Arrigo (2005) Marine Microorganisms and Global Nutrient Cycles Nature 437: Issues: Redfield stoichiometry Co-limitation N 2 -Fixation Anammox

N&P; Si&P; P&Fe Light or Fe / vary with cell size PO4 or Zn ? DOP use Add P+Fe = N2 fixers NO3 alone – non-N2 Fixers

PO 4 versus Nitrate (GEOSECS data) Insert shows the effect of nitrification, photosynthesis, N 2 fixation and denitrification. The solid line shows the linear equation P = 1/16 N (equivalent to N* = 0) Values to the right have positive N*, to the left have negative N* What is N*? N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9

N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9

a, Observations used by Redfield in his original study 1 (filled circles) and selected data from the global World Ocean Circulation Experiment survey (WOCE; small symbols) show a clear, approximately 16:1 relationship between nitrate and phosphate, with minimal deviation. b,c, Both denitrification (b) and N fixation (c) counteract any imbalances, with denitrification removing a nitrate excess, and N fixation adding more nitrate to the ocean. Together, these processes are expected to return nitrate concentrations back towards a mean content close to that of oceanic phytoplankton. 1 From Gruber and Deutsch (2014) Nature Geoscience

Feedbacks governing the marine N cycle

From Pahlow and Riebesell (2000) Science 287: 831 Does the Redfield ratio vary on geological timescales?

Gilbert (2013) Nature Climate Change 3, 954 and Toseland et al (2013) NCC, 3, 979 To overcome the low water temperatures (average of 2 °C) and concomitant reduction in efficiency, these cells just make more protein factories to maintain their productivity. As this requires more P, the N:P ratio in their cells is reduced. The model validated the hypothesis that under low temperatures the cells invested more in their cellular machinery to overcome the inefficiency of their factories; whereas under higher temperatures the cells invested in photosynthesis and hence biomass. Effect of temperature on physiology

Kim et al (2014) Increasing anthropogenic nitrogen in the North Pacific Ocean. Science 346, 1102=1106 Rate of deposition of AND has increased over past 100 years North Pacific receives a large input. Can it be seen in the data? An increase could impact the nitrogen cycle and biological food-web and productivity. This analysis: N* versus CFC-Age (N* = N – R N/P x P using R N/P = 16:1)

Fig. 1 Three trends of N* variation.This figure shows the relation between N* and the pCFC- 12–derived ventilation date (which is the year that the CFC-12 concentration in the water sample would have last been in equilibrium with the atmosphere) for seawater samples obtained in the East Sea (Sea of Japan) as part of the Circulation Research of the East Asian Marginal Seas II program in Published by AAAS Trend 1: Deep Ocean Southern Ocean Indian Ocean Trend 2: Low O2 regions Trend 3: N sources > N sinks Due to AND and N2 Fixation East Sea data

Fig. 2 Estimation of the rate of increase in excess N (N*).(A) Map showing the NPO. The color scale indicates AND in 1993 (15) overlaid with the mean wind vectors over the period 1990– 1999 (25). Published by AAAS Support for AND Date of increase Spatial pattern Ages best for 1960 to 2000

Fig. 3 Comparison of the N*-based rates with AND rates.Comparison of the N* values integrated over the water column [, where z is the depth at which the N* (N*OBS > N*DEEP) signal was observed, blue squares] with the modeled (circles) (15) and observed (stars) (16, 17) AND rates (see supplementary text S5 for the AND rates observed in the mid-latitude NPO). Published by AAAS

Fig. 4 Temporal variations of N*.(A) Distribution of N* in the western and central NPO (20°N to 40°N and 130°E to 180°W) determined from data in the PACIFICA database collected between 300 and 400 m in the period 1992 ‒ Published by AAAS HOT – Annual Mean N* Western NPO – 10° averaged