Long-term decreasing trend and seasonal shift in the abundance of Thalassiosira curviseriata in the Bahía Blanca Estuary, Argentina

GUINDER V. A.; POPOVICH C. A; FREIJE R. H.; MARCOVECCHIO J. E.

Mar del PLata

Congreso; The changing coastal and estuarine environment. A comparative approach; 2012

UNMDP

The monitoring of phytoplankton structure and composition is an accurate tool to track environmental changes in coastal ecosystems. We present here the long-term (1978-2008) variability in the abundance of the diatom Thalassiosira curviseriata Takano 1981, a key phytoplankton species in the Bahía Blanca Estuary (38º45´S, 62º20´W), Argentina, in relation to modifications in the pelagic conditions. For more than 20 years, 1978-2002, T. curviseriata was present throughout the annual cycle and has been a recurrent blooming species in winter-early spring. The maximal densities were always in winter (under a temperature range of 5-10 °C), over 1 x 106 cells L-1 (up to 12.7 x 106 cells L-1 in 1992), accounting for 60 - 96 % of the total phytoplankton abundance. In the last years (2003, 2006-2008), the population of T. curviseriata has significantly decreased in winter, with abundances lower than  0.8 x 106 cells L-1 in 2003 and  0.4 x 106 cells L-1 during 2006-2008. Moreover, it was not present all year-round and showed abundance peaks in spring and summer (up to 0.3 x106 cells L-1). Empirical studies of T. curviseriata isolated from the Bahía Blanca Estuary allowed characterize this species as eurythermal (5-20 ° C) and euryhaline (25 - 40 PSU) adapted to relatively low light intensities (growth became saturated between 70-80 µmol m-2 s-1 and inhibited at 150 µmol m-2 s-1). The modifications in water temperature, turbidity and pelagic trophodynamics observed over the past three decades in the Bahía Blanca Estuary seem to be the driving factors of T. curviseriata decrease and seasonal shift. We discuss potential niche modifications, ecological traits and species-specific interactions implicated in the exclusion of T. curviseriata from the winter period in this estuarine ecosystem.