43. Effects of river streamflow on summer-winter dynamics of pH/pCO2 in a Patagonia highly stratified fjord

José L. Iriarte (1,2)*, Michael DeGrandpre (3), Maximiliano Vergara (4), Cory Beatty (3), Rodrigo Torres (5,1), Luis A. Cuevas (6)

1 IDEAL, Universidad Austral de Chile and Instituo de Acuicultura, 5501558, Puerto Montt, Chile.
2 COPAS-Sur Austral, Universidad de Concepción, Concepción, Chile
3 Department of Chemistry & Biochemistry, University of Montana, Missoula, USA.
4 Programa de Doctorado en Ciencias de la Acuicultura, Universidad Austral de Chile, 5501558, Puerto Montt, Chile.
5 CIEP, Coyhaique, Chile.
6 Facultad de Ciencias Ambientales, Universidad de Concepción, Concepción, Chile

Background
Patagonian fjords are characterized by subantarctic surface water receiving high volume of freshwater (rivers, glaciers). The interplay between continental and marine influences affect the chemical stoichiometry, organic matter input and finally affect the plankton community in an “uncertain way” that make a difficult task the modelling efforts. In addition, fjords, will face major changes in the carbonate chemistry speciation which most probably will trigger changes in carbon budgets, impacting primary productivity and the entire biogeochemistry of this cold and rainy region.

Methods
In order to investigate the summer-winter pH/pCO2 dynamics related to freshwater streamflow, we analysed high temporal resolution observations of in situ pH and pCO2 (SAMI, at 3.5 m), river streamflow and bimonthly sampling of pH, total alkalinity (AT) at the middle section of Reloncaví fjord between January to July 2015.

Findings
Strong summer-winter variability showed that the influence of freshwater layer affected the pCO2 and pH dynamics in the fjord surface water. During summer months, data showed the lowest pCO2 (mean=200 μatm) and highest pH values (8.0 – 8.2), coincident with the lowest freshwater inputs. In contrast, we observed that increased winter freshwater inputs decreased pH (down to 7.6), AT (<1000 μmol kg-1), and increased pCO2 (mean=1000 μatm). The surface water shows a winter undersaturation of Aragonite (Ω < 1), contrasting with oversaturated levels of warmer waters of spring-summer period.

Conclusions
The summer-winter variability showed that freshwater streamflow affected the chemical features of the fjord, suggesting that increased winter freshwater inputs plays a role enhancing the shift from CO2 sink in summer (high community net production, higher temperature and higher buffer capacity) to a CO2 source in winter (high community respiration, lower temperature, lower buffer capacity). Furthermore, the system change from a supersaturated state in summer (Ω >1) to “corrosive waters” for Aragonite during winter (Ω <1).