Bioassay of the growth potential of the surface water of lower Narragansett Bay over an annual cycle using the diatom Thalassiosira pseudonana (oceanic clone, 13‐1 )1

The surface waters of unpolluted lower Narragansett Bay were bioassayed over an annual cycle ( n = 36) with Thalassiosira pseudonana (13‐1) at 16 different nutrient enrichments replicated three times. Omission of nitrogen usually markedly inhibited growth. This effect was followed in importance by adverse “water quality” properties associated with phytoplankton growth, which could be overcome by addition of the chelator EDTA in the presence of nitrogen and other elements. Silicon omission also limited growth. Limitation by omission of other nutrients was less pronounced. The following findings challenge the traditional view that phytoplankton growth is regulated by a specific limiting nutrient: seasonal variations in growth or yield limitation by a given nutrient occurred; the intensity of limitation by nutrients varied seasonally; several nutrients were usually limiting simultaneously, and with a different intensity; the combinations of limiting nutrients varied seasonally; a seasonal succession in type of nutrient limitation occurred. The results suggest that chemical changes in situ important to the occurrence, growth, and succession of phytoplankton species would not be detected by the routinely used nutrient and biological analyses.