Aquatic Systems and their Catchments

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Aquatic Systems and their Catchments
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• Small Catchments tend to have steeper slopes
• Soils tend to be thin, therefore lower mineral
  inputs
• Rapid runoff means detritus carried by streams
  has less time to decompose, therefore more
  decomposition occurs in the lake itself

Large Catchment, low slope

• Slower runoff means more contact time with
  substrates
• More processing of detritus in the stream

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• Maumee River Basin
• Highest rates of soil loss occur at the outer
  edge of the catchment because this is where slope
  is greatest.

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• Vegetation in the catchment is important
• Two adjacent catchments in Hubbard Brook
  Experimental Forest
• One had all trees removed (dashed line)
• One was untouched (solid line)
• Immediately after cutting the trees, large
  amounts of N, K, and Ca were exported (lost from
  the soil and transported away via flowing
  streams)
• Less vegetation to
• retain moisture (and reduce streamflow)
• Absorb nutrients (N,P,K)

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• Hydraulic Water Residence Time
• Calculated as HWRT lake volume/annual
  outflow
• But in reality, actual WRT can be very different
  from Hydraulic WRT. Why?

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• Relationship between WRT and Catchment AreaLake
  Area ratio
• These two lakes have the same area and are in the
  same ecoregion, but they have different catchment
  areas.
• Which lake has the greater CALA ratio?
• Which lake would have a greater WRT?
• What do you expect the relationship
• to be between WRT and CALA?

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Therefore, you can compare WRT of lakes within a
region using only topo maps But, there is a large
amount of scatter in this plot. What other
information would be helpful?
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Knowing lake volume would help to improve
predictions of WRT, but even that is not enough
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• WRT and CALA
• Lakes ranked in decreasing WRT
• How does CALA change with
• decreasing WRT?
• What is the trend with Secchi depth?

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WRT and Streams
WRT increases with Stream Order
Shorter WRT (hours)
Longer WRT (days)
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WRT and Trophic State

• Trophic State refers to the supply of nutrients
  (P,N) to a lake
• Eutrophic well nourished, high supply of
  nutrients
• Mesotrophic medium nourished
• Oligotrophic poorly nourished low supply of
  nutrients (see Ch. 2)
• The most important nutrient for primary producers
  in lakes is phosphorus, which usually enters
  lakes via tributary inflow.
• Because P-loading is positively related to
  tributary inflow, and WRT is negatively related
  to tributary inflow (relative to volume), THEN
  P-loading should be negatively related to WRT

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• Therefore, the higher the WRT, the lower
  Phosphorus loading and the lower the trophic
  state.

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• If the supply of phosphorus is the main factor
  that determines rates of primary production, then
  what will the relationship look like between WRT
  and primary production?
• The rate of respiration in a lake is positively
  correlated with the rate of primary production
  (everything that grows must eventually decompose
  respire). Therefore, what should the
  relationship between WRT and respiration look
  like?
• Lakes with long WRT and low primary production
  tend to be very clear (high secchi depth)

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• Some of the lakes with the longest WRT and
  clearest water are becoming more productive
• Lake Tahoe (development)
• Crater Lake (atmospheric Deposition)

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WRT and Biota

• If Flushing Rate (1/WRT) is high, planktonic
  communities can be affected.
• Macrozooplankton has higher growth rates than
  phytoplankton, therefore they will be more
  affected by rapid flushing (especially in the
  epilimnion).
• Heavy rain events coupled with a thin epilimnion
  may cause wash-out, where most of a planktonic
  population is flushed from the system.

Weeks
Months
Days
Days
Calanoid copepod
Phytoplankton
rotifers
Daphnia
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Retention of Dissolved Materials

• How long a substance added to a lake remains in
  the lake depends on two factors
• WRT
• Whether the substance tends flush through the
  lake without interacting (conservative) or
  whether it tends to be used by organisms or
  adsorb to sediment particles (non-conservative)
• Time to reach 90 of equilibrium is
• T90 equil 2.3 WRT (1-R) R is retention
  coef

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• Example
• If the annual input of road salt via Lake Erie
  tributaries were reduced by 50, the
  concentration of salt in Lake Erie would begin to
  drop and would reach a new stable point in about
• 2.3 2.6 (1-0) 6 years
• If the annual input of phosphorus via Lake Erie
  tributaries were reduced by 50, the
  concentration of P in Lake Erie would begin to
  drop and would reach a new stable point in about
• 2.3 2.6 (1-0.7) 1.8 years

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Sources and Sinks

• Substances such as nutrients (P,N) and many
  contaminants have high retention coefficients,
  meaning the water flowing out of the lake is
  cleaner than the water flowing in. In this case
  the lake is a Sink (net storage).
• In the case of contaminants such as PCBs, long
  after land and atmospheric sources are
  eliminated, lakes will continue to slowly flush
  out their stored contaminants. Lakes will have
  become sources.