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Revista Chilena de Historia Natural 78 (3): 469-488, 2005
20<sup>th</sup> century fluctuations in the abundance of siliceous microorganisms preserved in the sediments of the Puyuhuapi Channel (44°S), Chile
We present a 100-year reconstruction of siliceous export production from sediments of the Puyuhuapi Channel (44°S, 70°W) in the Chilean fjords. We use accumulation rates and concentrations of diatoms and silicoflagellates, organic carbon (Corg) and biogenic opal (SiOPAL) as proxies of export production, and fluctuations in the contribution of freshwater diatoms as proxies of rainfall in the hinterland and river runoff. Box core sediments collected at two sites within the Puyuhuapi Channel were analyzed: Station 35 (at the head of the fjord; 56 m water depth) and Station 40 (in the middle of the Channel; 270 m water depth). Surface sedimentation rates were 0.75 cm yr<sup>-1</sup> at Station 35 and 0.25 cm yr<sup>- 1</sup> at Station 40. SiOPAL content averaged ~ 4 % at both sites. Diatom accumulation rates as well as the contribution of freshwater diatoms were higher at the head of the fjord (1.59 1010 valves m<sup>-2</sup> yr<sup>-1</sup> and 22 %, respectively) than in its middle (1.08 1010 valves m<sup>-2</sup> yr<sup>-1</sup> and 14 %, respectively). Diatom abundances were two orders of magnitude higher than silicoflagellate contribution at both sites. In general, diatoms typical of high nutrient environments characterize the Puyuhuapi Channel sediments: at both sites, spores of the genus Chaetoceros dominated the diatom assemblage (> 40 % of total diatoms; spores of Chaetoceros radicans/cinctus, Ch. constrictus/vanheurcki, Ch. Debilis and Ch. diadema). Downcore analysis reveals an overall increase in the production of siliceous microorganisms from the late 19>sup>th</sup> century to the early 1980s, and then a decrease until the late-1990s. We associate a decrease in freshwater diatom contribution since the mid-1970s which we associate with a concomitant decline in rainfall in the Chilean fjords. We suggest that this decline is related to the global atmospheric and oceanic warming of the past ~ 25 years.
Key words:
siliceous microorganisms, diatoms, sediments, export production, Puyuhuapi Channel, Chilean fjords

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