Metal enrichment experiments in the Weddell-Scotia Seas: Effects of iron and manganese on various plankton communities

Abstract

During the European Polarstern Study (EPOS 1988(EPOS /1989 in the Weddell and Scotia Seas, five series of metal enrichment experiments were carried out with natural plankton communities under ultraclean conditions. Despite a clear stimulation of growth by the addition of Fe, control bottles (no additions) also showed rapid buildup of Chl a and complete utilization of a major nutrient within 2 weeks, indicating nonlimiting ambient Fe levels. Effects of Mn additions were less pronounced or absent, whereas extra additions of Zn and Cu in one experiment showed little or no effect. The species composition of the plankton community, monitored by HPLC pigment analysis and microscopic observations, changed in favor of diatoms when Fe was added. The addition of Fe also caused an increase in microzooplankton densities and concentrations of pigment breakdown products. However, metal-mediated shifts in the plankton community were minor compared to major changes resulting from incubation. Changes were most pronounced in experiments where microzooplankton was strongly developed, presumably as a result of excluding mesozooplankton from the bottles. Fe had an impact on plankton growth and species composition, but other factors seem to be responsible for keeping phytoplankton productivity far from its potential in these Antarctic waters.Over the past few years a lively discussion has come about on the matter of the presumed limiting role of Fe in various oceanic, high nutrient level ecosystems (e.g.

Responses of marine phytoplankton in iron enrichment experiments in the northern North Sea and northeast Atlantic Ocean

“…Martin and Fitzwater, 1988;de Baar et al, 1990 , major shifts in the plankton community towards predominance of small protozoa and large diatoms Ž . have been observed Buma et al, 1991 . Moreover, a general lack of response of bulk parameters within Ž the first 48 h has previously been noted de Baar et .…”

“…Wells, 1995 . Among these regions are the Antarctic Ž Ocean de Baar et al, 1990Baar et al, , 1995Buma et al, 1991;Scharek et al, . Ž 1997, the Equatorial Pacific Ocean Martin, 1992 .…”

“…Similar experiments, performed in the past, Ž lasted several days up to 1 week Martin and Fitzwa-. ter, 1988;de Baar et al, 1990;Buma et al, 1991 . Over this long time period enclosure artefacts are conceivable. In general, larger mesozooplanton grazers are excluded from the bottles, also the light regime differs from the in situ light spectrum and intensity, and notably the dissolved Fe may be lost due to adsorption to the bottle wall.…”

“…land and Wells, 1995 . Ž The effects of iron additions on phytoplankton de Baar et al, 1990;Buma et al, 1991;Timmermans et al, 1994;Scharek et al, . 1997 , the distribution of iron and other trace ele-Ž ments in the water column Saager et al, 1989;Nolting et al, 1991;de Baar et al, 1995;Loscher eẗ .…”

“…Ž . PII S 0 3 0 4 -4 2 0 3 9 7 0 0 1 1 0 -2 ( )Ž substantial attention in recent years de Baar et al, 1990;Buma et al, 1991;Price et al, 1991;Martin, 1992;Sunda and Huntsman, 1995;Takeda and Obata, 1995;Boyd et al, 1996;Coale et . al., 1996 .…”