Variations in Marine Optical Properties during the Period 2000-2009 at the Antares EPEA Station (Argentina)

LUTZ, V.A.; SEGURA, V.; NEGRI, R.M.; SILVA, R.I.; CUCCHI-COLLEONI, A.D.; GUERRERO, R.; MOLINARI, G.N.

Foz do Iguassu

Conferencia; American Geophysical Union (AGU) Meeting of the Americas; 2010

American Geophysical Union

The ‘Estación Permanente de Estudios Ambientales (EPEA)’, located at 38º28’S–57º41’W in the Southwestern Atlantic (Argentina), is a coastal station (45m deep) part of the Antares network, which nucleates time-series-studies around the Americas, and is a branch of the global ‘ChloroGIN’ network. Bio-optical measurements have been taken approximately once a month from February 2000.  These data, including light penetration and absorption by different components (phytoplankton, detritus and Chromophoric-Dissolved-Organic-Matter), provide information about water transparency, relevant for ecological studies, and essential for models of primary production. The analysis of the first annual cycle showed that the depth of the 1% surface irradiance [Z(1%Io)] followed a seasonal pattern, being shallower in winter (low Io) when the water column was mixed, and deeper in summer (high Io) with the development of the pycnocline. This was coincident with a high absorption by detritus at 440 nm [ad(440)] at the surface, probably increased by re-suspension of sediments from the bottom during winter. Re-analyzing now 10 years of data, we observed on the physical side a clear seasonal pattern of surface warming ranging from 22.1 ºC in summer to 9.7 ºC in winter. Looking at the optical data there was a low but significant relationship between Z(1%Io) and the percentage (on total particulate absorption) of  ad(440) (Spearman correlation=-0.28, P=0.016, n=72), suggesting again the effect of detritus on the attenuation of light in the water column. On the other hand, pulling all the data together values of Z(1%Io), ranging from 11 m to 40 m, scattered throughout the year showing no apparent seasonal variation. The surface absorption coefficient of phytoplankton at its maximum in the blue [aph(440)] ranged between 0.016 m-1 to 0.115 m-1 (mean 0.037 m-1) with two exceptions (0.202 m-1 and 0.394 m-1). These rare values corresponded to two events of unusually high phytoplankton concentrations, the first one dominated by a Haptophyte of the genus Prymnesium (November 2005), and the second one represented by a mixed community with dominance of chain diatoms (December 2008). The apparent lack of a seasonal trend in Z(1%Io) and aph(440), despite the clear temperate cycle of surface warming, evidence that physical – biological interactions are more complex than anticipated from the first year analysis at EPEA. Overlaying on these interannual variations there were extreme events resulting in anomalies in the total particulate absorption for the site. Variability in optical properties is going to be discussed in relationship to the phytoplankton composition and physical conditions.