Ecological dogma holds that mangroves are predominantly exporting and are sources for production of offshore systems including fisheries (the ‘outwelling hypothesis’). Yet, the amount of export, and thereby the economic benefits to fisheries, varies greatly between locations. Mangrove functional research has focused on tropical and subtropical areas, where rainfall-driven transport of terrestrial nutrients into mangroves makes forests productive and runoff flow makes export likely. It remains unclear to what degree the outwelling hypothesis applies to arid mangroves; and, at a more basic level, in the absence of rainfall-driven runoff, what are the sources of nutrients that sustain arid mangrove production? This presentation will address these fundamental questions and review the sources for contextual variation in mangrove outwelling; it will detail the findings of several recent multi-disciplinary ecological and oceanographic research projects on Qatar mangroves. Our research explored the notion that the extent of outwelling globally depends on interactions of coastal geomorphology, tidal regime and mangrove production. Findings show that outwelling is less prevalent than previously thought, particularly in areas where in-house herbivory is intense. In mangroves with low freshwater through-flow the net balance between import and export might well be lead to net inwelling: a net import of organic material from subtidal habitats. Thus, seagrass beds are source of inwelling at Al-Khor mangrove in Qatar. At Al-Khor, limited export of mangrove-derived carbon to adjacent food webs has been attributed to the arid environment where the lack of rainfall eliminates a significant flushing mechanism, reducing the opportunity for outwelling of particulate organic material. This presentation concludes that mangroves in the western Arabian Gulf are nutrient limited and dependent on localized retention and cycling of nutrients and inwelling of nutrients from adjacent habitats, partially driven by tidal asymmetry. We suggest that inwelling of particulate material from down-stream habitats, such as seagrass beds, makes a significant contribution in supporting mangrove resident fauna and sustains mangroves in arid countries.


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  • Received: 07 December 2015
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