Mixed layer depth

Global map of average mixed layer depths In (a) February and (b) August based on hydrographic data collected from 1941 through 2002. Source After de Boyer Montegut, C., ef al. (2004). Journal of Geophysical Research 109, Cl 2003. (See companion website for color version.)... 

At mid-latitudes (Westerlies domain), seasonal changes in light availability, mixed layer depth, and temperature support two plankton blooms, one in the spring and a lesser one in the fall (Figure 24.10). In the winter, phytoplankton growth is light limited. (The carbon fixation reaction is also slower at lower temperatures.) Thus as heterotrophic microbes remineralize detrital POM, DIN concentrations rise. 

By contrast, the gas transfer estimates utilizing Rn measurements assumes steady state between Rn production from radioactive decay of nonvolatile Rd and gas transfer with the atmosphere. This assumption is possible because Rn has a half-life of only 3.8 days, so accumulation and lateral ocean fluxes of Rn is assumed to be minimal. Again, a potential problem is the active, versus inactive layer of the ocean in this case, the mixed layer depth that may change during an experiment. 

Mixed layer depths (MLD) were calculated based on density criteria where MLD was defined as the depth at which density increased by 0.125 kg dm3 with reference to that at the sea surface. ML-DMS is the DMS averaged in the water column above the MLD. 

Figure 16.18 Distribution, abundance and temporal dynamics of N2 fixing bacteria at Station ALOHA. (A) Vertical profiles of <10 pm (left) and >10 pm (right) nifH phylotypes in December 2002 relative to upper mixed-layer depth (dashed line) and 1 % surface radiance isopleth (dotted line). Error bars are 1SD of triplicate QPCR (45 cycles) reactions. From Church et al. (2005a).

Krishnamurthy, A, Moore, J. K., and Doney, S. C. (2008). The effects of dilution and mixed layer depth on deliberate ocean iron fertilization 1-D simulations of the Southern Ocean iron experiment (SOFeX).J. Mar. Sys. 71, 112-130. 

Another difference arises from the way in which the tracers are introduced into the ocean. CFCs have the simplest boundary conditions, as they dissolve as inert gases, following gas exchange and solubility rules (Warner et ai, 1996 Warner and Weiss, 1985). A typical gas exchange timescale for CFCs is of order 1-2 months, depending on wind speed and mixed-layer depth. Radiocarbon also enters the ocean via gas exchange (as CO2), but its gas exchange timescale is amplified by the... 

Smith C. R. and Rabouille C. (2002) What controls the mixed-layer depth in deep-sea sediments The importance of POC flux. Limnol. Oceanogr. 47, 418—426. 

Eastern Canadian Arctic. In August 1980, ecological studies were conducted in Baffin Bay near the Greenland coast bounded by 75-76° N and 68-72° W. Figure 12 shows two typical profiles of temperature, salinity, chlorophyll, and total copepods from near surface to 100-m depth. The surface-mixed-layer depth extended to only 15 m with salinity values from 31.5 to 33.0 ppt, which indicate the influence of glacial runoff. The subsurface chlorophyll maximum was situated below a sharp thermocline that ranged from 1.0 to -1.5 °C. However, copepods were mainly situated above the chlorophyll maximum within the thermocline itself. 

Profiles of dissolved organic carbon (DOC) for four different times of the year from the Bermuda Atlantic Time-series Station (BATS). The arrow indicates the approximate mixed layer depth for these times of the year. The vertical shaded area represents the wintertime values. Redrawn from Carlson et al. (1994). 

Figure 1. Concentrations of different nitrogen pools for the case in which there is no algal coagulation. Shown are the concentrations of nitrate, phytoplankton (particulate) nitrogen, and total nitrogen (sum of nitrate and particulate concentrations). Decreases in total nitrogen concentration are caused by sedimentation of solitary algal cells. The algae had radii of 10 jun. The mixed-layer depth was 50 m. The shear was 0.1/s. (Reproduced with permission from reference 37.

The surface mixed-layer depth approximately equals the photic zone. 

Figure 1. Maps of mixed layer depth in the global ocean as monthly averages for January (upper panel) and July (middle panel). The lower panel shows typical profiles of sigma-t (a measure of density) for a polar (Southern Ocean) versus tropical (Pacific) area of the ocean. The maps use global ocean temperature and salinity data sets compiled by the U.S. National Oceanic Atmospheric Administration as processed by Kara et al. [94].

Fig. 6.8 Distributions of chlorophyll Huorescence (panels on the left) and POC (panels on the right) in the upper 150 m along the US JGOFS southern transect during different seasons. These properties were derived from in situ light transmission and fluorescence measurements. The dotted and solid lines indicate the mixed layer depth (identified by an increase in 0e by 0.03 relative to the surface) and the 0.5 pM NOj contour, respectively. Reproduced from Gundersen etal. (1998) with permission from Elsevier Science (see Color Plate 3).

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