Problems and Prospects of Marine Water Quality Monitoring Technology

The 21st century is the century of the ocean. Faced with extreme scarcity of terrestrial ecological resources and continuous environmental degradation, many countries have turned their economic development focus to the ocean. Exploiting offshore oil and gas resources, developing marine ranches, and aquaculture have become popular industries in the marine economy of coastal nations. However, while creating economic benefits, the development of marine industries has also triggered numerous environmental pollution problems.

Currently, the aquaculture environment in my country's coastal waters is deteriorating, and the marine water quality is severely damaged. Fishermen in coastal cities frequently encounter red tides while fishing in nearshore waters, seriously interfering with their safe operations. According to relevant statistics, the main sources of water pollution in my country's coastal areas are land-based discharges, including industrial waste, industrial wastewater, fertilizers and pesticides, petroleum, heavy metals, and untreated domestic waste. Before the 1970s, red tides occurred only in a few sea areas. In recent years, the frequency of red tides has increased, covering larger areas, lasting longer, and causing more severe damage, posing a serious threat to the survival environment of marine life and hindering the development of the marine economy.

If marine water quality monitoring technology is promoted and popularized in coastal cities, it can not only avoid huge economic losses, but also predict the development trend of the marine environment, and promptly grasp the patterns of marine disasters such as red tides, so that marine environmental departments and fisheries departments can make governance decisions. This is also in line with the requirements of adhering to the integrated development of land and sea, building a maritime power, and implementing the "Blue Sea" action.

Nearshore marine water quality monitoring methods

Traditional methods for monitoring nearshore marine water quality involve manual sampling, which involves navigating vessels to designated target points to collect water samples and transporting them back to onshore laboratories for analysis and processing. This manual method is limited by complex weather and sea conditions, resulting in low efficiency. It also presents challenges such as high risks to personnel, poor real-time monitoring, and limited monitoring areas. Furthermore, it requires a large workforce and extensive equipment, and cannot guarantee real-time data or reflect the dynamic changes in the water quality at the sampling points.

With the rapid development of marine engineering and Internet of Things (IoT) technologies, my country's nearshore water quality monitoring technologies primarily include buoy monitoring, satellite monitoring, airborne monitoring, and unmanned surface vessel (USV) monitoring.

①Buoy Monitoring

Real-time monitoring of water quality and ocean currents in nearshore waters using IoT and smart sensor technologies provides data support for research on marine water quality environment and hydrodynamic characteristics. Anchored buoy systems equipped with various parameter sensors are installed at selected monitoring points. The instruments on the buoy platform can be deployed at fixed points and moved via a vertical lifting system, enabling stratified monitoring in seawater. As shown in the figure, the buoy monitoring system integrates a power supply system and a data acquisition and transmission system. Relying on GPRS/BeiDou satellites, it transmits monitored water quality data to the land-based management station in real time. The system also has safety protection and self-testing functions to ensure stable operation of water quality monitoring in unmanned mode. The advantages of buoy monitoring are high automation, continuous monitoring, no need for manpower and material resources, and convenient maintenance and management. The disadvantages are that buoys can only be deployed in fixed monitoring locations, can only monitor water quality in small-scale sea areas, and surface equipment may be damaged in typhoon conditions.

②Satellite monitoring

Satellite monitoring is a popular new method for monitoring marine water quality in recent years. Compared with buoy monitoring technology, it has the advantages of fast monitoring speed, continuous monitoring, and wide monitoring range. This method is an effective means to study the occurrence time, coverage area, diffusion degree and direction, and occurrence pattern of red tides at sea. All substances in nature have spectral characteristics. Different substances have different spectral characteristics, and the difference in spectra is positively correlated with the difference in substances. The principle of marine water quality remote sensing monitoring is based on the fact that pure water and other water bodies have different spectral characteristics. Sensors on satellites receive radiation values ​​within a certain wavelength range, analyze the absorption and scattering of water quality parameters to light of specific wavelengths, and combine the mathematical relationship between the spectrum and the concentration of water quality parameters to finally deduce the concentration values ​​of various water quality parameters. As shown in Figure 2, the sensors on the satellite receive three parts of solar radiation during satellite remote sensing monitoring:

(1) Radiation reflected by the water surface;

(2) Radiation scattered by the atmosphere;

(3) Radiation scattered and reflected by sunlight after entering the seawater.

③Airborne monitoring

Airborne monitoring utilizes aerial vehicles such as helicopters as sensor mounts to remotely monitor water quality from the air. Airborne monitoring offers advantages such as high sensitivity, accuracy, and resolution, leading to its rapid development in water quality monitoring applications. Due to its greater mobility and flexibility, aerial monitoring is suitable for large-scale inspections and targeted surveys of key areas. Airborne monitoring can flexibly carry various types of sensors, including microwave, infrared, ultraviolet, and lidar, to acquire timely water quality information for marine areas, and is now widely used in marine environmental monitoring. Currently, unmanned aerial vehicles (UAVs) represent a new direction in airborne monitoring development. This method is reliable, can carry various high-resolution detection devices, and possesses advantages such as high mobility and low cost. It has already been deployed in nearshore marine water quality monitoring and coastline monitoring, and will play a significant role in nearshore marine disaster emergency monitoring in the future.

④Unmanned vessel monitoring

Unmanned surface vessel (USV) monitoring is a mobile monitoring method that involves mounting a water quality monitoring system on an unmanned surface vessel and transmitting the monitoring results back to a land-based base station in real time using wireless communication technology.

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