| | | 19.8 Moderate primary production, high respiration |
Plankton community production and respiration rates in relation to wind, solar radiation, biomass and nutrients revealed that the predominant limiting factor for phytoplankton growth is nutrient supply during spring and summer [351]. In autumn, the combination of low light levels and deep vertical mixing due to wind leads to conditions of light limitation. The growth conditions are in all three situations, however, strongly affected by the stabilising (solar heating) and destabilising (wind) forces acting on the system. Depending on the relative strength of these forces, day-to-day primary production varies by a factor of two. Under conditions of nutrient limitation mixing may lead to increased photosynthesis, which is a consequence of improved exploitation of incident light and of available nutrients in the deeper mixed layer.
Primary production and respiration measurements indicate that the gross primary production is generally high during the productive season, on average about 2 g C m-2 d-1 [351]. Assuming that this estimate is representative for a presumed growth period March/April to October (approximately 200 days), the total annual primary production is estimated to range between 300 and 400 g C m-2 [351]. High respiration estimates (on average about 4 g C m-2 d-1) demonstrates that the pelagic system is rather based on regenerated than on new production. This is supported by simultaneous vertical flux measurements [295] that indicate that the export of autochthonous matter out of the euphotic zone, relative to the standing stock and primary production, is small. Compared to many other coastal areas, the pelagic system of the Gulf of Riga appears exceptionally efficient in recycling and its ability to retain biogenic matter and nutrients in the upper layers.
| | | 19.8 Moderate primary production, high respiration |