Correction of the Chlorophyll a Fluorescence Quenching in the Sea Upper Mixed Layer: Development of the Algorithm

Purpose. Data from the immersion fluorescence sensors of the CTD complex and BioArgo floats are widely used for studying spatial and temporal variability of the chlorophyll a concentration on large scales. In these devices the fluorescence sensors are not equipped with a dark chamber; as a result, a part of the reaction centers of photosystem 2 is closed due to the sunlight effect. It is manifested in decrease of the measured fluorescence intensity in the upper mixed layer due to fluorescence quenching, while the chlorophyll a concentration can remain unchanged. The purpose of the work is to develop the algorithm for correcting the fluorescence quenching. Methods and Results. It is shown that photosynthetically available radiation decreases with depth within the upper mixed layer by almost an order, and the chlorophyll a fluorescence measured by the immersion sensor (without a dark chamber), increases with depth in this layer. Relationship between light intensity and share of open reaction centers of photosystem 2 in cell of microalgae was revealed. The relationship is described by the exponential function. The share of open centers effects on the degree of fluorescence intensity decreasing and therefore on fluorescence quenching induced by light. Conclusions. The universal algorithm for correcting fluorescence quenching in the upper mixed layer is proposed. Due to its correction, almost uniform fluorescence distribution is obtained in the upper mixed layer which is in a good consistence with the results of chlorophyll a concentration measurements in situ.