Acute and chronic toxicity of copper and zinc and environmental tolerances to the estuarine amphipod, melita zeylanica
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Date
2015
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University of Zululand
Abstract
Owing to economic growth and their geographic positioning, harbours and estuarine ecosystems have been considerably accelerated development around them over the past years. This has in turn induced excessive stress on these systems. Due to elevated metal deposition from surrounding industries, toxic metal concentrations are often evident in these systems as sediments tend to accumulate metals. Toxicity tests are essential in predicting the effects of contaminated water and sediments on biota. Acute bioassays refer to short term bioassay with mortality as an end-point, while chronic bioassays often take longer with sub-lethal end-points including growth and fecundity. In the past, acute toxicity tests were often reported as they were easier to conduct and generally more affordable than chronic tests. Chronic bioassays, however, are more relevant as chronic exposures are most likely to occur in the environment and they have higher sensitivity to metals sediment toxicity tests are often preferred over water-only tests, as metals tend to bind to sediment over prolonged periods of time , therefore, they provide a time-integrated measure of metal accumulation within estuarine ecosystems.
Due to their high metal toxicity, environmental tolerance, abundance, as well as ease to handle in the laboratory, amphipod bioassays are used worldwide in toxicity testing. In this study acute and chronic toxicity bioassays were undertaken with the aim to determine the suitability of Melita zeylanica as an estuarine benchmark toxicity test organism in sub-tropical ecosystems. Melita zeylanica amphipods were collected in Mzingazi Canal, Richards Bay Harbour, and cultured in climate control chambers in aerated culture trays. Exposure experiments of M. zeylanica to Copper (Cu) and Zinc (Zn) were conducted at 25⁰C and a salinity of 25 using a 6 by 5 grid. The LC₅₀ EC₅₀ as well as metal concentrations in tissue and sediment were determined following a microwave digestion method, while an ICP-MS was used for metal analysis of sediment and tissue concentrations.
An overview on amphipod toxicity bioassays using Cu and Zn over the past 10 years (2003-2013) was conducted with the aim to report on chronic toxicity publications during the past decade. General sensitivity of amphipods to Cu and Zn as well as the frequency of use of various endpoints (fecundity, accumulation, growth and behaviour) were reported. Results of the overview suggested that amphipods are generally more sensitive to Cu than Zn. Corophium volutator, however, showed a lower LC₅₀ value for Zn (10.03 µg/g-1) than that of Cu (26.07 µg/g-1). Fecundity was the most reported endpoint with C. volutator being the most used amphipod over the past ten years. It was concluded, however that no one amphipod species can represent all amphipod species as amphipods tend to adapt deferentially to their immediate environment.
A Cadmium (Cd) reference toxicity test is often used as a method of standardising interlaboratory results as it is non-essential in organisms. A Cd 96hr static water-only reference test was used in this study to determine relevant toxicity of M. zeylanica as compared to other amphipods. This study also evaluated the median lethal concentrations (LC₅₀) of ammonia on M. zeylanica during a static 96hr sediment toxicity bioassay. Survival of M. zeylanica across a broad range of salinity (5-40) was also determined during 96hr bioassay. The Cd water-only tolerance test showed that M. zeylanica sensitivity to metals is comparable to that of other amphipods, with LC₅₀ of 1.17 µg/g-1 a Melita zeylanica was found to be more sensitive to ammonia as compared to other studies with a LC₅₀ of 17 µg/g-1 a Melita zeylanica however, was found to be tolerant to a wide range of salinities (5-40) but intolerant to freshwater.
A 10-day acute sediment toxicity test with mortality as an endpoint and a 28 day chronic sediment toxicity test, with growth and fecundity as end-points, were conducted. Following the acute toxicity test, LC₅₀ values for Zn and Cu were found to be 9.15 and 11.76 µg/g-1 with LC₉₀ values of 238.5 and 78.6 showing that M. zeylanica is more sensitive to Cu at high concentrations. Tissue metal levels showed that M. zeylanica is able to accumulate both Zn and Cu in relation to sediment concentrations. This finding demonstrated that M. zeylanica is potentially a good biomonitor organism, as the ability to accumulate metals is one of the key requirements that qualify organisms as toxicity organisms the chronic bioassay showed that both Zn and Cu affected fecundity of M. zeylanica. Amphipods were, however, found to be more sensitive to Cu compared to Zn. EC₅₀ values of 2.7 and 0.8 µg/g-1 were recorded for Zn and Cu, respectively. Amphipod growth was also inversely correlated to Cu concentration.
South Africa has experienced accelerated growth in its economy over the past decade which has resulted in increased demand for cargo container handling capacity throughout South African ports. Richards Bay Harbour is currently the largest deep-water port in South Africa, used primarily for the export of coal. Furthermore, there have been proposals to expand the port over the next 40 years, resulting in 5 times increase in its surface area. In order to test the suitability of M. zeylanica as a bio-indicator of estuarine contaminated sediments in South Africa, a field validation study was conducted in Richards Bay Harbour with sediment samples being collected from 9 sites. Samples were analysed for aluminium (AI) Arsenic (As), Cd, Cu, Iron (Fe) Mercury (Hg) Nickel (Ni), (Pb) and (Zn) concentrations. Metal concentrations recorded at the Bulk terminal. Metal concentrations were found to be within standard quality guidelines as stipulated by the Department of Water Affairs as well as the Australian sediment guidelines. The data from this study was compared with historical data to identify contaminated areas. Elevated Zn concentrations within the harbour were contrary to historical data, as a result Polycyclic chlorine biphenyls (PCB) and Polycyclic Aromatic Hydrocarbons (PAH) analysis was conducted to identify or eliminate oil-spill as a potential reason for the high Zn concentration.
Metal accumulation in M. zeylanica following exposure to sediments showed that M. zeylanica is a good accumulator of metal, particularly for As, Cr, and Hg. When compared to other amphipods widely used in toxicity bioassays, M. zeylanica was found to accumulate metals to comparable concentrations. A range of 5-59 µg/g-1 and 98-227 µg/g-1 were recorded for Cu and Zn, higher than that of Melita matilda (140 µg/g-1 Zn), a closely associated species.
Richards Bay Harbour is of high ecological and economic importance as it serves as both a fully functional estuarine ecosystem and is also one of the busiest ports in South Africa. Richards Bay Harbour as an estuarine environment has already been put under immense pressure with increased cargo handling over the past ten years. Plans to expand the harbour will also result in re-suspension of metal causing secondary metal contamination, which may contribute to loss of organisms and thus hinder the role of the harbour as an ecosystem. Although several toxicity and biomonitoring studies have been conducted on the harbour, no Estuarine Management Plan (EMP) has been put in place to Richards Bay Harbour as yet. EMP should, therefore, be urgently designed and implemented, with all stakeholders accommodated.
National Sediment Quality Guidelines (SQG’s) should also be implemented as different areas do not necessarily react in the same manner, therefore, the use of international SQG’s would not be entirely appropriate. In terms of Estuarine Ecological Biodiversity, Richards Bay Harbour is Classified as a category C estuary, according to the National Water Act; no estuarine system should be allowed to degrade below D-class. The intended development of Richards Bay Harbour will, potentially, result in deterioration of the ecological integrity of the harbour. An EMP is thus of high importance within the harbour given the divergent roles of Richards Bay Harbour as both an estuarine ecosystem and a large cargo handling port.
Although, M. zeylanica was found to be sensitive to ammonia, it was overall, found to be a good accumulator of metal, relatively easy to handle in the laboratory tolerant to a wide salinity range and it was found in abundance in the Mzingazi Canal. Furthermore, accumulation of metals within amphipods did reflect sediment metal concentrations, particularly for Zn and Cu, following the 10-day toxicity bioassays. The findings of the study, thus suggest that M. zeylanica can and can be recommended as a suitable bio-monitor species in South African and can be recommended as a suitable bio-monitor species ins South African coastal waters.
Description
A thesis submitted to the Faculty of Science, Agriculture and Engineering in fulfilment of the requirements for the Degree of Master of Science in the Department of Zoology at the University of Zululand, South Africa [2015].