The party is over …

The phytoplankton bloom party in our mesocosms is over. The booze (i.e. inorganic nutrients) is gone and the crowds at the lower end of the food web are leaving. The exit door is the sediment trap at the bottom of the mesocosm, which is filing up quickly much to Tim’s delight. Surprisingly, our VIP bloomer, Coscinodiscus, doesn’t want to join the crowds. Hardly any of these “huge burgers in glass boxes” have made it into Tim’s samples. Instead, as Jana tells us, they are still increasing in numbers in the mesocosms. Didn’t they realize that the booze is gone? Well, they were smart to hide some schnapps from the rest of the crowd well before the others finished it all up. In their huge glass boxes they have large vacuoles, in which they can store plenty of nutrients. This way they can continue to party when all others are already running dry. And the water surrounding them still contains enough silicate to build more glass boxes.

Plankton_community

Sketch illustrating the plankton community inhabiting our mesocosms. All groups shown here, except the fish, are also present in the mesocosms.

What impact did the phytoplankton spring bloom have on the party animals higher up the food web? Did Pseudocalanus, the dominant zooplankton grazer in our mesocosms, have the feast of its life?

Much of the phytoplankton biomass accumulating during the bloom was small, in the nanoplankton size class. Not exactly the size class for Pseudocalanus to gorge. Indeed, the fact that Tim finds hardly any zooplankton feaces in his sediment traps suggests that Pseudocalanus and his mezozooplankton buddies may not have filled their stomachs to the limit. Well then, who made use of the abundant phytoplankton food? Was it the microzooplankton, ciliates, rotifers, radiolarians, and heterotrophic dinoflagellate? Or did viral infection terminate the nanoplankton party before the buffet was eaten all up? It will take a while until all the samples have been analysed and the storyline of our spring bloom party emerges. For the days to come I am curious to see how much longer Coscinodicus continues to party.

What’s coming up next, now that the heydays are over? Lots of dissolved organic matter is likely to have accumulated in the water during the bloom, plenty of food for the mixotrophs. Rapid remineralisation by bacteria will speed up the turnover of this organic matter. The so-called microbial loop will gain momentum and bring a rapid succession of the tiny guys in the pico- and nanoplankton size range, including the microzooplankton. Pseudocalanus and its offsprings will try to get their share. It’s time for the fish larvae to join the food web.

By Ulf Riebesell