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Populacijska dinamika bentičke foraminifere Ammonia tepida (Cushman): Podaci iz Rogozničkog jezera

The periods with low dissolved oxygen concentrations in bottom waters in coastal areas can have strong impacts on benthic ecosystems. Among the meiofauna living in these environments, benthic foraminifera is often the most tolerant of low oxygen levels. Some species (representatives of the genus Ammonia) are able to survive complete anoxia for weeks to months. Lake Rogoznica is located on the Gradina peninsula and is not directly connected with the open sea. There is, however, marine influence through the porous limestone rocks. Due to its position, the lake has very peculiar ecological characteristics, quite different from those of other coastal salt lakes in the Mediterranean. Seasonally stratified seawater characterizes this „lake“, the surface water is similar to waters of the adjacent Adriatic Sea in temperature and salinity, whereas bottom water is enriched in nutrients and dissolved organic carbon and deprived of oxygen (PJEVAC et al., 2014). How to describe the repetitive appearance of individuals of this species in the closed system, like Lake Rogoznica? In order to get some answers on survivorships and fecundity of foraminiferal species analysis of population dynamics of A. tepida tests was performed. Ammonia is an effective grazer of microalgae and its physiology, successful reproduction and survival are driven by factors including food availability, food quality or environmental temperature (MURRAY, 2006). In 2010 a short core (50 cm long) have been recovered from 9m of water depth. Sediments were sliced into 2cm thick subsamples and submitted to geochemical and granulometric analysis. Micopaleontological analysis on residues greater than 38 µm, were done on samples from certain levels only (intervals: 4 – 6, 16 – 18, 26 – 28 and 36 – 38 cm). The monospecific assemblage shows a similar pattern of size frequencies. Foraminiferal tests varies from 137 to 368 µm. Those tests with diameters between 220 and 260 µm make the largest portion of total tests in all population, the proportion of smaller and bigger tests are the same, with slightest increases of larger tests in population from the deeper part of the core. The bell-shape-size frequency graphs have been found in all studied populations, contrary to population dynamic typical for smaller benthic foraminifera. One possible explanation is that opportunistic foraminifera has fit their life strategy including reproductions to the seasons with a certain level of oxygen concentration. It is known that individuals of A. tepida are able to survive around 28 days of anoxia, and if we consider their generative time (season) then this scenario can be considered as one possibility.

Populacijska dinamika, Ammonia tepida, Rogoznica Lake

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