Bioaccumalation of metals in mullet from Mhlathuze Estuary and the effect of Cu and Pb on the uptake kinetics,haematology and acid-base balance of Liza dumerelii.
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Date
2001
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Abstract
Mhlathuze Estuary came into being when the original Richards Bay was divided into the northern part, which was dredged to form the habour and the southern part, which was reserved for the preservation of natural resources as a sanctuary. This sanctuary was however disturbed during development. The disturbance took the form of canalisation and redirection of Mhlathuze River and cutting of the new mouth after the berm wall was erected. These were drastic changes, which had a profound influence on the sanctuary (estuary). Resultant changes included siltation of the estuary and loss of water residence time.
The construction of the harbour was in line and enhanced the increase of industries in the Richards Bay area. The industries manufacture a number of metal products and also utilise other metal ores in the manufacturing process. The high population growth and concentration of industries in the area have a potential to pollute the sanctuary and the nearby harbour. Research was undertaken to assess the amount of metal pollution in the Mhlathuze Estuary and their possible effects on the biota.
In order for meaningful assessment of ecosystem health to be realised, the cause and also the effects of metals were investigated. Metals are non-biodegradable pollutants. Their persistence can result in them taken up by biota to concentrations far above ambient levels. This is termed bioaccumulation. The accumulation of metals in biota can result in stress, which can in turn induce changes in physiological parameters and other metabolic processes. The need to understand and predict the stress conditions, which metals will pose to fish, and extrapolate the effects of pollutants from laboratory to populations levels, have necessitated the search for physiological and biochemical indicators of health and sublethal toxicant effects. To determine the effects of these pollutants on the structure and functions of metabolic processes, it is important to monitor the bioaccumulation of such metals in the field and to expose the fish to environmental relevant concentrations of the same metal under controlled laboratory conditions.
In this study, this was achieved by conducting field observation and sampling, that involved the collection of water, sediment, fish (Liza diimerelii), benthic invertebrates (Paratylodiplax blephariskios and Apsendes digitalis) and submerged macrophytes (Zostera capensis) and analysing for metal accumulation. Seven metals, i.e. Al, Cr. Cu, Fe, Mn, Pb and Zn were analysed in the abovementioned abiotic and biotic compartments following standard nitric/perchloric acid digestion techniques and using a Varian AA50 atomic absorption spectrophotometer.
The study found elevated metal concentrations in the water and sediment samples from the Mhlathuze Estuary. High metal accumulation in water and tissues were found during summer months, which coincided with high rainfall and floods. The highest metal concentrations in the sediment were found during periods of low flow e.g. in winter and spring. A significant correlation was found between metals in water and fish liver tissue. Further significant correlations were recorded between metals in sediment and benthic invertebrates.
Metals occur in nature as ions and compounds. In order to understand their effects on biota, the effects of metal mixtures have to be investigated. In a metal mixture, the effects of metals can be additive, synergistic or antagonistic. Laboratory investigations involved the determination of effects of copper (Cu), lead (Pb) and their mixture on the haematology and acid-base balance of a mullet species, Liza dumerelii. Changes in physiological parameters measured, must be directly affected by the exposure to a toxicant, and the changes must not be because of other factors such as handling stress. Experiments were, therefore conducted in controlled environmental conditions to maintain constant temperatures and photoperiods.
Haematological and acid-base balance evaluation of fish blood provides valuable information concerning the response of fish to changes in the external media, and could be used to diagnose abnormal functioning of physiological mechanisms in fish. Haematological parameters measured were, red blood cell count (RBC), white blood cell count (WBC), haematocrit (Hct), haemoglobin (Hb), mean cell volume (MCV), mean cellular haemoglobin concentration (MCHC), mean cellular haemoglobin (MCH), blood glucose, and plasma lactate. Acid-base balance parameters measured were bicarbonate (HCO3"), pH, PC02, the plasma ions, sodium (Na), potassium (K), chloride (CI), and osmolality.
In the study of the effects of metals on physiology, the combination of Cu and Pb was found to induce more physiological stress as compared to individual metals. Copper binds to ligands in the gill lammellae of fish where it can disrupt the Na and CI balance. Lead on the other hand inhibits delta aminolevulinic acid dehydratase, an enzyme responsible for haemoglobin synthesis thus decreasing the potential for red blood cells to carry oxygen.
During stress the organism responds by either increasing or decreasing the particular parameter as a compensatory measure. The fish responded to the metal stress by increasing the RBC's, blood glucose concentration and haemoglobin. This was an attempt by the fish to increase the respiration potential, which was decreased by both the disruption of gill lamellae by Cu, and the inhibition of haemoglobin synthesis by Pb. Bicarbonates and PCO2 levels were also increased. The HC03" increase was probably due to increase in lactic acid that acidified the blood or, it was caused by the B -adregenic stimulation of red blood cells, which also slows the process of CO? elirnination from the blood.
The amount of metal accumulation in the Mhlathuze Estuary raises some concerns. While previous studies of metal accumulation in the water, sediment and fish from the Mhlathuze Estuary are comparable to this study, there seems to be a gradual increase in the amounts of metals from both the biotic and abiotic components of the Mhlathuze Estuary. The existence of metals in the water bodies as individual metals or as mixtures, as it was also shown in the study, has an effect on the physiology of the fish communities.
Description
Submitted to the Faculty of Science and Agriculture in fulfilment of the requirements for the Degree of Masters of Science in the Department of Zoology, University of Zululand, 2001.
Keywords
Marine biology, Estuaries, Heavy metals--Environmental aspects, Water--Pollution