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

Abstract

Short-term iron enrichment experiments were carried out with samples collected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The research area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml y1 . Maximum chlorophyll a concentrations varied from about 1.0 mg y1 Ž . l in a high-reflectance zone caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi and about 3.5 mg l y1 in a zone in which the phytoplankton were growing, to about 0.5 mg l y1 in the northeast Atlantic Ocean. From the high-reflectance zone to the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 mM to 6.0 mM. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high-reflectance zone to -1.0 nM in the northeast Atlantic Ocean. In the research area, no signs of true iron deficiency were found, but iron enrichments in the high-reflectance zone, numerically dominated by Synechococcus sp., resulted in increased nitrate uptake. Ammonium uptake was hardly affected. Strong support for the effect of Fe on cell physiology is Ž . given by the increase in the f-ratio. Net growth rates of the phytoplankton changes in cell numbers over 24 h were almost unchanged. Phytoplankton collected from the northeast Atlantic Ocean, did not show changes in the nitrogen metabolism upon addition of iron. Net growth rates in these incubations were low or negative, with only slightly higher values with additional iron. q

The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the northwest European shelf sea

“…Caveats were associated with the use of prolonged bottle incubations; while the photoperiod matched the geographical location sampled, the spectral quality of light within incubation bottles differed from natural light and may have influenced phototrophic activity; there was potential for progressive deviation in community composition and activity from that of natural communities due to bottle confinement, which prevented nutrient and metabolite exchange processes and may have selectively enhanced/suppressed the activity of specific community components (Timmermans et al, 1998;Calvo-Díaz et al, 2011). It is unclear how these factors influenced the results presented.…”

Enhancement of the reactive iron pool by marine diatoms

“…We described three experiments where the phytoplankton species, the Fe concentration and irradiance were varied. Short term experiments were conducted to allow observation of an immediate, or at least short-term, response of Fe speciation while the plankton community remained relatively uniform (Timmermans et al, 1998).…”

Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

“…Ship-board Fe addition incubation experiments have been widely used to assess the importance of iron as a factor limiting the growth of phytoplankton in HNLC waters (Boyd et al, 1996;Boyd et al, 1998;Coale et al, 2003;Cochlan et al, 2002;Fitzwater et al, 1996;Martin and Fitzwater, 1988;Timmermans et al, 1998;Zettler et al, 1996). Noiri et al (2005) conducted ship-board Fe addition experiments during the SEEDS experiments in 2001.…”