Structure%20and%20Productivity%20of%20Aquatic%20Systems

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Structure and Productivity of Aquatic Systems
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Functional Lake Zones
Pelagial
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Living Things in Lakes

• Distribution abundance of living things in lake
  controlled by physical and chemical conditions in
  different zones

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Organic Matter in Lakes

• Living things make up only small portion of
  organic matter in lakes
• Most is in form of non-living detritus
• Both particulate and dissolved

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Organic Matter in Lakes

• In most lakes, dissolved organic matter is 10 X
  more abundant than particulate
• Living things make up small portion of
  particulate
• Detritus is habitat energy resource for living
  things

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Organic Matter in Lakes

• Much of the organic production of photosynthesis
  within a system is not consumed, but becomes part
  of detritus reserve

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Primary Producers in Lakes

• 3 major categories of primary producers
• Phytoplankton
• Photic zone throughout lake
• Generally small, unicellular or colonial
  organisms

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Primary Producers in Lakes

• Emergent macrophytes
• Shallow portions of littoral zone
• Roots and lower portions in water, tops above
  water surface

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Primary Producers in Lakes

• Submersed macrophytes
• Deeper portions of littoral zone
• Completely underwater

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Productivity Hierarchy

• Emergents most productive (Carbon
  fixed/area/year)
• More productive than terrestrial grassland,
  forest
• Submersed much less productive
• Phytoplankton least productive

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Phytoplankton

• Cyanobacteria or blue-green algae
• Important nitrogen fixers
• High densities in late-summer
• Odor (and taste) problems

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Phytoplankton
Desmids

• Green algae
• Tremendous diversity
• Planktonic, but can be attached, benthic (often
  filamentous)

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Phytoplankton

• Golden-brown algae
• Low diversity, but can be important segment of
  phytoplankton
• Dinobryon important under low P conditions

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Phytoplankton

• Diatoms
• Very important group
• Planktonic and attached forms
• Cell walls with silica -- maximum abundance in
  spring when silica is most abundant

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Phytoplankton

• Cryptomonads
• Extremely small
• May reach high densities during cold periods with
  low light intensities (winter under ice)

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Phytoplankton

• Dinoflagellates
• Unicellular, flagellated, with spines
• Strict requirements for Ca, pH, temperature,
  dissolved organics

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Phytoplankton

• Some exhibit cyclomorphosis - seasonal change in
  size form
• Ceratium - more spines, longer spines, more
  divergent spines as water temperature increases
• Reduce sinking rate out of photic zone in less
  viscous water

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Phytoplankton

• Euglenoids
• Unicellular
• Most abundant in areas with high ammonia,
  dissolved organics
• Shallow farm ponds in cow pastures

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Paradox of the Plankton

• Lakes usually have a few dominant species and
  many rarer species
• Theoretically should have only single dominant
  species (niche overlap leads to competitive
  exclusion)

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Paradox of the Plankton

• Multispecies equilibrium in open waters
• 4 possible explanations

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Paradox of the Plankton

• Environmental change too rapid for competitive
  exclusion to occur
• Symbiotic relations among species (commensalism)
• Selective grazing on competitive dominants by
  zooplankton (size-based)
• Some species alternating between plankton and
  benthos
• Not truly competing with pure planktonic forms

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Phytoplankton andWater Quality

• Assemblage indicates level of nutrient enrichment
• Desmids and certain diatoms in nutrient-poor
  systems
• Different diatoms, greens, and blue-greens
  dominate as enrichment increases

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Phytoplankton andEnvironmental Factors

• Temperature and light control type, abundance of
  plankton
• Diatoms have lower temperature optimum,
  blue-greens higher optimum

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Phytoplankton andEnvironmental Factors

• Many can adapt to changing light intensities
• Chlorella changes pigments per cell to maintain
  same rate of photosynthesis
• Blue-greens regulate gas pressure in vacuoles to
  position themselves at depth with optimum light
  intensities

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Phytoplankton andEnvironmental Factors

• Some phytoplankton experience photoinhibition
• High light intensities near lake surface may
  temporarily destroy enzymes and decrease
  photosynthesis
• Sunny days - less photosynthesis near surface
  than at greater depths

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Phytoplankton - Seasonal Succession

• Changes in light, nutrients, temperature drive a
  shift in phytoplankton during the year

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Phytoplankton - Seasonal Succession

• Low growth in winter
• Diatoms and cryptophytes dominate in spring
• Greens take over in summer, joined or replaced by
  blue-greens as N runs low in productive lakes
• Less productive lakes - few greens, blue-greens,
  only peaks of diatoms spring and fall (silica)

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Phytoplankton - Seasonal Succession

• Seasonal abundance varies much more in temperate
  (1000 X) than in tropical (5 X) lakes, but total
  populations are much greater in tropical lakes
• Selective grazing by zooplankton can influence
  succession
• Eating some, providing nutrients for others

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Phytoplankton - Nutrient Enrichment

• Enrichment can greatly increase productivity (per
  volume) up to a point
• Eventually self-shading develops and thickness of
  photic zone reduced
• Inhibits further increases
• Productivity/m2 of surface remains virtually
  unchanged
• Photosynthetic efficiency low (lt1 of incident
  light)

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Phytoplankton - Variation in Production

• More production in littoral zones than pelagial
  areas
• Peak production during midday (except at surface
  - earlier in day)
• Seasonal production peaks in summer

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Macrophytes

• Restricted to the littoral zones
• In small, shallow lakes with no profundal zone,
  macrophytes may occur basin-wide

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Emergent Macrophytes

• Rooted in water or saturated soil with aerial
  leaves/stems
• Upper littoral - out to 1.5 m depth
• Typha - cattail

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Emergent Macrophytes

• Special category occupying mid-littoral region -
  0.5-3.0 m
• Floating-leaved plants
• Water lily

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Submersed Macrophytes

• All depths within photic zone down to 10 m for
  vascular plants
• Macroalgae - may occur slightly deeper
• Coontail, curlyleaf pondweed, Elodea

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Free-floating Macrophytes

• Not rooted
• May have well-developed submersed roots, or no
  roots
• Lemna - duckweed

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Aquatic vs. Terrestrial

• Aquatics mostly similar to terrestrial
  macrophytes
• One major difference - rooting tissues grow in
  anaerobic substratum

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Aquatic vs. Terrestrial

• Roots need O2 to respire
• Only can get it by transporting it from tissues
  in other parts of plant
• Extensive system of intercellular gas lacunae for
  gas transport, exchange

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Aquatic vs. Terrestrial

• Emergent macrophytes have leaf structure similar
  to terrestrial plants
• Linear, thick leaves - no problem obtaining
  light, CO2
• High transpiration - lose lots of water

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Aquatic vs. Terrestrial

• Submersed macrophytes often look much different
  than terrestrials
• gt70 of volume is intercellular lacunae
• Leaves very thin, divided and broadened to
  increase surface area to volume ratio
• Better absorb sunlight, CO2

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Aquatic vs. Terrestrial

• Some submersed forms also capable of assimilating
  bicarbonate for use in photosynthesis
• Based on relative scarcity of free CO2 in most
  environments

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Nutrient Needs

• Most nutrients required by macrophytes come from
  sediments
• Free floaters get it from water

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Nutrient Needs

• Interstitial waters generally contain much higher
  concentration of nutrients than waters above
  sediments (anoxic conditions)
• Most macrophytes can assimilate nutrients from
  water if concentrations rise (just like
  phytoplankton)

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Leaky Macrophytes

• Submersed macrophytes are very leaky
• Lose nutrients to surrounding water during active
  growth
• Developed on land and not adapted to water?
• Compromise - improved light, CO2 uptake at cost
  of losing some nutrients?

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Light Limitations

• Emergent macrophytes are seldom light-limited -
  tremendous capacity for production
• Submersed macrophytes are light-limited
• Depth distribution regulated by light, in part

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Depth Limitations

• Even in systems with light penetrating to great
  depths (unproductive systems), macrophytes only
  occur down to 10 m
• Results from hydrostatic pressure - doubles
  atmospheric pressure by 10 m
• Inhibits movement of gas through lacunae

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Macrophytes vs. Phytoplankton

• Phytoplankton productivity may be very low in
  littoral areas with many macrophytes - 3 reasons
• 1) Competition for nutrients
• 2) Shading
• 3) Release of inhibitory organic chemicals by
  macrophytes

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Macrophytes vs. Algae

• Productivity of some types of algae may be very
  high in close proximity to macrophytes
• Grow attached to macrophytes and live off
  materials leaking out