On the occurrence and ecological features of deep chlorophyll maxima (DCM) in Spanish stratified lakes

Deep chlorophyll maxima (DCM) are absolute maxima of Chlorophyll-a concentration among the vertical profile that can be found in deep layers of stratified lakes. In this manuscript I review the principal mechanisms that have been argued to explain the formation of DCM, which include, among others, in situ growth of metalimnetic phototrophs, differential impact of grazing between the different lake strata, and passive sedimentation to the layers where water density and cell density are equalized. The occurrence of DCM in Spanish lakes, as well as the main ecology characteristics of the oxygenic phototrophs that form DCM in these lakes is also reported. Cyanobacteria, either filamentous or unicellular, and cryptophytes, are the main components of most DCM found in the reported Spanish lakes, although diatoms, chrysophytes, dinoflagellates, and chlorophytes also contribute to these chlorophyll maxima. These organisms cope with strong physical and chemical gradients, among which those of water density, light and inorganic nutrient availability, and sulphide concentrations appear to be the most determinant factors influencing planktonic community structure.