Seasonal characteristics of size-fractionated phytoplankton community and fate of photosynthesized carbon in a sub-antarctic area (Straits of Magellan)

Phytoplankton community size drives the rates of biogenic carbon and the overall structure and dynamics of the marine pelagic food web. The Straits of Magellan, an inland passage between the Pacific and Atlantic Oceans, can be separated in three main sub-basin: the western-Pacific, the V-shaped central zone, and the eastern-Atlantic. To provide insights on the food structure of the phytoplankton community, size-fractionated chlorophyll a concentration and primary production rates were measured across the three sectors of the Magellan Straits in four periods between 1989 and 1995 in the Straits. Phytoplanktonic biomass and production ratios provided ecological insights on the food web structure, including the relevance of grazing on its largest fraction. The micro-phytoplanktonic fraction (>10 µm) in the western-Pacific sub-basin is significantly less abundant than in the Central and Atlantic ones. Conversely, the lowest abundance of the pico- fraction (< 2.0 µm) is encountered in the Atlantic sub-basin. The observed patterns agree with the diffusion of smaller-size fractions from the western towards the easternmost sector of the Straits, and suggest that the largest phytoplankton tend to accumulate in the inner stretch of the Straits, being constrained by a clockwise gyre generated by tidal phases or partialy spilling out into the Atlantic sector. The most active grazing activities occur in the central arm during the spring bloom and last even in summer. Our results pinpoint also that the basic levels of the planktonic food web rely on the nano-phytoplankton (10-2 µm) fraction, which is the main contributor to the continuum multivorous food web. When nutrient pulses from land freshwater and water mixing enter the system, the structure of the plankton in the Straits shifts towards the herbivorous food web and is characterized by the presence of large-size diatoms. The complex dynamics of the plankton comuunities in the Straits keep it in a persistent mesotrophic state, featuring a lower trophic status of than the Antarctic coastal waters but a much higher one than oligotrophic temperate areas.