Phosphorus and suspended matter retention in mangroves affected by shrimp farm effluents in NE Brazil.

Isabel Cristina S Araújo,Louize V Fonseca,Francisco A.T.F Silva,Luiz D Lacerda,Rozane V Marins

doi:10.1590/0001-3765202020200758

Isabel Cristina S Araújo, Louize V Fonseca + Show 3 more

Open Access

https://doi.org/10.1590/0001-3765202020200758

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Abstract

This study compares two mangroves with different land uses in the Jaguaribe River estuary, harboring large shrimp farms, and in the more pristine Pacotí River estuary. Normalized Difference Vegetation Index (NDVI) was used to compare the overall health of the forests. Measures of suspended matter (TSS), total (TP), particulate (PartP) and soluble reactive phosphorus (SRP) in the inflow and outflow waters of tidal channels draining the mangroves were performed during tidal cycles. NDVI varied from 0.65 in the Jaguaribe estuary to 0.85 in the Pacotí, suggesting the impact of shrimp farm effluents on mangrove canopy cover. The shrimp farm influenced site showed 10 times larger absolute ∑P (TP + PartP + SRP) = 1.2-5.2 kg.hr-1) flux than the pristine site (∑P = 0.22 kg.hr-1). Tidal balances showed smaller retentions of the total influx: 28-54%; 44-45%; 38-65% and 8-53% for TSS; TP; SRP; and PartP respectively, in the shrimp farm influenced site to over 93% of the total tidal input of TSS and all P fractions in the pristine mangrove. This suggests that mangrove phosphorus accumulation is decreased in the forest with lower NDVI and limits mangrove's potential as a natural barrier to the nutrient transport to adjacent estuarine and coastal waters.

Highlights

Population inhabiting the coastal region increased enormously in the last decades in Brazil, to a point that nearly 80% of the total country’s inhabitants live within the first 100 km from the sea
It is assumed that any difference in the P dynamics in the two sites will depend on the mangrove response to pressures from external factors, in this case shrimp farming
The results obtained in the mangrove sites at the northeastern Brazilian coast, clearly show the capacity of NDVI, even using a low resolution image such as Landsat, and at least for mangroves under semiarid climate, to respond to the stress resulting from shrimp farm effluents

Summary

Introduction

Population inhabiting the coastal region increased enormously in the last decades in Brazil, to a point that nearly 80% of the total country’s inhabitants live within the first 100 km from the sea. Rapid expanding intensive shrimp farming has increase enormously the environmental pressure on mangrove ecosystems (Lacerda et al 2019). Mangroves play a key role in the sediment balance and nutrient cycling in tropical estuaries and are key ecosystems to mitigate the impacts from global environmental changes, as well as from local anthropogenic drivers affecting estuaries (Valiela et al 2018). Due to their root morphology and high densities, they act as traps to suspended particles coming in with the tides. The effects of land uses neighboring mangrove areas on the capacity of these forests

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