Influence of nutrients and mixing on the primary production and community respiration in the Gulf of Riga

Rates of plankton community production and respiration in relation to wind, solar radiation, biomass and nutrients were measured in the mixed layer during a late spring, a mid-summer and an early autumn situation in the Gulf of Riga. System metabolism was estimated by the in vitro oxygen method and from oxygen mass balance directly in the water. Gross production (GPP) remained fairly stable throughout the investigation periods with a mean of 1.8 g C m−2 day−1 (range: 0.5–3.9 g C m−2 day−1). Community respiration varied from 0.5 to 6.4 g C m−2 day−1 during the two summer studies (the high respiration rates ranging from 3.0 to 10.4 g C m−2 day−1 found during the autumn cruise were probably overestimates). Respiration rates generally exceeded GPP indicating that the system was apparently net-heterotrophic during the three investigation periods. The predominant limiting factor for phytoplankton growth were nutrients during spring and summer. In autumn the combination of low light levels and high vertical mixing due to wind lead to conditions of light limitation. However, growth conditions were affected by the stabilizing (solar heating) and destabilizing (wind) forces acting on the water column in all three situations. Depending on the relative strength of these forces, day-to-day primary production could vary considerably. Under conditions of nutrient limitation mixing may increase photosynthesis; this is a consequence of improved exploitation of incident light and of available nutrients in the mixed layer. The strong dependency of mixing on primary production implies that measurements based on in vitro incubations may underestimate “real” production.