engleski

A contribution to the knowledge of macrozoobenthic species of Mrtvo more on Lokrum Island

Mrtvo more on the Lokrum Island consists of a pit hole and a submerged channel that connects the marine lake to the open sea. Despite the specificities of Mrtvo more geomorphological phenomenon which is under the EU ecological network Natura 2000 (habitat type 8330 - submerged or partially submerged sea caves), there is a lack of data considering macrozoobenthic species inhabiting this area. Aim of this research was to determine the composition and dynamic of macrozoobenthos in Mrtvo more and to determine whether there are seasonal changes in chemical composition of seawater. The research was carried out from June 2017 until June 2018, with 13 samplings being performed, species present in the supralittoral, mediolittoral and infralittoral area were recorded, abiotic factors were measured (temperature, salinity, dissolved oxygen concentration) and seawater samples were taken to chemical analysis for total ammonia nitrogen (TAN), nitrate (NO3-N) and nitrite nitrogen (NO2- N) and phosphate (PO4-P) concentrations. Mrtvo more is shallow, semi- enclosed area and as such is under more influence of atmospheric conditions. Changes in temperature and salinity are more rapid, especially during spring and autumn period. Dissolved oxygen concentration levels were mostly above 4 mg/dm3, except for the July and August when lowering of dissolved oxygen concentrations corresponded with higher sea temperatures and degradation of phytobenthic communities. This is the peak period of a tourist season, therefore, anthropogenic pressures are highest. TAN, NO3- N, NO2-N and PO4-P concentrations were determined with standard spectrophotometric methods and they are under allowed limits for the first quality waters according to Regulation on water classification (Official Gazette, NN/98 ; NN 137/08). Visual census method and identification keys were used to mark down and identify macrozoobenthic species in supralittoral, mediolittoral and infralittoral area. Average abundance of present species was estimated. A total of 25 taxa was recorded: 3 species of class Anthozoa, 6 species of class Gastropoda, 1 species of class Bivalvia, 2 species of class Polychaeta, 3 species of class Hexanauplia, 5 species of class Malacostraca, 1 species of class Echinoidea, 3 species of class Holothuroidea and 1 species of class Asteroidea. There were three species noted in the supralittoral zone, 17 species in mediolittoral and 10 species in the infralittoral zone, although some species inhabit both mediolittoral and infralittoral. During June, July and August there were high population mortalities of genus Patella Linnaeus 1758 recorded as well as withdrawal of some species from the mediolittoral to the infralittoral zone. Due to the partly isolated position of Mrtvo more marine lake, ecological conditions differ from conditions in the coastal area of the island, therefore stress caused by temperature and salinity changes is more expressed. Furthermore, seasonal anthropogenic pressures on such a small area with limited water exchange cause rising of stress levels for organisms inhabiting the marine lake. Data obtained from this research will contribute to the knowledge of biological and ecological characteristics of marine lake Mrtvo more as well as better and more successful management of Lokrum island protected area.

abiotic factors ; biodiversity ; littoral zone ; marine lake

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