Irena Ciglene c ki-Ju s i c Rudjer Boskovic Institute, Center for Marine Research Zagreb,

1. Rogoznica Lake as a unique karstic phenomenon of Adriatic coast: Protection and Coastal Management Irena Ciglenecki-Jusic Rudjer Boskovic Institute, Center for Marine Research Zagreb, 10001 Zagreb, Croatia

2. Thanks are extended to: Bozena Cosovic Damir Vilicic Marina Caric Frane Krsinic Marijan Ahel Franco Baldi

3. INTRODUCTION • Rogoznica Lake is a small naturally eutrophicated and intermittenly anoxic sea Lake • Anoxic conditions appear in the Lake mostly in summer months as result of increased stratification of the water column in spring with consequent curtailment of reoxygenation of the bottom waters across the halocline and benthic decay of organic detritus accumulated from plankton blooms which occurred during the previous spring and summer

4. Position of Rogoznica Lake

5. Rogoznica Lake 1996 Seawater lake Area 5300 m2 Max. depth 15 m - the Lake is surrounded with sheer cliffs (4-23 m) - there is no visible conection with the surrounding sea but lake tides are visible on the clifs (indication of undergroundwater conection through the porous karst) -permanent stratification – zone of well oxygenated surface water and zone of totally anoxic deep water ( the boundary is at 9-12 m depth)

6. Seasonal variations of oxic - anoxic conditions in Rogoznica Lake (depth 12 m)

7. S2O32- (10-5M) So (up to 10-6 M) 90% S2-

8. RogoznicaLake - chemocline

9. Rogoznica Lake - October 1997

10. Seasonal changes of nutrient concentration in vertical profiles of Rogoznica Lake

11. POSSIBLE CAUSES FOR TURNOVER OF THE WATER LAYERS IN ROGOZNICA LAKE (September 1997) • Natural - affected by the recorded hyperproduction of dense phyto- and zoo-plankton populations as well as the high activity of sulfur bacteria • - these populations could have consumed the dissolved oxygen in the water and therefore caused the occurence of the anoxic conditions throughout the whole water column • 2. Anthropogenic - affected by mine and deep drilling in karstic rocks which could caused the vertical vibration of water column and led to the overturn of water layers • - during the spring and summer months in 1997 series of construction works in the framework of nautical centre building which occurred close to the Lake undergone their maximum

12. Phyto- and zoo-plankton populations Due to extreme ecological conditions which prevail in this lake phyto- and zoo-plankton populations are represented by a small number of species, but some in denser population than in the surrounding sea. After the period of total anoxia the number of species and their abundance were significantly reduced in comparison with the pre-anoxia values. Diatoms were found to be the dominant microphytoplankton group, while copepods (Acartia italica), the heterotrophic zoo-plankton organisms, play an important role and control the biochemical processes in the lake, especially in the post-anoxic period.

13. Hermesinum adriaticum • rare non-photosynthetic flagellates • characteristic only for specific stratified conditions • mostly distributed around the oxic-anoxic interface

14. Rogoznica Lake - Spring 1998

15. Marine “Frapa” (1999)

16. RogoznicaLake – September 2001

17. Rogoznica Lake - May 2002

18. Rogoznica Lake - July 2002

19. CONCLUSIONS • Rogoznica Lake represents a unique model system in the Adriatic Sea for studying natural eutrophication processes, as distinguished from different parts of the Mediterranean Sea as well as in the northern Adriatic region with registered anthropogenic eutrophication. • It is a natural bioreactor with numerous and intensive biogeochemical processes whose equilibrium and dynamic is very sensitive on external influences • Due to natural and scientific significance the Rogoznica Lake deserves to be put in one of the natural protection categories