Synergistic Use of Synthetic Aperture Radar and Optical Imagery to Monitor Surface Accumulation of Cyanobacteria in the Curonian Lagoon

Francesca De Santi,Francesco Paolo Lovergine,Claudia Giardino,Mariano Bresciani,Giacomo De Carolis,Guido Pasquariello,Diana Vaiciute,Giulia Luciani

doi:10.3390/jmse7120461

Abstract

Phytoplankton blooms in internal water bodies are an unpleasant sight that often emerges on top like a layer of foam containing high concentrations of toxins (scum event). Monitoring the concentration of algae and the occurrence of scum in lakes and lagoons has become a topic of interest for management and science. Optical remote sensing is a validated tool but unfortunately it is highly hindered by clouds. For regions with frequent cloud cover, such as the Baltic region, this means loss of data, which limits the purpose of sensing to spatially and temporally characterize any scum for a comprehensive ecological analysis. The aim of this paper is to investigate whether the use of synthetic aperture radar (SAR) images can compensate for the weaknesses of optical images for cyanobacteria bloom monitoring purposes in the event of cloudy skies. A “ready to use” approach to detect cyanobacteria bloom in the Curonian Lagoon based on the level 2 ocean product of Sentinel-1 images is proposed. This method is empirically validated for the images of summer/autumn 2018 of the Curonian Lagoon.

Highlights

Cyanobacteria, known as blue-green algae, are aquatic bacteria affecting inland and coastal communities around the world and are some of the oldest living organelles on the Earth
Areas affected by cloud cover are masked out in chlorophyll a (Chl-a) maps as well as in S1 images areas where wECMWF is lower than 2 m/s
Given the unavailability of synchronous synthetic aperture radar (SAR)/optical images and the absence of in-field measurements in conjunction with the S1 acquisition, this research is focused to investigate whether the “anomaly” observed in SAR wind retrieval is compatible with a spatio-temporal evolution of scum observed in optical images

Summary

Introduction

Cyanobacteria, known as blue-green algae, are aquatic bacteria affecting inland and coastal communities around the world and are some of the oldest living organelles on the Earth. A mature blue-green algae bloom often surfaces atop as a layer of scum which is an unpleasant sight with potentially serious economic and health-related impacts [1]. The warmer is the water surface, the steepest is the vertical stratification of inland waters, with a consequent reduction of vertical mixing layer. Many cyanobacteria take advantage of these stratified conditions by forming intra-cellular gas vesicles, which make the cells buoyant and able to accumulate in dense surface blooms [4]. On top of water temperature, factors like turbulence levels, nitrogen fixation, long day-light and the buoyant ability of cyanobacterial cells play their part in the occurrence of scums [6]

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