Planktonic ciliate communities along an environmental gradient in the Nile Delta (Damietta region, Egypt).

The spatial patterns of planktonic ciliate communities were studied from May to June 2019 in the Nile Delta's Damietta region, southeastern Mediterranean. The ciliate communities were sampled from twenty-five sites of five stressed domains with spatial gradients of environmental status. A total of 32 ciliate taxa with six dominant species were identified, comprising 21 tintinnids and 11 aloricate ciliates. The abundance and richness of each ciliate group varied geographically and were most strongly influenced by salinity variations; tintinnid ciliates attained high abundance and richness at high salinity sites in the harbour and coastal region and decreased within the estuary upstream. Aloricate ciliates were poorly represented at most sites but were a substantial proportion of upstream estuarine sites. Multivariate/univariate analyses demonstrated that spatial patterns of the ciliate communities were significantly correlated with environmental variables, especially salinity, chlorophyll-a, and nutrients, either alone or in combination with one another. These results indicate that the ciliates can be useful bioindicators in stressed environments while also allowing the detection of impacts on short time scales by rapidly responding to environmental variations.