I. Trophic Equilibrium
A. Natural eutrophication
1. Changes over time with
the filling in of lakes – the epilimnion volume doesn’t change as fast
as the
hypolimnion – the mixing depth is set by the SA of the lake, the fetch
and the wind speed
2. Increased AHOD –
aerial hypolimnetic oxygen deficit
3. Decreased volume of
hypolimnion
B. Artificial Eutrophication
1. altered watershed
conditions
a.
fire (changes evapotranspiration, water budgets, runoff)
b.
clearcutting – changes runoff
c.
agriculture
2. addition of nutrients
a.
fertilizers
b.
waste treatment plants
1) primary
treatment – settling
2) secondary
treatment – digest organic matter;
3) tertiary
treatment – any additional treatment of secondary effluent to
improve its quality;
for example, alum to remove P (expensive)
B. Madison Lakes, Wisconsin
Mendota, Monona, Waubesa,
Keyonsa, Yahara River
Early 1900’s – settlement,
clearcutting, agriculture
1912 – Monona and Mendota
eutrophied; to kill algae added CuSO4
1930’s -- Monona was still
so bad that they were diverting sewage to Waubesa
1962 – so bad that diverted
sewage into the Yahara River
For these lakes 88% of the P
and 75% of the N had come from sewage
C. Lake Washington
1933 – oligotrophic lake; 65
m deep
Lake Washington is a
monomictic lake in Seattle, with a residence time of ~3 years
Seattle population in 1865 –
300; 1965 – 1.2 million
In the 1950’s, the secchi
depth was >3 m; lake was dominated by diatoms
1955 Oscillatoria
was
found in the lake
Early 1960’s – “Lake Stinko”
T. Edmondson at the U.
Washington
At the maximum of
eutrophication, Lake Washington was receiving 20 million gallons of
secondary effluent each day
Sewage diversion was started
in 1963 and was completed in 1968, with no more effluent entering after
that year.
In 1962, 72% of the
phosphorus was brought in by sewage; in 1966 it was 62%, and in 1967 it
was 26%
1962-1964 were even worse
algal blooms
1965-1966 dramatic algal
production decline and increase in transparency;
Food
web effects:
Early on in eutrophication, Daphnia were replaced by Diaptomus
(becauseOscillatoria
clogs the filtering apparatus)
In the mid-1970’s Daphnia returns; secchi depth increased to
record levels – 12 meters – due to Daphnia grazing
D. Arctic Lake Example: Lake N2 – divided
lake in the arctic, Fe binding of P, delayed effects of fertilization
E. Tropical Lake Example: Lake Victoria
| Year | Secchi | Chl. a (mg/L) | Prim. Prod. (g O2/m2/day) |
Reference |
| 1960 | 7 m | 3 | 1 | Talling |
| 1990 | 1-2 m | 15 | 4 | Hecky |
Lake had become eutrophied and no one knew what had happened. The
algal species had changed from diatoms to
blue-green algae. There were many changes in the system.
1. Addition of nutrients
from basin – now in Lake Victoria there is constant N limitation,
especially inshore; there have been increases in P loading; N fixation
is not enough to relieve N limitation.
30 years ago the bottom waters were rarely anoxic, today they are often
anoxic – 70% less space for the fish to live in.
2. Food web changes
–
introduction of Nile Perch (Lates); introduced in the early
1960’s; haplochromines,
the cichlid fishes, included many guilds and trophic specialists; many
of the 300 endemic species have been eaten
by the Nile Perch.
3. Climate changes
causing different mixing – in the past 50 years the climate has warmed,
causing changes in mixing
and
the strength of stratification; affects light, nutrients, oxygen (blue
greens need more light and have an advantage
at
shallow mixing depth)
F. Practice of moving sewage outflows to rivers
has improved the nutrient condition of lakes, but caused many
problems in estuaries and nearshore ocean environments -- lots of Nitrogen
loading
leads to dead zones
III. Oligotrophication – concern about too low phosphorus
levels in the Laurentian Great Lakes – are we actually
decreasing fish production?
Proposals to start fertilizing lakes in a balanced N:P ratio to increase fisheries production levels. Some fisheries managers see the issue as a choice between productive ‘greener’ lakes and streams and unproductive aesthetically clear waters.
IV. Other Pollution
A. Types
1. Thermal Pollution – power
plants; industry
2. Radioactivity
3. Toxic contaminants
a.
Types
1) POPs – persistant organic pollutants -- Pesticides and
organic toxins – dioxins, furans, benzopyrene,
DDT/DDE, dieldrin/aldrin, hexachlorobenzene, alkylated lead, mirex,
mercury, PCB’s, toxaphene,
heptachlor, chlordane, endrin
i. Nearly 80,000 synthetic organic chemicals are in daily use
ii. Endocrine disrupters
2) Metals – cadmium, mercury, arsenic, lead
b.
Characteristics
1) Toxicity
i. Acute – quick death
ii. Sublethal/chronic – impairment of growth, reproduction..
iii. Carcinogenicity – impairment of function, death
iv. Mutagenicity/teratogenicity – effects on future generations
2) Bioaccumulation
i. Capacity to enter the food chain
ii. Biomagnification
3) Persistence – resist degradation in the environment
4) Volatility – easily evaporated and transported in the atmosphere