Primary productivity: seaweeds/seagrasses hot buttons for primary producers part of BCCR hot button for cyanobacteria productivity part of BCCR hot button for phytoplanktonp productivity part of BCCR hot button for invertebrates primary productivity part of BCCR hot button for seaweeds/seagrasses productivity part of BCCR

There are several routes of entry for the sun's energy into the coral-reef ecosystem. These include cyanobacteria, phytoplankton, seaweeds, and seagrasses. A number of sessile/sedentary invertebrates, such as corals, gorgonians, sea anemones, and sponges, and even some motile forms, such as certain nudibranchs and clams, host photosynthesising symbionts. This section deals with phytoplankton; other topics can be accessed via the icons below.

NOTE information on photosynthesis in corals can be found elsewhere in the ODYSSEY: CORALS: A CASE STUDY

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of coral-reef seaweeds taken from a video "Hmmm! Nice growth of this funny-looking green alga. Quite shallow here. I always thought that green algae favoured shallow areas because they rely so heavily on the red part of the sun's spectrum. I wonder if that's true..?." - St. Thomas 2005
  figure showing
Why are plants GREEN?
Plants such as green algae and seagrasses have two main types of chlorophylls. Type b absorbs more at the blue end of the spectrum, while type a absorbs more at the red end. The unused portion of the light spectrum, mostly green wavelengths, is reflected and this gives plants their green colour.
photo collage of different marine plants

figure showing absorption spectrum for red seaweeds
Why are some seaweeds RED? Photosynthetic
pigments in red seaweeds maximally absorb green, violet, and blue wavelengths, and reflect mostly red light. The ability of red seaweeds to use deep-penetrating blue/violet wavelengths for photosynthesis suggest that they are capable of living deeper than green seaweeds, which rely more on shallow-penetrating red wavelengths for photosynthesis.

NOTE in addition to possessing chlorophyll pigments for photosynthesis, as in terrestrial plants, seaweeds have auxiliary photosynthetic pigments known as phycobilins, and these contribute to the different shades of red found in red seaweeds

photo collage of red seaweeds
  figure showing absorption spectrum for brown algae
BROWN seaweeds
absorb more in the blue-green portion of the spectrum, and the broad mix of wavelengths reflected makes them appear brown. On the basis of their ability to use a broad range of wavelengths, but not as efficiently as red seaweeds, we might expect brown seaweeds to live at depths shallower than reds.
photo collage of brown algae

graph showing maximum depths of different types of Caribbean seaweeds
So, based on portions of the light spectrum used in photosynthesis, our overall prediciton is that green seaweeds should be restricted to shallow waters, red seaweeds could live at deepest depths, and brown seaweeds would be intermediate. How well does our prediction match the facts?

Here are data for estimated maximum depths of occurrence of green, red, and brown seaweeds in the Caribbean region. Data from Littler et al. 1989 Marine plants of the Caribbean. Smithsonian Inst Press.


So, it seems that our predictions are NOT supported by the data. All 3 seaweed types can apparently live from surface waters to the depths. What did we fail to consider? Think about which of these possibilities may be correct, then CLICK HERE to see explanations.

Respiration rates are not taken into account.

Physical features of the seaweeds are not considered.

Certain biological factors are not included.

Effects of ultraviolet light are not dealt with.

Other habitat preferences of a seaweed are ignored.

NOTE plants, like other living things, need to respire to survive. In so doing they oxidise glucose created by their own photosynthesis. The depth at which the amount of oxidisable matter produced by photosynthesis equals the amount used in respiration is known as the plant's compensation depth. A plant cannot survive deeper than its compensation depth