Phyto- and zooplankton dynamics in two ICOLLs from Southern Portugal

Abstract

Foz do Almargem and Salgados are two small, intermittently closed and open lakes and lagoons (ICOLLs) located in the Portuguese southern coast, both remaining isolated from the sea throughout great part of the year. Based on environmental and biotic parameters, the first lagoon was classified as mesotrophic, with a greater marine influence, while the second one was classified as hypereutrophic. The plankton relations and seasonal variation were examined in both lagoons aiming at understanding the biological patterns here present and relating them with the environmental conditions, as well as inferring possible top-down or bottom-up control in the communities, determining also in which alternative trophic state they are: a clean water lagoon, with submerged macrophyte vegetation; or a turbid water lagoon, with only phytoplankton as primary producer.The plankton community greatly differs in the two lagoons. Salgados lagoon presented extremely high concentrations of phytoplankton, which form blooms throughout the year, mostly composed by potential harmful organisms. This lagoon also had very high densities of zooplankton. Phytoplankton and zooplankton density was always lower in Foz do Almargem. Rotifera, Ostracoda and Cladocera dominated the Zooplankton community in Salgados. In Foz do Almargem, Rotifera also dominates, together with other groups, such as annelids, crustaceans and mollusks larvae, which denotes a stronger marine influence. In both lagoons, organisms with body size inferior to 100 μm mainly composed the zooplankton. In Salgados lagoon, excess nutrient load induced a high density of phytoplankton, which is utilized by a high density of zooplankton, following a bottom-up control model. The lagoon clearly showed a permanent turbid-water alternative state due to excess nutrients. On Foz do Almargem, the frequent openings of the inlet throughout the wet season caused strong physical disturbance, which prevented the development of a stable planktonic community.