Effect of large marine diatoms growing at low light on episodic new production

Mesoscale eddies have been shown to be a common feature of open ocean regions such as the Sargasso Sea. By lifting nutrients up to the lower portions of the euphotic zone, these eddies can cause episodic phytoplankton blooms that can lead to substantial new production. In the Sargasso Sea, it has been estimated that such blooms can account for 35%–50% of annual new production. In the present study, it was shown that a common large diatom Stephanopyxis palmeriana is capable of growing in a low-light environment typical of the bottom 50 m of the euphotic zone at rates sufficiently high to sustain the contemporary estimates of new production that have been attributed to mesoscale eddies. The diatom was grown in laboratory batch experiments at irradiance levels from 11 to 79 mmol photons m22 s21, equivalent to irradiance levels occurring in the Sargasso Sea during the summer at depths from ;100 m (the 1% light level) up to 50 m. Resulting growth rates were compared with growth rates from the literature for similar large diatoms, and a simple model of new production was developed to show the dependency of new production on specific growth rate. Included in the model were other important parameters such as the depth of nutrient incursion resulting from lifting of the thermocline when an eddy passes, the duration of the bloom, and the size of the prebloom diatom population. Using realistic ranges for these parameters, it was evident from the model that there is no physiological constraint on these large diatoms from growing fast enough at very low light levels to meet the new production estimates resulting from eddies. There is growing recognition among oceanographers that mesoscale eddies occur frequently in oligotrophic regions and that, through the introduction of colder, nutrient-rich water into the lower layers of the euphotic zone, they can lead to greatly enhanced new production (Jenkins and Goldman 1985; Woods 1988; McGillicuddy et al. 1998; Oschlies and Garcon 1998; McNeil et al. 1999; Siegel et al. 1999; Mahadevan and Archer 2000; Levy et al. 2001). This form of new production, which, because of its episodic nature and because it occurs deep in the euphotic zone, frequently goes unnoticed. In the Sargasso Sea, for example, episodic eddies have been estimated to account for ;35%–50% of total new production, which is estimated to be ;0.5 mol N m22 yr21 or 3.3 mol C m22 yr21, under the assumption of a Redfield C : N ratio of 6.6 by atoms in phytoplankton (Jenkins and Goldman 1985; McGillicuddy et al. 1998; Siegel et al. 1999). Important questions then arise as to whether marine phytoplankton are capable of growing in a low-light environment at rates that would support the contemporary new production values. One of us has shown previously that a small seed population of large marine diatoms residing at the base of the 1 Corresponding author ([email protected]).