Biological Oceanography
I. Introduction
Biological oceanography aims to explore the distribution, abundance, diversity, and ecological relationships of marine organisms. This relies on accurate and comprehensive data on physical and biological oceanographic characteristics. Acoustic Doppler Current Profilers (ADCPs) were originally designed to measure ocean currents. Due to their ability to simultaneously measure multiple physical and biological parameters, they have played an important role in biological oceanographic research.
II. Principles and Functions of ADCPs
Fundamentally, ADCPs operate based on the Doppler effect. They emit acoustic signals into the water column and measure the frequency shift of backscattered signals from particles and organisms in the water. From this frequency shift, ADCPs can derive current velocities at different depths. The intensity of the backscattered signal also provides information on the abundance and size of suspended particles and organisms.
III. Applications in Studying Marine Biological Activity Patterns
1. Fish Migration
Many studies related to fish migration have been improved by the use of ADCPs. Deploying ADCPs at key locations such as estuaries, bays, and along migration routes allows the tracking of fish shoal movements. Consequently, current velocity measurements from ADCPs help indicate the direction and speed of fish migration. For instance, they can monitor the return migration of salmon as they swim from the ocean back to their natal rivers to spawn. Furthermore, backscatter data provide information on the size and density of fish schools, which serve as input for studies on the population dynamics of migratory fish.
2. Zooplankton Dispersal
As a critical component of the marine food web, zooplankton exhibit complex spatiotemporal dispersal patterns. Their movements can be detected from backscatter signals generated by their small bodies. Such information improves understanding of zooplankton distribution in the water column and how they are transported by ocean currents. For example, in marine environments, ADCPs can quantify relative dispersion driven by tidal and wind-driven currents, thereby revealing zooplankton dispersal. This can improve predictions of zooplankton availability to higher trophic levels such as fish, and overall contributes to understanding the functioning of marine ecosystems.
IV. Estimation of Marine Organism Abundance
1. Biomass Estimation of Fish and Plankton
Backscatter intensity measured by ADCPs is related to the biomass of fish and plankton in the water. In principle, biomass within a given volume can be readily estimated by calibrating backscatter signals against biological samples of known size and density. In fisheries management, for example, biomass estimation using ADCPs provides approximate assessments of the population size of commercially important fish species. For plankton, biomass estimation is essential for studying primary productivity and energy flow in the marine food chain.
2. Population Density Mapping
ADCPs can be used to map the population density of marine organisms. By measuring the spatial distribution of backscatter signals and converting them into biological density maps—either by towing ADCPs over large areas or deploying a network of bottom‑moored ADCPs—they produce high‑quality results in mapping benthic distributions. For example, ADCP surveys of coral reef areas can identify boundaries of high and low coral cover, which is valuable for conservation and management.
V. Marine Habitat Mapping
1. Locating Seabed Features and Coral Reefs
Using ADCP backscatter data, the presence of topographic features such as rocky reefs, shipwrecks, and submarine caves can be identified. Many marine species depend on these diverse features. By mapping the location and characteristics of these underwater habitats, ADCPs help understand the spatial distribution of marine biodiversity. For example, ADCP surveys in coastal marine protected areas may reveal previously unknown coral reef habitats deserving special protection.
2. Mapping Seagrass Beds and Kelp Forests
Seagrass beds and kelp forests are major coastal habitats that support abundant marine life. ADCPs can map the extent and density of these habitats. The leaves and stems of seagrass and kelp generate unique backscatter signatures detectable by ADCPs, enabling assessment of habitat health and extent over timescales relevant to environmental changes such as coastal development and climate change.
VI. Interactions Between Marine Organisms and Ocean Currents
1. Larval Dispersal and Recruitment
Ocean currents drive predictable patterns in the movement of marine larvae. By providing information on currents carrying larvae, ADCPs assist research into the dispersal of fish and invertebrate larvae. Understanding larval dispersal is critical for predicting adult population recruitment and addressing connectivity among populations. In coral reef systems, for example, ADCP data can track the sources of coral larvae and identify their settlement and growth locations, which is vital for the long‑term survival and recovery of coral reefs.
2. Pelagic‑Benthic Coupling
ADCPs improve understanding of pelagic‑benthic coupling—the interactions between organisms in the water column and those on the seabed. ADCPs measure currents transporting organic matter from surface waters to the seabed and vice versa. Deploying ADCPs reveals how pelagic productivity influences benthic communities and how benthic processes affect the water column. For example, the export of organic matter produced by phytoplankton to the seabed is part of the marine carbon cycle, and ADCPs provide key information on the mechanisms and rates of this transport.
VII. Integration of ADCP Data with Other Oceanographic and Biological Data
Generally, to better understand biological oceanography, ADCP data are often integrated with other data types. For example, combining ADCP current velocity measurements with temperature and salinity data from CTD (Conductivity‑Temperature‑Depth) instruments helps explain relationships between the physical marine environment and marine organism distribution. ADCP data can also be combined with other biological sampling data such as catch records and plankton net tows to validate and refine interpretations of acoustic backscatter. Essentially, this interdisciplinary approach further advances knowledge of the ocean as a complex ecosystem.
1. Species Identification
Although ADCPs provide information on organism size and density via backscatter, species‑level identification remains challenging. Multiple variables affect backscatter signals, primarily organism size, shape, and composition. Distinguishing between species with similar acoustic properties is difficult.
2. Signal Interference
ADCP signals are subject to interference from other acoustic sources in the ocean, mainly vessel noise and marine mammal vocalizations. This distorts backscatter and current measurements, severely reducing accuracy in noisy marine environments.
3. Calibration and Validation
Reliable biological interpretations from ADCP data require robust calibration and validation. Calibration methods are often complex and require numerous samples of known organisms. Furthermore, relationships between backscatter and biological characteristics often need ongoing validation across different marine regions and seasons.