The Study of Diatoms variability along the Vembanad wetland, Kerala and implications to limnology

Diatoms are tiny, unicellular organisms that can be identified by their frustule, a siliceous cell wall. They can be used to compare streams, lakes, wetlands, oceans, estuaries, and even certain ephemeral aquatic environments because they are present in nearly all aquatic habitats. Their value as potent and trustworthy environmental indicators has long been acknowledged. Their high abundance and species diversity are ascribed to their usefulness. Surface sediments from the coastal and continental regions of southwest Kerala, which include wetlands, river, and estuarine complexes, have been used for the current diatom study. Thirty grab samples were gathered for the investigation from the Vembanad wetland, Alleppey back waters, Asthamudi lake, Peruman, Poovar estuaries and, Periyar, and Manakudi rivers. The diatoms taxa recorded includes both planktic and benthic forms viz. Cyclotella, Thalassiosira, Campylodiscus, Actinoptychus, Aulacoseira, Triceratium favus, Biddulphia, Nitzschia, Cocconeis, Gomphonema, Tabellaria, Navicula, Ulnaria ulna, Caloneis, Luticola, Neidium, Achnanthes, Achnanthidium, Gyrosigma, Encyonema, Surirella, Stauroneis, Sellaphora, Pinnularia, Grammatophora, Eunotia and many others to count. In the study it was observed that the diatom assemblage varied on different scales either for river sediments, wetland or estuarine complexes. It was noted that in the river sediments freshwater benthic diatoms with very high diversity and frequency were recorded and also indicate organic pollution activity in the vicinity of the river channel. In the wetland and back water sediments, the planktic diatoms were recorded in high frequency both for the freshwater and marine forms while the benthic diatoms were in low counts. This was probably due to the higher water levels in the wetland and back waters. The change in freshwater and marine diatoms was reflected where the marine incursion was either limited or freshwater inundation was high. The estuarine samples however recorded varying frequencies of freshwater and marine diatom assemblages owing to their proximity to sea. Thus it can be postulated that the diatoms are one of the major biological entity that can record and witness the variable limnological aspect of environment and climate change. In a broader sense it can be visualized that the proxy can be helpful in fine tune observations for palaeolimnological and palaeoenvironmental reconstruction in geological times.

Keywords: - Diatoms, Kerala, wetland, limnology, palaeoenvironment