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Taxonomy and distribution of benthic foraminifera and ostracods in an intertidal zone of the Adriatic Sea: Lokunja salt marsh (Pag. is.)

Intertidal zones due their conditional instability and great anthropogenic pressures are considered the most vulnerable and stressful ecosystems in the world. The Lokunja salt marsh, located in the town Pag and near to saline basins, is well known by its healthy mud. Most of the areas present very shallow water depths with microtidal (0.25 m, HHI data), semidiurnal tidal regime. Once natural spit protect the area from the open sea. Four sediments samples were collected manually in July 2018. The aim of this study is to describe the composition and structure of Foraminifera and Ostracoda in detail in order to investigate the distribution of each group in this restricted marine environment. Micropalaeontological analyses were performed on samples washed through a 63μm sieve and residue was then split into small proportions containing at least 300 specimens of benthic foraminifera. To characterize the biodiversity of total assemblages, five faunal parameters were calculated: (1) species diversity and the number of species in each sample ; (2) dominance, (3) Shannon–Weaver index, (4) Pielou’s evenness and (5) Fisher - α index. The size frequency graphs were constructed for dominant species in order to recognize the status of assemblages. A total of 1038 benthic foraminifera, belonging to nine taxa, have been counted and identified. Species richness in all studied samples varies from 4 - 8, dominance shows a maximum value of 0.43, the lowest value of Shannon–Weaver index is 0.77, the lowest value of Pielou’s evenness is 0.39 whereas Fisher α index reaches the greatest value of 1.59. Ammonia parkinsionana forms generally dominate the foraminiferal fauna (63 to 77%), with subordinate Haynesina depressula (2.7 - 15% and Cribroelphidium granosum (9.5 – 16%). The population of A. parkinsoniana is characterized by a Gaussian (normal) size distribution, peak in abundances occurred approximately at range 260-300 µm. It clearly point out that even in this restricted area certain hydrodynamic parameters occur and they control over the distribution of the tests. Also, the dominance of the opportunistic, oligohaline species confirms relatively high nutrient inputs, represented by labile and refractory organic matter. All ostracod assemblages are dominated by Cyprideis torosa, a widespread, euryhaline taxon, commonly found in transitional environments such as lagoons, estuaries, and inland ponds. This an opportunistic species, able to live from almost freshwater to hypersaline conditions, irrespective of substratum type, vegetation cover, temperature, and oxygen concentration. C. torosa presents two morphotypes with different ornamentation of the valves, and both of them are recorded within this assemblage: noded and un-noded (although this type prevails). The co-dominance of noded and un- noded reflects deposition in a restricted, oligohaline environment, like a low-salinity inner lagoon possibly with oxygen deficiency, where only this opportunistic species is able to thrive. Interestingly, based on ratio W/L, all found valves of C. torosa belonged to male individuals and along with uniform size suggest a post-mortem transport. Dealing with the area separated from the open sea where foraminifera and ostracods are re-distributed, suggest the impact of seasonal strong waves.

Ammonia parkinsoniana, Cyprideis torosa, salt-marsh, size-frequency graph.

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