Interannual variability of planktic foraminiferal populations and test flux in the eastern North Atlantic Ocean (JGOFS)

Planktic foraminiferal assemblages vary in response to seasonal fluctuation s of hydrographic properties, between water masses, and after periodical ch anges and episodic events (e.g. reproduction, storms). Distinct annual vari ability of the planktic foraminiferal flux is also known from sediment trap data. In this paper we discuss the short-term impacts on interannual flux rates based on data from opening-closing net hauls obtained between the oce an surface and 500 m water depth. Data were recorded during April, May, Jun e, and August at around 47 degrees N, 20 degrees W (BIOTRANS) in 1988, 1989 , 1990, 1992, 1993, and during May 1989 and 1992 at 57 degrees N, 20-22 deg rees W. Species assemblages closely resemble each other when comparing the mixed layer fauna with the fauna of the upper 100 m and the upper 500 m of the water column. In addition, species assemblages > 100 mu m are almost in distinguishable from assemblages that are > 125 mu m in test size. The stan ding stock of planktic foraminifers at BIOTRANS can vary by more than one o rder of magnitude over different years; however, species assemblages may be similar when comparing corresponding seasons. Early summer assemblages (Ju ne) are distinctly different from late summer assemblages (August). Signifi cant variations in the species composition during spring (April/May) are in dependent of the mixed layer depth. Spring assemblages are characterized by high numbers of Globigerinita glutinata. In particular, day-to-day variati ons of the number of specimens and in species composition may have the same order of magnitude as interannual variations. This appears to be independe nt of the reproduction cycle. Species assemblages at 47 degrees N and 57 de grees N are similar during spring, although surface water temperatures and salinities differ by up to 10 degrees C and 0.7 (PSU). We suggest that the main factors controlling the planktic foraminiferal fauna are the trophic p roperties in the upper ocean productive layer. Planktic foraminiferal carbonate flux as calculated from assemblages reveal s large seasonal variations, a quasi-annual periodicity in flux levels, and substantial differences in timing and magnitude of peak fluxes. At the BIO TRANS station, the average annual planktic foraminiferal CaCO3 fluxes at 10 0 and 500 m depth are estimated to be 22.4 and 10.0 g m(-2) yr(-1) respecti vely. (C) 2000 Elsevier Science Ltd. All rights reserved.