19.9 Phytoplankton and vertical export of cells

19.9 Phytoplankton assemblages and the limited vertical export of phytoplankton cells

During spring the Gulf is characterised by an intensive dinoflagellate (mainly Peridiniella catenata) dominated spring bloom while the diatom Thalassiosira baltica contributed most (80 to 90 %) to the settling phytoplankton biomass. As mineral nutrients are abundant during the bloom it is suggested that the differential sedimentation of species is caused by physical factors [356].

During summer the phytoplankton biomass is dominated by high abundance of the filamentous cyanobacterium Aphanizomenon flos-aquae, autotrophic nanoflagellates and picoplankton. The primary vertical flux of phytoplankton is mainly due to non-motile species and aggregate-bound picoplankton and A. flos-aquae. Below the pycnocline, resuspended dormant diatom populations cause a significant secondary flux. Large heterotrophic dinoflagellates (Gyrodinium/Gymnodinium) are abundant in the middle layers and associated with ammonium regeneration. The phytoplankton assemblage appears to have three major functional groups, which build up roughly equal parts of the total biomass [356]:

1. Large filamentous cyanobacteria (Aphanizomenon flos-aquae). These are mainly inedible by the majority of the ambient zooplankton community in the Baltic Sea [415][416]. As with the grazing losses, the sedimentation loss rates of this group are very low. Consequently, the fate of this biomass is probably to disintegrate in the water column and to fuel the detrital and microbial pathways of the food web.
2. Nanoflagellates (e.g. Teleaulax spp., Plagioselmis prolonga, Pyramimonas spp., Pedinelle spp. and Pseudopedinella spp., Chrysochromulina spp., Heterocapsa rotundata). These are usually considered as readily available for mesozooplankton grazing. Their abundance points at the importance of the grazing food chain and represents the `efficient' part of the energy transfer in the food web to higher trophic levels. The sedimentation of this compartment is also insignificant. It is suggested that most of the nanoflagellates are grazed, and that their abundance is the result of an equilibrium between their growth and herbivory.
3. Picoplankton. This includes picocyanobacteria and pico-sized eucaryotes. The abundance of picocyanobacteria is quite common everywhere in the Baltic during summer. It is suggested that this compartment is an important food source for ciliates and cladocerans. Surprisingly, the sedimentation of picocyanobacteria was consistent. A likely mechanism is sedimentation within detrital aggregates.

The only significant sedimentation of fresh phytoplankton biomass took place in spring, in particular for T. baltica. In the other seasons phytoplankton sedimentation appears unimportant. The matter that sedimented was detritus. Consequently, there must be mechanisms that prevent vertical export of phytoplankton cells and promote recycling of bioelements in the pelagic ecosystem of the Gulf of Riga.

19.9 Phytoplankton and vertical export of cells