Effects of hydrologically confined fishes on bacterioplankton and autotrophic picoplankton in a semiarid marsh

An enclosure experiment was conducted to assess the separate effects of exotic benthivorous (Cyprinus carpio L.), planktivorous (Gambusia holbrooki Gir.), and omnivorous (Lepomis gibbosus L.) fish on the dynamics of bacterioplankton (BAC) and autotrophic picoplankton (APP) in a semiarid Spanish marsh. Special emphasis was put on simulating the effect of natural fish density situations occurring under periods of hydrological confinement in the wetland, i.e. during summer draw downs or artificial water-level reductions, when fish biomass can reach high levels. The resulting simulation of such a scenario (C. carpio, 5000 to 6000 kg ha(-1); L. gibbosus, 1300 to 1700 kg ha(-1), and G. holbrooki, 115 kg ha(-1)) revealed that C. carpio and L. gibbosus significantly increased the trophic level in the enclosures. This resulted in a significant increase of BAC in the respective treatments. G. holbrooki, on the other hand, failed to fuel BAC growth. Considering APP, composed of phycocyanin-rich picocyanobacteria Synechococcus sp., no fish treatment effects were detected, suggesting that the fishes, independent of biomasses used, were not important in driving APP. Results indicate that microbial communities respond in different ways to changes in fish biomass in fluctuating wetlands.