Motility of freshwater zooplankton from a temperate lagoon from Patagonia (Chubut, Argentina) throughout a year cycle: Influence of solar radiation

R. J. GONCALVES; E. S. BARBIERI; V. E. VILLAFAÑE; E. W. HELBLING

Río Grande (Puerto Rico)

Congreso; American Society of Photobiology; 2006

American Society of Photobiology

We investigated the impact of solar ultraviolet radiation (UVR, 280-400 nm) on motility (i.e., swimming speed) of dominating zooplankton species (cladocerans and copepods) from a freshwater lagoon of Patagonia, Argentina, during a yearly cycle (February 2005 to February 2006).  Zooplankton samples were collected with a net (200 mm mesh size) from surface waters every 2-3 weeks; samples were put in UVRtransparent containers and exposed to solar radiation, inside temperature-controlled water bath under three radiation treatments: 1) PAB (280-700 nm), 2) PA (320-700 nm), and 3) P (400-700 nm), and a control (no radiation).  Swimming speed of free motile individuals was measured every 2 h of exposure with an image-analysis system from video recordings.  Optical characteristics of the water body, i.e., the attenuation coefficient of PAR (400-700 nm) (KdPAR) and mean irradiance (Im) in the upper mixed layer, were estimated from the measured irradiance and published models taking into account the concentration of chlorophyll-a (Chla) and chromophoric dissolved organic matter (CDOM). Chl-a was variable between 4 and 800 mg l-1, and it was found to be the most important variable controlling KdPAR (range: 2-17 m-1). Carnivorous-cyclopoid copepods (Metacyclops sp) and herbivorous cladocerans (Daphnia sp.) dominated the zooplankton community at different times showing a negative relationship in their abundance.  Additionally,the abundance of cladocerans was negatively correlated with Chl-a concentration, suggesting a top-down control of the phytoplankton population.  Swimming speed of both zooplanktongroups increased relatively when KdPAR was low (autumn / winter) and it decreased when KdPAR was high.  Solar ultraviolet radiation had relatively little impact on swimming speed and these changes in motility seem to be more related to factors such as the underwater radiation field/ KdPAR and prey concentration, than to the incident solar radiation.