| | | 19.11 P retention |
Although increased vertical export of C and N was recorded along with the export of phytoplankton cells at the end of the spring bloom [356], pelagic retention of P is already high in late spring, resulting in low vertical loss rates of P. It is suggested that increased export of C, N and P to the benthos may take place during the spring bloom before the pelagic system develops sufficient complexity to greatly reduce vertical losses. The main supply of phytoplankton biomass to the benthos in the Gulf of Riga is thus determined by the time window characterised by low trophic complexity in spring. Retention of P was also high during summer and early autumn. In regions where nutrients are continuously supplied both from allochthonous and autochthonous sources, C and N are lost in excess while P is recycled from the pelagic system throughout the productive season [226]. However, species-specific differences in bloom dynamics and sedimentation (i.e. dinoflagellates vs. diatoms) give rise to episodic events which are crucial for the further channelling of the assimilated nutrients and produced biomass into pelagic or benthic food webs [225]. Seasonal and annual variations in nutrient supply through rivers [282], in particular with regard to dissolved silicate, will possibly contribute to the differences in dinoflagellates vs. diatoms prevalence and thus induce interannual changes in the pelagic-benthic coupling.
The efficiency of P retention depends on the complexity of the pelagic food web and is enhanced by the increasing complexity of the trophic structure in the system over time. Increased P retention will also counteract the P deficiency induced by river runoff [282] and contribute to the complex N vs. P limitation scenarios recorded from the southernmost Gulf [419]. Through this response P limitation may be eliminated and the P requirements of phytoplankton are adequate. The trophic complexity of the planktonic community in the southern and central Gulf of Riga and the variable, but continuous supply of nutrients by rivers such as Daugava give rise to a high `buffering capacity' of the pelagic ecosystem throughout the productive season. Thus retention rather than export food chains are favoured and the effect of external perturbations, such as increased nutrient supply, is reduced.
| | | 19.11 P retention |