Sediment properties and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux from intact and cleared temperate mangrove forests

R H Bulmer,L Schwendenmann,C J Lundquist

doi:10.5194/bg-12-6169-2015

Abstract

Abstract. Temperate mangrove forests in New Zealand have increased in area over recent decades. Expansion of temperate mangroves in New Zealand is associated with perceived loss of other estuarine habitats, and decreased recreational and amenity values, resulting in clearing of mangrove forests. In the tropics, changes in sediment characteristics and carbon efflux have been reported following mangrove clearance. This is the first study in temperate mangrove (Avicennia marina) forests investigating the impact of clearing on sediment CO2 efflux and associated biotic and abiotic factors. Sediment CO2 efflux rates from intact (168.5 ± 45.8 mmol m−2 d−1) and cleared (133.9 ± 37.2 mmol m−2 d−1) mangrove forests in New Zealand are comparable to rates measured in tropical mangrove forests. We did not find a significant difference in sediment CO2 efflux rates between intact and cleared temperate mangrove forests. Pre-shading the sediment for more than 30 min prior to dark chamber measurements was found to have no significant effect on sediment CO2 efflux. This suggests that the continuation of photosynthetic CO2 uptake by biofilm communities was not occurring after placement of dark chambers. Rather, above-ground mangrove biomass, sediment temperature and chlorophyll a concentration were the main factors explaining the variability in sediment CO2 efflux in intact mangrove forests. The main factors influencing sediment CO2 efflux in cleared mangrove forest sites were sediment organic carbon concentration, nitrogen concentration and sediment grain size. Our results show that greater consideration should be given regarding the rate of carbon released from mangrove forest following clearance and the relative contribution to global carbon emissions.

Highlights

Mangroves are generally confined to the tropics, between latitudes 30◦ N and 30◦ S
A landward expansion of mangroves into salt marsh has been observed in Australia and the USA (Cavanaugh et al, 2014; Saintilan et al, 2014), while mangrove expansion into tidal flats is typically observed in New Zealand (Stokes et al, 2009; Lundquist et al, 2014a)
The magnitude of dark sediment CO2 efflux in intact Avicennia marina forests measured in this study (168.5 ± 45.8 mmol m−2 d−1) is similar to values reported for intact Avicennia marina forests in New Zealand (Lovelock, 2008; Lovelock et al, 2014), Australia (Livesley and Andrusiak, 2012) and tropical locations (New Caledonia: Leopold et al, 2013, 2015; Table 2)

Summary

Introduction

Mangroves are generally confined to the tropics, between latitudes 30◦ N and 30◦ S. Temperate mangrove forests mainly occur in Australia, New Zealand, the USA and South Africa (Morrisey et al, 2010; Giri et al, 2011). These forests are subject to colder and generally more variable climatic conditions, and are typically associated with lower diversity of tree species and lower faunal abundance and diversity than in the tropics (Alfaro, 2006; Morrisey et al, 2010). The expansion of mangroves in New Zealand has been linked to increased sedimentation leading to vertical accretion of tidal flats (Swales et al, 2007; Stokes, 2010), increased nutrient inputs (Lovelock et al, 2007), and climatic factors (Burns and Ogden, 1985)

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Discussion

Conclusion