Sasthamcotta Lake managed to remain a freshwater body for long owing to the virtual absence of nutrients in it.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/sasthamcotta-lake-faces-yet-another-threat
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-thiruvananthapuram
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/kerala
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lakes
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lake-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/sasthamkota