Direct and indirect effects of fertilizers on experimental aquatic communities

Author(s)

Bianca Gonçalves dos Santos

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Biociências (IBIOC/SB)
• Defense date: 2013-07-12

Examining board members

Luís César Schiesari; Evaldo Luiz Gaeta Espindola; Paulo Roberto Guimaraes Junior

• Advisor: Luís César Schiesari

Abstract

The human demand for food, fibers and biofuels imposes a pronounced growth in intensive agriculture and, with it, in the consumption of fertilizers. Fertilizer management promotes dramatic changes in the quantities and proportions of nutrients in agroecosystems which, in turn, influence biological communities through changes in the biomass, nutritional quality, and composition of producers. A recent trend in the Brazilian productive scenario is the expansion of biofuel crops - notably sugarcane (ethanol) and soybean (biodiesel) - over the cerrados. These crops, covering no less than 30 million hectares in the country, demand contrasting fertilization patterns: sugarcane requires high proportions of nitrogen relative to phosphorus, whereas soybean requires high proportions of phosphorus relative to nitrogen. The objective of this thesis was to test the direct and indirect effects that changes in the quantities and proportions of N and P that follow conversion of native habitats to sugarcane and soybean fields have on biological communities. These objectives were achieved by means of two mesocosm experiments manipulating nutrients, algae, tadpoles and dragonfly naiads. The first experiment followed a full factorial design crossing three concentrations of N (Nreference, Nsoy, Ncane) and three concentrations of P (Preference, Psoy, Pcane), as well as two trophic levels (algae and Physalaemus cuvieri larvae). Nutrient levels simulated conditions measured in water bodies of the cerrado, or patterns of fertilization known for sugarcane or soybean crops. In the second experiment we paired these concentrations (Nreference Preference, Nsoy Psoy, Ncane Pcane) so as to reduce dimensionality and as such to permit inclusion of a third trophic level (algae, Hypsiboas faber larvae, dragonfly naiads). We included an additional treament with caged naiads to separate the effects of actual tadpole predation from those of changes in tadpole traits. The main response variables in both experiments were cholophyll a concentrations as na indicator of phytoplankton biomass, and tadpole mortality, final mass and final developmental stage as indicators of consumer performance. A comparison in the concentration of chlorophyll a before and after the introduction of tadpoles tested the effect of consumers on producer biomass. As expected, measured TN and TP responded strongly to N and P manipulations. In the weeks preceding the introduction of tadpoles, algal biomass increased in response to the concentration of N, but not to the concentration of P; this trend was maintained throughout the experiment. This probably happened because measured TN varied from oligotrophic to hypertrophic conditions; in contrast, measured TP consistenly corresponded to hypertrophic conditions. Tadpole introduction had no evident effects on chlorophyll concentrations, possibly indicating that food was abundant and competition among tadpoles, if present, was weak. Consumer performance was significantly influenced by nutrient manipulations. P-rich treatments led to a reduction in growth, and to a reduction trend in development and survivorship, of Physalaemus cuvieri. Similarly, the scenario of soybean fertilization - therefore P-rich - led to a significant reduction in Hypsiboas faber survivorship. The scenario of sugarcane fertilization led to a 30% increase in mass gain in Hypsiboas faber. We hypothesized that this reduction in tadpole growth and survivorship in P-rich environments was caused by a proliferation of unpalatable or even toxic cyanobacteria, which are favored in low N:P ratio environments. Contrary to our expectations, dragonfly naiads did not cause a direct or indirect reduction in tadpole growth or survivorship, nor influence phytoplankton biomass. We conclude that changes in nutrient levels that accompany the conversion of native habitats to agricultural fields can have strong effects on freshwater food chains (\'bottom-up\' control), and that these effects are much stronger than those mediated by predator presence (\'top-down\' control). However, bottom-up effects are not necessarily intuitive as certain fertilization patterns negatively influence the performance of intermediate consumers and, as such, the transmission of biomass to higher trophic levels. (AU)

• FAPESP's process: 10/14767-3 - Direct and indirect effects of fertilizers on experimental aquatic communities
• Grantee: Bianca Gonçalves dos Santos
• Support Opportunities: Scholarships in Brazil - Master