[Johann Mahne]

is it Kelp, Diatoms, Krill, algae?

[Don_1]

It might be dependant on the time of year.

[CliffordK]

I suppose it would depend on the definition, and how you measure. 

Doesn't Plankton cover algae, diatoms, and perhaps other stuff. 

[Nizzle]

Yea, are you talking about number of individuals, or tons of biomass?

I thought the total mass of krill organisms was the largest,
but phytoplankton (being at the very bottom of the food chain) are the most abundant in number of 'individuals'

[CliffordK]

What about prokaryotes?

At least in numbers, there is an awful lot of prokaryotes around the globe.

[Devilmunkey]

I think in terms of number, diatoms are quite likely to come on top, though in terms of overall biomass possibly krill (I think I recall being told in a lecture, althought this was some time ago and marine life is not my area). Additionaly, from my own research i'm well aware of the vast amount and diversity of protozoa in general wherever water is found, such as Blepharisma. spp, Paramecium and other ciliates.