Seasonal variability of fractionated phytoplankton, biomass and primary production in the Straits of Magellan

The results on the distribution of phytoplankton biomass (expressed as Chla) and primary production (14C assimilation), during three oceanographic cruises carried out during Austral spring and at the end of the summer and the au~mu in the Straits of Magellan, suggest a strong variability of trophic levels for this ecosystem. Seasonal evolution of the biorwss concentration goes from the spring maximum of 2.33 bg/l through a sharp decrease, 0.49 pg/l, observed at the end of summer, until the minimum of 0.24 pg/l measured during the autumn. The trophic conditions are dependent on hydrograpbic, meteo-climatic and geo-morphological characteristics: at the Atlantic entrance and between the two Angosturas the strong mixing of water column limit the development of phytoplankton; at the Western opening and along the Pacific arm the complex exchange mechanisms with the ocean. the glacio-tluvial contribution and the presence of a thermohaline front near the Isla Carlos III influence both biomass and primary production distributions. The maximum values are reached in the Central Zone (Paso An&o) characterizedby high stability of the water column. Primaryproductionrangedfroma minimumof 12.3to a maximumof 125.9mgCm-* h- '. The overall trend seems to be a progressive and simultaneous increase from the Pacific and Atlantic openings to the Central Zone of Paso Ancho where the maximumvalue was reached,In general, biomass and primaryproductiondistributionscorrespondquite well except for the area of Isla Carlos III where biological and chemico-physical causes tend to limit 14Cassimilation. Contribution of pica-phytoplankton (< 2 km) to total biomass appears to be time dependent: in the blooms observed during spring a very modest incidence (< 6%) was observed whereas became more significant (> 50%) during the summer-autumnseasons when total biomass was decreasing. Within the Straits, at the end of summer,the contributionof pica-phytoplanktonprimaryproductionis 59%, whereas nano and microplankton contribute 39% and 2%, respectively. At the oceanic external stations the photosynthetic activity of the bigger size-fraction ( > 2 pm) is predominant( > 50%). These findings supportthe hypothesis that the pica-phytoplankton( < 2 pm) is substantiallyconstant,whereastemporal variationsaredue to the larger(> 10 km) cells only.