WETLANDS, Vol. 17, No. I, March 1997, pp. 1 4 © 1997, The Society of Wetland Scientists
C : N : P RATIOS A N D STABLE C A R B O N I S O T O P E C O M P O S I T I O N S AS I N D I C A T O R S OF O R G A N I C M A T T E R S O U R C E S IN A R I V E R I N E W E T L A N D SYSTEM ( M O J I - G U A ~ U RIVER, SAO P A U L O - B R A Z I L ) Ana Luiza S. Albuquerque ~ and Antonio A. Mozeto
LaboratOrio de Biogeoqufmica Ambiental Departamento de Qufmica Universidade Federal de S6o Carlos (UFSCar) Caixa Postal 676, I3.565-905-S~o Carlos (SP), Brazil Email:
[email protected] Present Address: Laboratdrio de Paleoecologia--Departamento de Ecologia e Biologia Evolutiva Universidade Federal de S6o Carlos (UFSCar) Caixa Postal 676 13.565-905-S~o Carlos (SP), Brazil Email:
[email protected] Abstract:
Particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP) concentrations, and stable carbon isotope compositions (8~3C) are reported for Moji-Guaqu River (suspended matter) and Infern~o Lake (suspended matter and bottom sediment) samples collected during the entire 1990/91 hydrologic year. POC and PN concentrations for both systems and riverine PP concentrations peaked during the wet season, but only riverine PP concentrations peaked together with the river discharge, thus characterizing the regional watershed (main river) as the principal source of this phosphorus. Lacustrine POC, PN, and PP displayed maximum concentrations before discharge peak, suggesting that local sources (lake basin, wetlands, and internal production) are more important in the lake case. A less pronounced increase in riverine PN than in POC or PP increments also was documented, possibly due to denitrification occurring in the river-wetland system. The principal source of organic matter in the Moji-Gua~u River is watershed inputs. The origin of suspended matter in the oxbow lake is mainly pbytoplankton, while bottom sediments are composed by a more refractory fraction of organic maner derived from local runoff, lake macrophytes, and t-iverine POM, with some contribution of lake POM subjected to alteration by early diagenetic processes.
Key Words: Organic matter sources, C : N : P ratios, stable carbon isotope, tropical river wetlands, oxbow lakes
INTRODUCTION The composition of paniculate and sedimentary organic matter in river-wetland systems is determined by the intermixing of terrestrial, lacustrine, and riverine sources, subjected to additional physical, chemical, and biological modifications in the channel (Hedges et al. 1986). Moreover, the intensity and direction of water exchange between river and floodplain seem to have a direct relation with the balance of paniculate organic material in floodplain wetlands. The role of wetlands in retaining and transforming organic matter has been demonstrated in various studies (Newbold et al. 1981, E l w o o d et al. 1983, Lowrance et at. 1984, Pinay et al. 1990, Puckett et al. 1993). Even so, complete organic matter budgets for wetlands have proven difficult to evaluate. One o f the most difficult problems has been to identify the major sources of organic car-
bon in these systems, mainly because of the many molecular forms and sources, as well as great temporal variability in production and release, typical of biologic materials (Hedges et al. 1986). Floodplain lakes and associated wetlands are generally sinks for carbon, nitrogen, phosphorus, and other elements as they accumulate organic matter (Drcamps and D r c a m p s 1989, Lugo et aL 1990). In spite o f this, the exchange o f these elements among landscape boundary components contributes to the establishment of a highly dynamic environment, which is the major difficulty in identifying sources and diagenetie processes in these systems. Few detailed studies of organic matter composition and dynamics have been conducted in river wetlands, especially in tropical systems (Lesack and Melack 1995), which in South American wetlands and associated rivers, cover about 753,000 km 2 (Neiff 1990). Some biogeochemical stud-
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W E T L A N D S , V o l u m e 17, No. 1, 1997
ies have been carried out in o x b o w lakes o f the central section o f the Moji-Guaqu River (Mozeto et al. 1991, Santos et al. 1995), but identification of different particulate organic sources o f the sediments of these systems remains still unsatisfactory. Thus, we report here data on the composition of particulate organic matter collected from the MojiGuaqu River and one of the associated floodplain lakes (Inferngo Lake) during a complete hydrologic cycle for the purpose of investigating the potential contribution of these organic carbon sources to lacustrine and riverine particulate balance. In order to do that, we used environmental tracers such as carbon stable isotope (g~3C) and elemental ratios ( C : N : P ) , which have proven to be valuable tools in identifying the sources and diagenetic state of organic matter in a wide range of environments, such as those found in marine (Deuser 1970, Faganelli et al. 1988) and freshwater systems (Fry and Sherr 1984, Mozeto et al. 1988, Matson and Brinson 1990).
IU Scrub [ ] Area subject fo flooding r~'ILakes Atlantic
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MATERIALS AND METHODS Study Area and Sampling Design The studied area was within the Jatai Ecological Station (21 ° 37' S, 47 ° 45' W), Luiz Antonio, SR southeastern Brazil at the central part of the Moji-Gua9u River basin (Figure 1A). This region is located at the Upper Paran~ River basin. The elevation of the Ecological Station is between 670 and 510 m above sea level, covering an area of 4,532 ha; o f this, 95.2 % is covered by terrestrial ecosystems (upland area). The native vegetation covers 91.2 % of the upland system and is represented by " C e r r a d g o " (a kind of closed canopy dry forest) and Atlantic Forest. The terrestrial area remaining is covered mainly by Eucalyptus and Pinus plantation. The lowland area represents 4.8 % of the Ecological Station and is formed of perennial water bodies (1.2 %), such as the Moji-Guaqu River, streams, and o x b o w lakes, and temporary aquatic systems (2.5 %) in the wetlands (Ballester et al. 1995). Moji-Guaqu is a third order river, and its wetland within the limits of the Ecological Station contains 15 o x b o w lakes. During the wet season ( D e c e m b e r to April) local and regional precipitation increases cause a corresponding river-discharge increase, resulting in discrete pulses of river-water overflow reaching the oxbows. Hydrologically speaking, inundation intensities reach different levels mainly depending on the distance between river and lake. Thus, during overflow periods, material exchange between the river bed and adjacent wetlands is an intense process. Land-use in the Moji-Gua~u River watershed is characterized by
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b ' 2bo ' 46o' 6d0 ' e6o// ' 16bo ' Distancefrom river(m] Figure 1. (A) Jatai Ecological Station, Sao Paulo State, Brazil (scale: 1:1,250,000). (B) Schematic geomorphological profile of the Moji-Gua~u River wetland and Infernao Lake. intensive agricultural activities, principally sugar cane and other plantations. Infern~o is a small (3.05 ha) and shallow lake (maxi m u m depth of 4 m ) - - a typical subtropical oxbow lake situated about 500 m from the bank of the Moji-Guaqu River at 520 m above sea level (Figure 1B). Its watershed is 5 km 2 with a great declivity ranging from 520 to 645 m. Vegetation cover is c o m p o s e d of dense Atlantic Forest with gallery-forest species dominating. The lakeshore is colonized by wide banks of emergent aquatic macrophytes, dominated by Cyperaceae (mostly Scirpus cubensis Poepp & Kunth), while the limnetic zone is partially covered by floating and submerged aquatic macrophytes, c o m p o s e d of Eichhornia azurea Kunth and Cabomba cf piauhyensis Gardner, respectively, as the dominant species (Nogueira 1989, Carlos et al. 1993). Scirpus cubensis biomass was determined by Carlos et al. (1993) and corresponds to 2,476 and 1,321 g of dry mass m -2 during wet and dry periods, respectively. Although not measured, the biomass of other macrophyte species is not significant in comparison to the S. cubensis, as they cover a much smaller area.
Albuquerque & Mozeto, C : N : P RATIOS A N D '3C C O M P O S I T I O N S IN M O J I - G U A C U R I V E R W E T L A N D S
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