Ecology of Marine Phytoplankton

Transcription

1 Ecology of Marine Phytoplankton LECTURE NOTES WILL BE POSTED AT hvp://ocean.mit.edu/~mick/ens- S Mick Follows: Tuesday 5 Nov 2013 Introduc@on to marine phytoplankton Type II func@onal response: Encounter- handling processes Wednesday 6 Nov 2013 Seasonal blooms Cell size and equilibrium resource compe@@on Thursday 7 Nov 2013 Equilibrium: Top down control and co- existence Resource supply ra@os and co- existence Nitrogen fixers

2 Surface ocean chlorophyll from space NASA MODIS Global marine primary ~ 50 Pg C year- 1 Mid- and high- la@tudes: Strongly seasonal insola@on and ver@cal mixing Tropics and subtropics: Low seasonality, permanent nutrient deple@on

3 Why do they live there? Carbon is Supply of N, P, Fe, etc is for PaVerns of wind- driven upwelling and downwelling control the delivery of deep, nutrient rich waters from below

4 Why do they live there? Dashed line indicates light at 1% of surface incident flux of light Phytoplankton need light and dissolved nutrients Chlorophyll maximum follows boundary between high light and high nutrient regions Nutrients are stripped from the surface layer by phytoplankton sink as organic remineralized at depth by microbes Figure: Anna Hickman in Williams and Follows (2011)

5 Diverse types of phytoplankton Many groups of phytoplankton with different physiological Each group consists of thousands of species

6 Biogeography on Meridional Transect - diatoms - coccolithophores pico- cyanobacteria - Prochlorococcus - Synechococcus Different phytoplankton groups are adapted for different ocean environments Aiken et al (2000)

7 Laroche and Breitbarth (2005) For example, nitrogen fixers (diazotrophs) like Trichodesmium are found mostly in tropical and subtropical oceans where fixed nitrogen is scarce

8 Subpolar blooms The mid- and high oceans are characterized by vigorous spring blooms of phytoplankton Dale et al, Sarsia (1999)

9 Blooms growth and loss decoupled Riley (1946) demonstrated that blooms are a consequence of the decoupling of popula@on growth and loss Low light and deep mixing inhibit growth in the winter, but bring nutrients to the surface High light, weak mixing and high nutrients promote growth in spring Nutrients are quickly depleted and then grazing by zooplankton dominates and reduces popula@on dp dt = µ o R R + K R P mp gpz theore@cal curve observed J F M A M J J A S O N D

10 Blooms are dominated by Diatoms G. Riley, Phytoplankton of the central Sargasso Sea, L&O, 2, (1957) Diatoms other phytoplankton * Month * before the discovery of picocyanobacteria

11 Resource replete, low mortality (bloom db dt = µ o R R + K R B mb 1 B Bloom db dt ~ µ o Resources replete: R>>K R Low mortality: μ o >> m Per capita growth determined by max growth Periodic resource injec@ons select for fastest μ o

12 Maximum growth rate: to cell volume and taxonomic group. Max growth rate of diatoms faster than other types at same cell size Diatom grow fastest, hence typically dominate blooms Cell Volume F. Monteiro, S. Dutkiewicz, B. Ward

13 Subtropical oceans, low seasonality, low nutrients all year

14 for nitrogen in the REVIEW ARTICLE tropical and subtropical a b 80 Nutrient amendments: Green dot = N limited 40 Shading surface nitrate Experiments at sea demonstrate that phytoplankton popula@ons are generally limited by nitrogen in Atlan@c, and by iron in S. Ocean, Tropical Pacific and Subarc@c Pacific E 150 E 110 W 10 W t al, Nature Geo (indicate 2013) Figure 3 PatternsMoore of nutrientelimitation. Backgrounds annual average surf assist comparison, nitrate is scaled by the mean N:P ratio of organic matter (that is, and secondary (outer circles) limiting nutrients as inferred from chlorophyll and/or p

15 Monod resource limited growth In the low seasonality, tropical and subtropical oceans, equilibrium coupling between growth and loss) is ouen a reasonable and useful approxima@on Equilibrium db dt = µ o R R + K R B mb R * = mk R µ o m In theory, the organism which has the lowest subsistence concenra@on for the limi@ng nutrient, R*, outcompetes all others (e.g. Tilman, Ecology, 1977) R R+K R K R R

16 Resource db dt = µ o R R + K R B mb Equilibrium R * = mk R µ o m R* = subsistence concentra@on Organism with lowest R* excludes all others at equilibrium Ambient concentra@on of limi@ng resource set to (lowest ) R* at equilibrium Tilman, Ecology, (1977),

17 Cell size and K R for nitrate Litchman et al (2007) Empirical evidence K N 0.17 V cell 0.27 Theory and empirical data suggest that the smallest cells have the lowest R* Hence Prochlorococcus, the smallest photo- autotroph should be the best compe@tor in the stable, nutrient depleted subtropical gyres

18 Prochlorococcus: Smallest cells with lowest R* for nitrogen abundant in very low nutrient regime Nitrate Prochlorococcus Synechococcus AMT 15 Phytoplankton: log(cells ml - 1 ) Nitrate (micromoles kg - 1 ) Johnson et al, Science (2006)

19 Diverse diazotrophic Image: WHOI phytoplankton Trichodesmium Croccosphaera watsonii 1 μm Image: AnneVe Hynes 1 mm Use N 2 gas as source of nitrogen Benefit: Not limited by fixed nitrogen availability Enzyme nitrogenase breaks triple bond Cost: energy iron demand O 2 control

20 Prochlorococcus abundant in very low nutrient subtropical gyres Nitrate Prochlorococcus Synechococcus AMT 15 Phytoplankton: log(cells ml - 1 ) Nitrate (micromoles kg - 1 ) Johnson et al, Science (2006)

21 However, grazing pressure on the smallest cells increases their R* and allows co- existence of larger size classes dp i dt = µ oi R R + K Ri P i m i P i g i P i Z i R i * = Equilibrium (m i + g i Z i *)K Ri µ oi (m i + g i Z i *) Presence of grazing increases R* Size- based differences in grazing can compensate for increase in K R with cell volume

22 Larger cells become abundant where total (nutrient supply) is highest picophytoplankton (<2 μm) nanophytoplankton (2-20 μm) microphytoplankton (>20 μm) Size on AMT: Ward et al. J. Plankton Res. (2013) total chl a (mg chl m- 3) total chl a (mg chl m- 3)

23 resources/resource supply enables co- existence nutrients also allow the co- existence of phytoplankton types For example, a popula@on of iron/phosphorus limited nitrogen fixers can co- exist with a popula@on of nitrogen limited Prochlorococcus Formalized in Resource Ra@o Theory Tilman (1980, 1982, ) See graphical approach in David Claessen s lectures

24 Nitrogen/iron system S Fe AT High iron supply Non- diazotrophs nitrogen limited Control ambient N Diazotrophs iron limited Control ambient Fe Co- existence S Fe = w Fe S N = w NO 3

25 Trichodesmium consistent with regions where Prochlorococcus are limited by nitrogen, and Trichodesmium by Fe or P. Where both are limited by iron (e.g. Tropical Pacific) diazotrophs are excluded due to slow growth rate LaRoche and Breitbarth (2005)