THE EFFECT OF DAYLENGTH AND LIGHT INTENSITY ON THE GROWTH RATE OF THE MARINE DIATOM DETONULA CONFERVACEA (CLEVE) GRAN1

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Abstract
SUMMARY: The influence of 40 combinations of temperature (2, 7 C), light intensity (50, 200, 600, 1200, 1800 ft‐c), and photoperiod (24, 15:9, 12:12, and 8:16 LD) at 30% salinity on the rate of cell division of the Narragansett Bay clone of Detonula confervacea (Det‐1) was examined following appropriate preconditioning. At 2 C Detonula is a long day species (24 L) and prefers low light intensities (200–600 ft‐c); poorest growth occurred at 12:12 and 8:16 LD, and the compensation intensity was about 10 ft‐c. Increasing temperature to 7 C increased the mean growth rate, reduced the optimal daylength (15:9 LD), even though Detonula remained a long day species and increased the optimal light intensity (600–800 ft‐c). The compensation intensity varied with daylength and ranged from about 10–50 ft‐c. Photoperiods of 12:12 and 8:16 LD were least favorable for growth at both temperatures; light limitation and inhibition were observed at 50 and 1800 ft‐c. respectively; inhibition was less pronounced at 7 C. There is some indication that the conditions of growth that the stock cultures were exposed to prior to preconditioning for use in the experiments may have sometimes influenced response. Detonula produced resting spores without nutrient depletion at 2 and 7 C at all light intensities when the photoperiod was lengthened. Auxospore formation was also observed.Although short daylengths (9:15 LD) limit Detonula's growth during the early stages of the winter bloom, it competes successfully against Skeletonema costatum initially. This results from its higher rates of growth and of photosynthesis at the prevailing temperature and light conditions and a lower compensation intensity than reported for Skeletonema. The main causes of Detonula's growth inception and termination in Narragansett Bay differ.