engleski

The influence of environmental factors on diatom colonization on glass slides in the marine lake Mrtvo More (island of Lokrum, Adriatic Sea coast)

In this study we investigated diatom colonization during period of first 6 months of artificial substrate (glass slides) exposure. From April to October 2016 microscopic glasses were submerged in the marine lake Mrtvo more (island of Lokrum, Dubrovnik area, South Adriatic, NE Mediterranean, Croatia). This semi-closed shallow marine ecosystem is connected with the open sea by an underwater passage, and is during summer season under significant anthropogenic influence (swimmers). The parallel oriented glass slides (against the surface) were placed on a seafloor at 1 m beneath the water surface. The samples were collected weekly. For light microscopy (LM) observations diatom frustules were cleaned from the organic material using method with hydrogen peroxide and hydrochloric acid. In order to determine the relationships between diatom community and environmental variables, water samples for analysis of physicochemical variables were taken weekly from the same place where diatom sampling was carried out. Water temperature ranged from 18.3°C (May) to 27.3°C (July). Salinity varied from 26.6 (October) to 37 (August). Oxygen saturation (O2/O2′) ranged 0.58 (September) to 1.3 (June). TIN ranged from 0.96 (May) to 10.2 (September) µM. Phosphate (PO4) ranged from 0.112 (May) to 0.578 (July). Silicate (SiO4) ranged from 3.925 (May) to 13.016 (July) µM. The benthic diatom flora was investigated and diversity was determined in relation to the concentrations of phosphate, silicate, and nitrogen compounds. The composition (% relative abundance) of a total of 133 diatom taxa within 47 genera found in the benthic diatom community of studied artificial substrates were identified in 21 samples. Species Cocconeis scutellum Ehrenb. var. scutellum, Cocconeis dirupta var. flexella Grunow, Pinnularia Ehrenb. sp., Diploneis crabro (Ehrenb.) Ehrenb., Navicula salinicola Hust., and Licmophora paradoxa (Lyngb.) C. Agardh were the most abundant. Genera with the greatest number of taxa were: Nitzschia Hassall (20 taxa), Mastogloia Thwaites ex W. Sm. (11), Amphora Ehrenb. ex Kütz. (9) and Cocconeis Ehrenb. (9). Altogether, only three taxa were presented in more than 85% of samples: Cocconeis scutellum var. scutellum, Cocconeis dirupta var. flexella and Cocconeis pseudomarginata W. Greg. In total, 60 taxa were found only once (sporadic) with average relative abundances lower than 1.3%. The number of taxa per sample ranged from 9 (May, June) to 52 (August), with an average of 25.4. The species diversity index varied from 0.74 to 4.51. Pielou’s species evenness ranged from 0.23 to 0.86 (the average 0.63).The amount of biofilm increased through study period. Diatom assemblages were significantly different (ANOSIM, P<0.05) between the beginning samples till July and samples afterwards. Throughout the period from end of July till end of September, the community was characterized by a stable composition and high species diversity (Shannon-Weaver, mean H’=4.22). Different environmental conditions such as sea temperature, salinity, oxygen saturation and nutrient concentrations may have influenced unequal colonization between months. This study indicates that the early diatom biofilm formation and composition are affected by different environmental conditions. Additionally, it clarifies what parameters contribute to observed differences between months.

benthic diatom colonization ; artificial substrate ; South Adriatic ; NE Mediterranean ; physicochemical variables ; biodiversity

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