<P> In a reversal of the pattern on land, in the oceans, almost all photosynthesis is performed by algae, with a small fraction contributed by vascular plants and other groups . Algae encompass a diverse range of organisms, ranging from single floating cells to attached seaweeds . They include photoautotrophs from a variety of groups . Eubacteria are important photosynthetizers in both oceanic and terrestrial ecosystems, and while some archaea are phototrophic, none are known to utilise oxygen - evolving photosynthesis . A number of eukaryotes are significant contributors to primary production in the ocean, including green algae, brown algae and red algae, and a diverse group of unicellular groups . Vascular plants are also represented in the ocean by groups such as the seagrasses . </P> <P> Unlike terrestrial ecosystems, the majority of primary production in the ocean is performed by free - living microscopic organisms called phytoplankton . Larger autotrophs, such as the seagrasses and macroalgae (seaweeds) are generally confined to the littoral zone and adjacent shallow waters, where they can attach to the underlying substrate but still be within the photic zone . There are exceptions, such as Sargassum, but the vast majority of free - floating production takes place within microscopic organisms . </P> <P> The factors limiting primary production in the ocean are also very different from those on land . The availability of water, obviously, is not an issue (though its salinity can be). Similarly, temperature, while affecting metabolic rates (see Q), ranges less widely in the ocean than on land because the heat capacity of seawater buffers temperature changes, and the formation of sea ice insulates it at lower temperatures . However, the availability of light, the source of energy for photosynthesis, and mineral nutrients, the building blocks for new growth, play crucial roles in regulating primary production in the ocean . Available Earth System Models suggest that ongoing ocean bio-geochemical changes could trigger reductions in ocean NPP between 3% and 10% of current values depending on the emissions scenario . </P> <P> The sunlit zone of the ocean is called the photic zone (or euphotic zone). This is a relatively thin layer (10--100 m) near the ocean's surface where there is sufficient light for photosynthesis to occur . For practical purposes, the thickness of the photic zone is typically defined by the depth at which light reaches 1% of its surface value . Light is attenuated down the water column by its absorption or scattering by the water itself, and by dissolved or particulate material within it (including phytoplankton). </P>

Which is the major limiting element in marine primary production