The significance of water inputs to plankton biomass and trophic relationships in a semi-arid freshwater wetland (central Spain)

This study attempts to describe changes in plankton biomass (including bact eria, phytoplankton, ciliates, rotifers, and crustaceans) in the semi-arid, hypertrophic, freshwater wetland, Las Tablas de Daimiel National Park (cen tral Spain), in relation to water inflow. An inter-annual comparison of 199 6 and 1997, with contrasting rainfall, reveals that inflows in the form of flooding act as a bottom-up structuring force on total plankton biomass. Ho wever, the responses of plankton biomass to flooding were different in stre ngth between the two years, which can be attributed to shifts in plankton c ommunity structure. Crustacean zooplankton conditioned total plankton bioma ss in 1996, based on the relationship between the high individual biomass a nd community development. Crustaceans decreased in 1997, while bacteria, ph ytoplankton and rotifers increased. The quantitative increase of the latter groups resulted in a higher total plankton biomass in 1997. The key positi on of crustaceans in the wetland plankton is also reflected in their capabi lity for exerting significant top-down control, which was clearly evident i n 1996 but weaker in 1997, based on their numerical reduction. Statistical analyses provide evidence that the decline of trophic interactions in 1997 is related to the strength of flooding in that year, thereby highlighting t he significance of the disruptive action of physical disturbance on biotic interactions in the plankton. Flooding and areal inundation were also shown to be significant for spatial heterogeneity. In 1996, site-specific develo pment took place in the absence of prolonged flooding effects, resulting in high spatial heterogeneity. In 1997, however, remarkable homogenization of plankton biomass occurred along the major water flow path. Thus, wetland l andscape heterogeneity depends on inflows which condition areal inundation. This, in turn, influences plankton dynamics.