Optical and Biological Complex
It is a well-known fact, that the dynamics of phytoplankton abundance or biomass is determined by the processes of photosynthesis, consisting in the construction of organic matter from mineral compounds. The solar energy plays the main role in the dynamics. In this connection, the main attention in the study of the dynamics of the primary producer of organic matter in various water bodies was given to seasonal, annual and interannual variations. At the same time, little attention has been paid to small-scale processes, which can have a significant effect on the dynamics of the phytoplankton-zooplankton-fish-marine animals chain. The small-scale processes include the processes that lie in the range from daily tides to wind waves, i. e. the processes that lie in the range of periods from 24 h to 1 s. There are various hydrophysical processes of periodic and quasiperiodic in this range of periods characters: tides, inertial oscillations, seiches, internal waves, swell, regional wind waves. To study the relationship of these processes to the biomass of the primary producer of organic matter, marine animals and fish, a complex was created consisting of a laser instrument, measuring hydrosphere pressure variations, a fluorimeter and an underwater video monitoring system that is capable of recording any periodic and quasiperiodic variations of the main hydrophysical parameters (pressure, temperature) in the period range under consideration, as well as the variations in the abundance of the biomass under study. One of the investigated problems of the complex is the task of determining the threshold level of hydrophysical parameters oscillations to which biomass responds.
DESCRIPTION OF THE COMPLEX
As it was said above, the optical and biological complex consists of a laser instrument, measuring hydrosphere pressure variations, a fluorimeter and an underwater video monitoring system. In addition, it is equipped with power lines and telecommunication lines for transmitting information from the stations to the shore laboratory premises, a multi-bit ADC, an exact time system, a computer designed for primary processing and recording of the experimental data on solid carriers.
Currently, the laser instruments, measuring hydrospheric pressure variations and their modifications have been developed [1–3], designed to measure variations in hydrosphere pressure with an accuracy of 1 mPa to 1 µPa in the frequency range from 0 (conditionally) to 1 000 Hz. Furthermore, these units are equipped with high-sensitivity temperature sensors for parallel measurement of temperature variations. With respect to the frequency range and the accuracy of measurements of the parameters under study, these installations are most suitable for conducting monitoring works on the effect of hydrophysical processes on various bioobjects. Moreover, these laser-interference reception systems are used in conjunction with onshore laser deformographs to study various hydroacoustic and hydrophysical processes and the laws of their transformation at the media interface [4–6].
The fluorimeter C3 is fixed in the setting frame next to the laser instrument, measuring hydrosphere pressure variations (see Figure 1, right-hand side). Submersible fluorimeter C3, manufactured by Turner Designs, is equipped with two in vivo photosensitive sensors for measuring the luminescence of phytoplankton chlorophyll in blue 460 nm and red 635 nm radiation spectra. The range of blue radiation allows to fix the concentration in the range of 0.025–500 µg/l. The range of red radiation is over 500 µg/l. The instrument is also equipped with pressure and temperature sensors. The durable housing is made of Delrin poly-formaldehyde, which allows operating in aggressive marine environment. The surface of the housing near the optical sensors is equipped with copper inserts to reduce the biological contamination of photosensitive elements. Weight of the instrument is 1.64 kg, its length is 23 cm, its diameter is 10 cm, temperature range from –2 °C to 50 °C. Depth of immersion is up to 600 m. The maximum discreteness of the data acquisition is 1 Hz.The fluorimeter has the ability to output digital data in ASCII format, or obtaining analog data in the presence of an appropriate adapter.
The underwater video surveillance system is based on TANTOS TSi-Ple2VPZ network video camera, which has 1920 Ч 1080 pixel frame resolution with a video frame rate of up to 30 per second. The camera is equipped with a motorized lens with a variable focal length of 2.8–12 mm, which allows for remote control of the viewing angle of the observed scene in the range of 27–91 degrees. Together with the control unit, it is placed in a waterproof cylindrical metal box, one of the sides of which is made of durable glass, through which the camera observes underwater scenes. The box is attached to a massive base and, together with it, descends into the sea at the place of setting. The power and communication cables are connected to the box, allowing placing the video system at distances up to 400 m from the shore. Trials by descending the box into the sea from the vessel have demonstrated its ability to withstand immersing, at least up to 100 m, which is sufficient for use on virtually any coastal water area of the Peter the Great Bay. To monitor at night and at great depths, the system is equipped with flashlights that can be remotely switched on and off. The coastal communication point is connected to the nearest node of the telecommunication network of the scientific monitoring system of the Peter the Great Bay. This provides real-time transmission of underwater video surveillance data to monitoring information stores in Vladivostok, as well as live video transmission from the camera to the Internet. Fig. 1 (left) shows a similar underwater video observation system, established in 2014 in Alekseev Bay (Popov Island, the Peter the Great Bay).
The laser instrument, measuring hydrosphere pressure variations, together with a fluorimeter was first installed in the Vityaz Bay 200 m from the shore near the pier of the Marine Experimental Station "The Cape of Shultz" on June 23, 2016. Subsequently, the complex of these two instruments was re-installed several times in different points of the Vityaz Bay at distances from 100 to 250 m from the shore. The underwater video system was first installed on October 24, 2016, 100 meters from the shore in the immediate vicinity of the fluorimeter. In order to exclude the influence of the night illumination of the camera on the parameters recorded by the fluorimeter, the latter was later moved 20 m from the camera towards the sea. In total, you can distinguish two settings, when for fairly long time all three devices of the complex were located in approximately the same place. The first setting at a distance of about 100 m from the shore and a depth of 5 m was carried out from October 2016 to early January 2017, the second, at a distance of 200 m from the shore at a depth of 15 m – from May 16, 2017 to the present. Fig. 2 shows the location of the instruments during the first and second settings: circles – underwater camera, diamonds – a hydrophone-fluorimeter complex. Note that in both cases the optical axis of the camera was oriented from the shore towards the open part of the bay.
All information received from the measuring settings of the complex through the cable lines goes to the onshore laboratory building, where it is entered into the created database of experimental data after preliminary processing. The data from all the near-real-time monitoring devices enter the file storage of the Peter the Great Bay scientific monitoring system in the premises of POI FEB RAS and become available to the Institute’s scientific specialists. The data of the laser instrument, measuring hydrosphere pressure variations, are presented in the storage by one-hour files with records of hydrospheric pressure variations digitized at a frequency of 1000 Hz.The fluorimeter data are presented by daily files of signals of changes in the specific content in chlorophyll-a water, with the sampling frequency is 1 Hz.These video surveillance systems are represented by snapshots and small video recordings of the underwater scene, reproduced with time intervals of 1 and 30 minutes, respectively. Underwater video surveillance system is used: for visual control of the situation at the location of the first two devices; for the registration and accumulation of a database of video materials for the purpose of their subsequent analysis by expert biologists and submission to all interested persons, including scientists, teachers and students; for the development and debugging of techniques for automatic description of the state of biodiversity based on video analysis; for the development and debugging of techniques for recording the hydrological characteristics of the medium (disturbances, sea level fluctuations, currents, turbidity and water color).
PROCESSING AND ANALYSIS OF FIRST EXPERIMENTAL DATA
For a primary analysis of the experimental data obtained, a series of about one month was chosen. The main purpose of this processing is to study the capabilities of the complex to use it to study the possible dependence of the dynamics of marine biomass on the variations of the world’s mid-scale fields in the frequency range corresponding to oscillation periods from diurnal to 1s. Furthermore, the synchronous data of the laser instrument, measuring hydrosphere pressure variations, and the fluorimeter were processed. The data of underwater video monitoring at the first stage were analyzed only qualitatively.
The processing and analysis of the obtained experimental data will be carried out in a sequence from the highest-frequency components to low-frequency components. Given that the sampling frequency of the analyzed series was 1 Hz, the highest frequency of the series under analysis, according to the Nyquist theorem, is 0.5 Hz (period T=2 s). Thus, high-frequency range "rests" in sea wind waves, the main periods of which are in the range from 2–3 s (local wind waves) to 15–16 s (powerful wind waves caused by prolonged action of typhoons over the water of the Sea of Japan with a wind speed of around 25–30 m/s). When processing the obtained experimental data, it was established that the concentration of primary biomass at a particular horizon correlates with the hydrospheric pressure variations caused by the present sea waves. Thus, the top graph of Fig. 3 shows the dynamic spectrogram of the recording area of the fluorimeter, and the bottom graph of Fig. 3 shows the dynamic spectrogram of a synchronous recording section of a laser instrument, measuring hydrosphere pressure variations. The main period of the allocated oscillations is approximately 7.3 s.
The main oscillations of the lower frequency range, which were allocated during earlier research work in the Vityaz Bay, correspond to the seiches, i. e. own vibrations, of the Vityaz Bay, where the period of the main mode varies with time in the frequency range corresponding to the periods from 16 to 18 minutes . When processing the experimental data of a fluorimeter and a laser instrument, measuring hydrosphere pressure variations, it is established that these fluctuations do not affect the dynamics of phytoplankton. Thus, Fig. 4 shows the spectra of synchronous recording sections of the laser instrument, measuring hydrosphere pressure variations, where a powerful peak is isolated at a period of 17 minutes 04 s, corresponding to the fundamental mode of natural vibrations of the Vityaz Bay, and that of the fluorimeter where this peak is not observed. It should be noted that the natural oscillations by analogy can be attributed to standing waves, where the pressure does not change with depth at the same time.
In the tidal range, both in the fluorimeter records and in the records of the laser instrument, measuring hydrosphere pressure variations, powerful peaks corresponding to the daily and semidiurnal tides are isolated (Fig. 5). In the spectrum of the fluorimeter, a diurnal tidal harmonic with a period of about 8 hours is isolated, and a powerful peak between the diurnal and semidiurnal tides in a period of 18 hours 34.5 minutes is isolated in the spectrum of the laser instrument, measuring hydrosphere pressure variations, which is apparently associated with the inertial oscillations of the water masses on given latitude.
When the created optical and biological complex was tested, it was established that the dynamics of phytoplankton concentration at a certain depth depends on hydrospheric pressure variations. Apparently, the phytoplankton tracing the change in hydrosphere pressure and moving vertically is on horizons with constant hydrosphere pressure. This dependence is traced in the range of surface sea waves and tides, which at certain horizons cause variations in hydrosphere pressure with an amplitude proportional to the amplitude of the wave. In standing sea waves, i. e., in seiches, the pressure with depth does not change. This leads to the fact that the dynamics of phytoplankton on specific horizons is not associated with standing sea waves.
The work was partially supported by the RSF (agreement No. 14-50-00034, processing and analysis of experimental data) and the Far East program.