Non-marine epilithic diatom communities in Malta and Gozo

Abstract

Diatoms are used as bio indicators to assist mainly in investigations related to ecological, agricultural, forensic and archeaological issues. Diatoms are found in aquatic environments such as marine, brackish and freshwater systems. This was the first qualitative study performed in order to understand and identify non-marine epilithic diatom communities in certain valleys found in Malta and Gozo. Samples of epilithic diatoms were collected from 10 valleys in Malta and Gozo and a pond in Gozo. Four collections were taken in two seasons, autumn and winter, to compare habitats under dry and wet conditions. Four methods using drain cleaners, hydrogen peroxide and hydrochloric acid were investigated to choose the best method for all the diatom sample cleaning in local samples. The best method with hydrogen peroxide and hydrochloric acid was chosen and identification was done under the Light Microscope Motic BA410. Using Motic Image Plus 2.0 ML, diatom images were captured, and then qualitative and semiquantitative analysis was carried out accordingly. All physical and quantitative data was analysed statistically at a significance level of p<0.05. Out of all 11 sites, 51 species were identified and 171 taxa made up the overall community. Most of the taxa were found in Wied ta’ Marsalforn, Gozo and the most common genus was Nitzschia sp. Other common genera/species were Hanztschia abundans, Amphora sp., Luticola sp. and Gomphonema parvulum. The latter species exhibited the greatest length to width variation (R2 = 0.1674) compared to the rest. All diatoms were strongly related to alkalinity (pH=8.15±0.13), conductivity (425.5-5365 μS cm-1) and suspended solids like rocks. The temperature was relatively constant between the 20-metre apart sampling sites. Nitzschia sp and Amphora sp were commonly found between Wied il-Qlejgħa, Wied ta’ Marsascala and Wied il-Għasel. Wied ta’Marsalforn, Wied il-Lunzjata and Wied is-Sewda had Navicula sp.1, Nitzschia palea and Gomphonema parvulum in common. Diatoms, quoted in previous studies to grow normally in acidic conditions, were found in alkaline conditions in Malta. This was observed with Frustulia sp. (Wied il-Lunzjata), Neidium sp.(Wied Qannotta) X and Eunotia sp. (Wied il-Qlejgħa). Two rare species (Planothidium delicatulum and Fallacia pygmaea) were only found in Wied Mistra during both autumn and winter. Furthermore, it may be concluded that the presence of these species reflected their environment and that they were able to adapt to the degree of pollution present in the area. This qualifies the diatom populations as indicators to specific sites and to the environmental status. However, further research should be carried out to consolidate the present findings.