Abstract: An ecological study of the effect of various quantities of crude oil spills on the soil microarthropod fauna was conducted. Three stations P.Q and R were polluted with 0.5, 1.5 and 3.0 L, respectively. The control station C, was not polluted. A total of 553 microarthropods were collected for a period of 6 months. The microarthropod populations collected were classified into 3: Insecta, Acari and Myriapoda which were further divided into 13 families, Soil pH, temperature, moisture content and Total Hydrocarbon Content (THC) were measured. Abundance of microarthropods correlated positively with increasing moisture content and pH and negatively with increasing THC and temperature in the upper 10 cm of soil in stations P, Q and R. Of the total microarthropod, the Acarina and hymenopterans were the most abundant groups. The least abundant were the Isopterans and Myriapods.
INTRODUCTION
The soil is made up of an amazing diversity and complexity of life which include protozoa, nematodes, enchytraids and various arthropods which include Acari, Isopods, Diplopods and insects (Curry, 1994). This arthropods can be divided into micro, meso and macro groups, depending on the size of individuals involved. The microarthropods is grassland soils generally consist of Acari, Coleoptera, Collembola, Diplura, Dipteran larvae, Homoptera, Protura and symphyla. It is generally agreed that the Acarina and collembolan are by far the most important groups of microarthropods in the soil (Bardgett et al., 1993; Curry, 1994). Seastedt (1984) stated that Acari and Collmbola usually account for 95% of the soil arthropod fauna. Collembolans.
Collembolans are omnipresent members of soil fauna, they inhabit both the surface and depth of the soil and usually occur in aggregation. The commonly seen snow fleas are aggregations of millions of Collembolans from the family Hypogastruridae (Hopkin, 2000).
In Nigeria quite often there are accidental oil pipelines breakages and vandalization resulting in oil spillage which in un-dates vast areas of farming land leading to poor productivity in such soils.
This study aims at studying the distribution of soil micro arthropods across specific depths and their response to various dosages of crude oil spills.
MATERIALS AND METHODS
There were four sampling stations marked P.Q.R and C. The stations are interspersed by distances of 2 m and measure 1x1 m in area. They are all of the same soil type (sandy-loam), well drained, uncultivated and having plants such as guinea grass and Sida acuta as coverage. They are described as follow: Station P: This station was polluted with 0.5 L of crude oil (pure blend). Station Q: This station polluted with 1.5 L of crude oil (Pure blend). Station R: This station was polluted with 3.0 L of crude oil (pure blend). Station C: This station was unpolluted and served as control.
The split core sampler was used for collection of samples. The cylinder was driven to a depth of 10 cm and the soil sample trapped within was removed. The cylinder was placed again into the hole and then driven to another depth of 10 cm (collecting soil sample between 10 and 20 cm below ground surface) and the soil within removed. The volume of soil in the cylinder is 282.9 cm3. After each soil sample was taken, they were transferred to the black cellophane bags and labeled accordingly.
The Berlese-Tulgren funnel extractor which is the best for extracting soil microarthropods (with efficiency of about 90%) as suggested by Hopkin (2000) was used. The samples were placed in the funnel for about 48 h and the microarthropods collected in containers with 70% alcohol. The sampling exercise for this study was carried out twice a month between the hours of 10 and 11 am in the morning.
After the micro-arthropods were extracted and collected, sorting was done with the aid of a binocular dissecting microscope. Individual species were removed from the debris and other species by suction using a sucking pipette. They were place in glass specimen bottles containing 70% alcohol. These specimen bottles were also labelled accordingly. Slides of the small arthropods were prepares and the insects were identified by use of keys.
Soil pH, temperature, total hydrocarbon and moisture contents were determined.
RESULTS
The collection of soil microarthropod lasted for a period of six months (January to June 2003). Thirteen families, were recorded from 3 classes of Phylum Arthropoda. Of the 13 families, 16 species were represented. Every family had a species representation except for families Formicidae and Mesostigmatidae that had 2 representative species each (Table 1 and 2).
The results of the collection showed a relationship between occurrence and abundance of microarthropods with the variation in physical and chemical factors of weather and crude oil spills. These factors include soil temperature, pH, moisture content and Total Hydrocarbon Content (THC).
| Table 1: | Monthly distribution of soil microarthropods families in the upper 10 cm of stations P, Q, R and C |
| Table 1: | Continued |
| Table 2: | Monthly distribution of soil microarthropods families in the upper 10 cm of stations P, Q, R and C |
Soil pH was almost uniform in all stations in the upper and lower 10 cm of soil. There was a progressive increase in soil pH from January to June with a range of 6.05 to 6.68 in the upper 10 cm and 6.17 to 6.82 in the lower 10 cm. Soil pH correlated positively with all microarthropod groups in upper and lower 10 cm of soil.
Soil temperature did not vary much across the stations. However, there was a general decrease in temperature values from February to June in both levels of soil (Table 1 and 2). Temperature values were between 34.0 and 25.0°C in the upper 10 cm and 32 and 24°C in the lower 10 cm. Temperature also correlated negatively with all microarthropod groups in both levels of soil in all the stations.
Soil moisture content was low in January and February, but increased from March to June in both levels of soil. Soil moisture content values recorded, were between 2.71 and 23.0% in the upper 10 cm and between 3.20 and 22.0% in the lower 10 cm.
Soil moisture correlation positively with all microarthropods of all stations in the upper 10 cm. It also correlated positively with Hymenoptera Acarina Myriapoda in Station C which is the control and negatively with Myriapoda in the lower 10 cm of stations Q and R.
Total Hydrocarbon Content (THC) showed a different pattern compared with the other parameters. Station C (control) recorded an increase in THC from January to June in the upper 10 cm. The crude oil spilled stations P(0.51), Q(1.51), R(3.01) recorded higher THC than the control with R having the highest values and P the least. However these values decreased progressively from January to June in the upper 10 cm of stations P, Q and R. In the lower 10 cm, THC values increased in stations P, Q and R from January to March and a decrease began in April and continued till June. Range of THC values was between 1012.1 and 39.7 ppm in the upper 10 cm. Generally THC correlated positively with microarthropod groups in the upper 10 cm and between 88.40 and 38.4 ppm in the lower 10 cm of station C and negatively with Collembola, Isoptera, Coleoptera, Hymenoptera, Acarina and Myriapoda in the crude oil spilled stations. However, a positive correlation was established between THC and all microarthropod groups in the lower 10 cm. Test for significant difference was carried out using analysis of variance and only THC was found to be significantly different across the upper 10 cm of the stations. In terms of microarthropod abundance, a total of 553 microarthropods were collected for the six month period. From January to March, a relatively low abundance (8.50%) of microarthropods was recorded. April to June recorded a much higher abundance of 91.50%. The upper 10 cm of all stations recorded higher microarthropod abundance than the lower 10 cm (Fig. 1 and 2). In the upper 10 cm of all stations, all the microarthropod groups were presented (Fig. 1). Station C recorded the highest abundance of Collembola, Coleoptera, Acarina and Myriapoda. Hymenoptera was most abundant in station R and Isoptera in station Q. Termites were the most abundant microarthropod (Fig. 1 and 2) in all stations and was closely followed by Hymenoptera. The least abundant group was the Isoptera. In the lower 10 cm, orders Collembola and Isoptera made no representations in all stations through out the sampling period.
| Fig. 1: | Abundance of the microarthropod groups in the upper 10 cm of stations P, Q, R and C |
| Fig. 2: | Abundance of the microarthropod groups in the lower 10 cm of stations P, Q, R and C |
| Fig. 3: | Monthly variation in abundance of Collembola and Isoptera in the upper and lower 10 cm of soil |
The Coleoptera was absent in stations C and R. The Acarina, hymenoptera and Myriapods were present in all stations with station C recording the highest abundance in station R and least in station C (Fig. 2).
Figure 3 and 4 show the monthly variation of microarthropod groups in the upper and lower 10 cm of soil. There was a general increase in the upper 10 cm and a decrease in the lower 10 cm in terms of abundance of micrarthropods through the sampling period. The only exception to this was observed in orders Collembola and Isoptera which were absent in the lower 10 cm, through there was a progressive increase in their abundance in the upper 10 cm (Fig. 3). The Collembola was observed for the first time in April (represented by families Isotomidae and Entomobryidae) in stations C, P and Q. Station C recorded the highest percentage abundance of Collembola (50%) in April and Q the least abundance (16.67%). It was absent in Station R at this time. In June, it was present in all stations with station C recording the highest abundance in Collembola again with 33.3% and station R, the least (13.33%). The Isoptera on the other hand was first seen in March (represented by Rhinotermitidae) in Station C only.
| Fig. 4: | Monthly variation in the abundance of Coleoptera, Hymenoptera, Acarina and Myriapoda in the upper and lower 10 cm of soil |
In April and May it was observed in stations P and Q only, being relatively more abundant in station Q. The total abundance of Rhinotermidae dropped slightly in May but soon increased in June.
In the other microarthropod groups, the time of increase in abundance in the upper 10 cm roughly coincided with the time of decrease in their abundance in the lower 10 cm in which case they occurred about the same time between March and April.
The Coleoptera represented by Curculinoidae, Tenebrionidae, Chrysomellidae and Carabidae made its first appearance in March only in station C and P. After this, there was a continuous increase in all stations till June in the upper 10 cm of soil. A decrease in abundance of Coleoptera in the lower 10 cm was noticed in April and continued till June.
Order Hymenoptera represented by formicidae was observed from January to March in the upper 10 cm stations had higher number of hymenoptera than in station C. At this time, it was absent in stations P, Q and R but present in the lower 10 cm of these stations. In April when a sharp increase was observed (in the lower 10 cm), the crude oil spilled station C. The abundance of hymenopterans in the lower 10 cm was relatively low except in stations C and R in June.
The mites (Acarina) represented by family Oribatidae and Mesostigmatiidae and an unidentified family were first observed in January with only family Oribatidae making a representation in the upper 10 cm of stations C and Q (Table 1). In February and March, it was present in the upper 10 cm of station C and the lower 10 cm of stations P and Q. A sharp increase in abundance was noticed in the upper 10 cm. Of all stations in April coinciding with a slight decrease in the lower 10 cm. This trend continued till June (Fig. 3).
The Myriapods represented by families symphyllidae and Polydesmidae were absent in the upper 10 cm of all stations from January to March and present only in the lower 10 cm of station Q during this period. In April, they made their first appearance in all stations being relatively more abundant in station C than in any other station. At about the same time a continuous decrease was observed in the lower 10 cm till May and then a slight increase in June.
DISCUSSION
A square foot of soil may be inhabited by hundreds of species of soil microarthropods, many barely visible to the naked eye. Their intricate relationship with soil ecosystem and role played by individual species remains fundamental in ecosystem balance. Their abundance is determined by four major factors and these are the availability of resource materials such as organic matter, the macro and micro climatic disposition and degree of disturbance that is directed at them (Peterson and Luxton, 1982).
Soil pH has acidic values in both depths of soil and was noticed to correlate positively with all microarthropods groups in the upper and lower 10 cm of station P, Q and R. This could be as a result of the dilution potential of water with increasing moisture content, activities of microorganisms and microarthropods (Kandeler and Elder, 1993).
Soil temperature showed negative correlation with all microarthropod groups in upper and lower 10 cm of all stations and decreased from January to June. This was as a result of seasonal change from dry to rainy season. Rainy season is characterized by reduced temperatures.
The positive correlation shown between moisture and the microarthropod groups was also as a result of seasonal change leading to increased moisture content caused by increased rainfall. The parameters discussed above did not show any significant difference across the stations and so may not have affected the abundance and distribution of the microarthropod groups. However, THC showed significant difference caused by the crude oil and may have been response for the variation in abundance of the microarthropods.
The negative correlation between THC and the microarthropod groups in the crude oil spilled areas (Stations P, Q ad R) invariably meant that as THC values decreased, the number of microarthropods increased. As was observed from January to June. From January to March, the microarthropod were absent in the upper 10 cm of station P, Q and R probably because of the effect of the effect of the crude oil due to oxygen depletion because of increase in demand of oxygen by hydrocarbon degrading microbes of metabolic activities. Duncan et al. (2002) have shown that petroleum products are toxic to various species of microarthropods. Station R recorded no microarthropod (from January-March) in the lower 10 cm probably because the concentration of crude oil was highest and some of it may have percolated downwards making this level intolerable to the organisms. From April to June, the significant increase in microarthropods in the upper 10 cm and decrease in the lower 10 cm was probably as a result of increased nutrients cause by bioremediation activities of hydrocarbon degrading microbes causing them to migrate upwards. There seems to be a mutual relationship between the microarthropods and the microbial community, such that an increase in one gives a corresponding increase in the other. This was probably the reason why station R had the highest abundance of microarthropod in June.
The positive correlation in the lower 10 cm between THC and microarthropods meant that an increase in microarthropods led to an increase in THC values in stations P, Q, R and C. This was probably due to the addition of hydrocarbon to the soil, through the activities of microarthropods such as such as excretion, death and decomposition by microbes. Seatedt (1984). This resulted in a slight progressive increase in THC values in the lower 10 cm of stations P, Q, R from January to March and a decrease in the lower 10 cm of the same stations from April to June because of the upward migration of the microarthropod. The same activities by microarthropods such as the collembola were probably responsible for the positive correlation between soil THC and microarthropod abundance in the lower 10 cm of station C (Filser, 2002).
The general low abundance of microarthropod from January to March can be attributed to lack of food (microbes), high temperatures and reduced moisture. According to Swift et al. (1979) and Curry (1994) microbes (fungi and bacteria) are a food source for various arthropods including Acari, Isopoda, Diplodpoda and a number of Insecta. With increasing moisture as it began to rain (April to June) and temperatures becoming optimal (15-30°C), a good food base of inorganic nutrients, microbial growth is sure to increase (Zuberer, 2000) which probably led to the increased microarthropod abundance during the period.
The upper 10 cm of all stations recorded higher microarthropod abundance than the lower 10 cm and could be as a result of favourable conditions in the upper 10 cm.
The most abundant group of microarthropods was the Acarina (mites) especially the Oribatid mites. They were present in the dry as well as the wet periods. They were able to survive in the dry period probably because of their structural features. According to Markwiese et al. (2001) adult Oribatid mites usually have strong exoskeleton and are consistently active in extreme and environment. Another basis for their abundance in soil ecosystem could be due to their Omnivorous nature (MacFayden, 1963). They have a diversity of feeding habits, hence it may be difficult for them to run out of food (Hartenstein, 1962).
Next in abundance was the Hymenoptera. This high record of abundance is similar to records of Badejo (1982) who highlighted the high level of social organization in the group, a basis for their aggregated number. Hence, a numerical strength record of this group is not a true expression of their dominance. The collembolans were relatively absent from January to March but their numbers increased from April to May. This is probably because they rely upon moist habits for terrestrial existence and are particularly adapted to these conditions (Cloudsley-Thompson, 1988). They were low in abundance in the upper 10 cm of soil from January to March, suggesting that they succumbed to the adverse conditions. The general low abundance of collembolans could also be as a result of their high degree of selectivity in feeding (Shaw, 1988).
The myriapods were also relatively absent from January to March in all stations (i.e., P, Q, R and C). These myriapods may have migrated deep into the soil during unfavourable conditions (like low moisture content, high temperature, crude oil spill in stations P, Q and R). Cloudsely-Thompson (1968) reports that the first instar of symphyla never migrate to the soil surface and older symphyla which do visit the soil surface retreat rapidly into the soil if disturbed. The rapid recovery in the rainy period suggests that suitable conditions prevailed to maintain the population.
From this study, oil spillage in the environment affected the microarthropods that inhabit the soil. They are usually found in the upper 10 cm of soil, but when the habitat is disturbed (with crude oil) they either go into dormant phases or migrate downwards to avoid the effect of the unfavourable conditions above. The collembolan may have also played a role in the carbon and Nitrogen cycle as was observed by Filser (2002) and can be used as agents of biomediation.