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Thermal structure of a whitewaters lake in the Solimões/Amazon River Basin,
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Central Amazonian
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Fabio M. Aprile *
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Visitor Researcher Instituto Nacional de Pesquisas da Amazônia - INPA, AM, Brazil
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Instituto Oceanográfico, Universidade de São Paulo, SP, Brazil.
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E-mail address:aprilefm@hotmail.com
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Short title: Thermal structure of a whitewaters lake in Amazonian
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Abstract
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The thermal structure of the Poraquê Lake (03º57’33.4”S; 63º09’48.1”W) a shallow whitewaters
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lake of the Solimões/Amazon River basin has been conducted. Temperature and PAR radiation
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were measured bimonthly in the period from February 2004 to July 2006 at 0.25 m intervals
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from the surface to bottom of the lake. The temperature of the water column varied between 26.3
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and 28.8 ºC in the surface, and from 25.2 to 27.8 ºC in the bottom, between flood-crest and dry
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periods, respectively. Classic thermal stratification was not observed in the lake. The turbulent
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kinetic energy from action of winds and of the flood-pulse mixes the masses of water and
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homogenizes the temperature. The heating of the water column was influenced by the high rates
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of SPM present in whitewaters systems. The limit of the euphotic zone ranged from 1.36 to 1.77
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m in the period studied. The results of the transmission curves for the sampling sites show that,
*
Corresponding Author: Tel: +55 11 38149949 Instituto Oceanográfico, Universidade de São Paulo, Rua
Doralice P. Teixeira 48/13 São Paulo 05417-070 Brazil.
E-mail address:aprilefm@hotmail.com
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in general, less than 0.01% of the surface light reached the bottom. Mathematical model of
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limnological series were used for sequential generation of limnological data for simulation
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purposes. Stochastic elements were considered dependents, because the direct relation between
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temperature in the water column and the seasonality. The model developed in this research can
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facilitate the understanding of the limnological and ecological processes in lentic systems of
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whitewaters of the Central Amazonian.
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Keywords: Amazon floodplain; thermal model; hydrological cycle; thermocline; solid particulate
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matter; PAR radiation; euphotic zone; limnology of lake; whitewaters; Solimões/Amazon River.
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1. Introduction
Poraquê Lake (03º57’33.4”S and 63º09’48.1”W) is a small and shallow lake with
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whitewaters located upstream of Coari City (Fig. 1), and that is connected with the Solimões
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River by a “paraná” (natural flow channel). It is a young lake of quaternary origin (Sioli, 1984),
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with processes and materials associated to transport and deposition by running water. The
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“paranás” as igapós both have a very important participation in the hydrological processes in the
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Amazon floodplain or “várzea” controlling the ictiofauna (Junk, Bayley & Sparks, 1989; Junk,
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Soares & Saint-Paul, 1997). The floodplains extension associated to the Amazon River is
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expected to alter the transport of water from upland watersheds through river systems to the sea
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(Junk, Bayley & Sparks, 1989). During the rising water stage, river floodwaters temporarily fill
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wetlands connected to the “paraná” of the river, inundating an immense area for the whole basin
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(Junk, 1982). Amazon lakes are strongly influenced by periodic supplying of organic matter
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(Darwich, 1995) of dissolved and particulate forms from rivers and “paranás.” This supply is
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responsible by deposition of nutrients in surface sediments of the lake. The Poraquê Lake is
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formed by various “igarapés of forest” a typical Amazonian flowing rivulet or small stream of
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water, which serves as the natural drainage course for a lake drainage basin. Water supply to the
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lake and igapós varies in space and time. In fact, the great seasonal variability of the
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limnological parameters is characteristic of aquatic floodplain ecosystems (Sioli, 1984), and the
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annual inundation of the central Solimões/Amazon River floodplain causes profound
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modifications in the aquatic environment and provides a variety of habitats where shelter and
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food become available to fish (Soares, Menezes & Junk, 2006).
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Average annual depth of the Poraquê Lake is approximately 3.0 m at the central site.
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Changes in water supply in Amazon floodplain occur over diverse time scales, associated with
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daily and seasonal variations precipitation. The lake water level has its flood-peak between June
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and July with average of 6.4 m, and maximum dry between November and December with
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average of 0.9 m both at the central site. According to Köppen classification, the climate is
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equatorial hot and wet. The thermal structure of the Poraquê Lake has been studied in the
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hydrological cycle from February/2004 to July/2006, and a thermal model to whitewaters lakes
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was developed. Information of this nature to offer valuable contribution in studies on abundance
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and distribution of fishes, and to permit a much better understanding of the limnological
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processes, in particular, in the lentic systems of the Solimões/Amazon System.
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Figure 1-
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2. Methodological proceeding
Temperature was measured bimonthly at 0.25 m intervals from the surface to bottom at
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the centre of the lake (stations P1 and P2, Fig. 1) with a WTW OXI-197 thermistor electrode of
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accuracy  0.1 ºC, from February 2004 through July 2006. PAR radiation measurements were
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made with a Quantum Radiometer in the water column. The results were utilized to calculate the
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transparency (Zds), euphotic zone (Zeu) and attenuation coefficient (K) according to
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methodological procedures decrypted by Wetzel & Likens (2000). A correlation analyzes
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between temperature and PAR radiation has been made in order to express the temperature
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association with the seasonality in the region. Based on the thermal and light profiles, differential
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equations were calculated, and a thermal model to whitewaters lakes was developed.
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3. Thermal structure
In the study period depth varied from 1.5 m in the dry period to 6.1 m in the flood-crest at
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the centre of the lake (stations P1 and P2, Fig. 1). The warmer of the water layers in the lake
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cause a typical condition of structure of whitewater lakes, which is presented in the Fig. 2. The
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thermal profile of the hydrological cycle is represented with the respective standard deviations.
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The water temperature in the flood period (between February and March) ranged from 27.6 to
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28.3 ºC (average 28.0  0.49 ºC) at the surface layers, and keeping on 26.0 ºC at the bottom.
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Temperature levels in the flood-crest (June-July) ranged from 26.3 to 27.5 ºC (average 26.9 
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0.85 ºC) in the surface, and ranged from 25.2 to 25.6 ºC (average 25.4  0.28 ºC) between 4 and
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6 meters. The higher variations of the temperature were found in this period, with a maximum
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difference between surface and bottom at 2.3 ºC. During September to October, in the ebb
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period, the water temperature ranged of 27.7 - 27.9 ºC (average 27.8  0.14 ºC) in the surface,
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and stayed stable in 26.1 ºC at the bottom. The maximum depth obtained in the ebb was 3.1
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meters. The water temperature in the dry period (November – December) ranged from 28.6 to
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28.8 ºC (average 28.7  0.14 ºC) in the surface, and ranged of 27.7 - 27.8 ºC (average 27.8  0.07
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ºC) at the bottom (Fig. 2). Pearson Correlation was determined to the temperature and PAR
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radiation profiles with base on 10 samples and 2 sampling sites (P1 and P2), and significance at
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p < 0.004. Temperature correlation was mean to high for obtained light (Lobt r2 = 0.6607) and for
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calculated light (Lcalc r2 = 0.7228) profiles. The stronger correlation between Lobt and Lcalc
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profiles (r2 = 0.9868 to p < 0.0001) shows a confidence in the calculated data.
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Figure 2-
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The difference of temperature between surface and bottom never exceed 2.3 ºC in all
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study period, including the flood-peak phase when is winter in the South Hemisphere. A mean
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profile to the temperature was as given in Fig. 3A. Weak thermal stratification in the flood
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period was observed. However, it was not observed in the other periods. In fact, I believe that
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shallow tropical lakes of whitewaters are permanently heated by a diffuse radiation that occurs
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very slowly due to the high concentration of suspended matter from geological processes. The
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turbulent kinetic energy from action of winds and of the flood-pulse mixes the masses of water
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and homogenizes the temperature. This process occurs in almost whole lake. The absence of a
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significant different in thermal gradient to indicate there is not a typical stratification, common in
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lakes of black waters of the Amazonian (Rai & Hill, 1981; Darwich, Aprile, Robertson & Alves,
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2005). Therefore, the water temperature in the hypolimnion is higher in whitewater lakes than
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that in black water lakes. Studies have been suggested that many shallow tropical lakes stratify
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and mix on a daily basis (Lewis, 1983; Lampert & Sommer, 1997). In the Amazon, however,
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stratification and mixing events in floodplain lakes vary throughout the year mostly because of
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the seasonal changes in depth and lake morphology (MacIntyre & Melack, 1984). The depth and
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area of the lakes change according to the flood-pulse of the main rivers (Melack, 1984; Junk,
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Soares & Saint-Paul, 1997). The thermal structure is an important parameter to identify the
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degree of eutrophication and its effects in a lake (Chandler, 1942; Sawyer, 1969; Wetzel &
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Likens, 2000).
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Transparency, euphotic zone, and attenuation coefficient for the Poraquê Lake are
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showed in the Table 1. The transition between flood and flood-crest periods showed higher
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transparency. The limit of the euphotic zone ranged from 1.36 to 1.77 m (average 1.55 m) in the
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study period. The compensation level that usually occurs at the depth of 1 percent light
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penetration and forms the lower boundary of the Zone of Net Metabolic Production was more
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pronounced between flood and flood-crest periods. The most of the bottom receives relatively a
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very low percentage of the light that reaches the surface. The results of the transmission curves
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for the stations P1 and P2 (see Fig. 1) show that less than 0.01% of the surface light reached the
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bottom, exception to the dry period when the lake is shallow. These results approach a typical
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exponential curve (see Fig. 3B), what means that the water was optically heterogeneous from top
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to bottom. Changes in transparency may alter the depth of the euphotic zone; affect the primary
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production of phytoplankton and activities of diverse organisms, including benthonic organisms
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(e.g. phytobenthos and zoobenthos). In fact, the distribution of the organisms in water column
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depends on this submarine daylight and conceivably its spectral quality.
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The distribution of suspended particulate matter (SPM) is not homogeneous in the water
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column. In whitewaters lakes of the Amazonian, the lower layers contained higher amounts of
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SPM (average of 110 mg.l-1) than the upper layers (average of 65 mg.l-1). The concentration of
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suspended particulate matter in water column varies also seasonally, influenced by flood and dry
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periods. These facts explain the variation of the extinction coefficient (K) in the hydrological
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cycle (Table 1). The K confirmed the heterogeneity in the optical quantity observed in the light
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profile (Fig. 3B).
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Table 1-
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4. Thermal model to whitewaters lakes
Mathematical models of limnological series are used for sequential generation of
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physical, chemical, and biological data for simulation purposes. Mathematical models are
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commonly considered by contain a deterministic and a stochastic element. The deterministic
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element may be composed of a trend and/or a periodic component. The trend was recognized
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using polynomial regression. The periodic component may be modeled using different methods
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such as Euler, Runge-Kutta, Milne, or Series of Fourier. In this research were used differential
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equations (1st and 2nd), where a number of harmonics represents the means and standard
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deviations of the limnological variables (temperature and PAR radiation). Based on the
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continuous observations of the natural conditions a periodic component was modeled. Results
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were based on the period equivalent to hydrological cycle from February 2004 to July 2006.
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Stochastic elements were considered dependents, because the direct relation between
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temperature in the water column and the seasonality. The application of mathematical models in
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the Amazon ecosystems is not usual. Furch (1999) developed a theoretical model for estimation
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of groundwater input and ionic flux to a floodplain lake in the Solimões River. Lesack (1995)
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presented an empirical seepage model for a floodplain lake of whitewaters further up the
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Solimões River. This model accounts for seepage flux rates and describes the dynamics in detail.
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Figure 3-
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Based on the light obtained (Fig. 3B) and calculated (Fig. 3C) profiles differential
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equations to the model were developed. Temperature model was explained by an equation of
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first order. The condition to the validation of the light model is that the light obtained (Lobt) and
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light calculated (Lcalc) profiles will be equivalent (yobt  ycalc). Light model was explained by an
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equation of second order.
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Temperature : N ( z )  A . e k . z
Equation1st order: y  y0  A1 . e
ytemp  24.94  3.13 e


( x  x0 )
t1
 Model validated
x
2.60
Light : N ( z )  A . e k .z
to I z  I 0 . e  k .z and yobtained  ycalculated
Equation1st order: y  y0  A1 . e
yobt  ycalc  1262 .86 e

x
0.20

( x  x0 )
t1
 1328 .20 e
Equation 2nd order: y  y0  A1 . e
yobt  ycalc  1237 .47 e
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

x
0.18



( x  x0 )
t1
 65.18 e

x
0.31
 e
 A2 . e
x
0.59


x
0.20
.
( x  x0 )
t2
 566 .32 e

x
0
.
e 31
 e 0.05
 Model validated
x
0.20
 814 .40 e

x
0.37


e x 1237 .47 e  6.6 x  65.18 e  2.7 x  566 .32 e  6.0 x  814 .40 e  3.7 x  0
to e x  0
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Daily the radiation flux reaching the lake increases and the upper layers become warmer.
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The wind effect associated to flood-pulse and the penetrative convection, transport the turbulent
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kinetic energy generated down into the lake, mixing the column of water (Fig. 4). In shallow
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whitewater lakes of the Amazonian, the mixing is almost complete. Eventually there are some
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places, corresponding to the deeper zones in the lake, where the mixing is impeded.
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Limnological results no published about others whitewater lakes at the Solimões River basin
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(e.g. Preto Lake, Anana Lake, Araçá Lake, Maracá Lake and Aruã Lake) confirm the trend of
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mix of the various layers of water. Studies on abiotic and biotic processes and nutritional fluxes
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in floodplain of whitewater lakes at the Solimões/Amazon River basin have been described
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(Furch, Junk, Dietrich & Kochert, 1983; Furch, 1984; Junk, Bayley & Sparks, 1989; Piedade,
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Junk & Long, 1991; Furch & Junk, 1997; Cullmann, Junk, Weber & Schmitz, 2006). There is a
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strong correlation between the limnological/ecological processes of areas permanently inundated
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and of areas temporarily connected by a paraná (Fig. 4). The fluvial transport and storage of
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nutrients and sediments within channel-floodplain systems, and oxygen distribution to the more
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deep layers of the lake are examples of this important connection between river and lake. The
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Solimões/Amazon River basin is formed by many floodplain lakes, with physical and chemical
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characteristics very similar to the Poraquê Lake. Thus, the developing and application of
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mathematical models can offer valuable contribution to the ecological studies in the
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Solimões/Amazon River basin, in particular to lentic systems.
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Figure 4-
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5. Conclusions
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In general, typical thermal stratification was not observed in the lake during the study
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period. The heating of the body water is influenced by the high rates of suspended particulate
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matter present in whitewaters systems. The compensation level that usually occurs at the depth
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of 1% light penetration and forms the lower boundary of the Zone of Net Metabolic Production
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was more pronounced between flood and flood-crest periods. The results of the transmission
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curves for the sampling sites show that, in general, less than 0.01% of the surface light reached
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the bottom. The model developed in this research can facilitate the understanding of the
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limnological and ecological processes in lentic systems.
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Acknowledgements
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The author extends his thanks to the Dr. Assad Darwich and Dr. Edinaldo Nelson dos
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Santos at the Instituto Nacional de Pesquisas da Amazônia - INPA for valuable help with the
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sampling and discussion. This research was partially supported by the Piatam Project - UFAM
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and CNPq/Finep (Project numbers # 301746/1996-6 and # 505085/2004-6).
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Figure legends
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Fig. 1 - Location of the Poraquê Lake in the floodplain on the Solimões/Amazon River basin
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(A), and Poraquê Lake with the sampling sites (B), Central Amazonian - Brazil
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Fig. 2 - Temperature profiles at the centre of the Poraquê Lake for the hydrological cycle
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February/2004 – July/2006.
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Fig. 3 - Mean temperature (A), light obtained (B) and light calculated (C) profiles at the centre of
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the Poraquê Lake, with the respective coefficients of the differential equations, for the
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hydrological cycle February/2004 – July/2006.
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Fig. 4 - Light radiation and warmer model to the Poraquê Lake in Solimões River basin (Central
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Amazonian).
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