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 du ring 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) rema ined 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 (th e high respiration rates ranging from 3.0 to 10.4 g C m(-2) day(-1) found d uring the autumn cruise were probably overestimates). Respiration rates gen erally exceeded GPP indicating that the system was apparently net-heterotro phic during the three investigation periods. The predominant limiting facto r for phytoplankton growth were nutrients during spring and summer. In autu mn the combination of low light levels and high vertical mixing due to wind lead to conditions of light limitation. However, growth conditions were af fected by the stabilizing (solar heating) and destabilizing (wind) forces a cting on the water column in all three situations. Depending on the relativ e strength of these forces, day-to-day primary production could vary consid erably. Under conditions of nutrient limitation mixing may increase photosy nthesis; this is a consequence of improved exploitation of incident light a nd of available nutrients in the mixed layer. The strong dependency of mixi ng on primary production implies that measurements based on in vitro incuba tions may underestimate "real" production. (C) 1999 Elsevier Science B.V. A ll rights reserved.