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Oceanography 101

The following page is a general overview of the fundamental concepts of oceanography! Dive in to learn about Ocean Properties, Technology, Ecosystems and Threats.

• Ocean Properties

• Ocean Technology

• Ocean Ecosystems

• Ocean Threats

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Ocean Properties

Chlorophyll

Phytoplankton have photosynthetic pigments which convert solar energy into chemical energy through the process of photosynthesis. The most abundant photosynthetic pigments produced are chlorophyll. Since phytoplankton depend upon sunlight to grow, chlorophyll concentrations rapidly decrease with depth in the ocean. Chlorophyll concentrations are measured with a fluorometer and are often used to approximate the level of primary production. The amount of primary production present provides oceanographers with important information as to how much energy is available for animlas further up the marine food chain (Lalli and Parsons, 2010).

Density

The density of an object or solution refers to its mass per unit volume and is measured in kilograms per cubic meter (kg/m3). Solutions of varying densities will arrange themselves in order of increasing density down. Similarly, objects that are less dense than the surrounding solution are positively buoyant and will float. In the ocean, the primary determinants of density of seawater are temperature, salinity and pressure. As temperature decreases and salinity and pressure increases, seawater density will increase accordingly (Lalli and Parsons, 2010).

Oxygen

Sea surface water is saturated with oxygen from a continual gas exchange across the atmosphere – ocean interface. Oxygen production by phytoplankton also increases oxygen saturation in surface waters. Ocean oxygen concentrations are commonly measured in millilitres per litre (ml/l). The vast majority of marine life depends upon oxygen to survive. Oxygen is consumed both by respiring organisms and by the bacterial oxidation of organic detritus; this causes the oxygen saturation below surface layers to gradually decrease with depth.

Pressure

Measured as the force per unit area exerted perpendicular to the surface of an object, pressure is often expressed as kilopascals or decibars (Talley et al, 2011). In the ocean, pressure is measured by instruments such as conductivity-temperature-depth (CTD) sensors or bottom pressure recorders (BPRs), which measure the weight of the overlying water column. Measuring ocean depth provides an estimate of the pressure (1metre is equivalent to 1 decibar). Pressure measurements are also used to view daily tidal cycles. During high tide, the water column height above the BPR increases and a greater pressure is exerted onto the recorder, leading to a larger reading. During low tide, the water column height decreases, and a smaller pressure is recorded.

Salinity

Refers to the concentration of dissolved salts in seawater and can be (and was traditionally) defined as the total weight (in grams) of inorganic salts dissolved in 1 kg of seawater. Since measuring the weight of salinity is a tedious task, a salinometer is commonly used to determine the salinity of seawater instead. A salinometer measures the electrical conductivity of water, which increases proportionally with increasing salt content. Both sodium and chloride ions are the predominating constituents of salt content; however, several other inorganic salts including sulphate, magnesium and potassium contribute to the salinity of seawater.

Surface salinity values are usually provided in Practical Salinity Units (PSU) based on a internationally agreed upon standard solution value. In the open ocean, salinity generally ranges from 32 - 37 PSU with an average salinity of 35 PSU, and is influenced by global climate. Salinity is increased through evaporation, and is decreased by the addition of freshwater as rain or river inflow. Within enclosed seas such as the Red Sea, high evaporation rates produce salinity values around 40 PSU, whereas seas with high river inflow such as the Baltic Sea have surface salinity values around 7 PSU. Salinity rapidly changes with depth within an area known as the halocline. Below the halocline at a depth of about 1000 m, salinity is 34.5 - 35.0 PSU at all latitudes (Lalli and Parsons, 2010).

Temperature

Water temperature is very important as it strongly influences many physical, biological, chemical and geochemical events. A continual exchange of heat and water between the ocean and the atmosphere establishes sea surface temperature. Because solar radiation intensity varies with latitude, sea surface temperatures range from temperatures exceeding 30° C in the tropical oceans to temperatures as low as -1.9° C in the polar oceans.

Turbulent mixing produced by winds and waves transfers heat downward from the surface and can create a mixed surface layer with uniform temperature. Below the mixed layer, water temperature begins to rapidly decline with depth. This area within the water column is known as the thermocline and generally occurs between 200 - 1000 m. Apart from hydrothermal vents, water temperatures never rise above 4° C below the thermocline (2000 - 3000 m), regardless of latitude (Lalli and Parsons, 2010).

Ocean Technology

Acoustic Doppler Current Profiler (ADCP)

Measures water current velocities (speed and direction) using the Doppler effect of sound waves scattered back from particles within the water column. ADCP’s can either be permanently deployed on the ocean seafloor, or deployed on a moving ship. Backscatter data from ADCP’s can also be analyzed to provide information about fish, plankton and bubbles in the water column.

Bottom Pressure Recorder (BPR)

This instrument measures the pressure of the water column sitting above it. The instrument is sensitive enough to predict millimetre changes in the water level above it. The instrument can be used to measure the increase in water during tidal activity, and can also be used to detect tsunamis. When a tsunami passes over a bottom pressure sensor, the instrument detects that the water pressure above it is significantly greater than normal, and can alert researchers to the anomaly.

CTD

A CTD is used to measure the Conductivity, Temperature, and Depth in the water. A CTD can be deployed in a fixed location on an observatory (e.g. VENUS and NEPTUNE underwater cabled observatories), or it can be lowered over the side of a ship to collect a continuous profile of the water properties through the water column. The sensors in the instrument actually measure conductivity, temperature and depth, which can be used to derive salinity, pressure, density and sound speed. CTDs are used to characterize basic water properties and are fundamental in a wide range of oceanographic research areas. Additionally, CTD’s can be equipped with an oxygen sensor to detect the oxygen concentration of seawater.

Echo-sounder

Acoustic signals emitted by an echo-sounder reflect (echo) off of bio-matter and bubbles in the water column and then propagate back to the instrument, where they are measured and recorded. For example one type of echo-sounder is the Zooplankton Acoustic Profiler (ZAP), which produces an image using recorded acoustic backscatter to asses s how the vertical distribution of zooplankton and fish vary over time.

Fluorometer

Used to identify the presence and amount of chlorophyll in seawater. A fluorometer detects electromagnetic waves (e.g. light) which at depth usually is a measure of the fluorescence of a substance. Fluorescense is the emission of light by a substance that has absorbed light or other electromagnetic radiation.

Hydrophone

An underwater microphone used to detect and record ambient sound in the ocean. Using hydrophones, scientists study marine mammal vocalizations, noise pollution such as that caused by shipping traffic, and seismic activity.

Secchi Disk

Created in 1865 by Pietro Angelo Secchi, the Secchi disk is a circular disk separated into black and white quarters. It is used to gauge water clarity (or turbidity) by measuring the depth at which it is no longer visible from the surface.

Sediment Traps

Used to collect samples of falling marine particles, sediment traps are like rain gauges for oceanic materials. The instrument is placed on the sea floor with the open end pointed to the surface. As marine materials (nutrients, dead plankton, dead animals and plants) sink to the bottom, they become ‘marine snow’. These particles are collected in sediment traps and studied by scientists.

Seismometers

A seismometer measures the movement of the earth. Seismometers work on the principle of inertia. A small mass of metal within a coil inside the seismometer doesn’t move until another ground shaking acts upon it. When the earth moves, it moves the seismometer and the metal mass generates a current in the coil which is then recorded. Very sensitive seismometers can even detect movements smaller than a millimetre.

Sonar

Sound travels better through water than radar or light, so it is often used to determine where objects are underwater. The instrument emits a signal or pulse of sound and if an object is in the way of the sound, some of the sound will hit the object and bounce off of it back and return to the sonar device. The instrument then uses this information to determine where the object is the water and create a ‘sound map’ of the area. In nature, sonar is called ‘echolocation’.

Turbidity Sensors

Turbidity is a measure of how much matter is dissolved in water. A water sample with a high turbidity has large amounts of particles in the water, which can make it difficult to see through. Water with low turbidity has a small amount of particles in the water and looks clear or nearly clear. Turbidity can be used to determine water conditions which can be used to monitor the health of an area. For example an area with lots of plankton growth has a lot of nutrients and a lot of turbidity. And area with minimal nutrients and not a lot of plankton growth would have a low turbidity. Also, underwater landslides generate a lot of turbidity and the sediment plumes can be seen with turbidity sensors, too.

Video Camera

An underwater video camera system consists of a network video camera, typically mounted on a pan and tilt mechanism, with lights and lasers for estimating the size of objects in the field of view. Video data is used by biologists to study organisms including their diversity, distribution, feeding and behaviour. Other visual aspects of the deep sea such as geological features and environmental changes can also be observed in video.

 

Ocean Ecosystems

The ocean supports a great diversity of ecosystems spanning from the deep sea to the shallow intertidal zone. Community composition and species richness varies widely.

Deep Sea

The deep-sea environment includes a wide range of depths identified as the bathypelagic (1000 - 4000 m), abyssopelagic (4000 - 6000 m) and hadalpelagic (6000 - 11000 m) zones. At these depths, seafloor environments are relatively stable and homogenous with respect to both physical and chemical parameters compared to the shallow waters. Water temperatures remain quite low (-1° to 4°C) and salinity values average around 35 PSU. The seafloor consists primarily of soft bottom sediments and clays, originating from both land and the sinking of dead planktonic organisms.

Marine organisms occupying deep-sea environments must endure low temperatures, darkness and high pressures. Organisms acquire nutrients primarily from marine snow – a shower of organic detritus from the photic zone. Consequently, life in the deep sea is highly dependent upon surface primary production as a food source, even though only a small percentage (~ 1 - 5%) of the phytoplankton produced in the euphotic zone are transferred to the ocean bottom. Food limitation in the deep sea is the primary reason why population density and biomass are much less than shallow water areas (0 - 200 m).

Unlike species abundance, species diversity increases with depth from about 200 - 2500 m. The wide range of organisms observed at these depths includes small crustaceans, Molluscs (snails and clams), Cnidaria (sea anemones and sea pens), primitive crinoids (stalked sea-lilies) and echinoderms (sea stars and sea-cucumbers). Deposit-feeding infaunal organisms generally dominate over other organisms in the deep sea because of the abundance of soft organic-rich sediments on the seafloor. Organisms that feed on suspended particles are much less abundant in the deep sea and are usually restricted to particular localities. This is because most of these organisms (i.e. sea anemones and barnacles) require hard substrates to which to attach and high concentrations of suspended food particles (Lalli and Parsons, 2010).

Hydrothermal Vents

Hydrothermal vents form along ocean spreading centres and back-arc basins where seawater percolates through the thin ocean crust to form hydrothermal fluid. Seawater becomes enriched in sulfur and dissolved minerals (e.g. iron, zinc and copper) through reactions with superheated rock within fractures and permeable zones in the seafloor, near the magma chamber, and is released as superheated (250° - 400°C) buoyant plumes of hydrothermal fluid. Once the vent effluent mixes with the cold seawater, minerals precipitate and form black metal sulfide deposits and tall chimneys. When the seawater does not penetrate deep enough into the ocean crust, chemical reactions are partial and the fluid is released as diffuse flow characterized by lower temperatures (20° - 50°C). The mixing of hydrothermal fluid with seawater generates steep heat and chemical gradients, sometimes at the scale of a few centimetres.

Vents are home to an endemic faunal community independent of energy from sunlight and photosynthetic organisms. The vent food web fully relies on chemical fluxes as a source of energy, through a process known as chemosynthesis. Vent organisms including limpets and snails graze upon dense mats of sulphur oxidizing bacteria, whereas tubeworms obtain nutrients is through symbiosis with sulphur oxidizing bacteria that live in their gut or trophosome.

 

Ocean Threats

Coral Bleaching

Bleaching occurs when stress causes corals to expel symbiotic algae (zooanxthelle) resulting in a loss of colour (hence the appearance of being bleached). Several factors can cause bleaching, many tied to climate change, including: seawater warming, ocean acidification, sea-level rise, or changes in storm intensity. However, the biggest driver resulting in mass bleaching events is warming temperatures. By 2080, 80 to 100 per cent of the world's coral reefs will suffer annual bleaching events due to global warming.

Eutrophication

The release of excess nutrients (primarily nitrogen and phosphorus) into a water way arising from human activities (industry, mining, farming, storm drains etc.). Excess nutrients promote increase algal growth, sometimes resulting in massive and lethal "blooms."

Habitat loss

The destruction of ecological structures and functions vital to maintaining the richness and abundance of species native to an area. While habitat loss can be caused by natural hazards such as lightning strikes or avalanches, the human-caused (anthropogenic) loss of habitat is of greatest concern. Activities include bottom trawling, crude oil spills, pollution and industrial, urban and agricultural development. These activities are incredibly destructive to a variety of marine environments, particularly estuaries, swamps, marshes, and wetlands, which serve as breeding grounds or nurseries for nearly all marine species (Lalli and Parsons, 2010).

Hypoxia

Low oxygen environments (hypoxic zones) are increasing throughout marine coastal ecosystems on a global scale (Diaz and Rosenberg, 2008). Hypoxic zones occur when oxygen levels fall under 1.5 ml/l, resulting in an environment where only a limited number of adapted species can thrive (Ocean Properties, 2013). Marine hypoxia can occur either naturally in deep basins, fjords and upwelling regions, or through anthropogenic disturbances. The main factors responsible for increasing hypoxic environments are eutrophication and climate change.

Eutrophication occurs when an excess of nutrients enters into the ocean either by natural or anthropogenic processes (i.e. industrial activities or mining practices) (Diaz and Rosenberg, 2008). The nutrient excess triggers massive phytoplankton blooms. When phytoplankton cells die, the high level of organic matter is decomposed by bacteria – a process which exhausts the oxygen supply (Ocean Properties, 2013). Climate change increases sea surface temperatures, leading to reduced oxygen solubility and increased water stratification. When the water column is stratified, oxygen saturated surface waters no longer recharge bottom waters at a sufficient rate, leading to hypoxic conditions.

Ocean hypoxia can pose harmful effects on the growth, survival, reproduction and behaviour of many marine species. Compared to other coastal and deep-sea environments, biodiversity is lower in hypoxic regions (e.g. Matabos et al., 2012). A number of coastal zones around the world are significantly impacted by hypoxic conditions. Saanich Inlet, located on the southeastern side of Vancouver Island is affected by ocean hypoxia. Saanich Inlet is a naturally occurring hypoxic estuarine fjord due to high productivity and the presence of a shallow sill located at the mouth of the inlet. This sill restricts movement of neighbouring oxygen-rich waters into the deep basin (Manning et al., 2010).

Ocean Acidification

A decrease in the pH (increase in the acidity) of the Earth’s oceans. Concentrations of anthropogenic carbon dioxide (CO2) in the Earth’s atmosphere have been rising at a rapid rate from human fossil fuel consumption (Guinotte and Fabry, 2008). An increase in atmospheric CO2 leads to a rise in oceanic CO2 levels through continual air-sea gas exchange. When CO2 dissolves in the ocean it reacts with seawater and carbonate, leading to a decrease in the amount of available carbonate in the ocean (Emerson and Hedges, 2008).

A decline in the available carbonate ions means that calcifying organisms must expend significantly more energy to build and maintain their hard shells. This will have a direct impact on the marine organisms which build shells composed of either biogenic calcium carbonate or aragonite. These organisms include tropical and cold water corals, mollusks (clams and mussels), echinoderms (sea stars), phytoplankton (foraminifera and coccolithophores), zooplankton (pteropods) and crustose coralline algae. In return, the decline of calcifying organisms can have a substantial impact on the marine ecosystem. Many calcifying organisms are an important source of nutrition and shelter for higher-trophic level organisms (Guinotte and Fabry, 2008).

Overfishing

Arguably the most serious and detrimental human impact on marine ecosystems, fishing removes more than 100 million tonnes of fish and shellfish every year. Large fishing vessels are now equipped with thousands of baited long lines and mid-water trawl nets with a mouth gape of 130 m and length of 1 km. These advances in fishing technology have made it easier to catch significantly more fish in a smaller amount of time, negatively impacting the species composition of both pelagic and benthic communities.Commercial fisheries discard about one of every four animals caught, although the percentage of by-catch may be much larger because the majority of it goes unreported. Fisheries discard species which have limited economic value or are too small. In particular, shrimp fisheries in the Gulf of Mexico catch and discard at least 5 million juvenile red snapper annually. Unfortunately, the majority of by-catch species are unable to survive after they have been released (Lalli and Parsons, 2010).

 

References

Lalli, C. M., and Parsons, T. R. (2010). Biological Oceanography: An Introduction. (2nd ed.). Burlington, MA: Elsevier Ltd.

Talley, L., Pickard, G. L., Swift, J., and Emery, W. J. (2011). Descriptive Physical Oceanography: An Introduction. (6th ed.). Burlington, MA: Butterworth-Heinemann.

Emersen, S.R., and Hedges, J.I. (2008). Chemical Oceanography and the Marine Carbon Cycle. New York, USA: Cambridge University Press.

Ocean Properties. (2013). Retrieved March 24. 2013, from http://venus.uvic.ca/research/ocean-properties/.

Diaz R., and Rosenberg R. (2008). Spreading dead zones and consequences for ecosystems. Science. 321(5891). Pages: 926-929.

Matabos M., Tunnicliffe V., Juniper S.K., and Dean C. (2012). A year in hypoxia: epibenthic community responses to severe oxygen deficit at a subsea observatory in a coastal inlet. PLoS ONE. 7(9):e45626, DOI:10.1371/journal.pone.0045626

Guinotte, J. M., and Fabry, V. J. (2008). Ocean acidification and its potential effects on marine ecosystems. Annals of the New York Academy of Sciences, 1134, 320-42. doi:10.1196/annals.1439.013

Manning, C., Hamme, R.C., and Bourbonnais, A. (2010). Impacts of deep-water renewal events on fixed nitrogen loss from seasonally-anoxic Saanich Inlet. Marine Chemistry. 122(1-4). Pages: 1-10.