Unraveling Flooding Dynamics and Nutrients’ Controls upon Phytoplankton Functional Dynamics in Amazonian Floodplain Lakes

Abstract

The processes in tropical floodplain lakes enable maintaining phytoplankton nutrient requirements over a hydrological year. The nutrients such as nitrogen, phosphorus and carbon compounds play an essential role in phytoplankton growth. However, the way that nutrients and phytoplankton interact and how this relationship varies seasonally in tropical freshwater ecosystems is not clear. In this study, we evaluate the relationship between phytoplankton–nutrients over the hydrological cycle in Amazonian floodplain lakes and verify if this relationship influences the biomass of cyanobacteria. We also check what factors linked to nutrients act in structuring phytoplankton community. Using the phytoplankton functional approach, we verified how their ability to respond to hydrological and environmental variations reflects the ecological conditions and investigated how these interactions work. The results show that the Amazonian floodplain lakes could maintain long-term nutrient enrichment status. The nutrients input conduces to cyanobacteria dominance, that allied to other factors, play an essential role in supporting the stability of the phytoplankton–nutrients relationship over the hydrological cycle.

Trophic state index validation based on the phytoplankton functional group approach in Amazon floodplain lakes

“…Affonso et al (2011) previously reported an increase in trophic state along the hydrological year for the Curuai floodplain, but their TSI T computation led to higher trophic state than in our study. The increase of the trophic state along the hydrological year was also suggested in Kraus et al (2019) and reflects the successive influence of river incursion and biogeochemical processes in the floodplain along the hydrological year. During the rising period, the river incursion across the floodplain is the major process that brings nutrients and organic matter into the floodplain (Junk et al 1989).…”

“…Therefore, despite a smaller TP concentration during the flushing period, the bioavailable P could be higher. But according to Kraus et al (2019), the proportion of bioavailable P (PO 4 ) during the flushing period is even smaller than during the rising period. Our results during this period are therefore more likely explained by the fact that P is not the main limiting factor of phytoplankton growth during this period.…”

“…As discussed, even if only part of these nutrients is available for biological uptake, the inputs satisfy the requirement to sustain a first peak of phytoplankton growth (Moreira-Turcq et al 2013, Bonnet et al 2016. Processes that occur in the floodplain participate in maintaining nutrients required for a second peak of phytoplankton during the flushing period (Kraus et al 2019). Especially, nutrient transfer from the sediment bulk into the water column is ensured by herbaceous plants that pump nutrients from sediments and release them into the water column during their decay (Junk et al 2010, Bonnet et al 2016.…”

“…Higher depth, shorter water residence time, and dilution limit primary production during the high-water period, whereas a second peak of primary production generally occurs during the flushing period (Junk et al 1989, Bonnet et al 2016. Extensive cattle ranching and increasing human settlements in the floodplain (Peres et al 2018) can contribute to nutrient enrichment or changes in their relative proportions and accelerate the eutrophication process (Kraus et al 2019), which is a serious and increasing problem worldwide (Dodds and Smith 2016).…”

Phytoplankton Genera Structure Revealed from the Multispectral Vertical Diffuse Attenuation Coefficient

“…The OACs vary widely and independently from each other, making freshwater optically complex environments [8,9]. The OAC concentration modifies the underwater light field, regulating the water transparency and impacting aquatic organisms such as fishes [10] and phytoplankton [11][12][13].…”