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Plankton Ecology and Productivity

Productivity and Plankton Abundance Limiting Factors

Spatial and Temporal Distribution

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Primary Production Primary Production:

The rate of formation of energy-rich organic products from inorganic material

Usually refers only to photosynthesis, although it also includes chemosynthesis

Gross Primary Production: The total amount of primary production

Net Primary Production: The total amount of primary production after the

algae respires (available for higher trophic levels)

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Measuring Primary Production Usually expressed as g C/m2/yr or

something similar (C/unit area/unit time) integrated over the entire water column to

the bottom of the euphotic zone Euphotic zone: the depth to which light

will penetrate (photosynthesis will occur)

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Measuring Primary Production

Oxygen Technique Oxygen released during photosynthesis is

used to estimate productivity Includes the addition from photosynthesis

and the subtraction from respiration But how do we separate photosynthesis

from respiration

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Light/Dark Bottle Technique

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Measuring Primary Productivity Oxygen Technique Radiocarbon

Radioactive 14C is used as a tracer in the uptake of bicarbonate during photosynthesis

Preferable technique in areas of low productivity

Bottles containing phytoplankton and 14C are placed under optimal light conditions (not in situ)

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Measuring Primary Productivity Oxygen Technique Radiocarbon Satellite Color Scanning

Satellite scanners estimate the relative standing stocks which are then used to estimate changes in production

Chlorophyll density is calculated from the ratio of the reflectance of blue to green light

Relationship between pigment concentration and primary production varies geographically

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Satellite Scanning

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Measuring Primary Productivity Oxygen Technique Radiocarbon Satellite Color Scanning Probe Fluorometer

Productivity is estimated by measuring the fluorescence obtained from phytoplankton

Photosynthetic pigments fluoresce when exposed to UV light

Deployed in the water column and measures photosynthesis directly

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Factors Affecting Primary Production

Limiting Factors: Terrestrial Systems Light Temperature Nutrient Concentration Soil Water

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Factors Affecting Primary Production Light (Quality and Quantity)

Light between 400-720 nm is absorbed by various photosynthetic pigments

Chlorophyll a Accessory pigments absorb at a wide range

of wavelengths

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Light Quality and Quantity

Light penetrates to different depths based on the angle of incidence (and seasonality)

Light of different colors penetrates differently Depth to which light penetrates is a function of

the depth of water, amount of phytoplankton, transparency of the water and the differential absorption by other things (e.g., sediments, organic matter)

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Light Quantity and Quality

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Photosynthesis vs. Light Intensity

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Differences Among Species

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General Trends

Light inhibition (photoinhibition) is caused by too much light saturating the photosynthetic centers (generating too much energy which then has to be disposed of) -- this can damage the cell.

Also ultraviolet radiation at surface is damaging.

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Depth vs. Production

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Compensation Depth

Depth where for a given algal cell, photosynthesis = respiration

Individual – not population level property Net Production = 0 Usually where light is 1% of the surface

intensity, maybe 150 m Varies spatially with water clarity

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Compensation Point/Depth

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Photosynthesis and light

Commonly, when faced with too much mixing below the compensation depth, cells will lower their metabolic rate or form cysts (resting stages) which can last through the poor conditions.

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Factors Limiting Primary Production Light Nutrients

Needed for enzymes, energy stores, energy carriers and structure

Nitrogen and phosphorus are often limiting; Diatoms also need SiO2

Uptake of nutrients is an active process – often works against a concentration gradient

Yet, it is concentration dependent

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Nutrients on Land versus Sea

Agricultural soil=0.5% N

This allows 50 kg dry wt wt P.P. per cubic meter

This N reservoir allows plants to live for many years

Ocean waters = 0.00005% N

This allows 5 g dry wt P.P. per cubic meter

This only allows for short-lived plants

Nutrients often become limiting

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Needs for Nitrogen Necessary for the production of proteins,

nucleic acids, and ATP In most habitats, N is the limiting nutrient Supply

Runoff or Atmospheric Deposition Recycled

Ammonium Phytoplankton Zooplankton

(Excretion)

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Phosphorous

Critical to energy cycling – i.e., ATP Usually less limiting than N, but there

are exceptions Coral reefs: carbonate sediments adsorb P

from the water column

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How do we determine if a nutrient is limiting?

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Uptake Rate vs. Concentration

At low external concentration – uptake depends on concentrationAt high external concentrations – uptake is saturated

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Seasonal Succession of Algae

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Restoring Nutrients Problem:

Light – available near the surface Nutrients– down deep where there is no

light How do we get the nutrients to the

Euphotic Zone? Thermocline/Pycnocline: influences the

degree of mixing between surface waters and high nutrient bottom water

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Thermocline Effects

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Tropical

Polar

Temperate

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High Nutrient (Nitrate) – Low Chlorophyll (HNLC)

Eastern Tropical PacificSub-Polar North PacificSouthern Ocean

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Evidence for Iron Limitation in ETP• Macro-nutrients at non-limiting concentrations• Small-scale bottle and microcosm experiments• Natural additions of iron from land nearby

Galapagos Islands

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IronEx IIronEx II

Southern Ocean

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Factors Limiting Primary Production Light (Quality and Quantity) Nutrients Turbulence

As water is mixed, not only will nutrients be carried up, but also algal cells will be carried downward

Wind induced turbulence often extends down to 200 m – yet, photic zone is shallower

If mixing extends below the critical depth, net production will be negative

Especially prevalent at high latitudes

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Depth of Vertical Mixing

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Compensation vs. Critical Depth Critical Depth

Depth where Gross Photosynthesis = Total Plant Respiration

It is a characteristic of the population Compensation Depth

Characteristic of individual cells As long as the population (on average) is

mixed above the level of the critical depth, the population will have a + net production

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Spatial Distribution of Phytoplankton Geographical Variation

Latitudinal variation

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Spatial Distribution of Phytoplankton Geographical Variation

Latitudinal Differences Regional Differences

Continental shelf and open ocean upwelling areas are most productive

Shallowness of coastal areas enables the regeneration of nutrients

Estuaries: High in nuts, but usually turbid which reduces the depth of photosynthesis

Central oceans and gyre centers are nutrient poor

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Relative Contribution

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Geographic Variation in Types

Oceanic environments are dominated by small species

Large Diatoms and Dinos are common near shores, but rare in the open sea

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Temporal/Spatial Distribution of Phytoplankton

Geographic Variation Seasonal x Geographic Variation

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Winter Spring

Summer Fall

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Factors Limiting Primary Production Light Nutrients Turbulence Zooplankton Grazing

What is the relationship between production and consumption?

Do herbivores remove microphytoplankton production as fast as it is formed?

What percentage of production is taken up by consumers?

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Production-Consumption Lag

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Nutrient Recycling How does zooplankton grazing

stimulate production? Metabolized algal cells –

releases nutrients Bacterial consumption releases

“nutrient stocks” Does herbivore pressure limit

plankton productivity – i.e., is there top-down control?

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Temperate Seas North Atlantic Light varies seasonally Thermal structure of the

water column changes seasonally

Mixing produces two blooms each year

PhytoZoops

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Tropical Seas Light is available year

round Thermal stratification last

year round Productivity is low, yet

constant Deepest compensation

depths What causes the brief

peaks and lags?

PhytoZoopl

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Polar Seas Productivity is restricted to

a short period in the polar summer

Snow cover disappears long enough to allow light to enter the water

When light is available for long periods-bloom occurs

Nutrients are not limiting and strong stratification never occur

PhytoZoops

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Phytoplankton Seasonal Succession Patterns

Temperate waters: small, rapidly growing diatoms in spring give way to larger diatoms in summer. Dinoflagellates dominate in late summer and fall, and small diatoms become dominant again in winter.

Tropical waters: dinoflagellates dominate year around

Polar waters: only summer diatom production

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Geographical Comparisons of Primary Productivity

Tropical Seas1) Well lit all year

2) Thermal stratification all year

3) Low nutrients in surface waters

4) Productivity low but constant year round

Temperate Seas1) Light varies seasonally

2) Seasonal stratification

3) Mixing in winter replenishes nutrients

4) Major PP spring peak, with minor peak in fall

Polar Seas1) Well lit in summer2) No stratification

3) Nutrients unlimited

4) PP only in ice free summer

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Temporal/Spatial Distribution of Phytoplankton

Geographic Variation Seasonal x Geographic Variation Small Scale Patches

Plankton tend to occur in patches Few meters to hundreds of km Samples are often highly variable – “True

Replicates?” What causes a patch????

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