Open AccessImportance of terrestrial subsidies for estuarine food webs in contrasting East African catchments
Author(s) -
Abrantes Kátya G.,
Barnett Adam,
Marwick Trent R.,
Bouillon Steven
Publication year - 2013
Publication title -
ecosphere
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.255
H-Index - 57
ISSN - 2150-8925
DOI - 10.1890/es12-00322.1
Subject(s) - estuary , trophic level , environmental science , ecology , drainage basin , food web , isotope analysis , biogeochemical cycle , δ15n , vegetation (pathology) , ecosystem , geography , δ13c , stable isotope ratio , biology , physics , cartography , quantum mechanics , medicine , pathology
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 δ 13 C, we anticipated that terrestrial subsidies would be reflected in a gradient in estuarine consumer δ 13 C values, following the relative importance of C3 (characterised by low δ 13 C) vs. C4 (characterised by high δ 13 C) 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, δ 13 C DIC , δ 13 C DOC , δ 13 C POC and δ 15 N PN . There were also significant positive relationships between δ 13 C POC and consumer δ 13 C and between δ 15 N PN and consumer δ 15 N 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.