Harmful planktonic and epiphytic microalgae in a Mediterranean Lagoon: The contribution of the macrophyte Ruppia cirrhosa to microalgae dissemination

•Planktonic and epiphytic microalgae in Ghar El Melh Lagoon.•70% of harmful species in the water column and on Ruppia leaves.•Pivotal role of Ruppia in the dissemination of harmful species.

Temporal and spatial distribution of planktonic and epiphytic microalgae coupled with environmental factors was studied at five stations in Ghar El Melh Lagoon (Tunisia). Phytoplankton was monitored for a year and epiphytic microalgae were sampled during phases of proliferation of the seagrass Ruppia cirrhosa in summer 2011. Microalgae assemblages were dominated by dinoflagellates and diatoms both in the water column and on Ruppia leaves. A significant difference was found in phytoplankton distribution among stations and seasons with (i) maximum proliferation in the inner parts of the lagoon, (ii) spring–winter association of the majority of species including the dominant harmful species Prorocentrum micans and (iii) summer diatom proliferation. Redundancy analysis indicates that temperature and nutrients have a substantial effect on distribution. Epiphyte distribution roughly follows the development cycle of R. cirrhosa. A list of harmful planktonic and epiphytic microalgae found in the water column and Ruppia leaves was compiled. This study highlights the role of Ruppia in the dissemination of harmful species in the lagoon's water column.