Guinea-Bissau : What fuels shorebird food webs in a pristine mangrove-bordered West African intertidal ecosystem?

Mangrove forests are dominant elements in many tropical and sub-tropical coastal and estuarine ecosystems and are among the most productive ecosystems in the world, providing a wide variety of services benefiting both the human well-being and nearby ecological systems. Africa is the continent with the second largest extent of mangroves, and Guinea-Bissau, in west Africa, is currently among the 15 countries with the largest extent of mangrove forest in the world, and the second in Africa.

Due to their high productivity, mangroves are regarded as major potential exporters of nutrients and organic matter for coastal and marine primary and secondary production. Intertidal flats adjacent to mangrove forests constitute a transitional habitat that supports a large variety of intertidal predators (e.g. shorebirds and fish) and their macrozoobenthic prey (Beninger 2018). These areas may benefit from adjacent mangrove productivity in two ways: (1) through direct carbon input, which may then be directly consumed by benthic macroinvertebrates, or (2) indirectly, with mangrove organic matter being exported through the tides to adjacent waters, where it can be transformed in inorganic nutrients (like nitrogen and phosphorous), which in their turn will promote the growth of other primary producers, like benthic algal cells, and then be used by benthic macroinvertebrates. In this study we investigate the first hypothesis, and assessed whether mangrove forests play an important role of in fuelling the intertidal food webs by providing direct carbon inputs macroinvertebrate consumers of adjacent intertidal flats. Additionally, we also assessed the magnitude of the contribution of other alternative primary food sources, like benthic microalgae, macroalgae, sediment organic matter (SOM) and suspended particulate organic matter in the water (POM), to benthic invertebrates. We do this in the Biosphere Reserve of the Bijagós archipelago, a group of around 88 islands and islets off the coast of Guinea-Bissau, which is a considerably pristine mangrove ecosystem and an important tropical intertidal area, namely as the second most important wintering site for migratory shorebirds of the East Atlantic Flyway, and where more than 700,000 of these birds feed on benthic macroinvertebrates.

All animals integrate the stable isotopic signature of the food they consume in their tissues. This means that benthic macroinvertebrates will have a carbon isotopic signature similar to that of the primary carbon source they consume. Mangroves have a very distinct carbon stable isotope signature, much more negative when compared to other primary carbon sources, and we used this characteristic in this study. Applying stable isotope analysis and mixing models, we compared intertidal flats with (mangrove sites) and without (control sites) adjacent mangrove forests regarding the carbon isotopic signature of benthic macroinvertebrates and sediment organic matter (SOM), and the relative importance of potential primary carbon sources in sustaining benthic macroinvertebrates.

We found no evidence that mangrove carbon sustains intertidal food webs directly. Mangrove leaves had the lowest relative contribution to the diet of benthic macroinvertebrates, while macroalgae, benthic microalgae and POM derived detritus showed variable but overall important contributions. Our results suggest that there is little direct input of mangrove carbon into the food webs of unvegetated intertidal flats. Still, this leaves open the possibility of mangrove forests acting as sources of inorganic carbon and processed nitrogen and phosphorous, on which other primary producers may feed and subsequently fuel adjacent food webs.

Auteurs : Mohamed Henriquesa,b*, José Pedro Granadeiroa, Theunis Piersmab,c, Seco Leãod, Samuel Pontese & Teresa Catrya

a Centro de Estudos do Ambiente e do Mar (CESAM), Departamento de Biologia Animal, Faculdade de Ciências da Universidade de Lisboa, Lisboa, Portugal.

b Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences, University of Groningen

c NIOZ - Royal Netherlands Institute for Sea Research, Department of Coastal Systems

d Village of Menegue, Island of Canhabaque, Bijagos archipelago, Guinea-Bissau

e Instituto da Biodiversidade e Áreas Protegidas Dr. Alfredo Simão da Silva - IBAP, Bissau, Guinea-Bissau