Nigerian Journal of Science and Environment Volume 6, 2007
MACROINVERTEBRATES FUNCTIONAL FEEDING GROUPS IN RIVER OROGODO, A SECOND ORDER STREAM IN SOUTHERN NIGERIA By Francis O. Arimoro Department of Animal and Environmental Biology, Delta State University, PMB 1, Abraka, Nigeria. E-mail
[email protected]
ABSTRACT: To examine the functional structure of aquatic biota in River Orogodo, southern Nigeria, benthic macroinvertebrate samples were collected from four ecologically distinct stations monthly for two years (Jul 2003-Jun 2005). Taxa recorded were allocated to functional feeding groups using published literature and examination of gut contents. Clearly, the spatial distribution of most species reflected morphological and trophic suitability to particular environmental conditions. Predators (e.g Hemiptera and Coleoptera) dominated the functional groups at stations I, II and IV. Collector-gatherers, particularly Chironomid larvae dominated the abundance at station III. Scrapers (e.g Melanoides, elmids) and Collector-filterers (Polypedilum) were only sporadically present mostly in the downstream reach (station IV). The paucity of shredders at all stations examined could be explained by the enhanced microbial activity replacing shredder activity at high temperatures common in most tropical streams. The functional organization did not however conform to the River Continuum Concept (RCC) model. This could be related to the degradation of station III with organic wastes from the Agbor Abattoir and deforestation activities at the head station. Keywords: Macroinvertebrates, Functional feeding group, River Continuum Concept, Predators, Collectors, Shredders, River Orogodo.
gatherers, scrapers/grazers, and predators (Rempel et al., 2000, Dobson et al 2002; Miserendino and Pizzolon 2000 and 2003). Food webs in forest stream ecosystems are typically driven by allochthonous organic inputs, mainly leaf litter (Vannote et al 1980; Cheshire et al 2005). In streams, leaf litter is readily leached, colonized and decomposed by microorganisms, and consumed by macroinvertebrates shredders (Gessner et al 1999). These processes lead to the
INTRODUCTION Functional group classification is useful in examining ecological relevant community-level associations with the physical habitat (Rempel et al 2000). A number of functional feeding macroinvertebrate groups (a nontaxonomic unit) have been identified as being involved in the processing of organic matter in streams (Rawer-Jost et al., 2000). These functional feeding groups are shredders, collector-filterers, collector45
Nigerian Journal of Science and Environment Volume 6, 2007 production of fine particulate organic matter (FPOM), which is consumed by a suite of collector organisms. The shredders and collectors are thus the major primary consumers in forest streams, providing the main link between the organic inputs and the predators. In developed countries of the world and particularly in Europe biomonitoring involves the use of the functional feeding groups aimed at assessing ecological integrity, and this leads to increase in the knowledge of complex dynamic processes in running waters ecosystems (Barbour et al 1997; Rawer-Jost et al 2000). The River continuum concept (RCC) classifies the zoobenthos in functional groups based on the feeding habits and provides a framework within which to interpret and predict longitudinal patterns in macroinvertebrate communities of river systems vis-a-vis wide changes in physical, energetic and metabolic parameters. The RCC proposes that the gradient of physical condition from the headwaters to the river mouth determines a series of responses resulting in a continuum of biotic adjustments and consistent patterns of loading, transport, utilization and storage of organic matter (Vannote et al., 1980). Tropical Africa has been particularly understudied, although recent studies from Kenyan streams (e.g Dobson et al., 2002) have provided an important contribution to the current debate regarding the similarities and differences in the functioning of tropical versus temperate
streams. Specifically, leaf litter in tropical streams is generally thought to be processed by microorganisms rather than by invertebrate shredders, organisms that play a main role in many forest temperate streams. However, recent studies show that this is not always the case, as shredders play an important role in some tropical streams (Cheshire et al., 2005), and thus the debate is still open. Data from multiple tropical sites are highly necessary to be able to describe global patterns. Therefore this study aim to discuss the occurrence and dominance of macroinvertebrate functional feeding groups in the different marked out stations of the River Orogodo, a tropical forest stream in southern Nigeria in relation to the RCC concept. This is necessary for the development of stream ecological theory, which is based mainly on temperate stream work. Materials and methods The Orogodo River is the longest river in Ika-axis of Delta state of southern Nigeria. It lies between 50.10-60.201N and longitude 60.10-60.261E (Fig.1). The Full description of the river and sampling stations are given elsewhere (Arimoro et al 2007 a & b). Four sampling stations were established within the river. They were chosen at intervals of 1km to 25km apart starting from the headwater region (station 1), station II, the middle reach (station III) with less vegetation and a receptor of effluents from a nearby abattoir and the downstream site (station IV).
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Nigerian Journal of Science and Environment Volume 6, 2007
measured in the mid channel on three occasions by timing a float (average of three trials) as it moved over a distance of 10 m (Gordon et. al., 1994). Depth was measured in the sample area using a calibrated stick. Nitrate-nitrogen (NO3-N) and Phosphate-Phosphorus (PO4-P) were measured spectrophotometrically after reduction with appropriate solutions
Water Sampling Water samples were collected monthly from each station. Surface water temperatures were recorded with a mercury-in-glass thermometer. Conductivity, pH, total alkalinity, dissolved oxygen and Biochemical Oxygen Demand (BOD5) were determined according to APHA (1985) methods. Water velocity was 47
Nigerian Journal of Science and Environment Volume 6, 2007 Mandaville 2002. Gut analyses were performed for un-easily identified macroinvertebrates for easy designation to the functional feeding group. Animals were placed into a microscope slide, their heads removed with a scalpel and the contents of the foregut carefully squeezed into a drop of water; they were then examined under the microscope for identifiable particles. Statistical analyses Community attributes and physicochemical parameters of the sampling stations were compared using repeated measures ANOVA. Fixed effect ANOVAs were performed using dates as replicates on log (x + 1) transformed data. Significant ANOVAs (p
