Francisco José Oliva Paterna

The effects of Aphanius iberus predation on an aquatic community: diel changes and the role of vegetation

The predation effects of a cyprinodontid fish (Aphanius iberus) on the aquatic invertebrate community structure of a Mediterranean shallow system with vegetation were investigated. Field studies were performed in March and July of 2009 in a pond adjacent to the salterns of San Pedro del Pinatar (Murcia, Spain) to evaluate two distinct settings of A. iberus population dynamics. In March, the A. iberus population had a low density, mainly dominated by large fish, whereas in July, the population had a higher density, mainly dominated by small, young-ofthe-year fish. To analyse the fish, vegetation and possible diel effects on the invertebrate community, we compared the aquatic invertebrate body size distribution (biomass size spectra) found under the following conditions: 1) during different seasons with different fish population structures to include possible ontogenic shifts, 2) in vegetated and non-vegetated areas to include sites with different resource availability, and 3) under day and night conditions to investigate possible diel cycles. We hypothesise that 1) because the main predator (A. iberus) has a high preference for vegetated areas and is expected to show a reduced mobility out of these areas, no diel differences in the aquatic invertebrate community structure would be apparent; 2) differences in the aquatic invertebrate size spectra would be found when comparing the non-vegetated area to the vegetated area (an area with higher resource availability but also higher predation pressure); and 3) differences in the aquatic invertebrate size spectra would be apparent when comparing different seasons due to an ontogenic shift in the A. iberus diet. Our results showed that in the vegetated areas, where fish were much more abundant, the assemblages of aquatic invertebrates had a characteristic size spectrum, showing a higher size diversity and a higher total biomass than the assemblages observed in the non-vegetated areas. Moreover, the observed differences were not affected by diel changes because none of the analysed parameters showed significant differences between the day and night samplings.