On the importance of water depth, macrophytes and fish in wetland picocyanobacteria regulation

Current paradigms contend abiotic regulation of wetland biota most important, although recent Studies show that biotic configuration can play an important role in wetland functioning within the constraints set by abiotic forces. Picocyanobacteria (PC) - although little studied in wetlands - may prove promising candidates for determining the response to experimental manipulation of important physical and biological components of wetland ecosystems (water depth, vegetation, fish) in the short term, because of their fast growth and high reproduction, and thus to test for the importance of biotic or abiotic regulation of PC in wetlands. In this study, the densities of the dominant fish species (Cyprinus carpio, Lepomis gibbosus and Gambusia holbrooki), and Submerged (charophytes) and emergent (Cladium mariscus) macrophytes were manipulated in enclosures that were installed in deeper (1.0 m) and in shallower (0.4 m) zones of the Spanish National Park, Las Tablas de Daimiel. Principal component analyses showed that the manipulation of biotic elements brought about significant differences in the limnological environment, but PC (chiefly Cyanobium sp.) abundances were relatively unresponsive to the biota-mediated effects. Instead we observed consistently higher PC abundances in all deeper site treatments compared with the shallower site treatments. Generalised linear models suggested that water depth has overridden any biota-mediated regulation of PC. Although the ultimate cause for PC regulation could not be revealed, results indicate that water depth may strongly mediate PC abundances in wetlands. Furthermore the marginal effect of biota-mediated effects, Suggests PC being regulated to a great extent by abiotic forces during our study, thereby matching Current paradigms of wetland functioning.