Shells of aquatic freshwater gastropods as archives for heavy metal contamination history of a river-coast transition zone – Proof-of-concept study in South India (Kerala and Tamil Nadu)
Mineralogy, Petrology and Geochemistry
Physical Geography
Final Report Abstract
This proof‐of‐concept study aimed at testing if rivers and coastal areas in South India are suitable as study areas for a future investigation of gastropod shells as archive organisms recording anthropogenic metal emissions in the history of rapidly growing urban agglomerations. Project work, which was carried out between 1st of January 2020 and 31st of March 2021, started with a field study in collaboration with partners from the Indian Institute of Technology, Chennai and the Annamalai University, Chidambaram, in February and March 2020. Instead of investigating three rivers in densely populated areas the two rivers Adyar and Cooum in the megacity of Chennai were studied as well the Vellar river as an example of an area with lower anthropogenic impact. This and an additional focus on numerous lakes and ponds that are characteristic of urban settling was shown by preliminary project work. In total 349 mollusc shells and shell fragments were obtained (among these 285 specimens of gastropods) from rural or urban sediments of Adyar, Cooum and Vellar rivers, coastal backwaters and in particular from stagnant water bodies (lakes, ponds, artificial reservoirs). Field observations were combined with a literature review on the historic development of Chennai since colonial times. In particular, historic maps allow to identify periods of existence of individual lakes in the history of the metropolitan area. The sediments accumulated in these different systems and gastropod shells contained in them could also archive anthropogenic emissions during the time of accelerated urban growth. Gastropod shells represent nine different families, which show markedly contrasting abundance. Shells of the species Pirenella cingulata, belonging to the family Paludomidae are dominant, followed by shells of the genus Bithynia (family Bithyniidae). Two major and non‐ overlapping groups of gastropods were observed, which either occur in rural and urban river sediments or in sediments of the estuarine river mouth or coastal waters. This invokes salinity as a major factor controlling gastropod distribution. Most shells are affected by mechanical, chemical or biogenic destructive processes after death of the shell forming organism. These post‐mortem effects were in detail investigated, individually ranked and averaged in a composite grade for each specimen. Microstructural features of gastropod shells were examined by optical microscopy, electron microscopy and Raman micro‐spectroscopy as an approach to an understanding of bio‐mineralization and shell growth. This will be crucial for a future study of metal intake to the shell during lifetime of a gastropod. Space resolved trace element analyses by laser ablation inductively coupled mass spectrometry. Complex element distributions occurring in accordance with microstructures show that intake into the shell is an element specific interplay of non‐environmental vital or physiological effects as well as of environmental effects. Results also show that post‐mortem alteration of shell material also leads to uptake of elements, which will carefully have to be evaluated prior to use of a shell as an environmental archive representative of living conditions of the organism. In this proof‐of‐concept study the initial research questions on the identification of appropriate aquatic systems and sites with long history of urban emissions for a future research were positively answered. Sampling and overview of shell inventory as well as microstructural and chemical investigation of gastropods from different riverine and coastal habitats was successful.
