Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

Abrantes, Kátya, Barnett, Adam, Marwick, Trent R., and Bouillon, Steven (2013) Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments. Ecosphere, 4 (1). 14. pp. 1-33.

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Abstract

Little is known on the degree to which terrestrial organic matter delivered to tropical estuaries contributes to estuarine consumers. Here, stable isotope analysis is used to constrain this contribution for contrasting east African estuaries whose catchments differ in relative C3/C4 vegetation cover. As these two types of vegetation differ strongly in δ¹³C, we anticipated that terrestrial subsidies would be reflected in a gradient in estuarine consumer δ¹³C values, following the relative importance of C3 (characterised by low δ¹³C) vs. C4 (characterised by high δ¹³C) cover. Five estuaries were sampled for aquatic biogeochemical parameters, primary producers and consumers of different trophic ecologies: the Zambezi (catchment with a C3/C4 cover of 61/39%) in Mozambique, the Tana in Kenya (36/64%) and the Betsiboka (42/58%), Rianila (85/15%) and Canal des Pangalanes (C3-dominated) in Madagascar. Sampling was done before and after the 2010/2011 wet season. There were positive relationships between the proportion of C4 cover in the catchment and turbidity, δ¹³C(DIC), δ¹³C (DOC), δ¹³C(POC) and δ¹⁵N(PN). There were also significant positive relationships between δ¹³C(POC) and consumer δ¹³C and between δ¹⁵N(PN) and consumer δ N-15 for all consumer trophic guilds, confirming the incorporation of organic material transported from the catchments by estuarine consumers, and implying that this material is transported up to high trophic level fish. Bayesian mixing models confirmed that C4 material was the most important source for the highly turbid, C4-dominated estuaries, contributing up to 61-91% (95% CI) to phytodetritivorous fish in the Betsiboka, whereas for the less turbid C3-dominated estuaries terrestrial subsidies were not as important and consumers relied on a combination of terrestrial and aquatic sources. This shows that the ecology of the overall catchment affects the estuaries at the most basic, energetic level, and activities that alter the turbidity and productivity of rivers and estuaries can affect food webs well beyond the area of impact.

Item ID: 31690
Item Type: Article (Research - C1)
ISSN: 2150-8925
Keywords: Africa; Bayesian mixing models; catchment; estuaries; stable isotopes; terrestrial subsidies
Additional Information:

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits restricted use, distribution, and reproduction in any medium, provided the original author and sources are credited.

Funders: European Research Council (ERC), Research Foundation Flanders, Rufford Small Grant, Marie Curie International Incoming Fellowship, 7th European Community Framework Programme
Projects and Grants: European Research Council (ERC-StG 240002, AFRIVAL), Research Foundation Flanders (FWO-Vlaanderen)
Date Deposited: 26 Feb 2014 09:47
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 100%
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