Recap on biological oceanography

CO2 changes in the last 50 yr Oceans Biosphere Rock Weathering 2

Carbon Cycle

The Carbonate System sources of inorganic carbon from dissolved CO2 gas from dissolution of Calcium Carbonate Total dissolved inorganic carbon

Carbon Dioxide and Carbonate system Carbonic Acid Bicarbonate Ion Carbonate

O2 CO2 pH (m) acid basic

Dissolved Gases in the Ocean Oxygen profile compensation depth Respiration: Animal, plants and microbial decomposition low oxygen environment

Distribution of Carbon species in water + -

from GLODAP climatology

Higher CO2 levels will generally be detrimental to calcifying organisms and that food web structures and biodiversity will likely change, Coccolithophores Forams Corals calcite calcite aragonite but it is not clear how this might impact overall productivity and top level predators (e.g. fish).

Biogeochemical Controls and Feedbacks on the Ocean Primary Production 3) Subpolar Gyres 2) Upwelling Regions 1) Subtropical Gyres Photosyntesis is the main contributor to the development of oxygen in the atmosphere. @.2 billion years ago organic C deposits. 4) HNLC regions What can explain the chlorophyll distribution?

What are the controls on Export Production? Ocean nutrient inventory Nitrogen appears to be the control during modern time. Where nitrogen can be brought at the surface there is higher productivity: Upwelling regions and subpolar gyres Exception: HNLC regions, particularly in the Southern ocean The Iron Problem

Map of annual average nitrate concentrations in Southern Ocean HNLC Map of annual average nitrate concentrations in the surface waters of the oceans. Data from Levitus, World Ocean Atlas, 1994.

Advanced Very High Resolution Radiometer (AVHRR) images of particle transport in the atmosphere between June and August. 

Biophysical interactions Wind-driven gyre circulation for surface distributions; MOC for concentrations at depth

North-south sections of (a) temperature, (b) salinity, and (c) oxygen along the 30oW transect in the Atlantic ocean. Note the salinity tongues indicating the interleaving of water masses from sources in the Antarctic and the North Atlantic.

Pacific

Summaring The large scale wind-driven circulation explains the (large scale) distribution of chlorophyll. We need to consider the biology to explain the distribution of nitrate (remember the high latitudes and the iron problem!) The thermohaline circulation explains the vertical distributions of oxygen and NO3 and the differences between the basins at depth

The Nitrogen Cycle The Demand Side. The Supply Side Denitrification. Alternative pathways to N2. The Supply Side Nitrogen Fixation: the usual suspects. “New” diazotrophs and how they work.

Who Cares? Broad reaches of the ocean are N-limited. Recycling of N within the water column supports biological production, but… Injection of new N into the upper water column is required to support export production. The N and C cycles are tightly coupled through biological production of organic matter (C:N ≈ 7). The N cycle therefore plays a key role in regulating the C cycle!

Oceanic N Cycle Schematic Fixation N2 Nitrification Mineralization NH4 NO3 Uptake Phytoplankton Grazing Mix Layer depth Chlorophyll Zooplankton Mortality Large detritus Water column Susp. particles Nitrification N2 NH4 NO3 Denitrification Aerobic mineralization Organic matter Sediment

Major Biological Transformations of Nitrogen (Inspired by Codispoti 2001and Liu 1979)

The major oceanic inputs and outputs of combined N are largely separated in space. Denitrification is restricted to sediments and anoxic water masses. N2-fixation is common in oligotrophic surface waters (e.g., the central gyres of the major ocean basins).

N* = 0.87( [NO3-] - 16[PO43-] + 2.9) (Gruber & Sarmiento 1997) N* Distribution Shows Interplay Between N2-Fixation and Denitrification N* = 0.87( [NO3-] - 16[PO43-] + 2.9) (Gruber & Sarmiento 1997)

What else? Time-dependence: waves, eddies, convection, ocean-atmosphere interactions and various modes of variability from intraseasonal to inderdecadal (ENSO, NAO, PDO, NPGO ….)

La Nina conditions El Nino conditions

in the North Pacific Pacific Decadal Oscillation

Mean Sea Level Pressure Anomaly PDO Negative Phase Anomaly PDO Positive Phase

+ - +

The NPGO (North Pacific Gyre Oscillations) Di Lorenzo et al. GRL 2008 North Pacific dominant mode of oceanic variability: the PDO and NPGO (left), are driven by the first two dominant modes of atmospheric variability evident in sea level pressure, the Aleutian Low variability and the NPO.

Variability of subsurface nitrate (NO3) Variability of subsurface nitrate (NO3). (A) Mean subsurface (150m) NO3 from ROMS model over 1975-2004. (B) First mode of variability for model subsurface (150m) NO3 anomaly inferred from EOF1. (C) Timeseries of NPGO index (black) compared to PC1 of NO3 (R=0.65, 99%), observed mix layer NO3 at Line-P [Peña and Varela, 2007], and observed NO3 from CalCOFI program (R=0.51, 95%).

The North Atlantic Oscillation Index The NAO index shows large variations from year to year. This interannual signal was especially strong during the end of the 19th century. Sometimes the NAO index stays in one phase for several years in a row. This decadal variability was quite strong in the second half of the 20th century. 36

SST Anomalies [C] Positive NAO Negative NAO Wet Dry Dry Wet Stronger Currents more storms Dry Dry fewer storms Wet Weaker Currents Negative NAO 37

Impacts of the NAO Martin Visbeck 38