Cole et al Science 265: Why this pattern?

Cole et al Science 265: Directly measured Autumn Full Seasonal Data Summer Tropical Africs

Lec 6: Nutrients and Nutrient cycling I. Storages and cycling II. Nutrient loading (more next lecture) III. Phosphorus IV. Nitrogen V. Other elements 1

I. Storages and cycling A. Energy versus nutrients -Energy flows -Nutrients cycle B. Closed system 1. Rate = cycles/time a. as rate increases, productivity increases b. total N or P versus the amount of inorganically available N or P 2. Pathways - In a closed system all the nutrients cycle within the system C. Open system - Boundaries 1. Rate 2. Pathways (e.g. internal cycling vs. nutrient loading) 3. Residence time: time spent cycling before being lost from the system a. residence time = amount of nutrient in the system/amount in output b. in an open system nutrient use depends on recycling rate and retention by the system (residence time) c. inputs and outputs do not necessarily balance 2

II. Nutrient Loading A. Estimates of critical amounts of nutrients for eutrophication (especially used for N and P) B. Amount of nutrient input per time and lake area called aerial loading C. Used to develop models of nutrient effects in lakes D. Must determine: 1. Volume of inflow and outflow 2. Concentration of nutrient in effluents and influents 3. Volume of lake 4. Loss rates to sediments 3

A. Except under polluted conditions, the only significant inorganic form of Phosphorus is Orthophosphate (PO 4 –3 ) B. Phosphorus often is a limiting nutrient in freshwater habitats C. Generally, >90% of Phosphorus is in or adsorbed to living or dead organisms D. Phosphorus is unique among the major inorganic nutrients in that its oxidation is not an important energy source (P always occurs in the oxidized form) III. Phosphorus (P) 4

C : N :P 106:16: C:10N:1P 6C:4N:1P 1st to become limiting 2nd to become limiting rarely limiting III. A. P as a Limiting Nutrient Elemental composition in plants (w/ balanced growth) Presence in environment Composition of sewage effluents ** Luxury Uptake 5

Weathering of Rock (Apatite) P adsorbs to particles III. B. Phosphorus Cycle 6

A. Particulate P 1. Organisms 2. Rocks, soil, sediments Igneous rocks are associated with low P Sedimentary rocks are associated with high P 3. Adsorbed B. Dissolved P 1. Orthophosphate (PO 4 –3 ) 2. Polyphosphates (from detergents) 3. Organic phosphates (mostly colloidal) Total Phosphorus must take into account all forms of P, including that incorporated into suspended matter and organisms. III. C. Forms of Phosphorus 7

A. Precipitation (Wet and Dry) Non-populated areas<30 ug/L Urban-Industrial areas>100 ug/L Range g/m 2 /year B. Ground Water 20 ug/L C. Runoff (fertilizers) varies Lake requirements ~ 0.07 g/m 2 /year: >0.13 g/m 2 /year may result in eutrophication if mean depth < 5m III. D. Sources of Phosphorus 8

Phosphate in living plant and animal tissue Phosphate dissolved in water Compensation Depth Aphotic Zone Photic Zone Phosphate in mud Thermocline In epilimnion, P rapidly is taken up by algae In sediments, P is removed by rooted vegetation and benthos III. E. Distribution of Phosphorus 9

Generalized P Profiles in Lakes of Low and High Productivity P, o C, O 2 O O 2 OLIGOTROPHICEUTROPHIC OO 2 P S P T P SP T P S = Soluble phosphorus P T = Total phosphorus Depth P, o C, O 2 10

Phosphorus in Sediments Depends on O 2 supply O 2 depends on trophic status and basin morphology P is retained by the oxidized microzone Breakdown of the oxidized microzone releases P (also Fe, Mn) P, Fe, and Mn concentrations are related P released from sediments under anoxic conditions (+ feedback of internal cycling) P also may be released from sediments by rooted vegetation and benthos 11

ProductivityTotal P Ultraoligotrophic<5 ug/L Oligo-Mesotrophic 5-10 Meso-Eutrophic10-30 Eutrophic Hypereutrophic >100 P generally is regarded as more important than other nutrients except in marine costal waters and under high P conditions. III. F. Epilimnetic Phosphorus and Lake Productivity *Note areal loading rate; influence of depth 12

TN TP TN:TP Generalized relationship between water clarity (Secchi depth) and algal concentration (Chl a). (OECD 1982). Hensley Reservoir and Fresno River Data

A. P levels often positively correlated with aquatic productivity B. Noxious algal blooms C. Hypolimnetic Oxygen Deficits D. P is difficult to remove from water III. G. Phosphorus and Water Quality 13

P Loading & Oxygen P Loading & Phytoplankton (Lake Washington, Seattle) Hypolimnetic O 2 deficit 14

A. Generally considered to be the 2nd most important nutrient in lakes in terms of limiting the rate of primary production (Phosphorus being 1st) B. Occurs in many forms and energy states (gas, organic and inorganic) Lithosphere97.6% Atmosphere 2.3% Hydrosphere + Biosphere0.1% C. Important both as a nutrient and (in some forms) for its toxicity to organisms IV. Nitrogen 15

A. Dissolved molecular Nitrogen (N 2 ) B. Organic Nitrogen Proteins Highest Energy Amino acids Amines Humic compounds C. Inorganic Nitrogen NH 4 + Ammonium NO 2 – Nitrite NO 3 – NitrateLowest Energy IV. A. Forms of Nitrogen 16

IV. B. Nitrogen Sources and Losses A. Sources 1. Precipitation (wet & dry) 2. Nitrogen Fixation 3. Runoff B. Losses 1. Outflow 2. Denitrification (NO 3 => N 2 ) 3. Sediments 17

IV. B. Nitrogen Fixation A. N 2 gas to ammonium, very expensive energetically ( Chemical “fixation” of molecular nitrogen (breaking the triple covalent bond) in the laboratory requires 500 O C and 100+ atmospheres of pressure) B. Only bacteria known to fix nitrogen C. Nitrogenase sensitive to O 2, a variety of adaptations protect it D. Lightning also fixes N 2 to NO 3 - in the atmosphere E. Nitrogen-fixing cyanobacteria can be very important in lake N cycles 18

A. Nitrification- oxidation of ammonium to nitrite (Azotobacter) and nitrite to nitrate (Nitrobacter) B. Denitrification- using NO 3 - as an electron acceptor for oxidation of carbon, yields N 2 O and N 2. Drives N loss from environment. Under very low redox, can go to ammonium C. Remineralization (ammonification) OrgN => NH 4 + IV. C. N Cycling 19

Nitrate (NO 3 ) Ammonium (NH 3 ) IV. D. N Distribution in a Lake 20

N, o C, O 2 NH 4 + NO 3 – O O 2 NH 4 + NO 3 – O O 2 OLIGOTROPHICEUTROPHIC N, o C, O 2 Depth Generalized N Profiles in Lakes of Low and High Productivity 21

pH NH 4 + : NH 4 OH 63000:1 7300:1 830:1 9.51:1 Least toxic Most toxic IV. E. Toxic Forms of Nitrogen A. Nitrate/Nitrite – concentrations in drinking water >10 mg/l can cause the disease Methemoglobinemia in infants (a problem in some agricultural areas) (NO 2 binds to hemoglobin more strongly than O 2 ) - Can be converted to carcinogenic nitrosamines in the stomach B. Ammonia (especially in the form NH 4 OH) is toxic to many organisms Amount of NH 4 + vs. NH 4 OH is pH dependent: 22

A. Silicon 1. Key element in diatom frustules 2. Can become limiting in lakes B. Iron 1. Ferric, Fe 3+, oxidized; ferrous, Fe 2+ reduced 2. Iron oxidation by microorganisms important chemoautotrophic pathway, but also will happen abiotically, so must occur at oxic/anoxic interface 3. Oxidized iron precipitates with phosphate, but dissociates again in anoxic conditions V. Silicon, Iron, etc. 23

Annual Silicon Cycle in a Lake 24