We propose studies on the effects of conversion from rainforest to pasture on the
biogeochemistry of small streams in the Amazon basin. Questions about how organic matter
and nutrients move from uplands to streams, how land use alters those inputs and controls,
and how terrestrially-controlled inputs are reflected in successively larger streams, are
central to our understanding of the ecological functioning and material fluxes in drainage
basin networks Our research will focus on three questions about how the dominant land use
conversion now occurring in the Amazon Basin-the conversion of moist tropical forest to
cattle pasture-changes the biogeochemistry and ecological functioning of small streams in
the Amazon Basin:
- Question 1: What is the role of uniform forest and pastureland use in the
drainage basin in determining the biogeochemical composition of stream waters?
- Question 2: What is the effect on stream organic carbon production of altered
nutrient inputs from the drainage basin?
- Question 3: Are patterns of nutrient concentrations, organic matter and control
of organic matter production associated with forest or pasture land use identifiable and
persistent in successively larger drainage basins?
We will measure stream chemistry in paired drainage basins with second order streams
and uniform land-use and in larger streams and rivers of mixed land-use. One drainage
basin in each pair will have land use exclusively of native forest, the other exclusively
of cattle pasture. Stream water measurements in paired basins will be made bi-weekly as a
minimum during two month-long field trips, one during the period of low flow and one
during the period of high flow. When possible, these periods will be supplemented with
collections during the wet-dry and dry-wet transitions. To characterize stream
biogeochemistry, we will measure pH, conductivity, NO3-, NH4+, PO43-
(as soluble reactive phosphate), dissolved organic N and P (DON, DOP), K+, Na+, Ca2+,
Mg2+, Al3+, Fe2+, Si, Sr, Cl- and SO42-,
total suspended solids, suspended chlorophyll a, and the C, N and P content of suspended
solids. We will add to these measurements the concentrations of total DOC and in different
size fractions determined by ultrafiltration. We will also measure the (13C
value of the POC and the DOC in the different fractions. To provide more detailed
information on stream discharges and element concentrations over a range of discharges, we
will install recording stage and temperature/conductivity sensors and autosamplers. To
test whether P limits algal growth in the forest streams if light limitation in removed,
we are deploying nutrient bioassays. We will measure the concentrations of inorganic
nutrients, other anions and cations, DOC and the (13C value of DOC in a series of slightly
Contribution to LBA-ECO
This research program will contribute LBA Ecology an understanding of the ways in which
changes in land affect stream biogeochemistry and influence downstream transport of
materials. The work in small basins will develop a detailed picture of the characteristic
concentrations of nutrients, other ions, DOC and (13C of DOC and POC derived from uniform
land-use forest and pasture watersheds. The work in larger streams, including streams
formed by the confluence of the small streams in our uniform land use basins, will allow
us to trace the persistence of patterns present in the uniform-use watersheds in
successively larger drainage. We will also conduct bioassay experiments of N and P
limitation in these larger streams draining mixed land use. These experiments will tell us
how changing nutrient concentrations and nutrient ratios influence stream organic matter
production and how much land use change in a mixed land-use basin must take place before
patterns of stream primary production are altered. This project will integrate an
understanding of upland- small stream linkages into a landscape and drainage-basin network
perspective that takes into account the extent of forest conversion in different-sized
basins and how the biogeochemical changes resulting from that forest conversion are
transmitted downstream to the larger rivers of the Basin.
Research Team Responsibilities
- Linda Deegan: Algal bioassays & stream nutrient chemistry
- Christopher Neill: Stream nutrient chemistry, soil chemistry and data-loggers for
- Reynaldo Victoria: Concentration, size fractionation and d13C value of DOC.
We would like to continue our work at Fazenda Nova Vida and extend this work to the LBA
paired drainage basins. We have over 3 years of work on two paired drainage basins on
Fazenda Nova Vida, a 20,000-ha cattle ranch in central Rondônia. These basins contain
permanently flowing second-order streams and are matched closely in area and discharge.
The forested basins contain moist terra firme forest typical of Rondônia. They have soils
(Kandiudults and Paleudults) that are typical of large areas of the western Amazon Basin.
First and second order forest streams in the region are typically 1 to 4 m wide, with
clear water and sandy bottoms, and a pH of 5.5 to 6.8. Forest vegetation consists of open
perennially evergreen broadleaf trees with a high number of palms. Pastures have been
planted to grasses that are widespread in Amazonia, including colonião (Panicum maximum)
and several species of the genus Brachiaria. Soils are well-drained ultisols (red-yellow
podzolic latosols in the Brazilian classification). Clay content ranges from 10 to 40%.
Forest soil pH is ~5. We will also pursue the possibility of making many of the same kinds
of measurements in another pair of small basins of uniform land use that will be outfitted
for intensive hydrological studies (Martin Hodnett, Institute of Hydrology, Wallingford,
UK, personal communication).
- We are assuming we can begin activities in mid-1998.
- Stream chemistry and bioassays in paired basins - biweekly during one-month periods in
the wet and dry seasons - June 1998 - Dec. 2000.
- Stream bioassays for primary production nutrient limitation - June 1998 - Dec. 2000.
- Stream chemistry in larger, mixed land-use streams - June 1998 - Dec. 2000.
- DOC characterization in paired drainage basins and larger streams - June 1998 - Dec.