Ocean science questions ANSWERED! post 6 - Salps and pteropods! Gelatinous zooplankton!

Q:  What is the overall niche of salps and pteropods? Are there any theories to predict where you get a food web with lots of gelatinous little zooplankton (I'm not counting medusae) instead of crustaceans?  (Neil)

A:  Salps have a particular niche, because there are only a few species groups and they tend to feed in a similar fashion to one another – they make large aggregates and feed and respire and excrete and die in a big cluster.  Pteropods on the other hand are varied, and diverse – from thecosome pteropods (the little “sea snails” with shells made so famous by ocean acidification research) to gymnothecates with no shell and soft bodies, they are varied in form, feeding and life cycle.  I guess there might be a similar niche, in the sense that shelled-pteropods make a mucous net to stick prey onto and then consume the house, and salps do something similar.  They both filter a large amount of water, although salps probably more because they are bigger in size. 

Theories to predict where you’d get a food web with lots of LITTLE gelatinous zooplankton!  Yes.  Making a theory up on the spot, I think there could be more LITTLE gelatinous zooplankton in the highly productive upwelling region food chains.  I’m saying that because we usually see those areas with a simplified food chain, with forage fish feeding on zooplankton, and large animals feeding on the fish – sort of skipping over the large jellies and some of the larger macrozooplankton.  Of course, systems are always more complicated than the model, but that could be a first crack at my theory.  Also, shelled pteropods have been shown to be a significant part of salmon’s diets (but only in some years).  So that’s not much to build a theory on, either.  Still a long way to go, in my view, for a unified theory on zooplankton ecology – especially gelatinous ones.  

Update:  i just read that Oikopleura (larvaceans) have mucous nets with such tiny openings that they can snare bacteria in there.  !  This seems mind-boggling to me, but what it means is that maybe Oikopleura are playing a bigger role in the microbial loop than we know at the moment.  If that's the case, perhaps there's a truncated food web going on when Oikopleura are present in large enough numbers.
Update #2:  it seems that some salps swarm and aggregate when other's (meso+macrozoop) don't, usually because of nutrient limitation.  This might also be a suggestion of a niche for salps:  if they can filter and feed on bacteria and the microbial loop, they might be able to subsist without phytoplankton if there is enough dissolved organic matter/dissolved organic carbon that's floating around in the water.  Such a scenario could happen if there were a phyto bloom, and then nutrients were limited and there wasn't a complete drawdown of nutrients by zooplankton, then phytos die off and there's just a slow accumulation of DOC for a period of months.  However, this doesn't really suggest a "niche" to me, per se, because it seems like salps could/would just as easily swarm in an environment replete with nutrients and things to eat.  There is probably a competitive advantage to being able to swoop in and eat when others can't, but i'm not sure that constitutes a niche.
Update #3:  i think this exploitation of resources by salps could be tied to disturbance - if there's disturbance and enough DOC is in the water, salps can ride in.