Simulation of upper-ocean biogeochemistry with a flexible-composition phytoplankton model: C, N and Si cycling in the western Sargasso Sea.

Transcription

1 Simulation of upper-ocean biogeochemistry with a flexible-composition phytoplankton model: C, N and Si cycling in the western Sargasso Sea. Mathieu Mongin 1,2, David M. Nelson 1, Philippe Pondaven 2, Mark A. Brzezinski 3 and Paul Tréguer 2. 1 College of Oceanic and Atmospheric Sciences, Oregon State University 2 Institut Universitaire Européen de la Mer, Plouzané, France 3 Dept. of Ecology, Evolution & Marine Biology, University of California, Santa Barbara

2 Models of nutrient-limited primary production in the ocean 1. Nitrogen-based models (e.g. Fasham et al., 1990, Martin et al., 2001) Simulate the transformations of one potentially limiting element (N). Simulate light limitation by making rates of NO 3 and NH + 4 uptake light-dependent. Usually consider just one phytoplankton group, or at most two (large and small cells). Do not explicitly consider limitation by other nutrients (e.g. Si limitation for diatoms). 2. Multi-element models in which the phytoplankton has a fixed elemental composition (e.g. Walsh, 1975, Pondaven et al., 1998, Chai et al., 2002) Explicitly allow for limitation by light, and by one or more nutrients. Usually consider more than one phytoplankton group (e.g. diatoms and nonsiliceous forms). Require phytoplankton to grow with fixed (usually Redfield) C:N or C:N:P ratios. Require diatoms (if considered explicitly) to grow with a fixed Si:N or Si:C ratio. Assume that productivity is controlled by the most limiting resource (i.e. the one that causes phytoplankton biomass to be produced at the lowest rate).

3 Biogeochemically significant processes that no single-element or fixed-composition phytoplankton model can consider 1. Growth of phytoplankton with altered elemental composition under either nutrient limitation or light limitation For all phytoplankton: higher C/N ratios when N is limiting lower C/N ratios when light is limiting For diatoms: lower Si/C and Si/N ratios when Si is limiting higher Si/N ratios when N is limiting higher Si/C ratios when light is limiting higher Si/C and Si/N ratios when Fe is limiting 2. Differential light dependence of C, NO 3, NH 4 + and Si(OH) 4 uptake, resulting in different depth distributions, day/night timing and depth/time-integrated rates of uptake absolute light requirement for C uptake strong light dependence for NO 3 uptake little or no light dependence for NH 4 + or Si(OH) 4 uptake

4 Difference between kinetics of nutrient uptake and kinetics of nutrient-limited growth V max µ max 1 1 Specific uptake rate (V) V = V max S/(K S + S) Specific growth rate (µ) µ = µ max S / (K D + S) K S K D Nutrient concentration (S) 0 0 Nutrient 10 concentration (S) 40 For many species, and several different nutrients, K d << K s

5 Severity of Si uptake rate limitation and estimated severity of Si limitation of diatom growth rates, as a function of latitude and time (a): V a :V max calculated from [Si(OH) 4 ] a and K s for Si uptake. (b): estimated µ a /µ max calculated in the same manner, but assuming that K µ = 0.15 K s. Symbol as in Fig.3. the location of the southern boundary of the APF is also shown in both panel. This analysis include only those experiments in which V was measurably limited by [Si(OH) 4 ] a (i.e. where E>1.2); thus all V a /V max values in (a) are constrained to be <0.83.

6 "Cell quota" model for nutrient-limited growth 1.4 µ inf µ max µ =µ inf (Q Q O )/Q Q O Q max Amount of the limiting element per cell (Q)

7 Fraction of externally controlled uptake rate 1 0 No N uptake Low cellular Si limits growth and N uptake Si uptake and N uptake are independent N uptake Si uptake Low cellular N limits growth and Si uptake No Si uptake Si/N ratio of living diatoms (log scale)

8 Fraction of externally controlled uptake rate 1 No N uptake Low cellular C limits growth and N uptake C uptake and N uptake are independent N uptake C uptake Low cellular N limits growth and C uptake No C uptake C/N ratio of living diatoms or picoplankton

9 Mean seasonal depth profiles of nitrate and silicic acid A & B from BATS data: (A) [NO - 3 ]; (B) [Si(OH) 4 ] C & D from model output (C) [NO - 3 ]; (D) [Si(OH) 4 ]

10 Time course of simulated surface [NO - 3 ] and [NH + 4 ] (mmol N m -3 )

11 Contour plot of measured and simulated Chlorophyll-a, and simulated chlorophyll-a in picophytoplankton and Diatoms, (mg m -3 )

12 Measured and model-drived primary productivity at the BATS site, Primary productivity (mg C m -2 d -1 ) BATS Data Model results Time (year

13 Model derived C/N ratio of picophytoplankton and diatoms At the BATS site,

14 Mean C/N ratio in 25 m depth intervals at the BATS site (simulated and measured) Mean POC/PON (model outpu Depth range (m) Redfield ratio (6.6) Mean POC/PON (BATS dat Depth range (m) Redfield ratio ( Mean POC/PON (BATS dat Depth range (m) Redfield ratio (