Earth / Environmental Science

Ch. 15 – OCEAN WATER AND OCEAN LIFE Composition of Seawater

• Dissolved substances in seawater: sodium chloride, salts, metals, dissolved gasses

• The salt content of seawater makes in unsuitable for drinking or irrigating crops and causes it to be highly corrosive to many materials

• However, many parts of the ocean are full of life that has adapted to this environment

Salinity

• Salinity is the total amount of solid material dissolved in water (salinus = salt) • Because the proportion of dissolved substances in seawater is such a small

number, oceanographers express salinity in parts per thousand (%o) • The average salinity of seawater is 35% or 3.5 %o • Most of the salt in seawater is sodium chloride (aka table salt)

Sources of Sea Salts

1. Chemical weathering of rocks on the continents 2. Earth’s interior (volcanic eruptions)

Chemical Weathering

• Dissolved materials reach oceans through runoff from rivers and streams • Estimated rate of more than 2.3 billion metric tons per year

Earth’s Interior

• Principal source of water in the oceans • Through volcanic eruptions, large amounts of water vapor and gases have been

released into the atmosphere • About 4 billion years ago, the water vapor condensed and torrential rains filled

the oceans with water • Elements such as chlorine, bromine, sulfur, and boron were emitted along with

the water

Processes Affecting Salinity • Variations of salinity result form changes in the water content of the solution • Processes decreasing salinity:

– Precipitation – Runoff From land – Icebergs melting – Sea ice melting

• Processes increasing salinity: – Evaporation – Formation of Sea ice

Ocean Temperature Variation

• The ocean’s surface water temperature varies with the amount of solar radiation received

• Higher sea surface temperatures are found in low latitude regions (closer to the equator)

• Lower sea surface temperatures are found in high latitude regions Temperature Variation with Depth

• Surface waters are warmed by the sun so they have a higher temperature than deeper waters

• The thermocline is the layer of ocean water between about 300 meters and 1000 meters where there is a rapid change of temperature with depth

• It is very important structure because it creates a vertical barrier to many types of marine life

• A thermocline is not present in high latitudes. Instead the water column is isothermal (the same temperature no matter how deep)

Ocean Density Variation

• Density is defined as mass per unit volume. It is a measure of how heavy something is for its size.

• Density is an important property of the ocean because it determines the water’s vertical position in the ocean

• Differences in density cause large areas of ocean water to sink or float Factors Affecting Seawater Density

• Seawater density is influenced by two main factors: salinity and temperature • Increases in salinity add dissolved substances and result in a higher density • Increases in temperature result in a lower density • Temperature has the greatest influence on surface seawater density, except in

extreme polar areas where the water temperature is constant

Density Variation with Depth • When water temperature increases, its density decreases • The pycnocline is the layer of ocean water between about 300 meters and

1000 meters where there is a rapid change of density with depth • It presents a barrier to mixing between low-density water above and high-

density water below • Pycnoclines are not present in high latitudes, instead the water column is about

the same density throughout Ocean Layering

• Oceanographers generally recognize a three-layered structure in most parts of the open ocean

• Shallow surface mixed zone • Transition Zone • Deep Zone • No layers in high latitudes because there are no rapid changes in temperature

Surface Zone

• Shallow (300-450 meters deep) • Warmed by the sun so it has the warmest water temperatures • The mixed zone is the area of the surface created by the mixing of water by

waves, currents and tides • The surface zone accounts for 2% of ocean water

Transition Zone

• Between the surface and deep zones • Includes a thermocline and associated pycnocline • Accounts for 18% of all ocean water

Deep Zone

• Sunlight never reaches the deep zone • Temperatures are just a few degrees above freezing • Constant high-density water • Includes about 80% of water in the ocean

Diversity of Ocean Life

• Organisms range in size from microscopic bacteria to blue whales • Over 250,000 marine species in the world • Most marine organisms live within the sunlit surface waters because they need

sunlight to survive or because that is where most food is located

Classification of Marine Organisms • Marine organisms are classified by where they live or how they move • Plankton (floaters) • Nekton (swimmers) • Benthos (bottom dwellers)

Plankton

• Plankton include all organisms that drift with ocean currents including algae and bacteria

• Phytoplankton are the algae that undergo photosynthesis. Most are microscopic.

• Animal plankton are called zooplankton. Zooplankton include the larval stages of many marine organisms like fish, starfish, lobsters, and crabs

Nekton

• Nekton include all animals capable of moving independently of the ocean currents

• Able to determine their position within the ocean and usually able to migrate • Includes most adult fish and squid as well as marine mammals and reptiles • Fish are most abundant near continents, islands, and colder waters

Benthos

• The term benthos refers to organisms living on or in the ocean floor • Shallow coastal areas are the only places where seaweed is attached to the

bottom and where most benthos organisms are found because some sunlight can reach the ocean floor

• In the deeper parts of the seafloor where no sunlight can get through, it is an area of coldness, stillness and darkness and life progresses slowly

Marine Life Zones • The distribution of marine organisms is affected by the chemistry, physics, and

geology of the oceans • Three factors are used to divide the ocean into distinct marine life zones • availability of sunlight • distance from shore • water depth

Availability of Sunlight

• The clarity of seawater is affected by the amount of plankton, suspended sediment, etc. in the water

• The amount of sunlight varies with atmospheric conditions • The photic zone is the upper part of the ocean into which sunlight penetrates

(includes the euphotic zone and the lower photic zone) • The euphotic zone is the part of the photic zone near the surface where light is

strong enough for photosynthesis to occur (usually goes down 100 meters) • The aphotic zone gets no sunlight

Distance from Shore

• The intertidal zone is the area where the land and ocean overlap. The species that live in this zone have adapted to crashing waves, periodic drying out, and rapid changes in temperature, salinity, and oxygen concentrations

• The neritic zone is farther out than the intertidal zone and covers the gently sloping continental shelf. It is shallow enough for sunlight to reach all the way to the ocean floor, and is rich in biomass and organisms even though it only covers 5% of the world ocean

• The oceanic zone is the marine-life zone beyond the continental shelf. This zone has the lowest nutrient concentration which results in smaller populations.

Water Depth

• The pelagic zone is open water of any depth. Animals in this zone swim or float freely.

• The benthic zone is the marine-life zone that includes any sea-bottom surface regardless of its distance from shore.

• The abyssal zone is a subdivision of the benthic zone characterized by extremely high pressures, low temperatures, low oxygen, few nutrients, and no sunlight.

Hydrothermal Vents

• Seawater seeps into the ocean floor through cracks in the crust • The water becomes super-heated and saturated with minerals • At some vents water temperatures of 100° or higher support communities of

organisms found nowhere else in the world

Oceanic Productivity • Primary Productivity is the production of organic compounds form inorganic

substances through photosynthesis or chemosynthesis • Photosynthesis is the use of light energy to convert water and carbon dioxide

into energy-rich glucose molecules • Chemosynthesis is the process by which certain microorganisms create organic

molecules from inorganic nutrients using chemical energy Photosynthetic Productivity

• Two factors influence a region’s photosynthetic productivity • Availability of nutrients such as nitrogen phosphorus, and iron • Amount of sunlight/solar radiation • Productivity varies greatly because of the uneven distribution of nutrients and

the availability of sunlight due to seasonal changes Productivity in Polar Oceans

• High latitude surface waters typically have high nutrient concentrations because of the constant supply of nutrients rising from deeper waters

• The availability of solar energy is what limits photosynthetic productivity in polar areas

Productivity in Tropical Oceans

• Solar energy is available year-round and penetrates much deeper into tropical oceans than in temperate or polar oceans

• Lack of nutrients is what limits productivity in tropical waters • These areas have so few organisms that they are considered biological deserts

Productivity in Temperate Oceans

• Temperate regions are found at mid-latitudes • A combination of sunlight and nutrient supply controls productivity • Productivity is varied due to seasonal changes in temperate waters

Seasons in Temperate Oceans • Winter

– Low productivity – Days are short and the sun angle is low so productivity is limited due to

lack of sunlight • Spring

– Spring bloom of phytoplankton occurs because solar energy and nutrients are available

– Seasonal thermocline develops which creates a huge demand for nutrients – This causes the spring bloom to be depleted and productivity is limited

due to lack of nutrients • Summer

– Strong seasonal thermocline is created which prevents the mixing of surface waters and deeper waters

– Nutrients depleted from the surface can’t be replaced by deep waters – Phytoplankton population remains relatively low

• Fall – Surface temperatures drop and the summer thermocline breaks down – Nutrients return to the surface as increased wind strength mixes surface

and deep waters which causes a fall bloom of phytoplankton – The fall bloom is very short lived because sunlight becomes the limiting

factor

Organic Feeding Relationships • Marine algae, plants, bacteria, and bacteria-like organisms are the main organic

producers • The producer’s biomass in the ocean is many times greater than the mass of the

top consumers like sharks and whales • This is because only a small percentage of energy taken in at any level is passed

on to the next because energy is consumed and lost at each level Trophic Levels

• A trophic level is a nourishment level in a food chain • Plant and algae producers constitute the lowest level, followed by herbivores and

a series of carnivores at progressively higher levels Transfer Efficiency

• The transfer of energy between trophic levels is very inefficient • The average efficiency of algae is 2%, which means that only 2% of the energy

absorbed by algae is changed into a food source for herbivores Food Chains and Food Webs

• A food chain is a sequence of organisms through which energy is transferred, starting with the primary producer and ending with the top carnivore

• A food web is a group of interrelated food chains • Animals that feed through a food web rather than a food chain are more likely to

survive because they have alternative foods to eat should one of their food sources diminish or disappear