Relative importance of nitrogen and phosphorus emissions from shrimpfarming and other anthropogenic sources for six estuaries along the NEBrazilian coast

摘要

Shrimp aquaculture is a recent development of the Brazilian agribusiness but has increased by about 20% per year during the last decade along the semi-arid NE coast due to the optimal climate and environmental setting. The activity has been blamed to cause several environmental impacts mostly associated with the emission of large amounts of N and P to estuaries. Here we estimate, using an emission factor approach, the annual emissions of N and P from intensive shrimp farming and other anthropogenic sources and natural processes to six estuaries along the coast of Rio Grande do Norte State, NE Brazil, where about 40% of the shrimp farming area of the country is located. Emission factors for natural sources (atmospheric deposition and soil runoff) are 1 to 2 orders of magnitude lower than those from anthropogenic sources. Shrimp farming presents the largest average emission factors for N (1.9 t km super(-) super(2) yr super(-) super(1)), followed by agriculture (1.3 t km super(-) super(2) yr super(-) super(1)) and husbandry (0.7 t km super(-) super(2) yr super(-) super(1)). For P, largest average emission factors are from husbandry (0.9 t km super(-) super(2) yr super(-) super(1)) and agriculture (0.34 t km super(-) super(2) yr super(-) super(1)), although shrimp farming also presents a significant emission factor per unit of area for P (0.23 t km super(-) super(2) yr super(-) super(1)). Wastewaters and solid waste disposal and urban runoff present much lower emission factors per unit of area, due to the low level of urbanization and small population of the basins. Anthropogenic emissions of N and P are 20 to 50 times higher than natural emissions. Agriculture contributes with the larger fraction (40% to 63%) of the total annual N load to three of the rivers, whereas P emissions are dominated by husbandry (64% to 74%). Wastewaters contribute with significant loads of N and P to the more populated basin only (35% and 11%, for N and P, respectively). Urban runoff is practically negligible in all basins, (less than 5% of the total emission), with the exception of Guamare basin where 22% of the N emission come from this source. Aquaculture is the most important source of N to the Acu basin (58%), where the largest pond surface is observed. In the other basins N contribution from aquaculture ranges from 2% to 22%. Aquaculture contribution to the total P emission is small in all basins, varying from 2% to 14%. Notwithstanding the small area covered by shrimp farms relative to agriculture or husbandry, the location of farms adjacent to estuarine areas makes possible direct inputs to estuarine waters, while most other emissions go firstly to soils before eventually being transported to surface waters. In general, hydrochemical proxies of nutrient loads were consistent with the estimated loads.