2 The Oceanic Environment Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton ©Jeffrey S. Levinton 2001

2 The Oceanic Environment

Notes for Marine Biology: Function, Biodiversity,

EcologyBy Jeffrey S. Levinton

©Jeffrey S. Levinton 2001

The Ocean

Geography and Bottom Features



The Ocean and Marginal Seas

• The worlds oceans: oceans and marginal seas

• Oceans cover 71% of earth’s surface• Southern hemisphere 80%, Northern

hemisphere 61%• 84% deeper than 2000m• Greatest depth ~ 11,000 m in Marianas

Trench

Marginal Seas

• Examples: Gulf of Mexico, Mediterranean Sea• Affected strongly by 1. regional climate, 2. precipitation-evaporation balance, 3. river input of fresh water and dissolved solids, 4. limited exchange with the open ocean (e.g., sill

partially cutting Mediterranean from Atlantic)

Marginal Seas 2

• Often have recent history of major change

• Mediterranean: completely dry a few million years ago

• Baltic Sea: less than 11,000 years old

Marginal Seas 3

• Marginal Seas (Gulf of Mexico, Baltic Sea):• influenced more by drainage and local climate:• 1. River input (Baltic Sea) • 2. Precipitation - evaporation balance

(Mediterranean)• 3. Restricted circulation (Black Sea,

Mediterranean)• 4. Geological history (Baltic)

Ocean as a Receptacle

• Particulate mineral matter• Dissolved salts• Particulate organic matter (POM)• Dissolved organic matter (DOM)• Atmospheric precipitation• Volcanic sources• Water

Water Relationships in the Ocean

Topographic Features

• Continental shelf (1° slope)• Continental slope (2.9° slope)• Continental Rise• Abyssal Plain• Submarine Canyons• Oceanic Ridge Systems

Topographic Features 2

Coa

stal

pla

in

She

lf

Slo

pe

Co n

t in e

ntal

ris

e

Aby

ssal

pla

in

Sea

mou

nt

Mid

-oce

an r

idge

Aby

ssal

pla

in

Tre

nchVol

cani

c is

land

Mar

gina

l sea

Depth (m

iles)Dep

th (

km)

Earth’s surface is divided into plates: borders are ridge systems, faults

American

Eurasian

Pacific

Philippine

Cocos

American

Antarctic

African

Eurasian

Arabian

Nazca

Caribbean

The Oceanic Crust: Crust is formed at ridges, moved laterally, and destroyed by subduction,

which forms trenches

Intermediate, deep-focus earthquakes

Continentalcrust Inactive fault

Oceaniccrust

Ridge Fault

Continent

ContinentalCrust

Mantle

Trench

The Ocean

Seawater Properties


Water molecule

• Asymmetry of charge distribution on water molecule - increases ability to form bonds with ions - makes water excellent solvent

Water properties

• High heat capacity (0.9)• High heat of evaporation (590 cal/g)• High dissolving power• High transparency (absorbs

infrared, ultraviolet)

Latitudinal Gradient of Sea Water Temperature

Vertical Temperature Gradient: Open Tropical Ocean

Vertical Temperature Gradient: Shallow Temperate Ocean

• In shallow shelf regions with strong seasonal temperature change - get seasonal thermocline. Example: coastal NE United States - thermocline found in summer continental shelf, with surface warmer wind-mixed layer, deeper cooler water

• Winter: vertical temperature distribution much more uniform and cold, from surface to bottom. No strong thermocline

Salinity

• Definition: g of dissolved salts per 100g of seawater; units are o/oo or ppt

• Controlled by:+ evaporation, sea-ice formation- precipitation, river runoff

Salinity in open ocean is 32-38 o/oo

Important elements in seawater

• Chlorine (19,000 mg/l)• Sodium (10,500• Magnesium (1,300)• Sulfur (900)• Calcium (400)• Potassium (380)• Bromine (65)• Carbon (28 - variable)

Principle of Constant Element Ratios

• Ratios between many major elements are constant all over the ocean, even though salinity varies

Principle of Constant Element Ratios 2

• Why? Because residence time of elements is much greater than time to mix them evenly throughout ocean by water currents (ca. 1000 y)


• Residence time of Na, Cl, Sr is on the order of millions of years


• Principle does not apply to elements that cycle rapidly, especially under influence of biological processes (e.g., sulfur, phosphorous)

Measurement of Salinity

• Chlorinity: g of chlorine per 1000 ml of seawater

• Salinity = 1.81 x chlorinity• Measured by chemical titration

Measurement of Salinity 2

• Conductivity - increases with increasing salinity, has to be corrected for temperature

Measurement of Salinity 3

• Optical refraction - more salt, more refraction of light, uses refractometer

Temperature

• Oceanic range (1.9 - 40 °C) less than terrestrial range (-68.5-58 °C)

• Deep ocean is cold (2 - 4) degrees

Heat Changes in the Ocean

Additions Losses

Latitudinal gradient of solar heating

Back radiation of surface

Geothermal heating Convection of heat to atmosphere

Internal Friction Evaporation

Water Vapor Condensation

Seawater density (mass/volume)

• Influenced by salt, no maximum density at 4 °C (unlike freshwater)

• Density measure of seawater at temperature t

t= (density - 1) x 1000

t increases with increasing salinity

t increases with decreasing temperatureSpecial significance: vertical density

gradients

Latitudinal salinity gradient

Excess of evaporation over ppt in mid-latitudesExcess of ppt over evaporation at equator

The Ocean

Circulation in the Ocean


Coriolis Effect - Earth’s RotationLatitude Eastward Velocity

(km/h)

Equator 1670

30° N. latitude 1440

60° N. latitude 830

Coriolis Effect - Movement of fluids, in relation to earth beneath, results in deflections

Coriolis Effect and Deflection

• Surface winds move over water• Coriolis effect causes movement of water at an

angle to the wind (to right in northern hemisphere)

• Water movement drags water beneath, and to right of water above

• Result: Shifting of water movement - Ekman Spiral (actually friction binds water together and all water moves at a right angle to wind (right of wind in n. hemisphere)

Coriolis Effect and Deflection 2















Coastal Winds + Coriolis Effect = Upwelling

Southern hemisphere: water moves to the left of wind

Oceanic Circulation

• Wind-driven surface circulation• Density-driven thermohaline

circulation

Wind-driven Circulation

• Driven by heating of air near equator, which rises, moves to higher latitude, falls, creating circulation cells that are affected by Earth’s rotation. Air moves surface water

• Prevailing westerlies (40°N & S latitude)• Trade winds (toward the west)

Wind-driven Circulation 2

• Combination of wind systems and shapes of ocean basins create cyclonic flow known as gyres

• Rotation of earth tends to concentrate boundary currents on west sides of ocean - creates concentrated currents such as Gulf Stream

Wind-driven Circulation 3Wind systems Surface currents

Westerlies

NE TradewindsDoldrums

SE Tradewinds

Westerlies

Subpolargyre

Subtropicalgyre

Subtropicalgyre

West wind drift

Thermohaline Circulation

• Water in the ocean can be divided into water masses, identified by distinct temperature, salinity, and other physico-chemical characteristics

Thermohaline Circulation 2

• Thermohaline circulation is movement of ocean water controlled mainly by density characteristics

• Controlled by (1) Location of formation of water, (2) density, (3) Coriolis effect to a degree


• Water masses formed in high latitude surface waters - water is cold, often high salinity because of ice formation

• Waters sink, move at depth towards lower latitude

• Water masses each have a characteristic depth, because of their density, which is largely a function of their high latitude surface origin


AABW=Antarctic Bottom Water; AAIW = Antarctic IntermediateWater; NADW = North Atlantic Deep Water

Circulation Recap

• Coriolis effect - rotation of Earth, prop. to sine of latitude, Right deflection in N. hemisphere, Left deflection in S. hemisphere - upwelling, deflection of currents

Circulation Recap 2

• Surface circulation - driven by planetary winds, which are controlled by heating, convection, Coriolis effect - gyres, eastern boundary currents

Circulation Recap 3

• Thermohaline Circulation - driven by density, sinking, surface water brought to deep sea - water masses determined by density

El Niño - Global Phenomenon

• Periodic - every few years• Warm water moves easterward across

Pacific Ocean• Eastern tropical and subtropical Pacific

becomes warm, thermocline deepens• Causes mortality of clams, fishes, from heat

shock• Strongly affects weather in eastern Pacific,

storms increase; droughts in western Pacific

The Ocean

Coastal Processes


Waves

• Dimensions

Wave Length LAmplitude HVelocity V

Waves

• When depth < L/2: waves “feel bottom”• When H/L > 1/7: wave is unstable and

collapses (breaks)

Beaches

• Many beaches exposed to direct wave and erosive action

• Some sandy beaches are more protected, especially some that are very broad and dissipate wave energy at the low tide mark and only for a few meters from waterline

Beaches 2

• Profile more gentle in summer; fall and winter storms cause erosion and a steeper profile

Beaches 3

• Longshore currents, riptides are common features, causing erosion and transport of sand

Wave Refraction

Tides 1

• Gravitational attraction between ocean and moon + sun (moon has 6x the effect of the sun)

Tides 2

• Water on earth facing moon has net gravitational attraction, water facing away from earth has less attraction - centrifugal force causes high tides here

Tides 3

• Water on earth surface at right angles to moon is pulled either away or toward moon - results in low tides here

Tides 4

• Spring tides - greatest tidal range, highest high and lowest low - happens when earth, moon and sun are in line

• Neap tides - least tidal range - happens when earth, moon, and sun form approximate right angle

Tides 5

Sun

Sun

E

E

SpringTide

NeapTide

E = Earth

mm

m

m

m = Moon

Tides 6

• Tides differ in different areas; function of basin shape, basin size, latitude

• Results in strong regional differences in the amplitude of tides, the evenness of the expected two high and two low tides per lunar day (e.g., in Pacific NW, there is only one strong low tide per day, while the other “low” tide is nearly as high as the high tides

Tides 7

Connecticut - even tides Washington State - uneven

Estuaries 1

• Body of water where freshwater source from land mixes with seawater

• Often results in strong salinity gradient from river to ocean

• Salinity may be higher at bottom and lower at top, owing to source of river water that comes to lay on top of sea water below, or mixes with the sea water to some degree

Estuaries 2

Chesapeake Baywith summer surfacesalinity. Dark blueareas: tributaries have salinity< 10 o/oo

Estuaries - types 3

10

10

20

20 30

Sea water

Fresh water layer

Highly stratified estuary

Moderately stratified estuary (wind, tide mixing)

Vertically homogeneous estuary

Circulation in a coastal fjord

• Reduced exchange of fjord’s bottom waters, combined with density stratification and respiration results in low oxygen in bottom waters

Ocean Restricted circulation

Sill

The End

Documents

2 The Oceanic Environment Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton ©Jeffrey S. Levinton 2001