A comparison between quasi-horizontal and vertical observations of phytoplankton microstructure

Abstract

On 24 June 2013 and on 20 May 2014 near Joga-shima, Japan, we deployed two microstructure profilers, TurboMAP-L and the TurboMAP-Glider, to obtain vertical and quasi-horizontal measurements of turbulence and chlorophyll a fluorescence. The deployments were clustered at two stations (one near the coast and another further offshore). Both instruments carried two fluorescence sensors with different spatial resolutions: the light-emitting diode sensor (LED) with a spatial resolution of 2 cm and the laser sensor with a spatial resolution of 2 mm. We observed that vertical and quasi-horizontal averages as small as centimeters are statistically equal, regardless of the sensor. However, using the laser sensor, quasi-horizontal sampling showed higher fluorescence intensity peaks than vertical sampling at millimeter scale. We hypothesized that layered structures in the upper ocean and the existing shear tend to orient chlorophyll a patches horizontally. Also, we found that large patches tend to have a random distribution in the water column, while small patches are likely to be found near large patches in a non-random distribution. The understanding of the spatial distribution of fluorescence patches is an important step forward to achieve a reliable picture of phytoplankton microscale. Additionally, it provides observational evidence for those modeling phytoplankton microstructure.