Ecosystem changes after the SEEDS iron fertilization in the western North Pacific simulated by a one-dimensional ecosystem model Naoki Yoshie1, Masahiko Fujii2, and Yasuhiro Yamanaka1 1Graduate School of Environmental Earth Science, Hokkaido Univ., Japan 2School of Marine Sciences, Univ. of Maine, USA naoki@ees.hokudai.ac.jp I am Naoki Yoshie came from Hokkaido University, Japan.

Contents of today’s talk Introduction of iron in the ocean [3 sheets] Iron fertilization experiment in the western North Pacific “SEEDS-I” [2 sheets] Ecosystem model “NEMURO” [3 sheets] Simulation of SEEDS-I by NEMURO [4 sheets] Today, I would like to talk these contents. First, Introduction of iron cycle and the ecosystem model of PICES, “NEMURO” Second, Recent models including iron cycles. Two types models a and b. simple eco.+ complex iron and complex eco. + simple iron. Third, Simulation of an iron fertilization experiment in the western North Pacific “SEEDSI” by NEMURO. Forth, How to include the iron processes to NEMURO?

Importance of iron for phytoplankton Iron is an micro-nutrient and used for many biological and chemical processes in the cell of phytoplankton. Process Catalyst Photosynthesis & respiration: heme [cytochromec, bf6], Fe-S proteins [ferrodoxin], aconitase… Detoxification of reactive oxygen: superoxide dismutase Nitrogen metabolism: nitrate & nitrite reductase Availability of Iron controls, 1. productivity, 2. species composition, 3. trophic structure of planktonic communities.

Simplified iron cycle in the ocean Observed iron conc. in N Pacific (Morel and Price 2003) Aerosol Iron Upwelled Iron depletion in the surface 1000 2000 3000 4000 Uptaked by Phytoplankton Dissolved Iron Depth (m) Particulate Iron Left figure shows simplified iron cycle in the ocean. There are two main sources of iron in the open ocean, Dust and Upwelling. Right figure shows observed iron concentration in the North Pacific from Morel and Price 2003. Martin said that in the North East Pacific, relative contribution of upwelled Iron is smaller than that of aerosol iron. De Baar said that in the Southern Ocean, relative contribution of upwelled iron is larger than that of aerosol iron. Aumont et al 2003 upwelling 70% dust 30% Fung et al 2000 upwelling 40% dust 60% Parekh et al 2005 upwelling 40% dust 60% Climate change affects the fluxes of upwelling and aerosol. Major sources of iron in the open ocean are dust and upwelling. NE Pacific : upwelled iron < aerosol iron (from Martin) S. Ocean : upwelled iron > aerosol iron (from de Baar) Climate change affects the fluxes of dust and upwelling.

What’s role of aeolian dust in marine ecosystem? Satellite images of dust storm (from Uno) Mineral Dust Transport and Deposition (from Uematsu) 55N MODIS: Mar. 20, 2001 Dust stream over Korea SeaWiFS: Apr. 11, 2001 5N Left figure shows simplified iron cycle in the ocean. There are two main sources of iron in the open ocean, Dust and Upwelling. Right figure shows observed iron concentration in the North Pacific from Morel and Price 2003. Martin said that in the North East Pacific, relative contribution of upwelled Iron is smaller than that of aerosol iron. De Baar said that in the Southern Ocean, relative contribution of upwelled iron is larger than that of aerosol iron. Aumont et al 2003 upwelling 70% dust 30% Fung et al 2000 upwelling 40% dust 60% Parekh et al 2005 upwelling 40% dust 60% Climate change affects the fluxes of upwelling and aerosol. 165E 120E 85E Aeolian dust is an important iron supplier to the marine ecosystem in the western North Pacific.

What is “SEEDS”? SEEDS (Subarctic pacific iron Experiment for Ecosystem Dynamics Study) is an iron fertilization experiment in the western North Pacific. The general result of SEEDS-I was reported by Tsuda et al., 2003 in Science, and details were summarized in the SEEDS special issue (2005) in Prog. Oceanogr. Mesoscale iron fertilization experiments in the world What is SEEDS? SEEDS is an iron fertilization experiment. This figure shows the iron fertilization experiments in the world. SEEDS was done this region. Increased Chl.a by iron fertilization in SEEDS is highest compared with the other experiments. Courtesy of Boyd

Observed feature of SEEDS-I(From Tsuda et al., 2003) Time lag The time lag between iron-fertilization and start of bloom was observed. The reason of this time lag was initially considered as the delay of diatom’s physiological response. During observation period, a lot of particulate organic carbon (POC) was suspended in the surface water and was not exported to deeper layers. This figure shows a time change of surface concentration of chl.a. from day0 to day 13. The time lag between iron-fertilization and start of bloom was observed in SEEDS. The reason of this time lag was initially considered as the delay of physiological response of diatom. We analyzed this time lag by 1-D ecosystem model. The time lag and the fate of POC were demonstrated by ecosystem model “NEMURO for SEEDS”.

An ecosystem model “NEMURO” A lower trophic level ecosystem model “NEMURO (North pacific Ecosystem Model Used for Regional Oceanography)” was developed by MODEL task team of PICES (North Pacific Marine Science Organization) focusing on linkage between lower and higher trophic levels. Around 30 papers were published and submitted. 3Nut 3Det 3Zoo 2Phyt PICES has an ecosystem model the name of “NEMURO”. A lower tropic level ecosystem model “NEMURO (North pacific ecosystem model used for regional oceanography)” was developed by CCCC/MODEL task team focusing on linkage between lower and higher tropic. This figure shows the schematic view of NEMURO. NEMURO has 3 Nutrients 2 Phytoplanktons 3 Zooplanktons and 3 Detrituses and coupling with carbon-calcium cycle. There are already more than 20 papers using NEMURO, focusing on observatory stations, global 3-D model, under the global warming, iron fertilization experiments. NEMURO do not explicitly include iron cycles, because NEMURO originally focuses on ecosystem dynamics. NEMURO (3N-2P-3Z-3D) coupled with carbon-calcium cycle (Yamanaka et al., 2004) NEMURO do not explicitly include iron cycles, because NEMURO originally focuses on ecosystem dynamics.

Simulation of SEEDS-I by NEMURO (Yoshie et al., 2005) In addition to the iron cycle built into ecosystem model, we had to change in ecosystem dynamics for simulating SEEDS-I. That is, we divided diatom into two groups: sensitive and insensitive to iron fertilization, in NEMURO.

We extended NEMURO for SEEDS 2 diatoms centric pennate Our model including 3 Nutrients and 3 phytoplankton and 3 zooplankton. Our model explicitly represents 2 groups of diatom, one is centric, another is pennate. We assumed centric diatom has low potential in low concentration of iron, High potential in high concentration of iron. On the other hand, pennate diatom has medium potential regardless of concentration of iron. The iron fertilization experiment was conducted by changing P-I parameters of diatoms (e.g. Chai et al., 2002; Fujii et al., 2005) We assumed centric : low potential in low iron conc. high potential in high iron conc. pennate: med. potential regardless of iron conc.

Results of model simulations Time lag This figure shows the time changes of surface Chl.a simulated by models. Green line was simulated by typical previous 1 diatom model. Red line was simulated by our 2 diatom model. Our model successfully reproduced observed time change including the time lag. The other elements were also successfully reproduced. NEMURO for SEEDS successfully reproduced observed time change including the time lag.

Why SEEDS has the time lag? Upper figure shows the time changes of potential of diatoms. Lower figure shows the time changes of biomass of diatoms plotted in log scale. Red line shows centric diatom, blue line shows pennate, purple line shows total potential averaged by two diatoms. Averaged potential of 2 diatoms almost no change from day0 to day4, because contribution of centric diatom to total biomass is negligible small. It takes several days to shift from pennate to centric. Total potential averaged by two diatoms hardly changes from day 0 to 4, because contribution of centric to total biomass is negligibly small. Increasing the biomass of centric diatom takes several days.

Simulations after the observation period 100% 25% Phytoplankton biomass has its peaks during the observation period. Cumulative POC export flux increases gently, and its value at day13 (the last day of observation) reaches only about 25 % of its total change over 48 days. NEMURO for SEEDS demonstrates that the fixed organic carbon sinks to deeper layers after the observation period.

Summary Why SEEDS had the time lag between iron-fertilization and starting bloom? Because the time lag was caused not by the delay of physiological response of diatom (at least SEEDS-I), but by the delay of increment of high-potential diatom. Addition to introducing iron into simple ecosystem models, intermediate complexity ecosystem model representing ecosystem dynamics is necessary to reproduce the iron fertilization experiment. Why SEEDS has the time lag between iron-fertilization and start of bloom? Model simulation shows the time lag was caused by not the delay of diatom’s physiological response, but the increment of diatom with high potential in iron-rich environment. Thank you