The Limnology of Ohana Lake, a Potential Manmade Aquaculture System in Nigeria

Paul O. Ajah

Institute of Oceanography, University of Calabar, Calabar, Nigeria.

DOI: 10.4236/ojapps.2013.32031                

Abstract

The concentrations of heavy metals (Fe > Zn > Cu > Pb > Ag) in bottom sediments and fish gills in Ohana Lake, were found to be significantly high and far exceeded FEPA and WHO environmental standards for water quality by 1.5 to 18 times, respectively. Six classes of each of phytoplankton and zooplankton with a total of 35 phytoplankton taxa comprising 46 species i.e. 35(46) and 22(28) faunal were observed. The class Chlorophyceae dominated the phytoplankton community with 18(22) followed by Cyanobacteria 6(10). The aquatic fauna was dominated by the Rotifera 8(11), followed by the Copepoda 6(9). The benthic flora community consisted of five classes of phytoplankton made up of 28(36). The class Bacillariophyceae 11(15) dominated the group followed by Chlorophyceae 10(11). Benthic fauna were made up of seven classes of 13(13). The dominant class Nemata 4(4) was followed closely by Protozoa 2(3). Ohana Lake is fast turning to a eutrophic ecosystem with accompanied algal bloom due to very high nutrient contents. The equitability or evenness indices (J) for both phytoplankton and zooplankton were lowly indicating generally low species diversities as well as predominantly unstable ecosystem. The aquacultural implications of these parameters are discussed.

Keywords

Limnology; Aquaculture; Productivity; Pollution; Fishery; Benthos

1. Introduction

Residential development of lakeshores is expected to change a variety of key lake features that included increased nutrient loading, increased invasion rate of nonnative species, increased exploitation rates of fishes by anglers, and alteration of littoral habitats. All of these factors may alter the capacity of lakes to support productive native fish populations [1]. Ohana Lake being quite close to residential area witnesses a lot of anthropogenic activities such as bathing; canoeing and car/motorcycle wash, and consequently, is prone to suffer such fate.

Heavy metals are considered as major environmental pollutants and are regarded as being cytotoxic, mutagenic and carcinogenic [2].

Knowledge of the physicochemical regimes of a body of water is invaluable in the determination of the productivity and other characteristics [3,4] opined that fertility of water is related to its chemical properties and an understanding of the water chemistry serves as the basis for determining whether the water is rich or poor in biological production. Physicochemical factors are known to influence vertical and horizontal migration of organismsthe distribution and feeding regime [5]. The water quality of the lakes and rivers is assessed by the physicochemical and biological characteristics of their waters. It is known that several water quality parameters if not within normal range could act as stressors and adversely affect fish growth and reproduction [6]. Hence, regular monitoring of physicochemical factors is essential for assessing the status of lakes with references to fish culture [7,8] noted that density and diversity of flora and fauna were dependent on chemical regime of water. Fish yields from lakes and reservoirs are known to have strong correlation with primary production [9,10]. [11] studied the concentration of heavy metals in water and Hemichromis fasciatus of a waste pit influenced by petroleum activities and found that, seasonally, Cd, Cu, Pb, Mn, Co, Cr, V and Hg had higher mean values in water during the dry season while Zn, Ni and Fe were higher during the raining season. All heavy metals except Fe were higher in mean values during the dry season in H. fasciatus than during the rainy season.

A survey of the benthos makes it possible to establish if pollution has occurred in recent past and, if so, whether toxic or organic in nature. When organic pollution has occurred, the number of species is usually restricted, although the few species present may be present in very high numbers. On the other hand, pollution from toxic substances may eliminate almost all animals present except for a few highly resistant species. A survey of the benthos is, therefore often of more use than an analysis of a water sample, since the water sample represents only one sample taken at one particular point in time and tells you little about the integrated, long-term effects of water quality [12].

The study of Ohana Lake is aimed at assessing its limnology and possible aquacultural potentials. Studies like this make significant contribution to fishery management by identifying potential damage to fish populations, through low oxygen levels, toxic blooms or anthropogenic pollution [13].

Description of the Study Area

Ohana Lake is located between latitudes 05˚57''24''N and 05˚57''31''N and longitudes 008˚22'38''E and 008˚22'46''E. It is situated barely 30 m away from the major road linking Calabar and Ikom town and right at the centre of Ohana community in Obrubra Local Government area of Cross River State, Nigeria (see Figure 1).

Ohana Lake could be characterized as an oligotrophic perennial lake due to its nutrient poor status. It is also a limnetic lake due to its lack of rooted vegetation. The Lake covers an area of 3.0 km². The average depth of the Lake was 4.72 m with maximum depth of 6.5 m and minimum 0.3 m with a water volume of 14.173377 km³. It is of low lying vegetation comprising of shrubs and a few permanent trees surrounding it. Lemna minor dominated aquatic macrophyte.

2. Materials and Methods

Ohana Lake was sampled fortnightly in 2005 in the month of March. Sampling locations were gotten using a GPS instrument and the reading converted from longitudes and latitudes to Universal Traverse Mercado (UTM) coordinate system. Various water depth positions were first measured using a calibrated line with a lead and latter confirmed using an echo sounder. Heavy metals were analyzed using atomic absorption spectroscopy (A.A.S) following Unicam 919 Solar System. The fish species were identified using [14] while the mollusks were identified using [15]. Both phytoplankton and zooplankton were identified using [16-18]. Primary production of the lake was determined using light and dark BOD bottles at various depths and days in accordance to [19]. The benthic communities were first fractionated using screens with the following ranges; 0.105 mm, 0.50 mm, 0.30 mm, 0.50 mm and 0.80 mm stacked together and shaken vigorously and severally under a slow flowing tap water and latter identified under a microscope for the smaller groups and direct observation for the macro benthos using the appropriate taxonomic keys. Dissolved oxygen and temperature were analyzed by meter/probe methods [20], nitrate by cadmium reduction/diazotization method following [21]. Phosphate was analyzed by molybdenum blue method (spectrophotometrically) [21]. Total dissolved solids (TDS) and conductivity by meter/ probe methods (3000 HACH); turbidity was by transparency disc while Total suspended solids (TSS) and colour was measured using spectrophotometer (3000 HACH). Water hardness—Ca²⁺, Mg²⁺—by complexometric (titration) method [20,22].

The Shannon-Wiener’s index (H¹) was adopted in calculating the diversity indices of species (ShannonWiever, 1948) while the Equitability or evenness index (J) was used to compare the H¹ index.

3. Results

Iron (Fe) values from sediment sample averaged 359.8 mg/L. Likewise, the average values 10.01 mg/L, 10.79 mg/L, 5.9 mg/L and 37.42 ml/L, respectively, were realized for lead, copper, silver and zinc in the sediments. Results of analysis of fish gills showed that lead was not detected, the rest metals had the following values: iron— 35.17 mg/L, copper—0.315 mg/L, silver—0.45 mg/L and zinc—3.097 mg/L. The summary of the trace metals of Ohana Lake are represented in Table 1.

Table 2 summarizes the physicochemical parameters and nutrients of Ohana Lake, Nigeria during the sampling periods. The hydrographic data and nutrients in Ohana Lake are shown in Table 3. Temperature ranged from 31.7˚C to 35.0˚C with average of 33.12˚C ± 1.01˚C. Ohana Lake exhibited a mild thermal stratification. Both temperature and dissolved oxygen decreased with depth. Dissolved oxygen levels in Ohana Lake ranged from 3.9 to 4.1 mgO₂/L with a mean of 4.01 ± 0.099 mgO₂/L. Figure 2 shows the inverse relationships between dissolved oxygen and depth in metres in Ohana Lake. The pH of the water ranged from 6.90 to 7.79 with average of  

Conflicts of Interest

The authors declare no conflicts of interest.

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