Biology subjects

Gallagher, J. B.

Publications and source records attributed to Gallagher, J. B..

5 recordsLinked to original sources

bioRxiv2021

Missing the forest for the trees: A reappraisal of global seaweed carbon sequestration

Global seaweed carbon sequestration estimates are currently taken as the fraction of the net primary production (NPP) exported to the deep ocean. However, this perspective does not account for CO2 from the consumption of external subsidies. Here we clarify: i) the role of export relative to seaweed net ecosystem production (NEP) for a closed system and one more likely open to subsidies; ii) the importance of subsidies by compiling published estimates of NEP from seaweed-dominated ecosystems; and iii) discuss their impact on the global seaweed net carbon balance and other sequestration constraints as a mitigation service. Examples of seaweed NEP (n = 18) were sparse and variable. Nevertheless, the average NEP (-9.2mmol C m-2 day-1 SE {+/-} 11.6) suggested that seaweed ecosystems are a C source, becoming increasingly heterotrophic as their export is consumed. Critically, mitigation of greenhouse gas emissions was mixed relative to their replacement or baseline states, and where CO2 is supplied independently of organic metabolism and atmospheric exchange we caution a sole reliance on NEP or NPP. This will ensure a more accurate seaweed mitigation assessment, one that does exceed their capacity and is effective within a compliance and carbon trading scheme.

ecology↗

bioRxiv2020

Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes

Total organic carbon (TOC) sediment stocks as a CO2 mitigation service requires exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water- atmospheric equilibrium (PICeq). For the first time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots. Seagrass TOC stocks were similar to the salt marshes with soil depths < 1 m (59.3 {+/-} 11.3 and 74.9 {+/-} 18.9 MgC ha-1, CI 95% respectively) and sequestration rates of 1.134 MgC ha-1 yr-1. Both ecosystems showed larger BC constraints than their pristine counterparts. However, the seagrass meadows mitigation services were largely constrained by both higher BC/TOC and PICeq/TOC fractions (38.0% {+/-} 6.6% and 43.4% {+/-} 5.9%, CI 95%) and salt marshes around a third (22% {+/-} 10.2% and 6.0% {+/-} 3.1% CI 95%). The results demonstrate a need to account for both BC and PIC within blue carbon mitigation assessments.

plant biology↗

bioRxiv2019

Anthropogenic marine debris and its dynamics across peri-urban and urban mangroves on Penang Island, Malaysia

Mangroves act as sinks to a variety of anthropogenic marine debris (AMD) forms. However, knowledge of their distribution and accumulation dynamics is limited. To address this shortfall, abundance, sorting, and diversity parameters of AMD were evaluated across the canopy of Penangs urban and peri-urban mangroves. Two urban and two peri-urban mangroves were sampled at different periods over 2 months, with differences constrained by possible changes in their wind fields, and neap-spring tidal development. Debris were counted and classified across transects parallel to the coastline at progressively higher water marks. Plastics made up most of the AMD across all sites. More AMD was retained in the urban sites, consistent with their larger resident population density. Diversity of debris forms were consistent with the type of land use and population livelihood in each area. The greatest differences in abundance, diversity, and evenness were recorded between the lower tidal zones and the remaining inner transects consistent with sorting towards the coastal edge in favour of plastic items. Overall, differences across transects and sites suggested: 1) the canopy and root structure within the main body of the mangrove efficiently retained debris with little sorting; and 2) debris deposited closer to the edge is increasingly sorted and lost to the water body in favour of smaller plastic items, for a constant wind field and irrespective of neap-spring phases. The findings show that mangrove areas are vulnerable to a constant build of potentially harmful debris with selective leakage and sorting of materials back to the water body closer to their coastal edges. For Penang Island, the study highlights the areas in need of attention and prioritization, lists the types of debris needing proper management, and will aid in the future monitoring, mitigation and/or rehabilitation of these sensitive ecosystems.

ecology↗

bioRxiv2019

Blue carbon sequestration dynamics within a tropical seagrass sediment: Long term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales relies on several assumptions, such as no loss of particulate organic carbon (POC) after one or two years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates and removal of the stable black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49% and 51% after 100 years of burial and 20-22 cm horizons, respectively. However, the loss rate of POC was far greater than the release rate of CO2, both before and after accounting for CO2 from anoxic particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on {delta}13CO2, acidity and lack of sulphate reduction suggested methanogenesis. The results indicate the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

plant biology↗

bioRxiv2018

Carbon sink services for tropical coastal seagrass are far lower than anticipated when accounting for black carbon

Valuing the sedimentary blue carbon stocks of seagrass meadows in mitigating greenhouse gas emissions requires the exclusion of allochthonous recalcitrant forms, such as black carbon (BC) from the stock assessment. Regression models constructed across a tropical estuary predicted that carbon sinks within the more abundant sandy meadows of coastal bays likely support a significant but modest BC fraction. We tested the prediction by measuring BC fractions of total organic carbon (TOC) across three coastal meadows of the same region. One patchy meadow was located close to a major urban centre while the remaining two continuous meadows where contained in separate open embayments of a rural marine park, differing in fetch and species. In all cases, the BC/TOC fractions were significantly greater than predicted constituting a major component of the organic carbon content, 28% {+/-} 1.6, and 26% {+/-} 4.9 to 36% {+/-} 1.5 ({+/-}95% confidence intervals) for urban and marine park meadows respectively. The higher BC/TOC fractions were explained by site-specific variability in BC atmospheric supply, patchy coverage, and a presumed increase in the loss of seagrass litter, as determined by the canopy height and proximity to the meadows exposed edge.

plant biology↗