Determination of microalgae limiting nutrients in the Green Lagoon “Charco Verde de Los Clicos”, Lanzarote (Canary Islands)

Abstract

The “Charco Verde de Los Clicos” is a hypersaline green coastal lagoon located at the west coast of Lanzarote Island. It is separated from the sea by a black sand barrier, but it is connected with the sea by underground channels. It is part of a semi-submerged hydro-volcanic building. The surface and volume of the lagoon is 7.700m2 and 17.700m3 with a maximum depth of 3.5 m at the middle, being the green color due to high chlorophyll levels. The phytoplankton concentrations found in the lagoon indicates eutrophic characteristics. The aim of this work was to confirming that nitrogen is the main limiting nutrient of the primary production because phosphorus is provided by the volcanic substrate. Water samples from six different points of the lagoon were mixed and incubated as control and three different treatments with N, P and N+P were made with five replicates of each treatment. Biomass growth was measured by optical density and flow cytometry during twenty-two days. Results show that during an 11 days period, nitrogen was the limiting nutrient in “Charco Verde de Los Clicos” because the samples, which had received nitrate, present a biomass increase, contrary to the treatments without addition of nitrate where a biomass decrease was found. When both nitrate and phosphate were added it was observed a higher biomass increase. These results indicate that phosphorus started to be also limiting of primary production when the growth by nitrate concentration was high.