Many trophic indices have been constructed for temperate aquatic environments, but few have proved reliable for tropical environments. Indices constructed on the basis of nutrients may not be effective for describing the trophic state because the fractions of nitrogen and phosphorus differ in their potential to predict the nutrient limitation in tropical aquatic environments. We developed an index based on the phytoplankton Reynolds functional groups for an Amazonian floodplain lake from samples collected during 2 contrasting hydrological periods (rising and flushing) and compared it with the index initially proposed by Carlson in 1977 and further adapted to tropical environments by Toledo in 1990. The functional group and Carlson indices matched only 37% and 56% of the sample units collected during the rising and flushing periods, respectively. Our study confirms the difficulty of assessing trophic states using only phosphorus and chlorophyll a in tropical floodplain lakes. In this environment (1) nitrogen may significantly limit phytoplankton growth and (2) complex phytoplankton-nutrient relationships occur during the hydrological cycle that cannot be accounted for through a simple phosphorus and chlorophyll a formula.
The spatio-temporal land cover dynamics of a medium‐size floodplain
system along the Amazon/Solimões River (Janauacá Lake, 786 km
) and their hydrological impacts are studied through
remote sensing and modeling. Hence, the analysis of 5 satellite-derived
land cover maps (1972-2016 period) reveals a decrease in natural
environments (from 65% to 35%) to the benefit of anthropic classes
(from 17% to 51%) through deforestation vectors (two highways and lake
banks). Deforestation is a non-stationary process with significant
increase over specific subperiods (1972-1986, and 2005-2016). It occurs
in stages with conversions into secondary vegetation then into
non-natural environments. 7 land cover scenarios (5 satellite-derived, 1
deforested and 1 forest, used as reference) are used as inputs to run
simulations with the same meteorology over the 2006-2018 period. Beside
high ( ≥ 24%) and low ( ≤ 7%) interannual variability of
runoff-rainfall ratio (RRR) and evapotranspiration (ET), the numerical
experiments evidence, on an annual scale, the RRR decreases and the ET
increases with deforestation increases. Deforested scenario suggests a
convergence: for the RRR, around 0.34 (-87%) and for the ET, around
1146 mm.yr (+6%). At the seasonal scale, the
landuse/landcover changes (LUCC) induce positive wet season ET anomaly
(<9%) and large negative dry season RRR anomaly (-87%). The
highest LUCC-induced disturbances (from -15% to 18%) in the FP mixture
are recorded at seasonal scale, during LW and RW and, at interannual
scale, during dry and normal HY. The LUCC-induced disturbances patterns
of FP mixture mainly concern river and runoff. They are different
regarding the hydrological period or HY type. Our experiments suggest
the existence of a tipping point between present land cover (2016) and
fully deforested cover associated with reversal phenomena and enhancing
of seasonal and interannual LUCC-induced disturbance. At last, the model
shows the LUCC augment the vulnerability associated with drought
periods.
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