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Float no.6901521

    Temperature and salinity profiles   Time series plots
  Download the data   Using the float data with ocean analysis maps    



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This PROVOR_III float was launched by French scientists in the Labrador Sea at 58.7°N, 50.3°W on 15 May 2013. It has recorded a total of 69 profiles, and made its last report on 6 April 2014 from 63.7°N,  53.2°W.

float in the water

testing the float
Checking the float

testing the float
Deploying the float

This Bio-Argo float is one of several floats with optical and biogeochemical sensors.
It measures concentrations of chlorophyll-a and nitrate, a plant nutrient. It also measures light levels. Like all plants, phytoplankton need light and nutrients to grow.

The Labrador Sea spring bloom

The Labrador Sea is at the western side of the subpolar North Atlantic. At these high latitudes the spring phytoplankton bloom is extremely important for marine life. Many animals time their spawning so that their larvae can feed from the bloom.

During winter deep mixing replenishes surface nutrients, but the lack of light restricts growth, and plankton cells are mixed down to depths where there is no light for photosynthesis. With the arrival of spring, the wind-mixed surface layer shoals (becomes less deep) and available daylight increases. The bloom begins once phytoplankton growth exceeds losses by plant cells disappearing into the deep.

At the time when this float was deployed (15 May 2013) the phytoplankton had already started to grow, and the float recorded a moderate concentration of chlorophyll-a. The spring bloom started a little later, and reached its peak on 9 June (figure 1).


chlorophyll profile

Figure 1. Chlorophyll


At what depth do you find the highest chlorophyll-a on 9 June?

Argo float


What is the peak chlorophyll-a concentration there?


At what depth does the chlorophyll concentration drop to nothing?

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annotated chlorophyll profile
Figure 1. Chlorophyll-a on 9-Jun-2013

  Answers to Question 1


There is a chlorophyll maximum at about 19 m depth.


The concentration at the peak is over 20 milligrams per cubic metre of water (20 mg m-3). However, concentrations at the peak are only a little higher than in deeper layers. In fact the concentrations are high from about 12 m to well below 25 m. If you look carefully at the top of the main plot you will see that they remain high (>10 mg m-3) to almost 50 m depth. There are still relatively high chlorophyll contrations (>5 mg m-3) even deeper than this.


c) Chlorophyll concentrations only drop to near zero at around 150 m depth.


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What do you think are the reasons for Phytoplankton being present below 100 m depth?
You may get some clues from looking the time series of phostosynthetically available radiation (PAR), temperature, and nitrate below. (The time corresponding to the chlorophyll profile is indicated with an arrow.) When looking at the plots think about: Where is there enough light for photosynthesis? Where is there enough nutrients (nitrate) for growth? How would the plankton get from one depth to another? (They can't swim. Actually, they sink very, very slowly - but how do they get back up?)

Argo float


plot of photosynthetically available radiation   plot of photosynthetically available radiation   plot of nitrate   plot of photosynthetically available radiation 
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Answer to Question 2

In a bloom light available for photosynthesis drops faster with depth than usual. You can tell this from the time series of PAR. At the time of the bloom (white arrow) PAR is virtually 0 before you reach 20 m depth. This means that the phytoplankton below this depth do not have light for photosynthesis. Despite this chlorophyll concentrations are high. Something else is going on.

comparison of chlorophyll, PAR, nitrate and temperature plots Subset of the different time series plots focussing on the surface layer and the period of the bloom. Note different depth axis for PAR.

Nitrate levels are depleted in the surface layer occupied by the bloom. At the time when high chlorophyllis found at increasing depth the level of nitrate is increasing slightly in the surface layer above 100 m. At the same time it is slightly reduced in the layer 100-200 m. This could be an indication of mixing. Phytoplankton is mixed down, at the same time as nitrate is mixed up. The temperature profile supports this - parcels of higher temperature water is are found down to below 200 m.

The reason for the high chlorophyll below the sunlit surface region is mixing or overturning of the water above 200 m. This brings plankton down, but also replenishes nutrients. The light is so strong and the bloom so intense that chlorophyll concentrations stay high, despite the mixing.


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This bloom occurs every year in spring. and is essential for marine animal life. Many animals time their spawning so that their larvae can hatch in time to feed on the phytoplankton in the bloom.

At the end of June, the bloom is over. Chorophyll concentrations show that phytoplankton are present in the upper, warm layer throughout the summer, but these mini-blooms are more limited. In October the first storms begin to re-mix the upper water layer. Available light decreases strongly and plankton production is virtually non-existent until the next spring.

Combining the float data with satellite images

Since 1997 ocean colour satellites (SeaWiFS, MODIS and MERIS) have been making measurements of chlorophyll, PAR and bio-optical parameters. SeaWiFS stopped in 2010 and MERIS in 2012. However MODIS is still in orbit. This allows us to combine satellite data with the float profiles to get a spatial overview. The images below show MODIS chlorophyll around the time of the bloom. You can see clearly that phytoplankton concentrations are high in the region of the float profile.

Satellite chlorophyll is not as high as that measured by the float. But if you think back to the chlorophyll plot of figure 1, you will remember that the chlorophyll peak was below 5 m and the highest concentrations were found from 12 m downwards. Satellites measure what is know as 'depth integrated chlorophyll'. The chlorophyll concentrations they record are an average of chlorophyll at different depths, a concentrations in the top few m contribute relatively more to this signal than chlorophyll found deeper down.

MODIS chlorophyll for 8 days to 9 June 2013
MODIS chlorophyll for 8 days to 9 June.

MODIS chlorophyll for 8 days to 9 June 2013
Average MODIS chlorophyll for May

MODIS chlorophyll for 8 days to 9 June 2013
Average MODIS chlorophyll for June

How to obtain Bio-Argo data for this float

You can download the most recent plots of data from this float from
Besides chlorophyll the plots include time series of temperature, salinity, CDOM, particle backscattering, dissolved oxygen, nitrate concentration, and particle downwelling irradiance at three different wavelengths (380 nm, 412 nm and 490 nm), as well as phostosynthetically available radiation (PAR).

A guide to interpreting these plots can be found from the Bio-Argo web page .

Temperature and salinity profiles

Temperature profiles Salinity profiles

Profiles of temperature (left) and salinity (right) from Argo float 6901521. The profiles show how temperature (T) and salinity (S) change with depth from the surface to 2000m. Early profiles are dark blue, the latest profiles are deep red or brown. Click on the images for larger plots.     Source of plots: IFREMER/Coriolis.

Time series of temperature and salinity

Temperature section Salinity section

Time series of temperature (left) and salinity (right) from Argo float 6901521. The sections show all the temperature (T) and salinity (S) profiles measured by the float during its life-time side by side. Each profile is represented by a very thin column where deep red is the highest values and deep blue the lowest. The colour bars on the right relate the colours to actual data values. Profile numbers are given along the top of the plot, with corresponding measurement dates along the bottom. Click on the images for larger plots.     Source of plots: IFREMER/Coriolis.

Using the float trajectories with ocean analysis maps

Look at the float trajectory in Google Earth to see where the float has been. (If in doubt about how to reveal the float tracks, see our Google Earth screenshot for help.) Compare this to the maps of temperature and salinity for different depths available for example from Mercator ocean analyses.

Downloading the float data

The Argo Information Centre has more information about this float. You can also download the data from one of the Data Centres - just select Data > Data Downloads.

There are many different formats available. ASCII data can be viewed in spreadsheets such as Excel. The other data types may require more specialist software.


Link to the main Euro-Argo project website.