Equatorial Pacific HNLC region

Transcription

1 Equatorial Pacific HNLC region Another region with high nitrogen left over after the growing season Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis Landry et al. 1997

2 Growth & Grazing at the equator Landry et al. 1997

3 Equatorial Pacific Models during El Nino period, upwelling reduced: less Fe supplied from the equatorial undercurrent normal upwelling: higher proportion of bigger cells and more mesozooplankton grazing low Fe leads to dominance of small phytoplankton, & even these are Festressed despite relatively high growth rates Grazers remove most of daily production of phytoplankton Recycling of ammonium by grazers leads to suppression of nitrate uptake

4 Nature 383: , 1996

5 Results SF6 Fe Chl NO3 CO2

6 Community Response note: mesozooplankton abundance also increased in patch

7 Conclusions EqPac data are consistent with consensus perception of phytoplankton control in HNLC ecosystems. Fe limits stock size & affects community structure Ambient community displays signs of physiological stress, yet growth rates are high Grazing balances phytoplankton growth -- controls small forms, despite variations in Fe input -- does not control diatoms, which increase with higher rates of Fe upwelling High phytoplankton µ depends on Fe recycling -- new Fe upwelling is consistent with new production -- remineralization occurs with similar efficiencies for Fe & N Among open ocean regions of the Pacific, limiting resources may differ (Fe vs. N,P), but the underlying mechanisms of phytoplankton control are similar.

8 What is still missing? Fate of Carbon SOFeX: Southern Ocean Iron Fertilization Experiment, 15 kmx15 km patch size (Coale et al. & Buessler et al., Science, 2004, Volume 304) Using 234 Th (24.1 day half-life) as a tracer of sinking particles -- used to calculate net sinking flux of particulate carbon Day 20 1) Despite measuring flux for 28 days (17 days after last addition) -- probably still not the end of the bloom (longest experiment yet, though) 2) Key result: 7 mm C/m 2 /d difference translates into 900 tons if carbon (relative to 6.5 x 10 9 tons/y released globally) -- small difference

9 Ocean Policy Implications Large-scale Ocean Fertilizations planned by private companies, e.g., 6 projects to add 100 T of Fe to 10,000 km 2 patches of sea All countries internationally have a duty to protect and preserve the marine environment (e.g., Law of the Sea and London conventions) -- includes safe and environmentally responsible shipping and regulates ocean dumping -- ocean fertilization may be prohibited outright, currently under discussion. Dangerous precedent for general ocean research: might lead to regulation of all basic field research on an international basis

10 SubTropical Ecosystems DSR II 1996 Vol 43 DSR II 2001 Vol 48 DSR II 2006 Brix et al. Vol 53:

11 HOT -- Primary Production summer usually sees the highest phytoplankton biomass/production D. Karl 1999 Ecosystems 2:

12 BATS -- Primary Production Spring bloom (not summer like HOTS) Steinberg et al DSR II, 48,

13 Data Comparison

14 Trophic Cascades Microbial loop organisms the most important (recycling system) Size fractionation experiments suggest several trophic levels smaller than 20 µm

15 Mesozooplankton Biomass, HOT site Timing of maximum is the most puzzling: In the summer, when the water column is the most stratified (as opposed to the spring, after winter mixing) Coincides with blooms of N 2 fixers, such as Trichodesmium

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17 HOT/BATS MesoZP comparison Roman et al. 2002, DSR II, 49:

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19 Why the difference? Don t know for sure, but... Salps and sarcodines at BATS -- not quantified well with net tows (grazers and mixotrophs) Mesoscale eddies at BATS leading to episodic nutrient enrichments -- uncoupling of 1 producers and consumers L. Madin, WHOI

20 On the relationships between primary, net community, and export production in subtropical gyres, 2006, Deep-Sea Res. II, 53: , Holger Brix, Nicolas Gruber, David M. Karl and Nicholas R. Bates Export POC/Net Primary Production If ratio high, then export pathway ecosystem (larger phytoplankton) If ratio low, then regeneration ecosystem (microbial loop organisms dominate) Switch between these states by addition of increased nutrients 10 year data set at HOTS and BATS: BATS: Export pathway in Spring, Regeneration Pathway in Summer, Fall HOTS: Regeneration pathway all year round

21 Aside: Modern Primary Production Measurements vs. Historical Subtropical Gyres: mg C/m 2 /d (~ g C/m 2 /y) historical: <100 g C/m 2 /y Note that fisheries oceanographers still use the lower numbers, along with lower estimates of fish trophic levels -- the combined effect of these opposing trends may luckily end up with fisheries yields that aren t too far off...

22 Upwelling Zones Model 5 (Table 7.1 Kaiser et. al): Intermittent production at coastal divergences Classical coastal upwelling regions of eastern boundary currents and other coasts. Narrow shelf, river effluent influences minor. Primary producers: diatoms Primary herbivores: copepods, anchovies and/or sardines tornado.sfsu.edu

23 Small (<5 µm) Phytoplankton in Upwelling Zones Despite dominance of diatoms in upwelling zones -- smaller phytoplankton are present, too Sherr et al. 2005

24 Distribution of Small vs. Large Phytoplankton in Oregon Upwelling System Near Shore, relatively few small (<5 µm) phytoplankton -- mostly diatoms onshore offshore Offshore, phytoplankton biomass dominated by small cells Sherr et al. 2005

25 Food Webs in Chilean Upwelling System Biomass dominated by diatoms Copepods and appendicularians have high clearance rates on microzooplankton, too -- omnivory Vargas & Gonzalez 2004

26 More than one pathway for export flux... Clearance rates show optimum prey size Appendicularians -- consumers and responsible for passive particle scavenging Marine snow image by A. Alldredge Vargas & Gonzalez 2004