13.1 What is primary productivity?

Primary productivity = rate at which organisms store energy through the formation of organic matter (carbon-based compounds) from inorganic carbon (carbon dioxide), a process called carbon fixation.

• carbon fixation uses energy derived from solar radiation during photosynthesis or from chemical reactions during chemosynthesis

other organisms then eat primary producers

although chemosynthesis supports hydrothermal vent biocommunities along oceanic spreading centers, it’s much less significant than photosynthesis

• 99.9% of ocean’s biomass relies either directly or indirectly on organic matter supplied by photosynthetic primary productivity as its source of food, and only 0.1% of the ocean’s biomass relies on chemosynthesis

photosynthesis = reaction in which energy from Sun is stored in organic molecules

• plant, bacteria, and algae cells capture energy from sunlight and store it as sugars, releasing oxygen as a by-product

cellular respiration = animals consume sugars produced by photosynthesis and combine them with oxygen, releasing the energy stored in the sugars to carry on cellular tasks important for various life processes

Measurement of Primary Productivity

diff properties of ocean can be measured to given an approximation of the amount of primary productivity. one of the most direct at-sea methods is to capture plankton in cone-shaped nylon plankton nets

• fine mesh nets - resemble windsocks at airports - filter plankton from ocean as they’re towed at a specific depth by research vessels

• analysis of the amts and types of organisms captured reveals much about the productivity of the area

other methods include:

• lowering specially designed bottles into the ocean, collecting a sample of surface water, and measuring the uptake of radioactive carbon by phytoplankton in the sample

  • amt of labeled carbon incorporated into the phytoplankton sample in a day can then be used to estimate the total rate of photosynthesis in a particular region of the ocean

global primary productivity is best measured from the vantage point of space:

• monitoring ocean color from earth-orbiting satellites —> allows scientists to measure the concentration of chlorophyll in surface waters, which can be used to estimate phytoplankton abundance and productivity

ocean color is collected worldwide every two days by MODIS (moderate resolution imaging spectoradiometer) instruments aboard terra and aqua satellites, measures 36 spectral frequencies of light, including ocean fluorescence, which provides a wealth of information about ocean phytoplankton productivity, health, and efficiency

RECAP

primary productivity = rate at which carbon is produced by microbes, algae, and plants, mostly through photosynthesis, but also includes a small percentage of chemosynthesis

Factors Affecting Primary Productivity

two main factors limiting photosynthetic primary productivity =

(1) availability of nutrients

(2) availability of solar radiation

**some other variables i.e. amount of CO2 can limit it if they become scarce and human-caused climate change can affect it as well

Availability of Nutrients

the distribution of life throughout the ocean’s breadth and depth depends mainly on the availability of nutrients that phytoplankton need, i.e. nitrogen, phosphorus, iron, and silica. marine populations reach their greatest concentration where the physical conditions supply large quantities of nutrients. the sources of nutrients must be considered to understand where these areas are found:

• water in form of runoff erodes continents —> carries material to the oceans and deposits it as sediment on cont. margins

  • runoff also dissolves and transports compounds i.e. nitrates and phosphates = main nutrients for phytoplankton

**when nitrates and phosphate reach coastal areas, they cause eutrophication which is the enrichment of an ecosystem with chemical nutrients

• continents = major sources of nutrients os greatest concentrations of marine life are found along cont. margins

  • conc. of marine life decreases as the distace from the cont. margins into the open sea increases

  • marine life also decreases w/ increasing depth in the ocean bc sunlight doesn’t penetrate that far into the ocean

often, lack of certain nutrients can limit productivity —> result = these compounds are the most studied in chemical oceanography (nitrates, phosphates)

carbon is an important productivity element too bc carbon = basic component of all organic compounds - in ocean various forms are carbon are quite abundant —> no scarcity of carbon for photosynthetic production —> carbon doesn’t limit productivity

when nutrients aren’t limiting productivity, ratio of carbon to phosphorus in tissues of algae is in proportion of 106:16:1 = redfield ratio after american oceanographer alfred c. redfield

• ratio also observed in zooplankton that feed on diatoms

• phytoplankton take up nutrients in ratio they’re abailable in ocean water and pass them on to zooplankton in same ratio

• when these animals and plankton die, carbon, nitrogen, and phosphorus are recycled into water in same ratio

scientific studies in waters near antarctica and galapagos revealed photosynthetic production is low even though conc of all nutrients except iron is high

production is high only in regions of shallow water down-current from islands or landmasses where a signififcant amt of iron from rocks and sediments is dissolved in water —> lack of iron can severely limit PP

Availability of Solar Radiation

photosynthesis can’t proceed unless light energy is available

in clearest water, solar energy only been detected to depths of abt 1km and even then, the amt reaching these depths is inadequate for photosynthesis

• photosynthesis restricted to uppermost surface waters and those areas of the sea floor where water is shallow enough to allow light to penetrate

compensation depth for photosynthesis = water depth at which light is so limited that net photosynthesis becomes zero

euphotic zone extends from surface down to compensation depth ~100m in open ocean (near coast euphotic zone may extend to <20m bc water contains more turbidity or microscopic organisms that limit sunlight penetration)

how do two factors necessary for photosynthesis differ between coastal areas and open ocean?

• in open ocean: solar energy extends deeper into water column but conc. of n utrients = low

• coastal regions: light penetration is much less but conc. of nutrients is much higher

bc coastal zone is much more productive, nutrient availability must be most important factor affecting distribution of life in the oceans

Light Transmission in Ocean Water

most solar energy falls in range of wavelengths called visible light. this energy powerfully affects 3 major components of the oceansL:

(1) ocean winds: major wind belts of world, which produce ocean currents and wind-driven waves, ultimately derive their energy from solar radiation. wind belts and ocean currents strongly influence world climates

(2) ocean stratification: at ocean surface, a thin layer of water created by solar heating is warmer than the water below and overlies a great mass of cold water that fills most ocean basins —> causes ocean’s water column to be stratified into layers

(3) primary productivity: photosynthesis can occur only where sunlight penetrates the ocean water, so phytoplankton and most animals that eat them must live where the light is, in the relatively thing layer of sunlit surface water = life layer

The Electromagnetic Spectrum

sun radiates wide range of wavelengths of electromagnetic radiation comprising electromagnetic spectrum

lower energy = longer wavelength = red, orange, yellow

higher energy = shorter wavelength = green, blue, violet

The Color of Objects

light from the sun includes all visible colors. most of the light we see is reflected from objects. all objects absorb and reflect different wavelengths of light, and each wavelength represents a color in the visible spectrum i.e. grass absorbs everything except green and reflects green.

• ocean absorbs longer-wavelength colors

• true colors of objects in ocean can be observed in natural light only in the surface waters bc only there can all wavelengths of visible spectrum be found

red light is absorbed w/in uper 10m of ocean

yellow is completely absorbed before a depth of 100m

• shorter wavelength of visible spectrum is all that can be transmitted to greater depths (mostly blue, some violet, some green)

  • low intensity

in open ocean, sunlight strong enough to support photosynthesis only occurs within euphotic zone to depth of 100m and no sunlight penetrates below a depth of about 1000m

secchi disk = used to measure water transparency, and based on that, depth of light penetration can be estimated

• disk 20-40cm in diameter attached to a line marked off at intervals

• as disk is slowly lowerd into ocean, depth at which it can last be seen indicates water’s clarity

increased turbidity (includes microorganisms and suspended sediment) increases the degree of light absorption, thus decreasing the depth to which visible light can penetrate the ocean

Water Color and Life in the Oceans

color of the ocean ranges from deep indigo to yellow-green - why are some areas blue and others green? ocean if influenced by (1) amount of turbidity from runoff and (2) the amt of photosynthetic pigment, which increases w/ increasing primary productivity

coastal waters and upwelling areas = biologically very productive and almost always yellow-green in color bc they contain large amts of yellow-green microscopic algae and suspended particles. when these materials are present in surface waters, they scatter the wavelengths for greenish or yellow light

water in open ocean (particularly in tropics) is less productive and has less turbidity —> clear, indigo-blue color

• water molecules contribute most to scattering of light and scatter primarily blue wabelengths

photosynthetic algae and bacteria are microscopic but still occur in such large numbers that they can change the color of the ocean to such a degree that orbiting satellites are able to measure the changes from space

• satellite can view ocean chlorophyll concentrations

• areas w/ light green color = eutrophic - generally found in shallow-water coastal regions, areas of upwelling, and high-latitude regions

• oligotrophic = low chlorophyll concentration (low productivity) - found in open oceans of tropics

Why are the margins of the oceans so rich in life?

if stability of ocean is ideal for sustaining life, why are the richest concentrations of marine organisms in margins of the oceans, where conditions are the most unstable?

characteristics of coastal ocean include:

• shallow water depths —> greater seasonal variations in temp and salinity than open ocean

• water column that varies in thickness in nearshore region in response to tides that regularly cover and uncover a thin strip of land along margins of continents

• breaking waves in surf zone that release large amts of energy, which has been carried for great distances across open ocean

each condition stresses organisms but organisms have evolved by process of natural selection to fit every imaginable biological niche

Upwelling and Nutrient Supply

upwelling = flow of deep water toward surface that brings water from depths below euphotic zone. deep water = rich in nutrients and dissolved gases bc no phytoplankton at depths to consume compounds

• when cold water from below surface rises, it brings nutrients from depths to the surface where phytoplankton thrive and become food for larger organisms

surface warming and resulting stratification of ocean’s water column can limit upwelling and inhibit primary productivity

where does upwelling occur in oceans?

• coastal upwelling: ekman transport causes surface water to move away from these coasts so nutrient-rich water from depths of 200-1000m constantly rises to replace it

• equatorial upwelling

RECAP

photosynthetic productivity is limited in marine environment by amt of sunlight and supply of nutrients. upwelling greatly enhances conditions for life by lifting cold, nutrient-rich water to sunlit surface waters.

CONCEPT CHECK 3.1

(1) Discuss chemosynthesis as a method of primary productivity. How does it differ from photosynthesis?

• chemosynthesis is a way of using inorganic chemicals to make energy for an organism, releases water and oxygen

• photosynthesis uses sunlight and carbon dioxide to make energy for an organism, releases water and oxygen

• photosynthesis = 99.9% of biomass in ocean, chemosynthesis = 0.1%

(2) An important variable in determining the distribution of life in the oceans is the availability of nutrients. How are the following variables related: proximity to the continents, availability of nutrients, and the concentration of life in the oceans?

• continents = major source of nutrients —> greatest concentrations of marine life are near continents bc there’s the highest availability of nutrients.

(3) Another important determinant of productivity is the availability of solar radiation. Why is biological productivity relatively low in the tropical open ocean, where the penetration of sunlight is greatest?

• bc there is a lack of nutrients there and nutrients are the most important factor in primary productivity

(4) Discuss the general characteristics of the coastal ocean where unusually high concentrations of marine life are found.

• shallow water depths —> high variability in seasonal temps and salinities

• water column varying in thickness bc of tides

• breaking waves in surf zone releasing large amts of energy

• coastal upwelling - large amts of nutrients from runoff

(5) Explain why everything in the deep ocean below the shallowest surface water appears blue-green in color.

• longer wavelengths don’t penetrate that far, shorter wavelengths do

• yellow-green microorganisms at surface have chlorophyll and reflect green but below that, blue is most reflected

• less photosynthesis in clear open ocean bc there’s less nutrients far from continents

• less turbidity