Aquatic Ecosystems Ecosystem Ecology

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Aquatic EcosystemsEcosystem Ecology

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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Aquatic Ecosystems

• Governed by same principles as terrestrial
  ecosystems
• Radically different environmental controls

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Terrestrial and aquatic ecosystems have
fundamentally different physical environment
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Aquatic plants have No physical support
structures Small size to maximize diffusion
rates Rapid reproduction to cope with water
mixing and grazing
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lv K
Reynolds number
Where l length v velocity K viscosity
constant
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Size determines feeding strategy Small organisms
cant swim Depend on diffusion Large cells can
sink or float Move vertically Medium-sized
organisms can filter-feed Large organisms can
swim Can chase prey
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Some marine ecosystems are just as productive as
the most productive terrestrial ecosystems The
most widespread marine ecosystems are
unproductive
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What limits marine production?

• Water? (no, except intertidal)
• Strong contrast with terrestrial systems, where
  water is the dominant limiting factor
• CO2? (no, except sometimes intertidal)
• CO2-bicarbonate-carbonate equilibrium supplies
  CO2
• Light? (always at depth)
• Nutrients? (usually)

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More light penetrates to depth in oceans than on
land Marine production is restricted to top 200m
(Euphotic zone) Light limits production within
and below this zone
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Light quality differs between marine and
terrestrial Ecosystems 1. Ocean water enriched
in blue (high-energy wavelengths penetrate
deeper) 2. Forests enriched in red (plants
remove high-energy wavelengths for Ps
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Marine phytoplankton are like terrestrial shade
plants

• Low photosynthetic capacity
• Spend most time in light-limiting environ
• (due to frequent mixing)
• Sensitive to UV radiation
• Some specialized to use blue light
• e.g., kelps

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Ocean currents create radically different
environments Centers of gyres have little
mixing Off-shore currents cause upwelling Warm
oceans have high vertical stability (not much
vertical mixing)
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Nutrient concentrations of euphotic zone are
highest in upwelling currents Always depleted
at surface by algal uptake
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North Pacific is relatively nutrient-rich zone
of thermohaline-driven upwelling westerly storms
mix surface waters cold surface waters are
weakly stratified (especially in winter)
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Latitudinal gradients in productivity

• Polar oceans are most productive
• More effective mixing of nutrients from depth
• Polar lands are least productive
• Less rapid nutrient release from SOM
• Consequences
• Bipolar fish and mammal migrations
• Polar distribution of anadromous fish

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Major upwelling zones off Peru, Africa Outer
Banks, North Pacific California, North
Africa Wind-mixing off Antarctica
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Strength of tropical upwelling off Peru depends
on ENSO
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Strong recycling component to marine nutrient
cycling

• Most nutrients recycle within the euphotic zone
• Microbial loop
• Bacteria production contributes to food chains
  that feed fish

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What are the limiting nutrients in the ocean?

• NP ratio of ocean waters centers on 141 (ratio
  found in algae)
• Phosphorus is the master element
• N fixation adds N, whenever P is available

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NP ratio in oceans centers around 141 Redfield
ratio
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Water chemistry conforms to same element ratio
as in algae--141 N fixers always add N,
whenever this ratio drops
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Some regions of oceans have high N and P conc.

• Common in subtropical gyres and southern ocean
• Two processes contribute to this
• 1. Micronutrient limitation
• Iron addition experiments
• 2. Grazing

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Coastal oceans and estuaries

• More nutrient-rich
• Closer to terrestrial inputs
• Benthic decomposition more important
• Less time for decomposition to occur in water
• Often nitrogen-limited
• Perhaps due to denitrification in sediments

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Lakes

• Intermediate between oceans and land
• Pelagic zone functions like ocean
• Littoral zone is like wetland

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Latitudinal water balance
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Global Distribution of Lakes
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Global Distribution of Lakes
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Origin of Lakes?
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Origin of Lakes
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Light
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Light and Ice
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Stratification
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Stratification
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Water movements
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Lakes

• Production is seldom carbon-limited
• Groundwater is super-saturated with CO2
• Most lakes are net source of carbon to atmos

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Lakes

• Vertical mixing is critical
• Occurs in spring and fall, when lake isothermal
• Wind and wave action drive the mixing

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Lakes

• Nutrients are strongest limitation to NPP
• Lakes generally P-limited Why?
• Surrounding terrestrial matrix is N-limited
• Two basic explanations
• 1. P retained more effectively than N on land
• 2. Lakes add N through N fixation whenever P is
  available

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Lakes

• Lakes are vulnerable to eutrophication

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Lake Nutrients
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Lake Nutrient Limitation
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Lakes

• Lakes are vulnerable to eutrophication

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Lakes

• Lakes are vulnerable to eutrophication

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Lakes

• Lakes are vulnerable to eutrophication

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Lakes

• As in the oceans, N-fixation by cyanobacteria
  important

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Streams and rivers

• Directional movement of water in streams and
  rivers greatly influences their functioning
• Phytoplankton unimportant
• Would get swept away
• Periphyton are main producers

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River continuum concept Transition in
structure/function along length of river
system inputs, organisms, energy flow
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Nutrient spiraling

• Nu trients spiral horizontally
• Terrestrial systems vertical N cycling
• Nutrients held surprisingly tightly
• Flow downstream mainly in dissolved form
• Hyporrheic zone
• Flow also occurs within river bed

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Nutrient spiraling
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Nutrient spiraling
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Hyporheic flow