Changes in physico-chemical parameters and photosynthetic microorganisms in a deep wastewater self-depuration lagoon

Abstract Temporal changes in physico-chemical parameters and photosynthetic microorganisms were studied during a seasonal cycle (October-July) in an 8 m deep self-depuration wastewater lagoon in the south east of Spain. The mild climate of the area confers great potential on this type of water depuration. A first bloom of Chlamydomonas sp. dominated the system during the period of heavy organic contamination. This algal species disappeared without noticeable proliferation of the zooplankton grazer community. A second phase of water depuration was characterized by proliferation of small chlorophytes, mainly of the genera Golenkinia, Chlorella and Scenedesmus . Rotifers and ciliates grazed on this second bloom giving rise to a third one, dominated by a small cyanophyte (0.5–1.5 m) of the genus Synechococcus , when nutrient levels above the thermocline were low (2.5 mg PO 4 3− /1 and 1.0 mg NH 4 + /1). The cyanophyte population was controlled by rotifers, copepods and cladocerans up to the end of the study. Two blooms of photosynthetic bacteria were identified. The first one, occupying the whole water column, coincided in time with Chlamydomonas and an organic-rich phase of the water column. It was dominated by Thiocapsa sp. and Chromatium sp., both of them sulphur red photosynthetic bacteria. The second bloom, of the green photosynthetic bacteria Chlorobium , established at and below the thermocline when H 2 S concentrations reached 95 mg/l. The detection of all these easily identifiable organisms has proved to be very useful as bioindicators of the water depuration stage in deep lagoons of the area.