Chemical and Physical Environment

The Water in Seawater

No discussion of the chemical and physical environment of the ocean can start without a quick look at the structure of water.

Water is the most abundant substance in the ocean. Water has unusual physical characteristics. Water has a relatively high boiling point so most water on Earth is in liquid form. Water is essential for life because of its ability to dissolve most substances. Water stores and controls the distribution of heat.

Henry Cavendish discovered the composition of water in the 1760s when he synthesized it by combusting H2. Water is composed of an oxygen atom and two hydrogen atoms. Each of two atoms of hydrogen are covalently bonded to the oxygen atom.

Hydrogen Bonds

We just looked at the structure of water and saw that each of two hydrogen atoms is bonded to one oxygen atom. The higher electronegativity of the oxygen atom and its two pairs of unshared electrons cause the oxygen end of the molecule to have a small net negative charge. The hydrogen ends have small net positive charge. This unequal charge distribution influences the orientation of water molecules and allows for hydrogen bonding. Hydrogen bonding is a relatively weak intermolecular force caused by electrostatic attraction between the negatively charged end of one water molecule with the positively charged end of another water molecule.

Hydrogen bonding causes liquid water to have many notable physical characteristics, such as anomalously high boiling and freezing points. In the rest of this lesson, we will explore seawater's chemical and physical characteristics and their impacts on life in the ocean.

Water as a Solvent

Water is called the universal solvent because of its ability to dissolve at least a little of virtually every substance. Water is most effective at dissolving substances held together by polar or ionic bonds. Only small amounts of nonpolar substances, such as oil in oil spills, will dissolve in water.

Salts are composed of atoms held together by ionic bonds. The bonds result from electrostatic attraction between positively charged ions called cations and negatively charged ions called anions. When placed in water, salts dissolve because the cations and anions are electrostatically attracted to the water molecules. The figure below shows how sodium chloride (table salt) dissolves in water.

The cation (sodium, yellow circles) attracts the oxygen ends of the water molecules (red circles). The anions (chloride, green circles) attract the hydrogen ends (small white circles). When the water surrounds the ions, they are too far apart to exert a force of attraction on one another.

Dissolution of Gases in Seawater

Ions are not the only chemicals dissolved in seawater. The ocean also contains dissolved gases that are important to living organisms, such as oxygen (O2), carbon dioxide (CO2), and nitrogen (N2). Gases move between the atmosphere and ocean, helped by mixing from wind and waves. Although the ocean and atmosphere exchange gases, the relative amounts of each gas in air is very different from the amount found in the ocean.

The amount of each gas that can dissolve in the ocean depends on the solubility and saturation of the gas in water.

Solubility refers to the amount of a dissolved gas that the water can hold under a particular set of conditions (defined temperature, salinity, and pressure). The solubility of a gas increases with increasing pressure, decreased temperature, and decreased salinity.

Saturation refers to the amount of gas currently dissolved in the water, relative to the maximum possible content. If the water is undersaturated, more gas can dissolve. If the water is saturated or supersaturated, gas may be released.

Oxygen Concentrations

Water in deep ocean is separated from the atmosphere so circulation is important for mixing gases throughout the vast majority of the ocean. Deep water formation in the Weddell Sea and near Greenland supplies oxygen-rich water to the deep sea, spreading throughout the ocean by means of global thermohaline circulation (remember the conveyor belt). Upwelling and turbulence carry water low in oxygen from the deep ocean to the surface. These same circulation patterns also affect the exchange of CO2 between the atmosphere and ocean.

This figure from the journal Oceanography shows the concentration of O2 (µmol kg-1) at 1000 m for the Pacific Ocean basin and Japan/East Sea. High oxygen (purple) indicates more recent penetration of surface waters to depth.

Salinity I

As water moves through the hydrologic cycle, it interacts with rocks, soil and the atmosphere. Some materials are dissolved into the water and are added to the ocean through riverine run-off, precipitation, and hydrothermal activity along the mid-ocean ridge. The total concentration of all dissolved inorganic materials in water is called salinity.

Salinity II

Most major elements in the world ocean are in approximate steady state: the amount added is balanced by the amount removed when averaged over long time periods. The most abundant ions in seawater are present in constant proportions controlled by physical processes such as the addition and removal of water. These salts are called conservative ions. The rest of the substances dissolved in seawater are not present in constant proportions because their concentrations are altered by chemical reactions. These chemicals are called nonconservative ions.

The most abundant conservative ions are chloride, sodium, sulfate, magnesium, calcium, and potassium. You can see in the table below that these ions account for over 99% of the total salinity of seawater.

Average Sea Surface Salinity

Although the relative concentrations of the conservative ions do not change throughout the ocean, total salinity does. Salinities are highest in areas with high rates of evaporation and limited circulation. Salinities are lowest in areas of the ocean where precipitation rates are high and in areas where riverine discharge and run-off are high. This map shows average sea surface salinities during the summer in the Northern Hemisphere.

Temperature in the Ocean

Earth planet is heated by solar radiation from the sun. Because the Earth is round, the angle of the surface relative to the incoming radiation differs with latitude. At low latitudes, near the equator, direct overhead sunlight received all year warms surface waters. At high latitudes, ocean waters receive less sunlight – the poles receive only 40 percent of the heat that the equator does. These variations in solar energy mean that the ocean surface can vary in temperature from a warm 34°C in the tropics to a very cold -2°C near the poles (see figure; SST means sea surface temperature).

Temperature Ranges

In some areas, sea surface temperature is relatively stable while in others, it fluctuates depending on the season (and thus the amount of sunlight received).

Temperature is most variable in the surface ocean, including shallow coastal areas. This figure shows how the range of temperatures in the surface ocean as you move from the epipelagic to the neritic and the littoral zones. These areas all experience daily and seasonal variations in sunlight that drive changes in temperature.

Vertical Temperature Profiles

The temperature of ocean water also varies with depth. In the ocean, solar energy is reflected in the upper surface or rapidly absorbed with depth, meaning that the deeper into the ocean you descend, the less sunlight there is. This results in less warming of the water. Therefore, the deep ocean (below about 200 meters depth) is cold, with an average temperature of only 4°C. Cold water is also more dense, and as a result heavier, than warm water. Colder water sinks below the warm water at the surface, which contributes to the coldness of the deep ocean. The vertical structure in the ocean created by temperature differences has a large impact on how life is distributed in the ocean.

This figure shows vertical temperature profiles in the ocean at different latitudes: The tropics, midlatitudes and polar regions. The y-axis shows depth with the ocean surface at the top of the graph (0 meters deep). The x-axis of each panel is temperature. Red curves represent the profiles in summertime and the blue curves represent a typical winter profile. The two curves are furthest apart in the surface layers at mid- and polar- latitudes. These are areas with the largest seasonal temperature ranges.

Thermal Expansion

Most substances decrease in density as temperature increases and increase in density as temperature decreases. This pattern is a result of thermal expansion. Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature like the air in these balloons:

Water does not follow the same rules of thermal expansion. It responds to heat differently as a result of hydrogen bonding. As water cools and freezes, hydrogen bonds form between water molecules forming a crystal lattice (see the figure below). The ice has more open space than the liquid water so the ice floats on any remaining liquid water.

Freezing Point of Seawater

Adding dissolved ions (salt) interferes with hydrogen bonding between water molecules as water cools. The typical salinity of the open ocean (35) is high enough to cause cooling water to behave more like typical substances and get denser and denser as it approaches the freezing point. The salts also reduce the freezing point of water. Seawater freezes at approximately -1.9°C. We'll see later that the low freezing point can create challenges for organisms in polar regions.

Light

Light is electromagnetic radiation within the visible portion of the spectrum (range of 400–700 nanometers).

Light Penetration I

Light in the ocean decreases with depth. How far it penetrates is dependent on how much material is suspended in and settling through the water column. In the open waters of the tropics where the clearest waters are found, sunlight can penetrates the upper 200 meters of the ocean (epipelagic or photic zone) in sufficient quantities to support the growth of photosynthetic organisms. Depths below 1,000 meters receive no light from the surface. The only light available at these depths is generated by organisms.

Marine biologists define and name layers of the ocean by the amount of sunlight they receive:

• The euphotic zone is the upper part of the ocean that receives bright and clear sunlight. In clear tropical waters, the euphotic zone may extend to a depth of 80m. Sunlight does not penetrate as deeply near the poles, so in these areas the euphotic zone may be less than 10 m deep. Turbid, muddy waters may have a euphotic zone only a few centimeters in depth. The euphotic zone is the upper part of the photic zone.
• The disphotic zone is the water layer beneath the euphotic zone. In clear water it may extend as deep as 800-1000 m. The dim blue light that penetrates this zone is not sufficient to sustain photosynthetic organisms. The disphotic zones is the lower part of the photic zone.
• The aphotic zone is the water layer where there is no visible sunlight. Most of the water in the ocean lies in the aphotic zone.

Light Penetration II

Light interacts with water molecules so not all colors of light penetrate to the same depths. Red light is attenuated in water at very shallow depths and blue light penetrates the deepest. This figure shows depths of penetration in clear water. Remember, if ocean water is clouded by sediment or microscopic organisms then light cannot penetrate deeply at all.

Accessory Pigments

Photosynthetic organisms in the ocean have adaptations to capture as much light as possible and to survive in low light conditions. One adaptation is the use of accessory pigments that complement chlorophyll. The graph on the left below shows the wavelengths of light absorbed by different pigments. The graph on the right shows the wavelengths of light absorbed by different kinds of marine photosynthetic organisms. The figure shows the visible color of cultures of the organisms.

Pressure

Pressure is not on our list of learning objectives for this lesson, but it does impact organisms in the sea, especially those that change depth frequently. We'll touch on it briefly now and dwell on it a bit more in depth when we discuss marine mammals later in the course.

Pressure increases approximately 1 atm for every 10 m increase in water depth. Thus at the average depth of the ocean, approximately 4000m, the pressure is 400 times greater than at the sea surface. Due to the low compressibility of water, this large increase in pressure causes only a slight increase in density. This, too, is a function of hydrogen bonding which restricts how close the water molecules can be pushed together.

While seawater does not significantly compress with increasing pressure, gases do. Organisms with gasfilled organs must have adaptations to cope with the expansion and compression of gases as they change depth in the ocean. Have you ever been fishing and brought a fish to the surface too rapidly? Barotrauma is the injury caused to organs when gasses within their swim bladder and other organs expand during rapidly reducing pressure. The canary rockfish below is suffering from barotrauma, as shown by the bulging eyes (photo: https://barotrauma.ucsc.edu/research/research-sub/).

Response of Organisms

The Only Thing That Is Constant Is Change ― Heraclitus

As we discussed the physical and chemical environment, I hope a few things started to pop out at you. The deep ocean has relatively uniform conditions because it is separated from the sun and atmosphere. In contrast the surface ocean and coastal regions vary in conditions on short and long time scales. For example, the littoral and neritic zones are affected by tides, interactions with land and day-night cycles over the short term. Longer-term changes such as seasonal shifts in temperature and light affect the surface ocean everywhere outside the tropics.

Organisms living in a changing environment use a process called homeostasis to actively maintain fairly stable internal conditions necessary for survival. Homeostasis relies on regulating responses via multiple mechanisms:

Behavioral: Often the first response of organisms to an environmental change is to modify their behavior. A marine lizard will behaviorally thermoregulate by basking in the sun to warm up before swimming in the ocean.

Physiological: When behavioral mechanisms are insufficient, many organisms use physiological responses. During diving, marine mammals initiate a series of cardiovascular changes that include bradycardia (slowing heartbeat) and decreased peripheral circulation to conserve heat.

Biochemical: Animals that are constantly exposed to changing conditions, will often have stores of chemicals that maximize performance during change or stress. For example, many marine invertebrates maintain pools of fermentable amino acids for periods of oxygen limitation.

Genetic/Evolutionary: Permanent change in a population involves the selection of genetic variants that are best suited to meet new environmental challenges.

Patterns of regulation range from conformance to external conditions to complete regulation of internal conditions.

• Conformers allow the environment to determine their internal composition or condition for a certain range of environmental conditions (left panel figure).
• Regulators maintain constancy, or stability, in their bodies during environmental change (right panel figure).

Let's revisit our chemical and physical environmental parameters in the context of adaptative regulation and homeostasis.

Salinity and Osmoregulation

Salinity affects organisms through the process of osmosis and diffusion. In near-shore environments salinity can change rapidly, for example with precipitation events or shifting currents. Most cellular processes rely on specific ionic conditions within cells and across membranes. Changes in salinity can pose a major challenge to cellular function. Osmosis refers to the movement of fluid across a membrane in response to differing concentrations of solutes on the two sides of the membrane. Organisms in the ocean generally have bodies and tissues with lower concentrations of salts than the surrounding seawater. This means water is constantly diffusing across their membranes, dehydrating them. In the figure below, the flagellate in the far right panel represents osmosis in a salty environment.

Osmoregulation in Fishes

Organisms in the ocean have evolved many ways to cope with living in a salty solution. We'll spend more time on this topic later in the semester, but for now let's introduce some of the basics.

Osmoconformers match the concentration of solutes in their body to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater.

Osmoregulators tightly regulate the concentration of solutes in their bodies, maintaining constant internal conditions despite the salt concentrations in the environment. They are more common in the animal kingdom, including many marine fishes. Marine teleost fish (bony fish) have an internal osmotic concentration lower than that of the surrounding seawater, so they tend to lose water and gain salt. Many marine fish actively drink seawater and excrete salt out their gills to compensate.

Elasmobranchs (cartilagenous fish) have evolved the technique of reabsorbing and retaining urea and other body fluid solutes in their tissues so that serum osmolarity (solute/solvent concentration) remains just greater than that of the external seawater. Sharks, having slightly higher internal solute concentration, do not drink seawater.

We'll see many more examples of how organisms cope with living in a salty environment in the second part of the course as we survey diversity in marine organisms.

Oxygen

Nearly all eukaryotic organisms are aerobes, meaning they require oxygen for life. We generally think of oxygen being required for cellular respiration and the production of ATP, but many marine organisms can use alternative metabolic pathways that do not require oxygen. diving mammals may rely on anaerobic glycolysis to breakdown glucose or glycogen. As we mentioned a short while ago, some invertebrates use amino acids for metabolism.

Unicellular organisms, marine algae and small animals (a few millimeters thick) rely on diffusion for oxygen uptake. Larger animals have specialized organs such as gills, branchae and lungs for uptake. These larger animals use blood pigments (oxygen-carrying proteins; see list and figure below) to transport the oxygen from the organs to the rest of their bodies:

• Hemocyanin - copper-containing protein, found in molluscs, arthropods
• Hemerythrin - iron-containing protein, always in cells, found in sipunculids, some polychaetes, prapulids, brachiopods
• Chlorocruorin - iron-containing protein, found in some polychaetes
• Hemoglobin - protein unit (globin) and iron-bearing unit (heme), found in many phyla (chordates, molluscs, arthropods, annelids, nematodes, flatworms, protozoa)
• Myoglobin - iron-porhyrin protein, found in molluscs, also bound in skeletal and muscle tissue of some vertebrates

Respiratory pigments greatly increase the amount of oxygen blood can hold and transport. Many of these pigments are responsive to chemical changes in blood triggered by low oxygen conditions. For example, during shortages of oxygen the shape of the pigment molecules can change such that they have the capacity to bind more oxygen.

Temperature and Organisms

Internal temperature changes may adversely affect many aspects of animal physiology, including enzyme function, muscle activity, and energy metabolism. There are two primary responses to fluctuating ambient temperatures exhibited by animals: poikilothermy and homeothermy (figure).

Poikilotherms lack the physiological means to generate heat, so the body temperature of these animals tends to conform to that of the outside environment in the absence of any behavioral intervention.

Homeotherms have specific physiological adaptations for regulating their body temperatures; body temperatures of homeotherms do not fluctuate as much as those of poikilotherms. Homeotherms maintain high body temperatures in the range of 36 to 42°C.

Poikilotherms are also known as ectotherms because their body heat is derived exclusively from their external environments. This external thermal dependence enables them to employ behavioral thermoregulation by 1) shuttling between areas with lower and higher temperatures and 2) changing body positions to adjust heat exchange via conduction and radiation.

Homeotherms also use behavioral thermoregulation (i.e., habitat choice - think migration in whales to warmer nursery grounds) to adjust their body temperatures but, unlike poikilotherms, they do not depend solely on the outside environment as a source of body heat. Instead, homeotherms use physiological mechanisms to regulate their body temperatures independently from ambient temperatures.

Temperature and Species Distributions

Adapted from: Climate Change Indicators in the United States: Marine Species Distribution www.epa.gov/climate-indicators - August 2016

Changes in water temperature can affect the environments where marine species live. Certain fish species naturally migrate in response to seasonal temperature changes, moving northward or to deeper, cooler waters in the summer and migrating back during the winter. As climate change causes the oceans to become warmer year-round, however, populations of some species may adapt by shifting away from areas that have become too warm and toward areas that were previously cooler. As smaller prey species shift their habitats, larger predator species may follow them.

Marine species represent a particularly good indicator of warming oceans because they are sensitive to climate and because they have been studied and tracked for many years. Fish are especially mobile, and they may shift their location more easily than species on land because they face fewer physical barriers. Also, many marine species, especially fish, do not have fixed nesting places or dwellings that might otherwise compel them to stay in one place. Tracking data from many species is useful because if a change in behavior or distribution occurs across a large range of species, it is more likely the result of a more systematic or common cause.

In waters off the northeastern United States, several economically important species have shifted northward since the late 1960s. The three species shown, American lobster, red hake, and black sea bass, have moved northward by an average of 119 miles.

The dots on the map show their center of biomass, which is a point that represents the center of each species’ distribution by weight. As the populations shift generally northward, the center of biomass shifts northward as well. Lighter shading is further back in time; darker shading represents data from more recent years.

Organisms and Light

We've already touched on how light affects the distribution of photosynthetic organisms in the ocean - they have to live near the surface where light is abundant. We have not, however, talked about how organisms sense and produce light.

The diversity of eye designs and light sensing mechanisms that evolved in the ocean are more varied than on land, reflecting the greater range of light environments and lifestyles of the marine world. The variability is driven by a greater range of light habitats. Examples of optical mechanisms to optimize light capture at various light levels include simple sensory cells, lenses (octopus), concave mirrors (scallop) and the camera-like eyes of the nautilus.

This figure on the right shows how both chambered eyes (top) and compound eyes (bottom) form images using shadows (A and B), refraction (C to F), or reflection (G and H). Light rays are shown in blue, and photoreceptive structures are shaded.

Color Vision

Most animals in the ocean live in a dark environment and as a result are blind or color-blind monochromats. Many organisms in shallow water environments do see color. Common shallow water fishes and other crustaceans are dichromatic, probably reflecting the rapid, wavelength-specific attenuation of light with depth and distance. Mantis shrimp exhibit one end of the spectrum (pun intended) having a twelve-channel retina (figure below). For comparison, humans have 3 cone types.

Organisms and Light Production

Deep areas of the ocean are almost completely dark, light still plays an important role in these environments. The ability of an organism to create its own light could put it at a competitive advantage in situations such as during the hunt for food, evading predators, or finding mates. Marine animals produce their own primarily by three different processes:

Bioluminescence is the production of visible light by a chemical reaction without any prior absorption of radiant energy. While usually blue in color, because this is the light that travels best through the water, bioluminescence can range from nearly violet to green-yellow (and very occasionally red).

Fluorescence happens when a fraction of the light illuminating an object is absorbed and then reemitted as a different color. When you shine an ultraviolet light on a blacklight poster and it glows, that is fluorescence. Fluorescence in marine animals living in the upper to middle part of the water column is usually bright and concentrated in certain body parts. Scientists think that fluorescence in these animals may play a specific function, such as attracting prey or providing visual recognition. In contrast, fluorescence in organisms living on the deep-sea bottom seems to be more of a byproduct of particular tissue biochemistry and less likely to play any particular adaptive role.

Phosphorescence is similar to fluorescence in its chemistry; however, unlike fluorescence, phosphorescent materials continue to emit light for a much longer time after the external light that triggered the radiation is removed.