Contribution of seagrass plants to CO2 capture in a tropical seagrass meadow under experimental disturbance.

Diana Deyanova,Mats Björk,Matern S P Mtolera,Martin Gullström,Martin Dahl,Mariam I Hamisi,Liberatus D Lyimo,Silvia Mazzuca

doi:10.1371/journal.pone.0181386

Diana Deyanova, Mats Björk + Show 6 more

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https://doi.org/10.1371/journal.pone.0181386

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Abstract

Coastal vegetative habitats are known to be highly productive environments with a high ability to capture and store carbon. During disturbance this important function could be compromised as plant photosynthetic capacity, biomass, and/or growth are reduced. To evaluate effects of disturbance on CO2 capture in plants we performed a five-month manipulative experiment in a tropical seagrass (Thalassia hemprichii) meadow exposed to two intensity levels of shading and simulated grazing. We assessed CO2 capture potential (as net CO2 fixation) using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios (from the literature). To better understand the plant capacity to coping with level of disturbance we also measured plant growth and resource allocation. We observed substantial reductions in seagrass areal productivity, biomass, and leaf area that together resulted in a negative daily carbon balance in the two shading treatments as well as in the high-intensity simulated grazing treatment. Additionally, based on the concentrations of soluble carbohydrates and starch in the rhizomes, we found that the main reserve sources for plant growth were reduced in all treatments except for the low-intensity simulated grazing treatment. If permanent, these combined adverse effects will reduce the plants’ resilience and capacity to recover after disturbance. This might in turn have long-lasting and devastating effects on important ecosystem functions, including the carbon sequestration capacity of the seagrass system.

Highlights

Climax-stage ecosystems can generally cope with mild or occasional stress from factors such as light limitation and grazing pressure
The Leaf Area Index (LAI) was clearly lower in both clipping treatments than in the control, while no effects were seen in the shading treatments (Fig 2F)
This study has illustrated how prolonged shading and simulated grazing resulted in an overall decline in seagrass productivity, production, and stored carbohydrates, leading to a drastically lower areal productivity that will reduce the carbon sink capacity of the seagrass meadow

Summary

Introduction

Climax-stage ecosystems can generally cope with mild or occasional stress from factors such as light limitation and grazing pressure. Disturbance and seagrass productivity resist such short periods of light reduction, with no effects on species composition or ecosystem stability [2]. Mild grazing has been demonstrated to stimulate plant growth [3] and shallow-water coastal habitats such as seagrass meadows are adapted to low levels of grazing [4]. Prolonged periods of high-intensity stress in the marine environment could drive an ecosystem to a threshold at which change or adaptation will result, but with negative effects on important ecosystem services provided by the ecosystem [6,7]. There is, a clear lack of field studies assessing multi-intensity impacts of relevant stressors such as light limitation and grazing on seagrass meadow productivity linked to carbon sink capacity

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