Phytoplankton productivity in a highly colored shallow lake of a South American floodplain

Abstract

We described phytoplankton productivity in a floodplain wetland of the Lower Paraná River, Argentina. Four samplings encompassing periods of high and low water levels were conducted in a highly colored shallow lake. Photosynthesis-irradiance (P-E) curves and the areal photosynthetic rate (P_A) were estimated following the ¹⁴C assimilation technique. Likewise, physical and chemical variables and phytoplankton composition, density, and chlorophyll a concentration were measured. Phytoplankton assemblages shifted from cyanobacteria blooms in summer to cryptophycean dominance in winter, and co-dominance of cryptophytes and chlorophytes in autumn. Assimilation number P_max ranged from 3– 7.8 μg C (μg Chl a h)⁻¹ and peaked in early summer (low water level) when water color was highest, suggesting that phytoplankton productivity was not depressed by the high content of colored humic acids. Photosynthetic efficiency (α) ranged from 0.021–1 μg C (μg Chl a h)⁻¹ μmol photons⁻¹ m² s and reached its maximum value during winter even when the assemblage, dominated by cryptophyceans, did not achieve light saturation. In early and late summer and in autumn, optimal irradiance (E_opt) ranged from 544–1,397 μmol photons m⁻² s⁻¹. The highest P_A (207 mg C m⁻² h⁻¹) was registered in late summer (high water level) when the lowest mean irradiance (E_mean) was observed (341 μmol photons m⁻² s⁻¹). The lowest P_A (28 mg C m⁻² h⁻¹) occurred in winter when E_mean was maximum (1,432 μmol photons m⁻² s⁻¹). Our results indicate that productivity was similar to those recorded for other latitudes and appeared not limited by the humic content of the water because phytoplankton was dominated by algae well-adapted to low light conditions.