Aquatic-terrestrial linkages in Afrotropical lakes and rivers using stable hydrogen isotopes

Periodic Reporting for period 1 - AQUAHYDRO (Aquatic-terrestrial linkages in Afrotropical lakes and rivers using stable hydrogen isotopes)

Berichtszeitraum: 2016-09-01 bis 2018-08-31

Understanding ecological functioning of large aquatic ecosystems provides an essential framework for conservation and management of aquatic resources. The degree to which aquatic and terrestrial primary production fuel Afrotropical aquatic food webs remains poorly understood, since quantifying the relative contributions of these resources is methodologically challenging. Recently, the promising use of stable hydrogen isotope ratios (δ2H) has gained attention to complement the ‘traditional’ C and N isotopes (δ13C and δ15N) for aquatic food webs due to their greater power of separation between aquatic and terrestrial subsidies. However, measurements of δ2H of complex organic materials have faced several methodological issues related to the isotopic H exchange and the effect of residual moisture in samples. The aims of this research are: to test new preparation systems to control H exchangeability and absorbed water for H isotope analysis; and to quantify to which extent aquatic communities in the Congo River basin and Lake Edward depend on aquatic and terrestrial primary production using a multiple tracer approach over different temporal scales (recent versus historical, using museum-archived specimens). Given the importance of fisheries to support riparian human populations in these Afrotropical systems, our study point towards the significance of terrestrial sources for an adequate management of aquatic ecosystem and fisheries in the Congo River basin. In the case of Lake Edward, with a reported decline in fisheries yield, our results will bring a solid understanding of the energetic support of the fish food web and of the underlying mechanisms that have led to this decline, which will have direct implications for fisheries management, food supply and ultimately local stability.

The AQUAHYDRO project used cutting-edge stable isotope techniques and numerical models to provide well-supported conclusions. Sample treatment procedures were tested to control H exchangeability and absorbed water for the routine analysis of non-exchangeable hydrogen isotope ratios in animal tissues. During the project, the H isotope technique has been optimized and >1500 samples have been measured. Overall, our results show that we gained power to distinguish sources by using a multiple tracer approach including δ2H, δ13C and δ15N. We were able to discriminate sources by the measurements of stable carbon isotopes providing information on C3 and C4-based food webs, of stable nitrogen isotopes on trophic position and feeding groups, and of stable hydrogen isotopes on aquatic (autochthonous) vs. terrestrial (allochthonous) inputs as well as fish feeding habits.

The sample collection of primary sources and consumers from the Congo River basin from the supervisor of this project made during the COBAFISH project were measured for stable hydrogen isotope analysis to complement already processed C and N stable isotope data. We estimated the contribution of terrestrial and aquatic inputs to consumer (fish and invertebrate) diet by including C, H, (and N) isotopes within Bayesian mixing models. These models indicate a high diversity of food source preferences between the variety of invertebrate and fish species sampled, but at the community level it is clear that terrestrial C3 vegetation is an important contributor to the food web as a whole.

Direct sampling of potential sources and consumers to investigate the reliance on terrestrial, littoral benthic and /or pelagic-plankton resources were conducted in three field campaigns at Lake Edward under the HIPE project (October 2016, March 2017, January 2018). Stable isotope measurements of these samples for δ13C, δ15N and δ2H are finished and results have been presented in conferences and project meetings. Isotopic data indicate that aquatic and terrestrial production as well as methanogenic C can support the fish community of L. Edward, although the majority of data at first sight fit with a dominant reliance on aquatic (phytoplankton-based) food sources. More detailed quantitative modeling of the finalized dataset will allow identifying and quantifying the different inputs into the food web; this analysis is in progress.

Food web assessment for historical (i.e. 1930’s) and modern (HIPE project) samples in Lake Edward was conducted. Average fish δ13C values from Lake Edward were lower in the historical specimens compared to recent samples, which is consistent with trends in δ13C of sedimentary organic matter in sediment cores, such a trend would indicate a decrease in productivity and/or a change in the composition of terrestrial OM input to L. Edward over the last century.

The fellow has obtained a multidisciplinary set of research and transferable skills through high quality training to pursue his long-term career goal (i.e. become an independent scientist or research professor). The fellow has been directly involved in grant proposal and publications writing, teaching activities, networking opportunities, participation in inter-disciplinary projects, contribution to organization of conference sessions, transferable skills training and career development at KU Leuven, supervision of technical staff and students, and participation in editorial and review schemes. All these skills have enabled the fellow to reach a suitable starting point for setting up his own independent research career. The AQUAHYDRO project also had an impact by attracting new collaborations to the host research group, and for the fellow’s career, especially regarding the analysis of hydrogen isotopes.

The results of our project are expected to contain relevant management implications for tropical aquatic systems. In the case of the Congo River dataset, our results point towards a high diversity in trophic resource specialization within fish communities, but also towards an overall important role of terrestrial food sources for aquatic consumers. Hence, given the importance of fisheries to support riparian human populations, acknowledging the importance of terrestrial sources and an adequate management of riparian habitats is an important management implication. For the Lake Edward study, data analysis is still in progress but here too we expect our data and the HIPE project results in more general terms to be potentially highly relevant from a management perspective: Lake Edward is an extremely important source of dietary protein for the human population, and is an integral part of the Queen Elizabeth and Virunga National Parks (Uganda & DRC, respectively). A decline in fisheries yield has been reported, and is of major concern for food supply and local stability – a solid understanding of the energetic support of the fish food web, and of the underlying mechanisms that have led to this decline is of high importance for future fisheries management.

River sampling in Queen Elizabeth National Park

Periodic Reporting for period 1 - AQUAHYDRO (Aquatic-terrestrial linkages in Afrotropical lakes and rivers using stable hydrogen isotopes)

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