Sediment properties and CO₂ efflux from intact and cleared temperate mangrove forests

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.