Downloaded By: [Cerrano,] At: 08:53 7 February 2007 Chemistry and Ecology Vol. 22, No. 6, December 2006, 489–500 Marine lakes of karst islands in Ha Long Bay (Vietnam) C. CERRANO*†, F.AZZINI‡, G. BAVESTRELLO‡, B. CALCINAI‡, M. PANSINI‡, M. SARTI‡ and D. THUNG§ †Dip.Te.Ris, Università di Genova, C.so Europa 26, 16132 Genoa, Italy ‡DiSMar, Università Politecnica delle Marche, Via Brecce Bianche 60131 Ancona, Italy §Hai Phong Institute of Oceanology, 246 Da Nang Street, Haiphong City,Vietnam (Received 1 August 2006; in final form 11 September 2006) Ha Long Bay in North Vietnam is characterized by karst towers and bigger islands totalling more than 3000 isles. Karst processes carved hundreds of caves out of the limestone and contributed to the formation of many enclosed and semi-enclosed saltwater lakes. Here, we report the results of a general survey of several lakes and the first data on the Hang Du I lake, a small basin devoid of any apparent communication with the surrounding sea. Hang Du I is characterized by the presence of Rhizostomeae, genus Mastigias, suggesting strong similarities with the famous lakes described from the archipelago of Palau. Among the benthic organisms sponges are the most important group. Temperature and abundance of the monsoon rains are the main factors influencing remarkable seasonal variations in physical–chemical parameters and the community structure of the lake. A thermal crisis with water temperatures up to 36 ◦ C was recorded in September 2003. In this period, usually abundant medusae and sea anemones totally disappeared. Sponge populations showed fast growth rates in winter and spring and a partial degeneration to face the harsh conditions of the summer season. When isolated from the surrounding marine environment, the saltwater lakes share the condition of oceanic islands, representing spots of habitats surrounded by a completely different environment. The Ha Long Bay marine lakes are not easy to access, being surrounded by tropical forest, but local people usually exploit them for both fishery and oyster harvesting. There is an urgent need to develop measures of protection for these endangered and unique environments, natural laboratories that facilitate the study of evolution of marine organisms, where biodiversity has been until now totally unexplored. Keywords: Stratified lakes; Tower karst; Life cycle; Porifera; Tonkin Gulf 1. Introduction Ha Long Bay is one of the most extensive and best-developed areas of tower karst in the world. The eastern Adriatic, Greek, and Aegean coasts of Turkey provide other outstanding examples of tower karst although not of drowned tower karst, which is a style of karst landscape found mainly in the tropics and subtropics. Ha Long Bay is not unique in SE Asia for being a drowned tower karst; other sites where such a phenomenon occurs include the Mergui archipelago off the Andaman coast of Burma and other localities in northern Malaysia, Indonesia, and Thailand. *Corresponding author. Email: cerrano@dipteris.unige.it Chemistry and Ecology ISSN 0275-7540 print/ISSN 1029-0370 online © 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/02757540601024835