analyzed), accounting for 10 % of the animals in the diet. Copepoda, represented by nauplii and copepodites only, made up
