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

Kátya G. Abrantes,Trent R. Marwick,Adam Barnett,Steven Bouillon

doi:10.1890/es12-00322.1

Kátya G. Abrantes, Trent R. Marwick + Show 2 more

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https://doi.org/10.1890/es12-00322.1

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Journal: Ecosphere	Publication Date: Jan 1, 2013
Citations: 58	License type: CC BY 3.0

Affiliation: KU Leuven, James Cook University, Deakin University

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, δ13CDIC, δ13CDOC, δ13CPOC and δ15NPN. There were also significant positive relationships between δ13CPOC and consumer δ13C and between δ15NPN and consumer δ15N 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.

Highlights

The stability of biological communities depends in part on the availability of food and on the stability of trophic interactions among the different species (Polis and Strong 1996, Polis et al 1997)
D13CPOC was higher in the Betsiboka and Tana, indicating that the C4 grasslands from the catchment contribute significantly to the particulate organic carbon concentration (POC) pool (Bouillon et al 2007, Ralison et al 2008, Bouillon et al 2009), and even though there are large areas of C3 mangroves in the Tana estuary, d13CPOC at this site was higher than that from the C3-dominated sites
Because strong wind events are not regular, because no extensive macrophyte beds are present in the vicinities of the studied estuaries and because sampling was conducted in the inside of the estuaries, these marine subsidies are not likely to be important for the estuarine animal communities studied. This large-scale study demonstrates that terrestrial subsidies are important for aquatic food webs in east African estuaries

Summary

Introduction

The stability of biological communities depends in part on the availability of food and on the stability of trophic interactions among the different species (Polis and Strong 1996, Polis et al 1997). Rivers and estuaries are subjected to high human pressure, as their settings make them ideal for human settlement This pressure has been intensified over the last century, especially in tropical areas where increasing population numbers place increasing pressure on these environments (Junk 2002). Much research on the importance of terrestrial subsidies has been conducted on temperate lakes (e.g., Bartels et al 2012), rivers (e.g., Kendall et al 2001) and to a lesser extent on estuaries (e.g., Chanton and Lewis 2002, Sakamaki et al 2010), information on the main sources of energy for tropical estuaries is still lacking Processes in these areas are likely to be very different to those in temperate systems given the much higher seasonality, productivity and biological diversity of their waters, and the types of human impacts they are subjected

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