What is the average cholorphyll concentration for the ocean?

Question

For chlorophyll-a, measured in mg/m3, what are the average values for:

• Open ocean in general (global)
• Northen latitudes (N. Atlantic Ocean)
• Coastal waters around the UK (North sea, Irish Sea, etc)
• equatorial waters

Is there any pattern regarding plankton productivity and ocean climate that would help answer this? I.e. warmer waters with higher levels of sunshine (Mediterranean) have higher productivity, than colder waters (North Atlantic). Or are there too many factors involved (stratification, eutrophication) to generalise for the oceans?

Answer 1

Behrenfeld and Falkowski, 1997 have maps of cholorphyll concentration for the entire world on pages 11-13 of the linked paper. The maps are divided by into yearly concentration and seasonal concentration, with the maps filled out from experimental data by two different modeling methods.

These maps are in mg C / m$^2$ / year, but they offer the normalization of $$\frac{4.6 \text{mg C / hour}}{1\text{ mg Chlorophyll}}.$$ Chlorophyll production varies by optical depth, which is the distance that light can penetrate into the water. Optical depth depends on various characteristics of the seawater like sediment node, so it doesn't have an exact analogue to water depth. However, Fig 1A supports a conclusion that plankton production is reduced by an order of magnitude at depth of 20 meters, and by 2 orders of magnitude by 60 meters.

If we confine the chlorophyll to entirely the top 60m of the ocean, then we can convert $$\frac{1 \text{ mg C}}{\text{m}^2\cdot\text{year}}\cdot\frac{1 \text{ mg Chlorophyll}}{4.6\text{ mg C / hour}}\cdot \frac{1 \text{year}}{8760 \text{ hours}} \cdot \frac{1}{60 \text{m}}= \frac{0.41 \text{ ng Chlorophyll}}{\text{m}^3}.$$

Using this convertion specifically on map A on page 11 of the linked paper, I get for your answers:

• Open ocean in general: 75-150 g C/m$^2$/year $\rightarrow$ 30-60 $\mu$g/m$^3$
• N. Atlantic Ocean: 150-250 g C/m$^2$/year $\rightarrow$ 60-100 $\mu$g/m$^3$
• Coastal waters around the UK: 300-450 g C/m$^2$/year $\rightarrow$ 125-185 $\mu$g/m$^3$
• Equatorial waters: 100-200 g C/m$^2$/year $\rightarrow$ 40-80 $\mu$g/m$^3$

Answer 2

As @aretxabaleta points out in the comment above, chlorophyll concentrations are higher in polar regions as nutrient concentrations are higher due to the predominant low temperature conditions there.

Yoder et al (1993) also point out that algal, phytoplankton and other aquatic metaphyte productivity varies with latitudes and seasons (polar regions experience blooms in spring).

That being said, it is also important to know that chlorophyll concentrations vary with annual timescales as a result of temperature fluctuations so the average concentration is variable for that reason.

The NASA earth observatory website has database of global chlorophyll concentration changes from 2002 to 2016. There's a colour coded animation that shows these changes on the site which I hope you will find helpful.

Reference:

Yoder J.A., McClain C.R. Feldman G.C. and Elias W.E. (1993). "Annual Cycles of Phytoplankton Chlorophyll Concentrations in the Global Ocean a Satellite View" American Geophysical Union.