Deforestation alters the hydraulic and biogeochemical characteristics of small lowland Amazonian streams

Abstract

Abstract:We investigated how clearing of the tropical rainforest for cattle pasture along small, terra firme lowland streams in the Brazilian Amazon influenced stream hydraulic characteristics, solute concentrations and uptake of dissolved inorganic nitrogen and phosphorus. Measurements of stream channel structure, hydraulic properties and nutrient uptake were obtained from short-term conservative and non-conservative solute injections and an in-stream transport model in two watersheds with pairs of similar-sized forest and pasture streams in Rondônia. The pasture stream channels were deeper and had a lower cover of sandy bottom habitat and a higher cover of aquatic grass habitat than the forest streams. The pasture streams had larger transient storage zones, higher ratios of transient to channel storage and a shorter hydraulic uptake length than the forest streams. The pasture streams had lower concentrations of dissolved oxygen and NO 3 and higher concentrations of dissolved Fe 2C . Forest streams exhibited no NO 3 uptake, but one pasture stream took up NO 3 while the other did not. Uptake of NH 4 C was low and variable among streams. Uptake velocities and rates of PO 4 3 were greater in pasture streams compared to those in forest streams. In all streams, uptake lengths for NO 3 , NH 4C and PO 4 3 were long and indicated generally lower rates of uptake than in most comparably sized temperate forest streams. Uptake lengths or velocities were not correlated with stream transient storage, suggesting that other factors, such as hypoxia in pasture streams, controlled nutrient uptake. The structural differences are typical for the region, suggesting that deforestation may be altering stream hydrology and biogeochemistry over many thousands of kilometres of primary and secondary stream channels in the Amazon Basin. A better understanding of the extent and distribution of altered hydraulic and biogeochemical function in small streams is required to assess the importance of these changes for larger river networks in the Amazon basin.

Influence of land use changes on water chemistry in streams in the State of São Paulo, southeast Brazil

“…The effects of change in land use on the biogeochemistry have also been described in the literature for both major rivers and streams. It has been shown that these changes affect nitrate, ammonium, calcium, potassium and aluminum concentrations in streams located in temperate (Wang et al 2006, Kamisako et al 2008, Rai et al 2010) and tropical regions (Neill et al 2006, Andrade et al 2011.…”

Relationships between water transparency and abundance of Cynodontidae species in the Bananal floodplain, Mato Grosso, Brazil

“…In the Cerrado of Central Brazil and in southern Amazônia the process of deforestation for the establishment of agriculture and pastures has caused serious alterations in aquatic habitats (Williams et al, 1997;Neill et al, 2001Neill et al, , 2006. The removal of native vegetation can alter significantly the biogeochemical processes of soils and, as a consequence, increase the amount of particulate and dissolved material in streams (Williams et al, 1997).…”

Métodos e técnicas de pesquisa em bacias hidrográficas

“…No período de chuvas, os nutrientes presentes no solo são carregados para os corpos d'água e a serapilheira da fl oresta é uma fonte importante de DOC (SCHWENDENMANN; VEL-DKAMP, 2005). A presença de gramíneas nos riachos também leva a um aumento na carga orgânica (NEILL et al, 2006). Além disso, a entrada de matéria orgânica de dejetos animais está relacionada ao escoamento superfi cial (BIGGS; DUNNE; MURAOKA, 2006).…”