Transcript Nutrient Pollution and Eutrophication

Nutrient Pollution and Eutrophication
Outline of Topics
• Eutrophication
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What is it?
Limiting Nutrients
Causes of Cultural Eutrophication
Problems Associated with Cultural Eutrophication
• HABs
• Dead Zones
• Oxygen Depletion in Lakes
– Thermal Stratification of Lakes
– Types of Lakes
• Seasonal Stratification in Dimictic Lakes
– Specifics of Oxygen Depletion in Lakes
• Epilimnion (Diurnal Effects)
• Seasonal Depletion of Hypolimnion
• Effects on Chemical Composition
Eutrophication
• Lecture Question
– What is eutrophication?
– Eutrophication is a gradual increase in biological productivity of
an aquatic ecosystem with time
• “Productivity” here essentially refers to the net rate of
photosynthesis (the net primary productivity, NPP)
• This is a natural process
• The rate of productivity increase generally increases with time
– Cultural eutrophication is a human-induced acceleration of the
eutrophication process
Eutrophication
Primary productivity
• rate of photosynthesis
(carbon-fixing)
Three common productivity
levels are indicated in the figure:
• oligotrophic
• mesotrophic
• eutrophic
Eutrophication of Mirror Lake
Cultural Eutrophication
• Lecture Question
– What causes cultural eutrophication?
– NPP usually limited by some factor:
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Temperature
Sunlight availability/intensity
Predation
Nutrients
– Limiting Nutrients
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Chemicals that can control the productivity
Possible limiting nutrients: N, P, Si (for diatoms), Fe
Terrestrial NPP is often limited by N
Marine NPP is often limited by N and sometimes Fe
Freshwater NPP is often limited by P
– So cultural eutrophication is caused by “extra” sources of limiting
nutrients to aquatic ecosystems
• Major sources of nutrient pollution: sewage discharges, chemical fertilizer,
livestock waste, NOx emission followed by atmospheric deposition of nitrate
PM
Limiting Nutrients
• The limiting nutrient is
the chemical that limits
NPP
• Photos on the left show
the effect of phosphate
(the limiting nutrient) on
the algae populations of
two freshwater lakes
Cultural Eutrophication
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Lecture Question
– So what’s wrong with increased productivity?
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Oxygen Depletion
– Decomposition of organic material in the sediment causes hypoxia on the bottom
of water bodies (esp in summer)
– Larger day-night fluctuations in dissolved oxygen even in surface water
– Sediment may release toxic chemicals under anoxic conditions
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Algae Blooms
– A nuisance for recreation and water treatment
– Some algae blooms are toxic (HABs)
• HABs are not necessarily pollution-related (for most,
the connection is not obvious)
• some toxic algae definitely thrive in polluted waters
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Other Changes
– Decreased visibility (bad for submerged aquatic vegetation, SAV)
– Increased sedimentation rate (bad for bottom-dwellers, reproduction)
HAB Effects on Humans
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Amnesia Shellfish Poisoning
– Toxin: domoic acid (can be fatal)
– GI and neurological disorders
– Symptoms: nausea, vomiting, abdominal cramps, diarrhea
• Severe cases include neurological symptoms: headache, dizziness, seizures,
disorientation, memory loss, respiratory difficulty, coma
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Ciguatera Fish Poisoning
– Toxins: ciguatoxin/maitotoxin (usually not fatal)
– GI, neurological and CV symptoms
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Diarrhetic Shellfish Poisoning
– Toxin: okadaic acid (not fatal)
– GI symptoms
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Neurotoxic Shellfish Poisoning
– Toxins: brevetoxins (not fatal)
– Syndrome almost identical to ciguatera poisoning but slightly less severe
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Paralytic Shellfish Poisoning
– Toxins: saxitoxins (can be fatal)
– Rapid neurological symptoms
• Tingling, numbness, burning, drowsiness, etc
• Respiratory arrest can occur within 24 hours
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HAB events seems to be rising (may reflect better detection)
Coastal HAB Events in the US
Dead Zones
• Questions
– What are dead zones? What are some famous dead zones?
– Dead zones are very large areas with low oxygen levels. They usually
occur in oceans but have also been observed in large estuaries and
lakes.
• Can be caused by nutrient pollution (often associated with agricultural
fertilizers) and subsequent eutrophication
• Can also occur naturally
– Natural sources of nutrients
– River flooding empties into the ocean and creates a top layer of freshwater,
temporarily cutting off the oxygen supply
– Examples
• Most famous: Gulf of Mexico where the Mississippi River empties into the
gulf.
• Chesapeake Bay
• Black Sea
– Largest dead zone in the world; the bottom (below 150 m) is completely anoxic
– Not caused by pollution but by v slow exchange of water with the Mediterranean
through the narrow and shallow Bosporus Strait
• Kattegat straight (mouth of the Baltic Sea)
• Northern Adriatic Sea
Global Location of Dead Zones
Source: NASA http://daac.gsfc.nasa.gov/oceancolor/scifocus/oceanColor/dead_zones.shtml
Nutrient Pollution from the Mississippi
Satellite picture shows the effect of nutrient discharge
on algae levels (the green color reflects chlorophyll-a
concentration)
Formation of Gulf “Dead Zone”
• Gulf of Mexico “Dead Zone” forms
every summer
• When the algae die they settle to the
bottom and begin degrading (and
consuming oxygen)
• Oxygen is depleted (mostly below the
pycnocline) creating the dead zone
• The dead zone has grown in size
from 3200 mi2 (1985-1991) to 6200
mi2 (1993-2001)
• Currently about 7900 mi2 (approx
size of New Jersey)
Hypoxia in Lakes
• Question
– How exactly does hypoxia (low oxygen conditions) develop in
lakes in response to eutrophication?
– Hypoxia usually occurs at the bottom of lakes in the summer
• Prominent example: central basin of Lake Erie.
• But it can also occur at the top of lakes (especially overnight) and in
the winter.
– Steps to create summer hypoxia in lake bottoms:
• Eutrophication increases biological productivity over time
– Spring and summer blooms are common
• Algae die and settle to the bottom, where their decomposition
presents an oxygen demand.
• Oxygen demand is greatest in the summer, when biological
productivity and rate of decomposition are greatest.
• At the same time, summer stratification of lakes cuts off the
oxygen supply.
Lake Stratification (Usually in Summer)
• Inflection point in the thermal profile is called
the thermocline.
• Mixing across the thermocline is very slow.
Ocean Thermal Profile
3 main temperature zones:
•surface ocean: warm, 100-200 m
•thermocline: down to ~1 km
•deep ocean: cold, extends to floor
Development of Summer Stratification
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How does thermal stratification typically develop in lakes?
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Many lakes are stratified in the summer.
Start with a fully mixed lake in the springtime.
1. Lake is mixed by wind flowing over the surface, which pushes the top water
down.
2. Lake is mostly all the same temperature.
3. Late spring: top layer is warmed by the sun, but mixing “keeps up” so that
the entire lake is warmed.
4. A few hot, still days can occur…top layer warms rapidly but there is little
wind to mix the thermal energy. The top layer is warmer and less dense
than the bottom layer.
5. When the winds pick back up, the top layer is now too buoyant to be
“pushed” down and mixed. Stratification has occurred; mixing between top
and bottom layers is slow.
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Definitions
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Epilimnion: top layer in a stratified lake. It is warmer, less dense. It is in
contact with the atmosphere but not the sediment.
Hypolimnion: bottom layer in a stratified lake. It is cooler, more dense. It is
isolated from the atmosphere but it is in contact with the sediment.
Metalimnion: a middle layer in which the density gradient is larger. The
thermocline (inflection point of the density gradient) resides in this layer.
The thermocline can be thought of as the plane that separates the
epilimnion and hypolimnion.
Development of Summer Stratification
Lake Ontario in 1965
Development of Summer Stratification
Lake Ontario in 1965
Seasonal Mixing
• How often do lakes mix completely?
– Holomictic lakes are lakes that undergo annual mixing between
stratified layers.
– Dimictic lakes mix twice a year, usually in the fall and spring.
Ideal Development of Stratification in a Dimictic Lake
Observed Stratification in a Dimictic Lake
Measurements from Lake Lawrence (MI)
Seasonal Stratification in Lakes
• Do all lakes mix completely twice a year?
– Holomictic lakes are lakes in which the epilimnion and hypolimnion
undergo some type of annual mixing (ie, overturns)
– Meromictic lakes are permanently stratified. Often these lakes are fed
by underground saline springs, and the deepest water is too salty & cold
(dense) to ever mix. In these lakes, the upper layer can undergo
stratification “within” itself.
– What are the types of Holomictic Lakes?
• Amictic lakes are perennially ice-covered.
• Dimictic Lakes mix twice a year.
• Monomictic lakes undergo mixing once a year
– Warm monomictics undergo thermal stratification in the summer and mix freely
in fall, winter and spring.
– Cold monomictics circulate only in the summer (water temps never exceed 4 C)
and are stagnant during most of the year, when covered by ice.
– Some lakes are too shallow throughout for stratification to ever occur;
they are continuously mixed.
– Very deep lakes are “mostly permanently” stratified, but mixing can
occur every few years.
Geographic Tendencies of Holomictic Lakes
mixed types (mainly
varients of warm
monomictic)
Oxygen Depletion in Eutrophic Lakes
• Questions
– So…about oxygen depletion in eutrophic lakes? And what’s the
purpose of the aerators in Westhampton Lake?
– Oxygen depletion in eutrophic lakes tends to occur
• in the hypolimnion
• in the summer
– This is because
• that’s when the lake becomes strongly stratified (isolating the
hypolimnion from the atmosphere)
• the rate of biological production increases (causing more fresh
organic matter to be deposited onto the sediments) and
• the rate of organic decomposition in the sediments is greatest.
– The aerators serve two purposes:
• to give the bottom of the lake access to oxygen, and
• to increase lake mixing, disrupting summer stratification.
Idealized Seasonal Oxygen Depletion in Dimictic Lakes
Summer oxygen depletion in hypolimnion of
eutrophic, dimictic lakes
Observed Oxygen Depletion in Lake Michigan
• Contours give DO in mg/L
• Hypolimnion becomes anoxic by the end of the summer
• Note regions of supersaturation in epilimnion
Oxygen Depletion in Eutrophic Lakes
• Question
– Is the hypolimnion the only part of a stratified lake that ever
becomes hypoxic?
– No. The epilimnion of eutrophic lakes show greater swings in DO
concentrations than in less productive lakes.
• Caused by the day-night (diurnal) switch of the algae between
photosynthesis and respiration.
• Conditions (eg during a particularly intense algae bloom) may favor
development of extremely low DO levels overnight.
• Extensive algal mats (that may form during blooms) may interfere
with dissolution of atmospheric oxygen into the lake.
Idealized Diurnal Effects (Stream, Lake Epilimnion)
Effects of Oxygen Depletion on Chemical Composition
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Low DO favors reduced species
Common Chemical Species in Lakes
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Oxidized Forms
Reduced Forms
CO2, H2CO3, HCO3-, CO32-
CH4(g)
SO42-, HSO4-
H2S(g), HS-, S2-
NO3-, NO2-, HNO2
N2(g), NH3(g), NH4+
MnO2(s), etc
Mn (aq)
FeOOH(s), etc
Fe2+ (aq)
2+
Release of chemicals from sediment in reduced form
– Reduced form of many chemicals are more mobile than their oxidized form
• Release of gases: methane (CH4), hydrogen sulfide (H2S), ammonia (NH3)
– smelly!
• Release of some toxic metals
– Release of nutrients from sediment
• Increases effectiveness of nutrient recycling
• Feedback loop leading to further algae blooms, eutrophication
Effect of Productivity on Composition: Nitrogen
Effect of Productivity on Composition: Phosphorus
Sediment Release: The Oxidized Microzone
• How/why does the sediment release chemicals into the water
under reducing (low DO) conditions?
• When the overlying water is oxygenated, an oxidized microzone
exists in the top few mm of the sediment at the bottom of the
water body
• Anoxic conditions in the hypolimnion result in the disappearance
of the oxidized microzone, followed by the release of phosphate
(and various other chemicals in their reduced state)
Release of Phosphorus from the Sediment
• Phosphate doesn’t have a reduced form. Why is it released from the sediment
under reducing (low DO) conditions?
• Phosphate is bound by many metals – particularly iron – more strongly in their fully
oxidized form. The phosphate cannot pass through the oxidized microzone because
they are bound by the oxidized metal. When the metals are reduced as the
microzone disappears, phosphate is released.