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 bacteria, phytoplankton, ciliates, rotifers, andcrustaceans) in the semi-arid, hypertrophic, freshwater wetland,Las Tablas de Daimiel National Park (central Spain), in relationto water inflow. An inter-annual comparison of 1996 and 1997,with contrasting rainfall, reveals that inflows in the formof flooding act as a bottom-up structuring force on total planktonbiomass. However, the responses of plankton biomass to floodingwere different in strength between the two years, which canbe attributed to shifts in plankton community structure. Crustaceanzooplankton conditioned total plankton biomass in 1996, basedon the relationship between the high individual biomass andcommunity development. Crust-aceans decreased in 1997, whilebacteria, phytoplankton and rotifers increased. The quantitativeincrease of the latter groups resulted in a higher total planktonbiomass in 1997. The key position of crustaceans in the wetlandplankton is also reflected in their capability for exertingsignificant top-down control, which was clearly evident in 1996but weaker in 1997, based on their numerical reduction. Statisticalanalyses provide evidence that the decline of trophic interactionsin 1997 is related to the strength of flooding in that year,thereby highlighting the significance of the disruptive actionof physical disturbance on biotic interactions in the plankton.Flooding and areal inundation were also shown to be significantfor spatial heterogeneity. In 1996, site-specific developmenttook place in the absence of prolonged flooding effects, resultingin high spatial heterogeneity. In 1997, however, remarkablehomogenization of plankton biomass occurred along the majorwater flow path. Thus, wetland landscape heterogeneity dependson inflows which condition areal inundation. This, in turn,influences plankton dynamics.