Evaluating underwater light availability for Phytoplankton: mean light intensity in the mixed layer versus attenuation Coefficient

• Autor(a) principal: Domingues, Rita B.
• Data de Publicação: 2023
• Outros Autores: Barbosa, Ana B.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.1/19979
• topic: Phytoplankton; ; ;; Light measurements; Light limitation; Coastal ecosystems

The use of several light-related variables, such as the Secchi disc depth, the euphotic depth, and in particular, the diffuse attenuation coefficient (K_d), is deeply rooted in phytoplankton research, but these are not the most appropriate indicators of the amount of light available for photosynthesis. We argue that the variable of interest for phytoplankton is the mean light intensity in the mixed layer (I_m), which represents the mean light to which phytoplankton cells are exposed throughout their life cycle, while being continuously mixed in the mixed layer. We use empirical data collected in different coastal ecosystems in southern Portugal to demonstrate why I_m should be the preferred metric instead of the deeply rooted K_d. We show that, although the relationship between I_m and K_d is inversely proportional, it is not always strong or even significant. Different I_m values can be associated with the same K_d, but distinct I_m have different physiological effects of phytoplankton. Therefore, K_d does not capture the amount of light available for photosynthesis, given that, unlike I_m, K_d calculation does not consider the depth of the mixed layer. Therefore, we urge phytoplankton researchers to consider the measurement and calculation of I_m when evaluating light-related processes in phytoplankton ecology.