Carbon dynamics of Deep Bay, eastern Pearl River Estuary, China. I: A mass balance budget and implications for shorebird conservation

Deep Bay is a shallow embayment (112 km(2)) in the eastern Pearl River Estuary, China, and comprises 4 major wetland components: (1) a shallow brackish water body of 2.9 m average depth, (2) 2700 ha of intertidal mudflat, (3) 200 ha of tidal mangroves and (4) 300 ha of traditional tidal aquaculture ponds excavated in the mid-to-high intertidal region. A carbon budget is proposed for Deep Bay based on published information on the 4 major landscape components. Despite the usual emphasis placed on the role of mangroves as net carbon exporters in support of consumer (such as shorebird) populations, the carbon budget suggests that about 50% of all carbon available in Deep Bay originates from anthropogenic input from rapidly urbanising, as well as agricultural, areas in the local catchments. Mangrove production only contributes about 1.8% of the total carbon available in Deep Bay. Calculations based on the basal metabolic demands of the bird assemblage in winter suggest that the Deep Bay mudflats are close to carrying capacity. The annual carbon requirements of the shorebirds and availability (as demonstrated by the supply/demand ratio for the curlew) are lowest in the winter months, when bird numbers are large but production low. Considering the shorebird assemblage foraging on the mudflat, the total carbon consumption in January 1994 amounted to 16.9 t, while the carbon production during the same period was 27.8 t. These figures suggest that Deep Bay is near carrying capacity unless >50% of the invertebrate resources are made available to the birds. The results of this study also argue that organic enrichment in Deep Bay, which has usually been regarded as a 'pollutant', forms the basis of the major beneficial use of carbon (shorebird conservation), although further enrichment may result in negative impacts. This study highlights the importance of incorporating human influences into the study of the structure and function of ecosystems in urban settings.