An assessment of a sub-tropical seagrass, Zostera muelleri, as a potential bioindicator of trace elements

Semi-enclosed estuarine areas along the east coast of Australia accommodate industrial and shipping activities, but are also often areas of suitable seagrass habitat. Port Curtis is one such semi-enclosed estuary located in Gladstone, Queensland, and accommodates Australia’s fifth largest multi-commodity port. The local consortium, Port Curtis Integrated Monitoring Program Inc. (PCIMP) were looking for a local ecologically relevant trace element (TE) bioindicator to complement current sediment and water quality monitoring. The overarching aim of this research was to ascertain whether the locally predominant seagrass species, Zostera muelleri, could be a potential TE bioindicator. Zostera muelleri already meets some TE bioindicator criteria in that it is present where PCIMP monitors and is abundant enough to sample; however, further investigation of the ecology of Z. muelleri with respect to TE exposure was required to ascertain if the species was suitable. Specifically, the study examined Z. muelleri’s capacity to accumulate, partition and translocate TEs in relation to environmental TE concentrations over the spatial and temporal scale within the field and under manipulated experimental conditions. Spatial assessments were undertaken by assessing whole Z. muelleri TE concentration variability (Al, As, Cr, Cd, Cu, Fe, Pb, Mn, Ni and Zn) between and within five locations across Port Curtis during the peak growing period. It was expected that if Z. muelleri was a good indicator of differences in environmental TE exposure, TE concentrations would vary between locations more than within locations. Additionally, other factors such as plant morphology, sediment characteristics and epiphyte cover could drive location variation. Results indicated that each seagrass TE (except Zn) had significantly different spatial variability, suggesting that different natural or anthropogenic TE sources exist within Port Curtis. Additionally, localised meadow influences created significant withinmeadow effects for seagrass As, Cu, Fe and Ni concentrations. Seven of the ten TEs analysed in Z. muelleri had strong relationships with sediment TEs; however, no comparison to water TEs could be made due to low concentrations in water samples tested. Percent silt and % epiphyte cover explained the greatest variation in seagrass TE concentrations. Zostera muelleri TE concentrations demonstrated different location TE exposures, suggesting that it would be a good bioindicator of TEs. Zostera muelleri TE concentrations (Al, As, Cr, Cd, Cu, Fe, Pb, Mn, Ni and Zn) were observed over the active growing season of the Austral spring to summer. It was expected that seagrass TE concentrations within seagrass compartments would change over time due to either seasonal growth or external environmental TE exposures. Trace element concentrations in Z. muelleri were variable between seagrass compartments (e.g., Cu was greater in the above-ground compartment than in the below-ground compartment) and over time. Variations in seagrass TE concentrations over time were grouped and explained by either biological characteristics such as growth, or by external summer influences and did not appear to be due to environmental TE concentrations. It is evident that TE concentrations in Z. muelleri are influenced by season, limiting when and how often to sample Z. muelleri as a bioindicator. Light and salinity are two environmental variables that are dynamic within estuarine areas. It was suspected that these variables could influence the capacity of Z. muelleri to accumulate TEs and therefore its recommendation as a bioindicator. Salinity and light were manipulated within two individual laboratory experiments with exposure to one element, Cu, due to known effects on Z. muelleri. Copper exposures were control, low (5 µg L-1 ) and high (50 µg L-1 ) and the manipulated experiments were 1) variable salinities (normal 54 mS cm-1 and reduced 44 mS cm-1 ) and 2) low light (15.3 µmol photons m-2 s -1 ). Results of the experiments demonstrated that initial (24 h) leaf Cu accumulation was in proportion to exposure concentrations, irrespective of manipulated environmental conditions. This suggests that Z. muelleri leaves could act as a Cu bioindicator at times of reduced light and salinity (e.g., during a flood or along an estuarine gradient). During the low light experiment, the Cu concentrations in the belowground compartment of the seagrass significantly increased over time, suggesting active Cu accumulation to supply Z. muelleri with new Cu for metabolic requirements. Active Cu accumulation could influence the use of Z. muelleri as a Cu bioindicator in that Z. muelleri would not be displaying steady state Cu concentrations. The study provided new knowledge of Z. muelleri in relation to its use, partitioning and accumulation of a selection of analysed TEs, which was used to assess whether Z. muelleri can be proposed as a bioindicator. The results demonstrated that Z. muelleri can be a strong temporal and spatial accumulator of certain TEs from the environment. However, the interaction of age, growth, compartment tested, and specific TE uptake mechanisms influenced overall TE concentrations, and should be measured and used to interpret bioindicator results. Environmental variables such as light and salinity did not influence TE accumulation by Z. muelleri in an experimental environment. The results of the field study, however, showed that some environmental variables that vary between locations, such as silt and epiphytes, can contribute to TE concentrations in seagrass samples.