Irena CigleneckiJusic

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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
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Thanks are extended to

• Bozena Cosovic
• Damir Vilicic
• Marina Caric
• Frane Krsinic
• Marijan Ahel
• Franco Baldi

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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

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Position of Rogoznica Lake
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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 underground water 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)
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Seasonal variations of oxic - anoxic conditions
in Rogoznica Lake (depth 12
m)
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S2O32- (10-5M) So (up to 10-6 M) 90 S2-
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Rogoznica Lake - chemocline
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Rogoznica Lake - October 1997
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Seasonal changes of nutrient concentration in
vertical profiles of Rogoznica Lake
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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

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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.
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Hermesinum adriaticum

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

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Rogoznica Lake - Spring 1998
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Marine Frapa (1999)
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Rogoznica Lake September 2001
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Rogoznica Lake - May 2002
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Rogoznica Lake - July 2002
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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