Research On In Situ Size Distribution And Concentration Measurement System For Marine Phytoplankton

Phytoplankton is the major producer and energy transducer of marine ecosystems,and its excessive proliferation will worsen the marine environment and lead to more“red tides”. The in situ measurement of phytoplankton population structure andbiomass can be used to monitor the change in marine environment and to predict “redtides” and other natural disasters. As an important parameter, particile size distributionis related to the physiological properties of phytoplankton, such as metabolism,photosynthesis and energy use. In this thesis, an in situ measurement system isdeveloped to achieve real-time and automatic monitoring of phytoplankton sizedistribution and population density in coastal seawaters.The monitoring system combines laser-induced fluorescence detection and laserdifferential Doppler velocimetry. The former is used to identify the phytoplanktonparticles passing the detection zone, and to determine the time it take for thephytoplankton particles to transverse interference fringes in the laser intersectionvolume, while the latter is used to obtain the velocity parameter of the phytoplanktonparticles passing the detection zone, thus the size and concentration can be deduced.The system consists of a laser, a laser-fiber coupling device, an underwater opticalprobe and a data acquisition&processing part. In the thesis, the designing&debugging of all the whole system, the systematic calibration, laboratory tests, dataanalysis and in situ measurements was described. Specific works done are as follows.1. By in-depth research on the measuring method, an overall scheme of themeasurement system was determined. A532nm laser was selected; and a laser tosingle-mode fiber coupling device was made on the the basis of the characters ofGaussian beam and its transformation characters through an aspheric lens, which canachieve the coupling efficiency of50%for a100m single-mode fiber with a core diameter of3.5m and a numerical aperture of0.13.2. An underwater optical probe, a watertight chamber housing the optical sensingsystem, was developed. The laser transmitted by a single-mode fiber entered thechamber, then split into two parallel beams and intersected with each other outside thechamber. The beam intersection defines the measurement volume in the seawater. Thedirections of±90o deviating from the transmission direction of laser beams arerespectively chosen to receive laser-induced fluorescence and laser Doppler signals inorder to keep both signals synchronic to get accurate size distribution. A robustbeam-splitting device, with output intensity insensitive to the incident beam directionchange caused by slight incident inclination and sea waves, was designed, using alateral displacement beamsplitter. Lab and field experiments proved that the opticalprobe chamber with its support system met the measurement requriements.3. Photoelectric detection circuits and data acquisition&processing hardwareswere designed. DSP programs&PC software units for data processing, storage anddisplay were designed and completed.4．The measurement system prototype was debugged, tested, further adjusted inthe laboratory, and the error analysis was done. In the laboratory tests and calibrationexperiments,12phytoplankton species in3different kinds of shapes cultured in the labwere used. Gaussian curve-fitting method was adopted to study the phytoplankton sizedistribution data. The results of lab experiments were compared with that measured byCASY-TT cell counter+analyser system. Corresponding size distribution andconcentration errors for all kinds of phytoplankton measured in the tests were given.Statistical distribution errors of size distributions for spherical or ellipticalphytoplankton with a size range of5-50m were less than10%. The average size valuemeasured by the prototye was well linearly relevant with the one measured byCASY-TT, The concentration measurement errors of mixed phytoplankton were around5%.5. The prototype in situ experiments were done twice at different stations inQingdao coastal waters, and the results proved that the designed system can be used for in situ measurement of size distribution and concentration information of marinephytoplankton, The adjusted system can also do real-time monitoring at a fixedposition for a long time. The experimental data demonstrated no distinct change ofphytoplankton concentration in the measured waters. Compared with microscopesobservation results, the measurement errors are within an acceptable range.The system developed in the thesis is capable of real-time size distribution andconcentration measurement for phytoplankton particles. The underwater opticalsensing probe does not rely on electrical power, instead, it uses optical fibers totransmit laser and the measured optical signals. The sensing probe can stay submergedfor long period of time, making a long-term real-time in situ monitoring possible. In thesame time, compared with the other measurement methods, the system needs neithercomplicated optics nor complex algorithms, so it is of outstanding real-timeperformance. The system has a promising application in marine environmentmonitoring and evaluation,“red tide” research, marine ecosystem process research andwater quality monitoring.