Spatio-temporal distribution and ecological role of appendicularians off the La Plata River estuary (34°- 39°S, 52°- 58°W), Southwestern Atlantic Ocean

CAPITANIO, F.; SPINELLI, M.; NEGRI, R; VIÑAS, M.; ESNAL, G.

Hiroshima, Japón

Congreso; 4 th International Zooplankton Production Symposium; 2007

PICES, GLOBEC

The discharges into the continental shelf of the La Plata river, the second largest of South America, enhance high levels of biological production in the near-shore marine ecosystem.  Appendicularians, who are often implicated as mediators of carbon flux through the water column, graze on sub-micron colloids, lithogenic materials, cyanobacteria, bacterioplankton and nanoflagellates.  We present results on the distribution pattern of these tunicates in relation to the water masses and their ecological role, which were obtained during 6 spring cruises (1993-1996, 2003-2004).  The spatial distribution of salinity showed differences in the river discharges.  During 1993, low salinity waters (<33.3 psu) were displaced as far as 38°S and appendicularian abundances reached up to 3000 ind.m-3.  Oikopleura dioica was the most abundant and its pellet production, estimated by combining field measurements and literature data, reached 66.58 gC.m-2.d-1.  This value is considerably high when compared to those reported for other estuaries.   During 1995, the river scarce outflow and the detection of mid-shelf waters (33.4-33.7 psu) at the mouth of the estuary were coincident with O. fusiformis and Fritillaria borealis occurrences.  Both discharge anomalies may be associated with two climatologic events, El Niño and La Niña, registered in 1993 and 1995 respectively.  In the other cruises, especially in 1996, not only was the main influence of the river extended to the south but it also was displaced offshore to the northeast.  Therefore, the river’s outflow variability is reflected on the species spatial distribution and O. dioica may contribute significantly in the estuary’s carbon cycling.