Biological Oceanography  

1. Introduction

Biological oceanography seeks to know the distribution, abundance, diversity, and ecological relationships of marine organisms. It is based on correct and complete data of physical and biological ocean features. ADCPs, initially devised for oceanographic current measurement, have been helpful in biological oceanography because of their capability to measure a variety of physical and biological parameters simultaneously.

2. ADCP Principles and Functionality

Fundamentally, ADCPs work on the principle of the Doppler effect. They send acoustic signals into the water column and measure the frequency shift of the backscattered signals by the use of particles and organisms in the water. From this frequency shift, it is possible for the ADCP to interpret the speed of the water current at different depths. The intensity of the backscattered signal may also give information on the quantity and size of the suspended particles and organisms.

3. Applications in the study of marine organism movement patterns

3.1 Fish Migration

Numerous works related to fish migration have been complemented by the use of ADCP. Mounting an ADCP near strategic locations such as at the river mouths, estuaries, and along the migratory corridors have been able to track the moving schools of fish. The measured velocity from the ADCP would, therefore, be useful to indicate the direction and speed of fish migration. It would also be similarly capable of monitoring, for instance, the traffic of migratory salmon both ways as they go up from the ocean into their rivers to their birthplace to spawn. In addition, it will provide information on the size and density of schools through backscatter data which may be used as inputs to the studies of population dynamics of migratory fish species.

3.2 Zooplankton Dispersal

Being a major link in marine food webs, zooplankton present more complicated spatial and temporal dispersion patterns. The zooplankton movement can be detected from the backscattered signals that emanate from their small bodies. Such types of information provide insight into the distribution of zooplankton within the column of water and how they are conveyed by ocean currents. For instance, in the marine environment, it can quantify the relative diffusion due to tidal currents and wind-driven currents and hence the dispersal of zooplankton. This may likely result in improved forecasting concerning the zooplankton availability for higher trophic level organisms like fish but generally for the whole functioning of the marine ecosystem.

4. Estimating Marine Organism Abundances

4.1 Biomass Estimation of Fish and Plankton

The intensity of the backscattering measured by an ADCP originates from the biomass of fish and plankton in the water. In principle, it would be easily possible to estimate the biomass of fish and plankton present in a volume by only calibrating the backscatter signal versus well-known samples of organisms of known size and density. For example, biomass estimates by ADCP have enabled an approximation of the stock size of commercially important fish species in the management of fisheries. In addition, plankton needs biomass estimation for the study of primary productivity and the flow of energy in the oceanic food chain.

4.2 Population Density Mapping

The ADCPS can be used in the preparation of the population density map of marine organisms. This will enable the ADCPs to measure the spatial distribution of the backscattering signal, which may then be transformed into maps of density of organisms either by towing an ADCP over a large area or deploying a network of bottom-moored ADCPs. Results obtained are quite exceptional in charting the distribution of the benthic organisms lying in the seabed. Such an ADCP survey of a coral reef area can be used in determining boundaries on either high or low coral cover areas; very useful for conservation and management purposes, for instance.

5. Mapping of Marine Habitats

5.1 Locating Submarine Features and Reefs

It is possible to determine the presence of subsea topographical features like rocky reefs, wrecks, and submarine caves through the use of ADCPS backscatter data. Various marine species depend on a myriad of such features. By mapping the location and character of these submerged habitats, ADCPs are enabling us to understand the spatial distribution of marine biodiversity. For example, ADCP surveys of a coastal marine protected area may identify the presence of previously unknown reef habitats that should be given special protection.

5.2 Mapping Seagrass Beds and Kelp Forests

Seagrass beds and kelp forests form two of the main coastal habitats that provide homes to enormous amounts of marine life. The same ADCPs allow mapping of the extent and density of such habitats. Leaves and stems of both seagrass and kelp give a distinctive signature from the backscatter signal, which an ADCP is able to detect in such a way as to enable determination of health and extent of those habitats on timescales relevant to environmental changes such as coastal development and climate change.

6. Interaction of Marine Organisms with Ocean Currents

6.1 Larval Dispersal and Recruitment

The ocean currents make the movements of the larvae of marine organisms quite mechanical. ADCPs can help in studying the dispersal of fish and invertebrate larvae by providing information about the currents carrying them. Understanding of larval dispersal will be crucial for any prediction of recruitment into the adult population and for any resolution of connectivity among diverse populations. For instance, in a coral reef system, ADCP data can be used to track the origin of coral larvae and determine their settlement and growth locations, something vital for long-term survival and recovery of coral reefs.

6.2 Pelagic - Benthic Coupling

Due to this fact, the ADCPs can provide a better insight into the pelagic-benthic coupling, that is, the interaction between the organisms of the water column and those from the seafloor. The ADCPs will be used in measuring the currents carrying organic matter from surface waters into the seafloor and vice versa. Installing the ADCPs will understand how productivity in the pelagic zone influences the benthic community and vice versa. For example, the export of organic matter from the sea floor produced by phytoplankton is a part of the ocean carbon cycle, and ADCPs provide essential information with respect to the mechanics and rate at which that transport occurs.

7. ADCP Data Integration with Other Oceanographic and Biological Data

Generally speaking, in order to better comprehend biological oceanography, ADCP data is usually integrated with other forms of data. For example, the combination of ADCP current measurements and the CTD temperature and salinity data may be applied in interpreting how the physical oceans relate to the distribution of marine organisms. The ADCP data may also be combined with other biological samplings, like fish catch and plankton tows, in validating and developing further the interpretation of the acoustic backscatter signal. In essence, it is an interdisciplinary approach that will further enhance our knowledge regarding the ocean as a complex ecosystem.

8. Challenges and Limitations of ADCP Applications in Biological Oceanography

8.1 Species Identification

While ADCPs are capable of providing size and density of organisms by means of backscatter, identification of the species level for those organisms remains one of the challenges. Several variables will affect the backscattered signal, mainly by the size, shape, and even composition of the organisms, and discrimination among several species will have to be done under similar acoustic properties.

8.2 Signal Interference

These ADCP signals interfere with other acoustic sources in the ocean, mainly shipping noise and marine mammal calls. That interferes with the backscatter and velocity measurements-whose accuracy can be seriously deteriorated for noisy ocean conditions.

8.3 Calibration and Validation

This requires good calibration and validation to obtain dependable biological interpretation of data from ADCPs. The methods of calibration are usually complex and need a number of samples with known organisms. Further, the relationships of backscatter against those characteristics of organisms usually have to be continually validated for various regions of oceans and seasons.

9. Future Perspectives

Irrespective of the challenges, there is promise for the future in how ADCP applications are used in biological oceanography. This would be possible as some of the deficiencies in species identification can be overcome along with progress in both signal processing and machine learning techniques. For instance, machine learning algorithms in the analysis of backscatter signal patterns will give an increased ability to distinguish between species. In addition, new advanced ADCP sensors that are designed at higher resolution and frequency will enable the study of small and elusive marine organisms. The combination of ADCPs with other emerging technologies, such as AUVs and underwater gliders, would provide a comprehensive study of the ocean ecosystem in much greater detail.

Finally, ADCPs have established themselves as a useful tool in biological oceanography, while most of the applications ranging from the study of the motion of organisms, abundance estimates, habitat mapping, and ecological interactions have updated considerably the present information concerning the marine environment. Of course, some problems may be associated with its use, but further advances in technology and research works will most likely increase its role in unveiling secrets of the ocean's biological domain.

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Here is a table with some well known ADCP instrument brands and models.