Macro-invertebrate Colonization of Artificial Substrata II: Weeds and Plant Stems

INTRODUCTIOIN

The use of colonization samples for benthic macrofauna studies may have its advantages and disadvantages and is of great significance in the bioassessment of unwadeable streams and rivers. Some benthic groups have strong preference for or may be attracted by special artificial or natural substrates for immediate colonization. Colonization of macro-invertebrates following changes to the stream environment occurs rapidly through drift, oviposition, lateral movement and vertical migration (Fowler, 2002). Lakes (2000) observed that ability of invertebrates to colonize available habitats is influenced by disturbance type and also by the abundance and type of taxa present in the system (Death, 1996, 2000).

Colonization according to Mayr (1964) is defined as the invasion of species into disturbed and newly created habitats. Williams and Hynes (1979) discovered that the colonization of a disturbed habitat will be certainly be by an amount of original species which have been wiped out by perturbations or by new colonizers. Also, it has been discovered that the rate of colonization of invertebrates varies between taxa, as shown in the study carried out on the effects of shoot age on colonization of an emergent macrophyte (Typha latifolia) by microinvertebrates (Oertli and Lacharanme, 1995). Colonization studies lasting less than a week have distinguished early and late colonizers (Shaw and Minshall, 1980; Clifford et al., 1992). Macarthur and Wilson (1963) pointed out that a new habitat has a high invasion rate of species that have become established and a relatively stable equilibrium would be reached. Both authors proposed an equilibrium model for island faunas to describe patterns of species diversity amongst relatively homogeneous habitat “patches”. Minshall and Peterson (1985) developed the MacArthur-Wilson model further with respect to non-equilibrium phase as proposed in the colonization process where the number of species and individuals are less than or exceed the equilibrium number.

Artificial substrates have been employed and found to be attractive for sampling the macro-invertebrates of freshwater (Pearson and Jones, 1975; Boothroyd and Dickie, 1991) and many types have been used like the trays of substratum described by Moon (1935) and Egglishaw (1964) and the basket samplers used by Jacobi (1971) and Fowler (2002). These authors have recorded large number of the invertebrates colonizing the substrates, but little attention has been paid to the temporal pattern of colonization. Pearson and Jones (1975) have described the study of the benthic communities of the River Hill, East Vorkshire by using metal boxes.

The main objective was to evaluate the relationship between habitat and macro-invertebrate community and the colonization of 2 types of artificial substrates with emphasis on early and late colonizers and factors that may influence their colonization.

MATERIALS AND METHODS

Sampling technique: Ogba River is located at the southwest region of the outskirt of Benin City in Edo State. It lies between lat 6.5°N and long. 5.5°E. The River, which is about 42 km in length, takes its source at Ekenwan and flows in a South East direction through Ogba village and empties into Benin River, which in turn empties into the Atlantic Ocean. The municipal drains of Benin metropolis flow into the river at Ogba prison catchment area (Ezemonye and Kadiri, 1998).

The study area has the tropical wet and dry climate primarily regulated by rainfall. The rainfall season, which occur between April and October and the dry season, which occur between November and March, do not conform to any regular pattern. The selected sampling site is up and down streams of Ogba village (Ogbaneki station) that has a characteristic of narrow and shallow riffle.

Fifteen local trap artificial substrates were deployed in the Ogba River from June to December 2002. The Traps made with interwoven cane materials (from Raphia hockerii and Lacosperma secundiflorum) were used for sampling (Fig. 1). Fresh weeds and plant stems were used as artificial substrates. The trap has an opening at the base through which benthic organisms have access to the substrates. While the other end serves as opening through which materials are introduced into the trap. Each trap was half filled with substrate and closed by tying with an attached string and consequently anchored with a peg to the river bed. Recommended deployment period for artificial substrates are 4, 10, 21 and 40 days, respectively. This is a similar length of time to previous experiments using artificial substrates (Downes et al., 1998; Zimmermann and Death 2002; Fowler, 2002). Three replicate samples were collected per artificial substrate.

The traps were removed by hand picking and placed immediately in a polypropylene bucket containing water. Substrate materials were dusted and rinsed with water and were closely examined to ensure that no organism was still attached to them. The medium inside the bucket was sieved through a set of tyler sieves of varying mesh-sizes: 2 mm, 1 mm, 150 and 100 μm. The sieved contents were preserved in the field in labeled sampling bottles containing 10% formalin.

In the laboratory, sorting was done using the American optical dissecting microscope model 570 with magnification of 25-40x and HM-Lux 3 microscope. Sorted organisms were preserved in 4% formalin.

Identification of organisms was carried out using appropriate keys and works of Mellanby (1963), Macan (1963), Pennak (1978), Ward and Whipple (1958), Needham and Needham (1978) and Powell (1977, 1980) Olomukoro (1996). Due to the insufficiency of our taxonomic knowledge of the riverine fauna of Nigeria, some specimens have been assigned only to the lowest taxon within which they can be placed with certainty.

Fig. 1:	Trap used for sampling

In some cases identification can be carried out at generic level.

Estimation of total fauna diversity: Benthic Community diversity was determined in both substrates using biological indices such as Margalef, Shannon and Weaver (1949) and Evenness indices. Sorensen’s quotient (Q/S) and Jaccard Co-efficient (Sj) (Boothroyd and Dickie, 1991) were also used for the estimation of faunal similarities.

RESULTS

Physical and chemical parameters: Results of Table 1 shows that highest values of parameters were recorded in July, August and October, while the lowest values were obtained for Water temperature, Suspended solid, dissolved Solids and Conductivity in June and November, respectively. Other parameters had their lowest values between June and August.

MACROBENTHIC FAUNA

The total number of taxa recorded was 32 with 1070 individuals. These comprise 5 species of Oligochaeta, 3 species of Crustacea, 4 species of Odonata and 2 species of Hemiptera.

Table 1:	Physical and chemical parameters of the study area, Ogba River from June 2002-Dec, 2002
Note: Parameters are in mg L-1 except temperature in °C and conductivity in μs cm-1

Table 2:	The overall composition, distribution and abundance of macrobenthic invertebrate fauna in the substrate weeds and plant stems

Other groups are Arachnida (1 specie), Diptera (II), Mollusca (2) and Amphibia (Tadpole) (I). Hemiptera and Arachnida had the lowest population density of 0.30 and 0.24% in substrate weeds and plant stems, respectively. Of all the taxonomic groups identified, mollusca had no record in plant stems.

Table 3:	Relative percentage composition/abundance of the Macrobenthic Invertebrate of the submerged artificial substrates

Consequently, some groups e.g., Ephemeroptera, pleroptera, Coleoptera, Trichoptera, Lepidoptera etc, were not represented in the two artificial substrates. The number of individuals in weeds and stems substrates were 61.27 and 38.36%, respectively (Table 2).

The relative percentage composition of the macrobenthic organisms in the artificial substrate (Table 3), shows that Oligochaeta has the highest record of 29.15% in the substrate weeds and 55.34% in the plant stem

Table 4 and 5 show the relative colonization rate of individuals groups of organisms in each exposure periods in the substrate weeds and plant stems. Rapid invasion of substrates by the organisms occurred within the first four days of the experimental observation. The pattern of substrate colonization varied between the two substrates as few organisms e.g., Diptera and Amphibia attained their peak at 10 days exposure period in the trap containing plant stems. Consequently, in weeds all organisms except Odonata attained their peak at 21-day period before leveling off at 40-day period.

DOMINANT AND SUB-DOMINANT GROUPS

Taxonomic groups or genera comprising 15% or more of the total individuals collected are considered as “dominants” while those comprising 5-14% are considered as “subdominants” (Slaek et al., 1977). All other groups below 5% are regarded as the rare groups.

In all the submerged artificial substrates, the dominant invertebrates were the Oligochaeta, Diptera and Amphibia in Weeds substrate, Diptera 28.85% and Oligochaeta 29.15% occurred as the most dominant group.

Table 4:	The number of individuals recorded in each exposure periods in the substrate weeds and plant stems
W-Weeds, PS -Plant Stems

Table 5:	Pattern of colonization of artificial substrate by macro-invertebrate in each exposure period

Table 6:	The variations in Taxa richness and evenness (E), indices of macro-invertebrate in the substrates

Amphibi 9.47% was the subdominant group, while the rare groups were Hemiptera 0.97%, Crustacaens 0.49% and Arachnida with 0.24%, respectively.

DIVERSITY AND FAUNA SIMILARITIES

Table 6 presents the taxa richness (D), general diversity (H) and Evenness (E) for the submerged artificial substrates.

In the plant stems, Oligochaeta 55.34% was the most dominant group, followed by Diptera with 30.34% density. Amphibian 9.47% was the subdominant group, while the rare groups were Hemiptera 0.97%, Crustaceans 0.49% and Arachnida 0.24%.

Weeds substrate had high taxa richness 4.31% than plant stem. with 3.32 index, respectively. Simultaneous increase was also reflected in general diversity (H) and Evenness index (E) in weeds than plant stems.

Similarity indices estimated for the fauna of the artificial substrates, using Sorensen’s quotient (Q/s) and Jaccard’s Co-efficient (Sj), revealed that Q/S of 66.45% and Sj of 1.95 indicated that there was significant similarity in fauna abundance of the artificial substrates.