Net Photosynthetic Rates of Egeria Najas and Utricularia Breviscapa Changes Directed by Seasonal Hydrological Variations

We investigated the effect of hydrological dynamics of the Mogi-Guaçu River in a floodplain lake, with an annual unimodal flood pulse, on the net photosynthesis (NP) rates of two submerged aquatic macrophytes (Egeria najas Planchon 1849 and Utricularia breviscapa Wright ex Griseb. 1866). During the dry (fall/winter) and rainy (spring/summer) seasons, we monitored fifteen limnological variables (water temperature, atmospheric precipitation, total carbon, organic carbon, inorganic carbon, electrical conductivity, dissolved oxygen, coefficient of vertical light extinction, organic nitrogen, NH4+, free CO2, bicarbonate, total phosphorus, total suspended solids and pH. Except for nutrients (total phosphorus, NH4+ and organic nitrogen), dissolved oxygen, inorganic carbon and free CO2, the limnological variables differed between the dry and rainy seasons, where the water quality was influenced by the flood pulse. The NP rates of E. najas were significantly higher (p = 0.023) in the dry season. A marked difference was also observed for NP of U. breviscapa growing in dry and rainy seasons (p = 0.0135). The NP of E. najas and U. breviscapa during the rainy season was different from the dry season (p = 0.001). No significant differences in the NP between the two species were registered during the dry season. The hydrological regime changed the water chemistry and clearly affected the yield of NP of E. najas and U. breviscapa, although these macrophytes were influenced differently by the seasonal changes of physical and chemical variables. The higher NP of U. breviscapa compared to that of E. najas was related to the lower requirement of underwater radiation in photosynthesis than to E. najas. Moreover, the low availability of ions in the water column associated with lower temperatures in the dry season contributed to the decrease in NP of U. breviscapa during this period.