Recent Carbon Storage and Burial Exceed Historic Rates in the San Juan Bay Estuary Peri-Urban Mangrove Forests (Puerto Rico, United States).

Cathleen Wigand,Autumn J Oczkowski,Rose M Martin,Kenneth M Miller,Meagan Eagle,Stephen Balogh,Alana Hanson,Elizabeth B Watson,Joseph Loffredo,Benjamin L Branoff,Evelyn Huertas

doi:10.3389/ffgc.2021.676691

Abstract

Mangroves sequester significant quantities of organic carbon (C) because of high rates of burial in the soil and storage in biomass. We estimated mangrove forest C storage and accumulation rates in aboveground and belowground components among five sites along an urbanization gradient in the San Juan Bay Estuary, Puerto Rico. Sites included the highly urbanized and clogged Caño Martin Peña in the western half of the estuary, a series of lagoons in the center of the estuary, and a tropical forest reserve (Piñones) in the easternmost part. Radiometrically dated cores were used to determine sediment accretion and soil C storage and burial rates. Measurements of tree dendrometers coupled with allometric equations were used to estimate aboveground biomass. Estuary-wide mangrove forest C storage and accumulation rates were estimated using interpolation methods and coastal vegetation cover data. In recent decades (1970–2016), the highly urbanized Martin Peña East (MPE) site with low flushing had the highest C storage and burial rates among sites. The MPE soil carbon burial rate was over twice as great as global estimates. Mangrove forest C burial rates in recent decades were significantly greater than historic decades (1930–1970) at Cañno Martin Peña and Piñones. Although MPE and Piñones had similarly low flushing, the landscape settings (clogged canal vs forest reserve) and urbanization (high vs low) were different. Apparently, not only urbanization, but site-specific flushing patterns, landscape setting, and soil fertility affected soil C storage and burial rates. There was no difference in C burial rates between historic and recent decades at the San José and La Torrecilla lagoons. Mangrove forests had soil C burial rates ranging from 88 g m−2 y−1 at the San José lagoon to 469 g m−2 y−1 at the MPE in recent decades. Watershed anthropogenic CO2 emissions (1.56 million Mg C y−1) far exceeded the annual mangrove forest C storage rates (aboveground biomass plus soils: 17,713 Mg C y−1). A combination of maintaining healthy mangrove forests and reducing anthropogenic emissions might be necessary to mitigate greenhouse gas emissions in urban, tropical areas.

Highlights

Mangrove forests globally sequester significant volumes of organic carbon (i.e., “blue carbon”) because of the long-term burial associated with their high primary production and anoxic sediments, which slow or prevent carbon (C) remineralization (Donato et al, 2011; McLeod et al, 2011; Kauffman et al, 2020)
The site mean Sediment accretion rates (SAR) ranged from 1.96 mm y−1 at San José Lagoon (SJ) to 3.76 mm y−1 at Piñones Forest (Pin) in historic decades, and there was no significant difference among sites
Had similar SAR site means in recent decades, and these were significantly greater than the SARs at La Torrecilla (2.68 mm y−1) and San José (2.02 mm y−1) lagoons (Table 2 and Supplementary Table 3)

Summary

Introduction

Mangrove forests globally sequester significant volumes of organic carbon (i.e., “blue carbon”) because of the long-term burial associated with their high primary production and anoxic sediments, which slow or prevent carbon (C) remineralization (Donato et al, 2011; McLeod et al, 2011; Kauffman et al, 2020). Long-term trends in deforestation for development (e.g., aquaculture; timber) and the emerging threat of climate change, which is causing accelerated sea-level rise and increased storm intensities, are all expected to impact mangrove C sequestration (Valiela et al, 2001; Donato et al, 2011; Saintilan et al, 2020). Urbanization is a globally pervasive stressor to tropical and subtropical mangrove forests, as it is associated with extractive pressures, increasing levels of impervious surfaces, commercial and industrial development, and geomorphic changes (Bosire et al, 2014; Lugo et al, 2014; Ochoa-Gómez et al, 2019). In some peri-urban (i.e., adjacent to a city) mangrove systems (e.g., Mombasa and Kenya) extractive pressures (i.e., illegal wood extraction) are still a dominant cause for degradation (Bosire et al, 2014), while in other mangrove wetlands located next to urban areas (e.g., Puerto Rico), legal protections have reduced extractive pressures and allowed for mangrove expansion (Martinuzzi et al, 2009)

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