How does the Eastern How does the Eastern Mediterranean photosynthetic Mediterranean photosynthetic

community work community work

CYCLOPS Addition experiment CYCLOPS Addition experiment and resulting modeland resulting model

Why is the Eastern Med special?

• It is Ultra-Oligotrophic

• The annual productivity in the Sargasso Sea is 120-130 gC m-2 y-1

• The annual productivity in the E. Med is 60 gC m-2 y-1

The normal situation in the ocean: The Redfield Ratio

106CO2 + 16NO3-+1HPO4

2-

C106H263O110N16P1+138O2

Photosynthesis Respiration

N:P = 16:1

Progressively higher N:P ratio in deep water towards the E. Mediterranean

16:1

23:1

28:1

How do we know that the E. Med is P-limited in winter?

• In winter, when nutrients are mixed into surface waters, there is a phytoplankton bloom

• Bloom stops when system runs out of phosphate, but there is still nitrate present

Cyprus Eddy centre: Feb 1989

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Aim of the CYCLOPS programme

• To understand how the microbial food chain operates in the eastern Mediterranean

• To determine whether the Eastern Mediterranean is P-limited and what exactly that means

• To find out why the Eastern Mediterranean is P-limited

New data on Eastern Mediterranean

• The system is entirely P starved

• We knew that nitrate/phosphate in LDW (28:1)

• We found that PON:POP (30:1)

• And DON:DOP (~50:1)

• But both nitrate and phosphate are depleted in the photic zone

Nutrient Limitation elsewhere in the oceans

• Most of the ocean is N & P co-limited

• Some areas have high nutrients & low chlorophyll (HNLC)

• Iron limitation?

How these areas were shown to be Fe limited

• Dissolved iron was measured and found to be unusually low

• Bottle experiments showed an increase in chlorophyll when Fe was added.

Still not enough

• Designed and carried out an in-situ addition experiment

SOIREE iron addition experiment

Add Fe and an inert tracer (SF6) to a patch of water

Follow the chemical and biological effects

Fe addition in the Southern Ocean(SOIREE, February 1999)

CYCLOPS Addition CYCLOPS Addition experimentexperiment

May 2002

CYCLOPS addition CYCLOPS addition experimentexperiment

May 2002

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CYCLOPS P addition experiment: fertilized patch

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Biological uptake of P

Increase in bacterial growth after P addition

CYCLOPS PO4 Addition Experiment

May 2002, E Mediterranean

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F Mantoura Plymouth Marine Laboratory

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Abundance of copepod eggs in the patch after P addition

Conclusion

• When the ‘limiting’ nutrient is added to the E. Med the first rapid response is – an increase in bacterial activity

– a reduction in the phytoplankton

– an increase in micrograzers and in macrograzer egg production

Microcosm experiment on-board

4 treatments x 3 replicates

• Patch water only

• Patch water with ammonia

• Out of patch water only

• Out of patch water with ammonia

CYCLOPS Shipboard Mesocosm

The effect of NH4 addition on phytoplankcon

Chlorophyll in IN Patch (PO4 enriched) and OUT Patch (control) waters

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Mesocosm Treatment

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Nature of the nutrient limitation of the Phytoplankton

• How much N is available before the entire microbial system becomes N & P co-limited?

• Alkaline Phosphatase Bioassay

Slope of this curve is 17:1

Intercept is 230nM-N bioavailable-N

Heterotrophic Bacteria BL-DOC

Heterotrophic Flagellates, H

Pico and Nano-phytoplankton

Ciliates C

BCD

Diatoms D

Meso-zooplankton, M

phosphate & ammonium/nitrate

Silicate

Offshore microbes Coastal microbes

Schematic of the microbial ecosystem in the Eastern Mediterranean

Why is Productivity in the Why is Productivity in the Eastern Mediterranean Eastern Mediterranean Phosphorus-Limited?Phosphorus-Limited?

Is the high N:P ratio due to excess supply of N relative to P?

New Hypothesis:

• The N/P ratio of nutrients supplied to the E. Mediterranean is >> 16:1.

Calculated nutrient inputs to the Eastern Mediterranean basin

All values given in 109 moles/y

Source N input P input Ratio

Atmospheric input 111 0.95 117

Po and adjacent area 20 0.90 22

Nile input 15 0.25 60

Other river input 28 1.20 23

Black Sea 8 0 -

Total input 180 3.3 53

Calculate Steady-State Flux of nutrients through the Straits of Sicily

Calculated nutrient outputs from the Eastern Mediterranean basin

All values given in 109 moles/y

Source N input P input Ratio

Straits of Sicily 142 5.0 28.4

Sediment deposition 27 0.6 45.0

Denitrification 7

Total output 176 5.6 31.0

Total input 180 3.3 53.0

But this is not the complete story!

The GLOBAL Supply of N:P is >> 16:1

• The N:P ratio of nutrients supplied to the world’s oceans is > 50:1 (Smith, 1984)

• Yet most of the world’s oceans have a N:P ratio of ~16:1

Why?

Biological feedback mechanisms control N:P ratio in the oceans

• If there is excess P

– N fixing phytoplankton flourish and produce fixed N in the water column

• If there is excess N – Nitrate reduction in coastal sediments

or oxygen minimum zones which remove N (Seitzinger and Giblin 1996)

Why don’t global feedback mechanisms work in the E. Med?

• It is ultra-oligotrophic due to its unique anti-estuarine circulation

• Little excess organic matter and no pelagic oxygen minimum - almost no areas in which nitrate reduction occurs in the sediment.

Test of this hypothesis

• Are there any areas of the E.Med where the deep water N:P ratio is 16:1?

• Yes - the Northern Adriatic where nitrate reduction DOES occur

Why is the Eastern Mediterranean P limited?

• There is more N supplied to the basin than P

• The normal biological feedback mechanism does not work because the system is ultra-oligotrophic

• There is no evidence of N fixation

Summary

• P-limitation is not as simple as Fe-limitation; it is both time and organism dependant.

• Bacteria are P-limited throughout the year because they can access DON

• Phytoplankton recycle excess N to a form that is not bioavailable and become N & P co-limited in summer

• Grazers are P-starved and respond very rapidly to an external source of P

Summary: Why is the system is P-starved?

• N:P input >>16:1

• Normal global buffer mechanism does not work in the E Med

– the system is ultra-oligotrophic

– no excess organic matter to fuel nitrate reduction

Any Questions?