Oceanography - Nutrient Cycling (Shorter)

Biogeochemical cycles

Transfer of compounds or nutrients between living and nonliving parts of an ecosystem often converting nutrients into biologically usable forms

Important nutrients in biogeochemical cycles

Carbon phosphorus nitrogen silicon sulfur iron and others

Nutrient sources

Atmosphere land organisms and upwelling of deep nutrient-rich water

Nutrient sinks

Organisms temporarily store nutrients while export to the deep ocean and sedimentation provide other sinks

Macronutrients

Nutrients required in relatively large amounts such as N P and Si

Micronutrients

Nutrients required in smaller amounts such as Fe Mg and other trace metals

Constituent elements

Elements required for life but already abundant especially C H and O

Inorganic compounds

Compounds described in the lecture as lacking both C and H such as N2 and CO2

Organic compounds

Compounds containing C and H such as urea and organic matter

Dissolved compounds

Small compounds that pass through filtration including DOC DIC DON DIN and CDOM

Particulate compounds

Larger compounds and cells collected by filtration including POC and PON

DOC

Dissolved organic carbon

DIC

Dissolved inorganic carbon

DON

Dissolved organic nitrogen

DIN

Dissolved inorganic nitrogen

POC

Particulate organic carbon

PON

Particulate organic nitrogen

Biogeochemical pathways

Can involve phase changes inorganic to organic transformations and reduction-oxidation reactions

Reduction

Gain of electrons and often loss of oxygen

Oxidation

Loss of electrons and often gain of oxygen

Redfield ratio

The molar ratio of elements in average phytoplankton biomass that is also similar to average ocean nutrient ratios

Redfield C:N:P:Fe ratio

106:16:1:0.0075

Redfield N:P ratio

16:1

Importance of the Redfield ratio

Deviations from the expected ratio can indicate which nutrient may be limiting biological production

Nutrient limitation

Insufficient nutrient concentrations restrict primary production and can influence community composition

N:P greater than 16

Suggests phosphorus limitation

N:P less than 16

Suggests nitrogen limitation

Eutrophication

Accumulation of excess nutrients in an ecosystem

Effects of eutrophication

Can cause algal blooms increased decomposition oxygen loss lower pH and fish kills

HAB

Harmful algal bloom

Why can eutrophication cause hypoxia?

Extra nutrients stimulate algal growth and decomposition of the resulting organic matter consumes oxygen

Hypoxia

Conditions with very low dissolved oxygen

Anoxia

Conditions with essentially no dissolved oxygen

Oligotrophic

Nutrient-poor conditions with relatively low biological productivity

Largest carbon reservoir

The ocean which contains about 60 times more carbon than the atmosphere

Major forms of ocean carbon

DIC DOC and POC

Photosynthesis and carbon

Photosynthesis converts CO2 or inorganic carbon into organic carbon

Remineralization

Breakdown of organic matter that returns carbon and nutrients to inorganic forms and consumes oxygen

Two major ocean carbon pumps

Solubility pump and biological pump

Solubility pump

Physical transport of dissolved CO2 into the ocean interior through gas exchange cooling and circulation

Temperature and CO2 solubility

Cold water can hold more dissolved CO2 than warm water

High latitude CO2 flux

Cold high-latitude waters absorb CO2 from the atmosphere and can transport it to depth as the water sinks

Warm upwelling regions and CO2

Warming decreases CO2 solubility so upwelled water can release CO2 to the atmosphere

Biological pump

Biological transfer of carbon from surface waters to deeper waters through photosynthesis sinking organic matter and remineralization

Photosynthesis in the biological pump

Phytoplankton fix inorganic carbon into organic matter at the surface

Sinking POC

Particulate organic carbon sinks from surface waters and transports carbon toward the deep ocean

What happens to most sinking organic carbon?

Most is consumed or remineralized before reaching the seafloor

Carbonate buffering system

A series of reversible reactions among CO2 carbonic acid bicarbonate and carbonate that helps stabilize seawater pH

Carbonate buffering sequence

CO2 plus H2O forms H2CO3 which forms HCO3- plus H+ which forms CO3-2 plus H+

What happens to carbonate chemistry when pH decreases?

The equilibrium shifts toward bicarbonate and away from carbonate

What happens to carbonate chemistry when pH increases?

The equilibrium shifts toward carbonate

What drives most nitrogen cycling reactions?

Microorganisms

Major forms of dissolved inorganic nitrogen

NH4+ NO2- and NO3-

Nitrogen fixation

Reduction of N2 gas into ammonia making atmospheric nitrogen biologically available

Diazotroph

An organism capable of fixing N2

Nitrogenase

The enzyme responsible for nitrogen fixation

Why is nitrogen fixation difficult?

It requires substantial energy and nitrogenase is inhibited by oxygen

How can cyanobacteria protect nitrogenase?

They can fix nitrogen at night or use specialized cells called heterocysts

Assimilation

Incorporation of inorganic nutrients into biological biomass

Preferred nitrogen form when abundant

NH4+ because it can be incorporated more directly and requires less energy than nitrate or nitrite

Nitrification

Aerobic oxidation of reduced nitrogen from NH3 or NH4+ to NO2- and then NO3-

Does nitrification require oxygen?

Yes it is an aerobic process

Denitrification

Anaerobic reduction of nitrate or nitrite that ultimately returns nitrogen to N2 gas

Does denitrification require oxygen?

No it occurs under anaerobic conditions

Ammonification

Decomposition of organic nitrogen into ammonia or ammonium

Anammox

Anaerobic ammonium oxidation in which NH4+ and NO2- are converted into N2 and water

Where can anammox occur?

Anaerobic sediments and deep-sea vents

Simplified nitrogen cycle

N2 becomes usable nitrogen through fixation organic N returns to NH4+ through ammonification NH4+ becomes NO3- through nitrification and NO3- returns toward N2 through denitrification

Human impacts on nitrogen cycling

Fertilizers and runoff add nitrogen to coastal waters while fossil fuel combustion releases NOx

Major source of phosphorus

Earth's crust

How does phosphorus enter the ocean?

Weathering and runoff along with inputs from dust volcanoes and deep-sea vents

Major human phosphorus sources

Fertilizers sewage paper mills and other human discharges

Major phosphorus sink

Deposition and burial in sediments

DIP

Dissolved inorganic phosphorus

DOP

Dissolved organic phosphorus

Phosphorus cycling in organisms

Phytoplankton take up phosphorus and consumers excretion decomposition and cell lysis return it to the environment

Iron in marine ecosystems

An essential trace metal required for processes including enzyme activity photosynthesis and nitrogen fixation

Why is iron difficult for marine organisms to obtain?

Iron is poorly soluble under the oxidizing and relatively high-pH conditions of seawater

Major iron sources

Riverine sediments atmospheric dust and deep-sea sediments

Iron limitation

Insufficient Fe restricts phytoplankton growth even when macronutrients are available

HNLC

High nutrient low chlorophyll regions where macronutrients are abundant but phytoplankton biomass remains low because another factor such as iron is limiting

Example of an HNLC region

The Southern Ocean

Iron fertilization

Addition of iron to iron-limited waters to stimulate phytoplankton production

Effects of experimental iron fertilization

Increased chlorophyll increased primary production more diatoms decreased pCO2 and increased carbon export

Why is iron fertilization controversial?

Its long-term effectiveness and ecological consequences remain uncertain

General vertical nutrient pattern

Nutrients are often low in surface waters because of biological uptake and higher at depth because of remineralization

Role of upwelling in nutrient cycling

Upwelling returns deep nutrient-rich water to the sunlit surface and can stimulate primary production

Relationship between decomposition and oxygen

Decomposition consumes oxygen and can contribute to hypoxic or anoxic conditions

Relationship between decomposition and CO2

Decomposition produces CO2 as organic material is remineralized

Overall importance of nutrient cycling

Nutrient cycling controls nutrient availability primary production community composition carbon storage and many other biological and chemical ocean processes