Nutrient Pollution and Eutrophication

Outline of Topics Eutrophication Oxygen Depletion in Lakes 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 define the terms productivity and the trophic levels given above

Eutrophication of Mirror Lake

Cultural Eutrophication Lecture Question What causes cultural eutrophication? NPP usually limited by some factor: Temperature Sunlight availability/intensity Predation Nutrients Limiting Nutrients 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 Lecture Question So what’s wrong with increased productivity? 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 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 Other Changes Decreased visibility (bad for submerged aquatic vegetation, SAV) Increased sedimentation rate (bad for bottom-dwellers, reproduction)

HAB Effects on Humans 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 Ciguatera Fish Poisoning Toxins: ciguatoxin/maitotoxin (usually not fatal) GI, neurological and CV symptoms Diarrhetic Shellfish Poisoning Toxin: okadaic acid (not fatal) GI symptoms Neurotoxic Shellfish Poisoning Toxins: brevetoxins (not fatal) Syndrome almost identical to ciguatera poisoning but slightly less severe Paralytic Shellfish Poisoning Toxins: saxitoxins (can be fatal) Rapid neurological symptoms Tingling, numbness, burning, drowsiness, etc Respiratory arrest can occur within 24 hours 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) Gulf of Mexico dead zone: record is 8006 sq mi (about the size of Massachusetts) in 2001. Forms in the summer. Size fluctuates depending on nutrient load and weather. Has grown from 3200 sq mi (1985-1992) to 6200 sq mi (1993-2001) – about doubling in area Chesapeake has a problem with with hypoxia due to nutrients. An important input is atmospheric deposition the entire Black Sea is completely anoxic below the pycnocline generally: coastal areas have a real problem with nutrient pollution

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 How does thermal stratification typically develop in lakes? Many lakes are stratified in the summer. Start with a fully mixed lake in the springtime. Lake is mixed by wind flowing over the surface, which pushes the top water down. Lake is mostly all the same temperature. Late spring: top layer is warmed by the sun, but mixing “keeps up” so that the entire lake is warmed. 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. 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. Definitions 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 this is Lake Ontario in 1965

Development of Summer Stratification Lake Ontario in 1965 full stratification develops

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 mention the aerators in Westhampton Lake. Two functions: oxygenation and circulation (ie stirring) 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) sudden overnight oxygen depletion can result in fishkills

Effects of Oxygen Depletion on Chemical Composition Low DO favors reduced species 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 nitrate reduction (part of N-cycle, remember – catalyzed by microorganisms, eg in denitrification) occurs in hypolimnion again, very idealized. In reality, nitrate levels in the epilimnion of stratified eutrophic lakes may fall to low values due to assimilation

Effect of Productivity on Composition: Phosphorus release of phosphate from sediments is very significant – it constitutes an internal supply of nutrients. Basically, past nutrient (P) pollution comes back to haunt you such internal P sources becomes more prevalent as productivity increases (positive feedback) how does it happen? The P doesn’t change oxidation state; something else must be at work here

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. phosphate binds to the surface of hydrous iron (III) oxides, shown as Fe(OH)3.PO4 in the figure basically, reductive dissolution of Fe(III) is followed by release of phosphorus. Reduction can occur due to iron-reducing bacteria or due to reaction with sulfide produced by sulfate-reducing bacteria one way to immobilize P in lake sediment: treatment with Ca or Al, both of which bind phosphate and neither of which are reduced at typical pE values. Note, however, that both metals are mobilized by acidic conditions, so this only works well in a lake with decent alkalinity (definitely don’t want to add Al to an acidic lake – toxic!)