Fish predation has an important role structuring benthic and planktonic assemblages in shallow waters. Habitat
complexity may mitigate the effect of fish predation, thus reducing prey–predator encounters and prey
capture ability. In this respect, benthic organisms may be less affected by predation than planktonic organisms
because macrophytes would increase habitat complexity, thus providing a physical refuge. However,
certain small fish may move and live within vegetation, resulting in an increase in prey–predator encounters,
and so physical refuges provided by macrophytes would be less effective. To test the effects of small fish on
aquatic invertebrates, we performed a field experiment using mesocosms in a Mediterranean salt marsh,
using Aphanius iberus as the small fish species (b5 cm) and Ruppia cirrhosa as the dominant macrophyte. Although,
A. iberus preys on benthic and planktonic organisms, it has traditionally been described as a benthic
species. Hence, a stronger effect on benthos is expected if habitat complexity does not reduce the predation
ability of this small fish. We therefore analyzed and compared the strength of the effects of A. iberus on both
benthic and planktonic organisms. The fish was found to have stronger effects on planktonic than benthic organisms.
The presence of A. iberus was coincident with a decline in the abundance of large water-column invertebrates
such as gammarids and jellyfish, and an increase in the abundance of medium-sized plankton.
The total biomass of the benthic assemblage increased when A. iberus was present. These results suggest that
aquatic invertebrates of Mediterranean salt marshes may use macrophytes as refuge from small fish species.
Fish predation has an important role structuring benthic and planktonic assemblages in shallow waters. Habitatcomplexity may mitigate the effect of fish predation, thus reducing prey–predator encounters and preycapture ability. In this respect, benthic organisms may be less affected by predation than planktonic organismsbecause macrophytes would increase habitat complexity, thus providing a physical refuge. However,certain small fish may move and live within vegetation, resulting in an increase in prey–predator encounters,and so physical refuges provided by macrophytes would be less effective. To test the effects of small fish onaquatic invertebrates, we performed a field experiment using mesocosms in a Mediterranean salt marsh,using Aphanius iberus as the small fish species (b5 cm) and Ruppia cirrhosa as the dominant macrophyte. Although,A. iberus preys on benthic and planktonic organisms, it has traditionally been described as a benthicspecies. Hence, a stronger effect on benthos is expected if habitat complexity does not reduce the predationability of this small fish. We therefore analyzed and compared the strength of the effects of A. iberus on bothbenthic and planktonic organisms. The fish was found to have stronger effects on planktonic than benthic organisms.The presence of A. iberus was coincident with a decline in the abundance of large water-column invertebratessuch as gammarids and jellyfish, and an increase in the abundance of medium-sized plankton.The total biomass of the benthic assemblage increased when A. iberus was present. These results suggest thataquatic invertebrates of Mediterranean salt marshes may use macrophytes as refuge from small fish species.
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