Impact of sugar cane cultivation on biogeochemistry and phytoplankton dynamics in a tropical lagoon and estuary in Brazil

Abstract

Sugar cane cultivation has become the main land use in northeast and southeast Brazil and it is still increasing because of a growing national and international demand of sugar and biofuel. Shallow coastal lagoons and estuaries are the linkage between terrestrial and marine environments and are susceptible to anthropogenic modifications. Little is know about the impact of sugar cane cultivation in these systems. Aim of this study was to gain knowledge on the impact of effluents from sugar cane cultivation on the biogeochemistry and phytoplankton dynamics in two shallow coastal systems affected by sugar cane monoculture in their watersheds, a coastal lagoon and an estuary in Brazil. In the Mundau Manguaba Lagoon (MML), water samples were collected at the beginning and end of the dry season during five expeditions between September 2007 and February 2009. Additionally, sediment cores were taken in March 2007. In the Paraiba do Sul River (PSR), samples were taken during the rainy (January) and dry (September) season 2008. The main elements of this study were the determination and characterization of nutrients (nitrate, nitrite, ammonium, silicate, phosphate) and total suspended matter (TSM) as well as phytoplankton communities in the MML and the PSR. Particulate organic matter (Corg, PN) and stable carbon and nitrogen isotopes (13C, 15N) were determined in TSM of surface water in the MML and PSR and sediments of the MML. Inorganic nutrients were measured in the water samples. Phytoplankton abundance, biomass and community composition were identified in the MML. Effects of nutrient enrichment caused by fertilizer runoff from sugar cane fields on phytoplankton communities in the MML were experimentally simulated. Chlorophyll a (chl a) was used to estimate phytoplankton biomass in the PSR. Nutrients and organic matter derived from fertilizer runoff in the catchment area of the PSR were transported from the river and its tributaries to the estuary and subsequently to adjacent coastal waters during the rainy season because of high river discharge. Reduced river discharge and long residence time during the dry season led to an increase in chl a amount and retention of allochthonous and autochthonous material inside the system. Nutrient concentrations in the MML were mainly governed by the rivers which drain the sugar cane dominated catchment area. A seaward nutrient decrease coincided with high phytoplankton biomass inside the lagoons and was the result of fast nutrient uptake by algae. Parts of the organic matter were removed from the water column due to sedimentation and were recycled or stored in the sediments of the lagoons. Other parts of the organic matter were exported from the Manguaba and Mundau lagoons into adjacent coastal waters due to tidal currents. Phytoplankton community and diversity were highly variable in both lagoons because of rapidly changing salinity and nutrient concentrations as well as residence time. Thus, temporal and spatial phytoplankton distributions and shifts between cyanobacteria and diatom dominated communities were observed in the MML. Nutrient enrichment experiment conducted in order to simulate high nutrient input from fertilizer runoff showed a direct response of phytoplankton communities. Diatoms were the main profiteers due to permanently high silicate concentrations in tropical aquatic ecosystems. Alterations in the nutrient ratios affected by high nitrogen and phosphate inputs from sugar cane cultivation lead to a shift in phytoplankton communities and consequently influence the entire food web dynamics in the lagoons. It can be concluded that lagoons and estuaries play an important role for nutrient and organic matter cycles in the coastal ecosystems affected by sugar cane cultivation in the northeast and southeast regions of Brazil. Seasonal variations in fertilizer runoff, geomorphology of the system and biological processes are the main factors that control the material distribution, composition and finally the retention of nutrients and organic matter inside the system or the outwelling from the system into adjacent coastal waters.