1. 2013
Testing a new invertebrate-based river biomonitoring scheme for tropical rivers in Africa
Isabel Moore, Kevin Murphy
IBAHCM, University of Glasgow, Scotland
Introduction
In a changing world, the need to monitor water quality is crucial in order to protect and preserve this important natural resource. Benthic macroinvertebrates are a good measure of river water quality because of the impacts that polluted water can have on their populations, as well as having simple and inexpensive sampling procedures. Although sampling methods have been created for different ecosystems around the world, macroinvertebrate sampling protocols are still being developed for use in tropical rivers in Africa. The Southern African River Assessment Scheme (SAFRASS) encompasses the sampling of aquatic macrophytes, benthic diatoms, and benthic macroinvertebrates in order to assess river water quality (Kennedy et al., 2012). Within this scheme, collection and assessment of macroinvertebrates is done using the Zambian Invertebrate Scoring System (ZISS) which analyzes the presence and abundance of various taxa, to measure water quality.
Position and altitude were geolocated using a Garmin GPS. ZISS score, Average Score per Taxon (ASPT) and diversity (number of taxa per sample; S) were calculated for each site. The data were analysed using ordination (CCA) and classification (TWINSPAN) procedures, and the relationships between drag- and kick-assessed scores for ZISS, ASPT and S were determined by correlation and linear regression analysis.
Kick-sampling: Mulembo River, Zambia
Results and conclusions
In total 42 benthic macroinvertebrate taxa were recorded at the sample sites. The highest ZISS score recorded was 79 (indicating very good river ecosystem health), and the lowest (10) indicated poor water conditions at that site. Sites were classified into 4 site-groups, supporting different invertebrate assemblages, by TWINSPAN, and CCA analysis showed that the primary environmental controls influencing the benthic fauna were probably conductivity, alkalinity, altitude and flow rate (Fig. 1).
Objectives
The aim was to assess the use of a dredge-sampler as an alternative to standard kick-net sampling as a method to sample benthic macroinvertebrate populations when kick sampling is not a feasible option, due to the presence of dangerous wildlife, or other hazards, across a range of conditions in tropical African rivers and associated freshwater bodies.
Dredge-sampling in fast-flowing stream
Fig. 2. ZISS scores collected by both kick-sampling (r = 0.952, P<0.001***), and dredge- sampling ( r = 0.532, P = 0.034*) were significantly positively correlated to the overall ZISS score, with a regression for the latter of:
MaxSite = Drag
Fig. 1. CCA site ordination plot with TWINSPAN site-groups overlain, showing relation between environmental variables and 4 site-groups (I: characterised by abundant Gerridae, Gyrinidae, Culicoidae, and Ampularidae; II: Baetidae (single species), Oligochaetae; III: Coenagrionidae, Libellulidae, Baetidae (> 1 species), Atyidae and Leptoceridae; IV: abundant Hydrophilidae).
Sampling with an armed guard in crocodile-infested stream: Kasanka River, Zambia
Methods
References
Both kick-sampling (the default approach under the ZISS protocol) and dredge-sampling produced good results, but the latter was clearly a less satisfactory method, with a weaker (though still significant) correlation with overall ZISS scores attained at each site (Fig. 2). We considered dredge sampling a poorer option particularly in fast-flowing stony streams, and in rivers with dense aquatic vegetation. In both of these biotope types it was clear that the dredge was not able to collect invertebrates adequately, compared with the kick-net approach. However we conclude that use of the dredge remains a useful option in habitats where it is too dangerous to enter the water, because of risks from hazardous wildlife or for other reasons. The study also produced evidence to suggest that the ZISS approach can successfully be applied not only in river channels, but in associated waterbodies such as floodplain lagoons.
Within three areas of Zambia, during July 2013, 24 sites on rivers and associated water bodies were sampled following ZISS kick-net sampling protocols (Lowe et al., 2013) to collect macroinvertebrates from up to 3 biotopes (as present: gravel, sand, mud; aquatic vegetation; stones). At 17 of these sites dredge-net sampling was also undertaken to permit comparison with kick-sampling data. Both nets had 1 mm mesh size. Conditions were considered too dangerous to allow kick-sampling at the remaining sites. Environmental data collected for all sites comprised pH, conductivity, alkalinity, water clarity (using a SKYE underwater light meter), disturbance score, and flow class (slow, moderate or fast flow).
Lowe S. et al. (2013). SAFRASS Methodology Manual. SAFRASS Deliverable Report to the ACP Science and Technology Programme, Contract No. AFS/2009/ University of Glasgow, UK. 36 pp.
Kennedy, M.P. et al. (2012) Multiple-scale controls on physical and chemical habitat conditions and their influence on biotic assemblages in Zambian rivers. Proc. British Hydrological Soc 11th Nat. Symp.: Hydrology for a changing world, Dundee DOI: /bhs.2012.ns28.
Acknowledgements
We thank the Carnegie Trust for the Universities of Scotland for funding this study; and the Kasanka Trust for facilitating our fieldwork in Zambia.