Thesis
Heavy metals in the tissues and skeletons of scleractinian corals
Southern Cross University, Centre for Coastal Management
Doctor of Philosophy (PhD), Southern Cross University
1997
Metrics
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Abstract
Scleractinian corals are potentially useful indicators of environmental heavy metal loads, because their skeletons provide baseline records that date back hundreds of years. Nutrient availability over time can be traced by measuring concentrations of metals recycled by the biosphere (cadmium, zinc), in growth-bands of coral skeletons. The availability of several heavy metals associated with anthropogenic activities, that are toxic to some marine organisms (lead, copper), can similarly be measured over time. However, the use of corals as indicator organisms is not universally accepted, because processes of metal transfer through living coral tissues into skeleton are not understood, nor is it known how efficiently coral skeletons record environmental metal loads. Furthermore, there is no consensus on procedural consistency for measuring heavy metal concentrations in coral skeletons or tissues, and hence there are problems with the comparison of results between different studies.
To resolve problems with procedural inconsistencies, several published procedures for assessing concentrations of cadmium, lead, copper, zinc, nickel and chromium in tissues of scleractinian corals were reviewed, refined where possible, and compared to indicate relative strengths and weaknesses. Tissues that had skeletons removed by decalcification lost significant quantities of cadmium, copper and zinc by acid-leaching, compared to tissues extracted using a water-pik. Water-pik extraction had been refined by buffering the washings, to prevent dissociation and subsequent leaching of labile metals from separated tissues during extraction and collection. All six metals were highly concentrated in water-pik extracted tissues, compared to tissues extracted chemically from coral skeleton using hydrogen peroxide buffered at pH = 8.2. Although not conclusively proven, these procedural differences may result from consistent, positive errors in biomass estimates of chemically extracted tissues. The chemical extraction procedure had been refined to recover most metals lost from tissues to skeleton surfaces during tissue-digestion, by adsorption and precipitation processes, and to improve the precision of tissue-biomass estimates.
Both water-pik and chemical extraction of coral tissues are viable procedures. Water-pik extraction gives excellent measurements of tissue-biomass, and of metal concentrations in bulk tissue, but is costly and cannot remove filamentous tissues from within the skeletal mass. Chemical extraction gives precise, though less accurate, estimates of tissue-biomass, is cheaper, removes all tissues, and results in less varied metal concentrations between samples when representing coral populations. However, non-coralline organic and sulphidic components will be included as coral tissues, if present. Non-skeletal components such as organic or inorganic precipitates, endoliths, and borers, were removed from skeletons to isolate biogenic coral skeleton for analysis, as done in several modern studies, using a sequence of oxidising and reducing/chelating solvents.
Analyses of coral using these procedures indicated that heavy metals partition strongly into tissues, relative to skeletons, of Pocillopora damicornis, Acropora formosa, A. nobilis, Goniastrea aspera, and Montastrea annuligera. In near-pristine locations, individual metals rank from most, to least, partitioned into P. damicornis tissues as: Cd > Pb > Zn > Cu > Ni (Cr varies considerably). it is inferred that cadmium and lead may be fixed to sites in bulk tissue phases that reduce the toxicity of these metals to the polyp (i.e. adhering particles, mucus, or zooxanthellae), because these metals are generally non-essential and toxic to living tissue, but are concentrated in tissues relative to skeletons. Alternatively, accumulation of zinc and copper which are essential metals, may have reached threshold concentrations tolerated by coral tissues. Consequent partial elimination of zinc and copper from coral tissues, would lower concentrations in tissues relative to skeletons. Importantly, all six heavy metals are transferred from tissues to skeletons to some extent, regardless of tissue-bound metal concentrations.
Intra-colony variation of heavy metal concentrations in coral tissues and skeletons of P. damicornis is irregular, and can be large. Colonies are best represented by between two and six 1 gram skeleton samples, that can be homogenised. Species with large surface areas (e.g. dendroid forms) are the best type for representing metal concentrations in coral tissues: generally between 1 and 142 grams of coral are needed for chemical extraction of tissues, and between 1 and 40 grams of coral are needed for water-pik extraction of tissues. Because these large size requirements limit the number of samples that can be taken from colonies, only 1 or 2 samples for tissue analyses are recommended per colony. Heavy metal concentrations in coral populations can range from 2 to 4 times the population averages, and populations are best represented by between 13 and 20 samples. Using 15 replicate colonies, differences greater than 32 - 50% of mean skeleton-bound metal concentrations accumulated by most coral populations, are distinguished from metal concentrations accumulated by other populations. Differences greater than 26 - 42% of mean tissue-bound metal concentrations accumulated by most coral populations, are also distinguished by using 15 replicates. Hence, metal loads in coral populations can be represented by using appropriate sampling procedures, despite substantial variation within and between coral colonies.
Lead, copper, zinc, and probably also cadmium and chromium, accumulate in coral skeletons in response to increased biological availability. Nickel accumulation in skeletons appears to be modified by unknown environmental factors other than total nickel concentrations, indicating that coral skeletons are unsuitable indicators of environmental nickel loads. Inter-specific differences in the concentrations of skeleton-bound heavy metals are evident: Acropora nobilis concentrates more lead than G. aspera, which concentrates more copper than P. damicornis, which concentrates more copper and zinc, and less cadmium and chromium, than A. formosa.
Coral tissues can be used to measure short term copper and zinc accumulation. Lead is accumulated by coral, as indicated by the skeletons, but is eliminated from tissues in response to high availability. Zinc is partly eliminated from tissues, but concentrations in tissues still reflect increased availability well. Copper is not eliminated from tissues, and reflects increased availability very well. Accumulation of zinc and cadmium should be considered together, because there appears to be competitive interaction between these metals in tissues of several species from several sites. Tissue-bound cadmium concentrations are reduced in response to high zinc availability, although skeletons indicate increased uptake of both metals. Inter-specific differences in tissue-bound zinc concentrations were not observed, but P. damicornis tissues concentrate copper more efficiently than G. aspera, or A. nobilis.
Goniastrea aspera and P. damicornis are suitable sentinel organisms for monitoring metal loads in Australian waters, because they are common, readily identifiable, and reflect environmental metal loads. Coral skeletons record a larger range of metals than coral tissues over longer time frames, fewer samples are required to represent a population, smaller sample sizes are required, and metal concentrations in coral colonies are better represented. However, coral tissues can be used to represent short term accumulation of a limited range of metals.
Baseline metal concentrations in seawater, sediments, coral tissues and skeletons are presented for Darwin Harbour, the Townsville region, Heron Island, and the Solitary islands Marine Reserve, with recommended sampling and analytical procedures. The Solitary Islands is a temperate-water community that has low heavy metal concentrations. Heron Island is a subtropical outer Barrier Reef site, also with low heavy metal concentrations. Darwin Harbour is a tropical, tide-dominated estuarine harbour, with relatively minor anthropogenic activity. Reactive, sediment bound metal concentrations are higher than the Barrier Reef or Solitary Island sites, reflecting the muddy nature of the substrate. Townsvil!e Harbour is also a tropical estuary environment. High metal concentrations in water, sediment, and biota reflect shipping of heavy metal ores. Coral skeletons from reefs near Townsville Harbour also reflect intermittent heavy metal fluxes, resulting from periodic harbour activities such as dredging, which resuspends metal laden sediments.
Details
- Title
- Heavy metals in the tissues and skeletons of scleractinian corals
- Creators
- Graeme Esslemont
- Contributors
- David McConchie (Supervisor) - Southern Cross UniversityVicki Harriott (Supervisor) - Southern Cross University
- Awarding Institution
- Southern Cross University; Doctor of Philosophy (PhD)
- Theses
- Doctor of Philosophy (PhD), Southern Cross University
- Publisher
- Southern Cross University, Centre for Coastal Management
- Number of pages
- x, 257
- Identifiers
- 991012958500202368
- Copyright
- © Graeme Esslemont 1997
- Academic Unit
- School of Environment, Science and Engineering
- Resource Type
- Thesis