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Methods
The Project is composed by four subprojects, being developed
simultaneously:
1.
Online acquisition of micrometeorological and limnological data
The Integrated System of Environmental Monitoring – SIMA – is
composed of hardware and software designed for the collection of
data and the online monitoring of hydrological systems. For the
acquisition of data, the SIMA uses an autonomous system anchored
in the reservoir, composed of a toroid, where the sensors,
storage electronics, battery, solar panel and transmitting
antenna are set up. The data, collected periodically in
pre-programmed intervals, are sent by satellite almost
instantaneously to the user which may be up to 2500 km away from
the sampling station. The combination of these abilities results
in a powerful tool for the management and environmental control
of water resources. This system was developed by a partnership
between the University of Vale do Paraíba and INPE.
Since 1995 the project was transferred to Neuron Engenharia Ltda.
Through a partnership with the Agency of Hydrography and
Navigation (DHN), Neuron built a prototype of the SIMA, which
was anchored in the coastal waters of Rio de Janeiro for a year,
and the collected data were made available through the National
Buoy Program. The data collected in this period were then
compared with data collected in situ, confirming the good
performance of the system.
The selection of environmental variables to be
monitored took into regard the following aspects:
- its relevance for the characterization of aquatic environments
- its relevance as an indicator of environmental impact
(variables which respond very consistently to changes in the
functioning of the aquatic environment)
- its relevance to the processes of GHG emission
- the technical viability of measurement by automatic platforms
Following these criteria, the automatic system of
measurement is able to monitor the following variables:
- water: water temperature, pH and turbidity, dissolved oxygen
and CO2, conductivity, nitrate, ammonia, relative
depth.
- atmosphere: air temperature, atmospheric pressure, solar
radiation, direction and intensity of wind, direction and
intensity of currents.
These variables are measured through a kit of sensors linked to
the platforms. INPE is working alongside with the engineering
team of Neuron towards the specification of sensors, the
integration of the system and preliminary performance tests.
These activities are taking place in the laboratory of mechanics
and electronics of the program “Processes of the Hydrosphere”,
from the General Coordination of Earth Observation (OBT) at INPE.
All told there are 3 platforms, and it was necessary to acquire
also two step kits to be employed in the maintenance of the
system and to provide for possible changes of hardware.
Since it is a sophisticated equipment, the platform
is carried from INPE to the study site by adequate
transportation and packaging. Due to the weight of the platform,
the transportation from the reservoir margin to the site of
anchoring requires a large ship, which must be rented in
advance.
The strategy of platform distribution considers both
the safety of the experiment and the total period of sampling.
The platforms must be anchored in a safe place, preferably close
to the dam. The analysis of temporal series requires a large
number of data in the same series, as well as the absence of
missing values. Therefore, there must be one platform throughout
the year in each studied reservoir. During the execution of the
project there are visits to the platforms for maintenance and
for the installation, for one week, of a trace gas analyzer.
With this it is possible to establish, from the physical and
biological viewpoint, the relationship between the processes in
the water-atmosphere interface and the measured GHG flows (Lima,
2002).
In January of 2004 the two platforms were anchored at
UHE Serra da Mesa and at APM Manso. In January of 2005 they were
transferred to the UHEs Itumbiara ane Corumbá. In 2006 the
platforms will operate in the UHEs L.C.B. de Carvalho and
Mascarenhas de Moraes. In the beginning of 2007 one platform
will be transferred to UHE Funil and the other to APM Manso.
From 2004 to 2007 three field trips take place in each year, in
the phases of rising waters (November), high waters (March) and
drawdown (July).
2.
Estimates of Flows of CO2, CH4 and N2O
in the water-atmosphere interface and in the water column
While the combustion of coal, oil or natural gas in
thermoelectric power plants produces mainly carbon dioxide (CO2)
through chemical oxidation, in hydroelectric reservoirs the main
source of the gas is bacterial decomposition (both aerobic and
anaerobic), producing mainly CO2, CH4 and
N2O. The sampling of gas in the water-atmosphere
interface, both as bubbles and through diffusion, is set up
according to the region of the reservoir where the experiment
takes place and to the depth of the chosen area. The samples are
collected in the field in many different spots within the
reservoirs, with funnels for the entrapment of bubbles rising
from the bottom of the lake and through diffusion chambers which
measure the vertical transport of diffuse gases. Profiles of
methane concentrations in the water column are also measured.
Each field trip demands approximately 7 days of work, the first
and the last days being devoted to the transportation of the
team to the reservoir and to setting up and dismounting the
equipment. The team visits the reservoir twice in each day, in
the morning and in the afternoon. Funnel sets are placed in
areas with different depths in each studied reservoir, and the
set remains for about 24 hours in each spot, in equilibrium with
the water. After the funnels are set up the physico-chemical
variables are measured and the experiments of gas diffusion as
well as the profiling of CH4 take place. Basically,
the sampling sets are placed in significant areas in the studied
reservoirs, such as:
-
determine an emission pattern for very deep areas, where
there was, presumably, biomass removal prior to the
flooding);
-
presumably there was no biomass removal (the goal is to
establish an emission pattern for shallow areas with intense
biological activity);
-
opposed to the previous sampling (the goal is to determined
similarities or differences in the pattern observed
earlier);
-
(the goal is to determine an emission pattern in areas where
the presence of a higher organic load influences the
emission rates, as well as to evaluate the emission rate
found before the flooding of the reservoir.
-
pattern in water coming from the turbines, originally from
the deep areas of the reservoir).
As for the analysis, the best way to quantitatively estimate the
gas concentrations found in the samples is the transportation of
samples in adequate containers for later analysis in laboratory,
through gas chromatography.
Elements of
sampling:
The
gaseous components (methane, carbon dioxide, nitrogen, oxygen
and nitrous oxide) are quantified and the emission rates for
each gas are calculated in each site, expressed in kg/km2/day.
For each studied reservoir the emission rates are combined with
the percentage of each area to achieve the total emission rates
of the reservoir.
We
also measure, in situ, the gas flows, through a portable trace
gas analyzer.
3. Carbon Cycle in the water column
Within aquatic environments, most of the carbon is present under
the forms of dissolved inorganic and organic carbon. Particulate
organic carbon represents around 10% of the total organic carbon
and living organisms (biomass) are a small part of the
particulate fraction. Particulate organic carbon in the form of
detritus (necromass) is, in most cases, quickly mineralized, and
so it contributes to the general pool of dissolved carbon. Just
as the dominance of dissolved carbon in comparison with other
forms is defined by biological activities (intense decomposition
rates stimulated by high temperatures), the input (absorption)
and output (emission) flows are also regulated by biogenic
processes. The balance between the two flows is determined by
the magnitude of the rates of respiration and primary/secondary
production. The primary production converts inorganic carbon (CO2)
in particulate organic carbon (biomass). Nevertheless, this
process of primary production, through excretion processes that
take place simultaneously with photosynthesis, transfers
dissolved organic carbon from the cells to the environment. The
secondary production is the work of heterotrophic bacteria which
metabolize the dissolved organic carbon present in the
environment and incorporate it, therefore, in the food chain.
This ecological system of carbon transformation through the
generation of biomass has, obviously, a high energetic cost. The
respiration rates, expressed as the production of CO2,
represent this cost. The analysis of ecological efficiency,
expressed as the proportion between production and respiration,
allows us to identify the main routes of carbon and the
ecological status of the system. If the system acts
predominantly as an incorporator of carbon (production is
greater than respiration), it is considered as autotrophic. If,
on the other hand, the system exports carbon (respiration
greater than production), it is considered as heterotrophic.
This identification of aquatic systems as autotrophic or
heterotrophic is still a challenge for the aquatic sciences.
Within this context, the evaluation of the biological processes,
as well as the study of the energetic matrix involved – physical
and chemical parameters) – is of paramount importance for the
understanding of the carbon cycle and for the formulation of a
model of this cycle, as well as of its implications in observed
emissions.
Considering the above, the following data are being obtained:
- Biological stocks of Carbon
- Phytoplanktonic biomass is obtained through analysis in
inverted microscope, and the density is transformed to biomass
through the measurement of the cell volume.
- Bacterial biomass is obtained through analysis in
fluorescence microscope, and the density is transformed to
biomass through an image analyzer.
- Carbon Transfer Processes
- Phytoplanktonic primary production is estimated through
the incorporation rates of radioactive carbon (14C),
according to Wetzel & Likens (1992).
- Organic carbon excreted by the phytoplankton is obtained
through bubbling of the previously filtered sample in acid
solution, for the evaluation of the intracellular incorporation
of14C.
- Bacterial production is estimated through the
incorporation of radioactive leucine [3H] with the
centrifugation method proposed by Smith and Azam (1992).
Incorporation rates are calculated according to the modified
method of Bell (1993), originally established for the
measurement of timidine incorporation, and converted directly as
carbon production according to Simon and Azam (1989).
- Planktonic respiration – phytoplanktonic and bacterial –
are measured simultaneously through a Micro-Oxymax respirometer,
the oxygen uptake being measured through spectrophotometry as
described by Roland et al. (1999). Plankton separation for the
measurements of respiratory rates is achieved according to
Cimbleris and Kalff (1998).
- Environmental parameters
- Such as concentrations of dissolved inorganic carbon (DIC),
dissolved organic carbon (DOC), particulate organic carbon (POC),
total nitrogen, total phosphorus, concentration of
chlorophyll-a; 13C of dissolved CO2 and CH4 and 13C and
15N of the POC, aiming at distinguishing allochthonous and
autochthonous material, are determined according to different
methods. Limnological variables – pH, dissolved oxygen, electric
conductivity, turbidity and temperature – are measured in situ.
- Quantification of input of allochthonous material
- The main tributaries of each reservoir are identified, and
sampling sites are determined for the measurements of the
concentrations of DOC and POC (mg/L). The organic load is then
estimated through the measurement of the drainage areas
referring to each sampling site, and through the use of average
daily flow, registered in fluviometric stations present in the
basins of the reservoir. The work is developed with geographic
charts at the 1:50.000 scale, where the sampling sites are
chosen, aiming at outlining and measuring the respective
drainage areas. The average daily flows, obtained in the same
day when the sampling took place, are taken from fluviometric
stations, from which the drainage areas are known. After that,
the specific load (liters per second per square kilometer) is
estimated and multiplied by the drainage area of the sampling
site. The concentrations of DOC and POC are also evaluated
immediately downstream of each reservoir.
4. Estimate of Flows of CO2, CH4 and
nitrogen (N2) in the water-sediment interface
A large portion of greenhouse gases results from the
decomposition of organic matter present in anoxic sediments,
which are therefore one of the main components in the
transformations of carbon and nitrogen in aquatic systems.
Sediments are collected (Adams, 1994) in places close to the
sampling sites of water column and surface, and the
concentration of the following gases are measured in the
interstitial water: CO2, CH4, N2,
oxygen (O2) and argon (Ar). The samples, kept within
the sampling apparatus, are preserved in bags filled with helium
gas (Fendinger & Adams, 1986), and, in intervals between 1 and 2
cm, transferred to a system of syringes adapted to it. These
syringes are then stored in bags with helium and kept in ice
until the analysis of the gases is performed, through gaseous
chromatography, between 24 and 48 hours after the sampling. The
measurements of oxygen and argon are used as indicators of
atmospheric contamination of the samples.
The argon is also used for the enhancement of the performance of
N2 measurement.
The diffusive flows of CO2, CH4, and N2
are quantified in the sediments and in the interface
water-sediment, so as to evaluate the loss of these gases to the
water column and its potential of deoxygenation and
denitrification of hypolimnetic waters, as well as the flow to
the atmosphere. The isotopic composition of carbon and nitrogen
in the sediments are also analyzed.
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Overview