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Mangroves of the semiarid Caatinga region of northeastern Brazil are being rapidly converted to shrimp pond aquaculture. To determine ecosystem carbon stocks and potential greenhouse gas emissions from this widespread land use, we measured carbon stocks of eight mangrove forests and three shrimp ponds in the Acaraú and Jaguaribe watersheds in Ceará state, Brazil. The shrimp ponds were paired with adjacent intact mangroves to ascertain carbon losses and potential emissions from land conversion. The mean total ecosystem carbon stock of mangroves in this semiarid tropical landscape was 413 ± 94 Mg C/ha. There were highly significant differences in the ecosystem carbon stocks between the two sampled estuaries suggesting caution when extrapolating carbon stock across different estuaries even in the same landscape. Conversion of mangroves to shrimp ponds resulted in losses of 58%–82% of the ecosystem carbon stocks. The mean potential emissions arising from mangrove conversion to shrimp ponds was 1,390 Mg CO2e/ha. Carbon losses were largely from soils which accounted for 81% of the total emission. Losses from soils >100 cm in depth accounted for 33% of the total ecosystem carbon loss. Soil carbon losses from shrimp pond conversion are equivalent to about 182 years of soil carbon accumulation. Losses from mangrove conversion are about 10‐fold greater than emissions from conversion of upland tropical dry forest in the Brazilian Caatinga underscoring the potential value for their inclusion in climate change mitigation activities.

In addition to the largest existing expanse of tropical forests, the Brazilian Amazon has among the largest area of mangroves in the world. While recognized as important global carbon sinks that, when disturbed, are significant sources of greenhouse gases, no studies have quantified the carbon stocks of these vast mangrove forests. In this paper, we quantified total ecosystem carbon stocks of mangroves and salt marshes east of the mouth of the Amazon River, Brazil. Mean ecosystem carbon stocks of the salt marshes were 257 Mg C ha while those of mangroves ranged from 361 to 746 Mg C ha Although aboveground mass was high relative to many other mangrove forests (145 Mg C ha), soil carbon stocks were relatively low (340 Mg C ha). Low soil carbon stocks may be related to coarse textured soils coupled with a high tidal range. Nevertheless, the carbon stocks of the Amazon mangroves were over twice those of upland evergreen forests and almost 10-fold those of tropical dry forests.

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The potential of Blue Carbon as an integral part of global climate change adaptation and mitigation strategies requires quantification of drivers and rates of organic carbon (OC) burial and storage. However, there is limited recognition of how land use impacts benthic assemblages, their bioturbation potential, and OC burial in mangrove forests and tidal flats. Here we evaluated the effects of mangrove deforestation on benthic bioturbation potential (BPc), sediment accretion rates (SARs), and OC and total nitrogen (TN) burial in mangrove soils and tidal flat sediments from three estuaries in tropical Brazil. SARs based on 210 Pb dating varied significantly among undisturbed estuaries (2.1-18.6 mm yr −1) and the OC and TN burial rates were respectively 46% and 16% higher than global averages. The cleared mangrove sites had a twofold lower SAR, 40-fold lower OC burial, and over 100-fold lower TN burial when compared to undisturbed forests, revealing the pervasive impacts of land use. Variation in benthic assemblage structure and BPc among sites suggests that the composition and activity of macrofaunal communities may facilitate OC burial in some cases, yet the global extent and significance of bioturbation requires further study. Our work reveals a strong spatial variability in C burial in undisturbed mangrove forests and a decreased capacity of mangroves to accumulate C and sediments due to land use effects.