13.3 How does regional primary productivity vary?

primary photosynthetic production in oceans varies dramatically from place to place. typical units = mass of carbon gC per unit of area m² per unit of time yr, abbrevaited as gC/m2/yr

high variability in primary productivity = result of uneven distribution of nutrients and seasonal changes in availability of solar energy throughout ocean’s shallow photosynthetic layer

on avg, about 90% of organic biomass generated in euphotic zone of open ocean decomposes before descending below this zone

remaining 10% of material sinks into deeper water, where about 9% is decomposed. remaining 1% of this material reaches deep-ocean floor and accumulates there

way in which material is removed from euphotic zone to sea floor = biological pump bc it “pumps” carbon dioxide and nutrients from upper ocean and concentrates them in deep-sea waters and sea floor sediments

surface warming and resulting stratification of ocean’s water column also affects primary productivity. Ex. permanent thermocline and pycnocline develops in subtropical oceans, forming a barrier to vertical mixing, preventing resupply of nutrients to sunlit surface layer

• thermocline acts as impenetrable lid prohibintg movement of nutrient-rich deep water to surface, thereby inhibiting primary productivity

in middle latitude oceans, a thermocline only develops during summer, thermocline doesn’t usually develop in polar oceans bc of lack of adequate surface warming.

yearly productivity patterns of three open ocean areas: (1) polar or high-latitude oceans, (2) tropical or low-latitude oceans, (3) temperate or middle latitude oceans

RECAP

thermocline acts as an impenetrable lid that inhibits movement of nutrient-rich deep water to surface —> limits primary productivity

Productivity in Polar (High-latitude) Oceans: 60-90 degrees N and S latitude

polar oceans experience continuous darkness for 3ish months during winter and continuous illumination for 3ish months during summer

diatom productivity peaks in Barents Sea during May when Sun rises high enough in sky and there’s no deep penetration of sunlight into water. as soon as diatoms develop, zooplankton begin feeding on them —> zooplankton biomass peaks in June and continues at a relatively high level until winter warkness begins in October

in antarctic region, productivity is somewhat greater bc of upwelling of NADW, forming on opposite side of ocean basin, where it sinks and moves southward below surface, rising hundreds of years later to the surface near antarctica, carrying high concentrations of nutrients

when sun provides sufficient solar radiation in summer, there’s an explosion of biological productivity

blue whales eat mainly zooplankton and time their migration through middle latitude and polar oceans to coincie w/ max zooplankton productivity enabling them to have calves

density and temp change very little w/ depth in polar oceans = isothermal and there’s no barrier to mixing between surface waters and deeper, nutrient-rich waters. in summer, melting ice creates a thin, low-salinity layer that doesn’t readily mix w/ deeper waters. stratification is crucial to summer primary production bc it prevents phytoplankton from being carried into deeper, darker waters

nutrient conc. are usually adequate in high-lat surface waters so availability of solar energy limits photosynthetic productivity in these areas more than the availability of nutrients

Productivity in tropical (low-latitude) oceans: 0-30 degrees N and S latitude

productivity = low in tropical oceans bc sun is more directly overhead —> light penetrates deeper into oceans

productivity is low bc a permanent thermocline produces a stratification of water masses, preventing mixing between surface waters and nutrient-rich deeper waters, eliminating any supply of nutrients from deeper waters below

at about 20deg N and S lat, phosphate and nitrate conc are less than 1/100 of their concentrations in middle lat oceans during winter

**productivity in tropical oceans is limited by the lack of nutrients

generally, primary production in tropical oceans occurs at a steady but low rate. total annual production of tropical oceans = 1/2 of that found in middle lat oceans

exceptions to general pattern of low productivity in tropical oceans includes:

• equatorial upwelling: trade winds drive westerly equatorial currents on either side of equator —> ekman transport causes surface water to diverge toward higher latitudes. surface water is replaced by nutrient-rich water from depths of up to 200m

  • best developed in eastern pacific ocean

• coastal upwelling = prevailing winds blow toward equator and along western continental margins —> surface waters driven away from the coast and replaced by nutrient-rich waters from depths of 200-900m

• coral reefs: organisms that comprise and live among coral reefs are adapted to low-nutrient conditions

  • symbiotic microscopic algae living w/in tissues of coral and other species allow coral reefs to be highly productive

  • also retian and recycle what little nutirents exist

productivity in temperate (middle lat) oceans: 30-60 deg N and S latitude

productivity is limited by available sunlight in polar oceans and by nutrient suppluy in tropical oceans. in temperate or middle lat oceans, a combination of these two limiting factors controls productivity

Winter

productivity in middle lat oceans is very low during winter even though nutrient conc is highest at this time. water column is isothermal but sun is at its lowest position above horizong so a high percentaege of available solar energy is reflected, leaving only a small percentage to be absorbed into surface waters —> compensation depth for photosynthesis is so shallow that phytoplankton don’t grow much. absence of thermocline allows algal cells to be carried down beneath euphotic zone for extended periods by turbulence associated w/ winter waves

Spring

sun rises higher in sky during spring so compensation depth deepends

spring bloom of phytoplankton occurs bc solar energy and nutrients are available, and a seasonal thermocline develops due to increased solar heating that traps algae in the euphotic zone. this creates a tremendous demand for nutrients in the euphotic zone, so the supply becomes limited, causing productivity to decrease sharply. even though the days are lengthening and sunlight is increasing, productivity during spring bloom is limited by lack of nutrients. In most areas of the Northern Hemisphere, therefore, phytoplankton populations decrease in April due to insufficient nutrients and because their population is being consumed by zooplankton (grazers).

Summer

sun rises even high in summer than in spring so surface waters in middle lats continue to warm

strong seasonal thermocline created at depth of 15m preventing vertical mixing so nutrients can’t be replaced = low phytoplankton population

Fall

solar radiation diminishes as sun moves lower in sky so surface temps drop and summer themocline breaks down

nutrients return to surface layer as increased wind strength mixes surface waters with deeper waters —> fall bloom of phtoplankton, less dramatic than spring bloom. very short lived bc sunlight becomes limiting factor as winter approaches to repeat the seasonal cycle

Comparing Regional Productivity

polar regions: peak productivity in May

[middle lat = spring peak (April)

tropical remains constant

highest overall productivity in middle lats

steady, low rate of productivity year-round in tropical ocean

RECAP

in polar oceans, productivity peaks during summer and is limited by sunlight during rest of the year. in tropical oceans, producvitity is low year-round and is limited by nutrients. in middle lat oceans, productivity peaks in spring and fall and is limited by a lack of solar radiation in the winter and a lack of nutrients in the summer. despite this, middle lat oceans exhibit the highest overall productivity of the three regions

CONCEPT CHECK 13.3

(1) describe how a biological pump works. what percentage of organic material from the euphotic zone accumulates on the sea floor?

• 1%

(2) describe the yearly productivity pattern in polar oce