Aquatic Ecology
Dr. Mason MacDonald
mason.macdonald@dal.ca
The Hydrological cycle: a closed system
Like in the case of nutrient cycles (e.g. Carbon, Nitrogen) Earth is a closed system in
respect to the movement of distribution of water.
This drawing shows blue spheres representing relative amounts
of Earth's water in comparison to the size of the Earth. Each
blue sphere represents "volume." The volume of the largest
sphere, representing all water on, in, and above the Earth,
would be about 1,386,000,000 km3 in diameter.
The next largest sphere represents liquid fresh water in
groundwater, swamp water, rivers, and lakes. The volume of this
sphere would be about 10,633,450 km3 and form a sphere about
273 km in diameter.
The smallest sphere (look closely) represents liquid fresh water
in lakes and rivers. This is the main source of water for things
living on Terrestrial land. The volume is 93,113km3 or a sphere
with a diameter of 56.2 km.
What are the major reservoirs for
the water cycle?
Think About It
The biogeochemical cycles we discuss in
this lecture (and the next few lectures)
have two components: reservoirs and
transport.
A reservoir is part of a cycle that holds
water or elements for a long period of
time.
Transport is how water or an element
moves from one reservoir to another
Reservoirs of
the global
hydrological
cycle
Reservoir Distribution
Reservoir Distribution
Reservoir Distribution
Transportation Through the Water Cycle
Phase Changes
Movement
Evaporation (l -> g)
Infiltration/Percolation (l)
Transpiration (l -> g)
Groundwater Flow (l)
Sublimation (s -> g)
Surface Run Off (l)
Condensation (g -> l)
Precipitation (g -> l/s)
Irrigation
Deforestation
Urbanization
Agriculture
Climate Change
Climate change increases our risk of both heavy rains and extreme droughts. But
why – and how – is that? Aren't the two contradictory? What about the rising
temperatures affect on evaporation and transpiration?
Water evaporates from the land and sea, which eventually returns to Earth as rain and snow. Climate
change intensifies this cycle because as air temperatures increase, more water evaporates into
the atmosphere. Warmer air can hold more water vapor, which can lead to more intense
rainstorms, causing major problems like extreme flooding in coastal communities around the world.
At the same time that some areas are experiencing stronger storms, others are experiencing more dry
air and even drought. As temperatures rise, evaporation increases and soils dry out. Since the
air can hold more water vapor, the frequency of rain could decrease or become more variable.
Then when rain does come, much of the water runs off the hard ground into rivers and streams, and
the soil remains dry. What are we left with? More evaporation from the soil and an increased risk of
drought.
What is a
Watershed?
• A watershed is a land
area that channels water
to creeks, streams, and
rivers to eventual
outflow points
• Locally our watershed is
the “Salmon River
Watershed”
Several watersheds feed into the Cobequid Bay
Salmon River
North River
McLure Brook
Farnham Brook
McCurdy Brook
Fraser Brook
Drainage Area
(km2)
Drainage Density
(km/km2)
395.5
215.0
40.2
22.5
18.9
8.8
0.88
0.99
1.07
0.75
1.62
1.62
Shubenacadie drainage area is (2800 km2)
Stream Profile
Gross Slope
Mean Slope
(m/km)
(m/km)
6.76
9.93
11.25
10.23
19.32
16.10
4.11
6.86
7.03
7.20
9.79
15.15
•
•
•
McCurdy Brook runs from the Onslow
Mountain area into Salmon River
McClure’s Brook runs from Hilden into Salmon
River
Farnham brook runs from Valley, through Bible Hill, into Salmon River
Characteristics of a Healthy Watershed
Ideal Parameters at a Glance (NS Fish Habitat)
Parameter
Temperature
Optimum (if known)
15°C
Tolerable Range (if known)
1°C - 24°C
Dissolved Oxygen
pH
Conductivity
≥ 7 mg/L
6.5 – 8.0
200 – 1000 μS/cm
≥5 mg/L
4.0 – 9.5
n/a
Turbidity
Velocity
0.25 – 35 mg/L
≤ 0.15 m/s
n/a
≤ 0.25 m/s
Ideal Parameters at a Glance (NS Fish Habitat)
Ion
Ammonium
Nitrate
Phosphate
Iron
Concentration (mg/L)
0.002 (sublethal), 0.2 (lethal)
25 (sublethal)
n/a
9 (sublethal)
Heavy metals are another important criteria: e.g. iron, zinc, arsenic, lead, cadmium,
mercury, cobalt
Digger Log – Margaree, NS
Digger Log – Antigonish, NS