| Abstract: | Phytoplankton growth rate in response to irradiance can be approximated by a hyperbola defined by three coefficients: i) initial slope (α); ii) asymptote (μm); and, iii) X-axis intercept or compensation irradiance (Ic). To mathematically represent the interaction of temperature and irradiance on growth rate, one must describe the relationship between these constants and temperature. The marine diatom, Skeletonema costatum (Greville) Cleve, was grown in unialgal culture at different levels of irradiance and 2-3 photoperiods at 0, 5, 10, 16 and 22 C. The value of Ic is ca. 1.0 ly·day?1 or less at all temperatures. The initial slope (div·ly?1) is a “u-shaped” function of temperature described by the second degree polynomial, α= 0.25–0.02T+0.001T2. Within the range 0–10 C, μm (div·day?1) is an exponential function of temperature described by the equation, μm= 0.48 exp (0.126T). At each temperature and selected levels of irradiance, cell size and cellular content of C, N and chl a were determined. The C:chl a and N:chl a ratios increased with irradiance because of increases in C and decreases in chl a. At lower temperatures (0, 5, 10 C), the rate of increase of both ratios with irradiance was greater than at the higher temperatures (16, 22 C). Cellular content of N was independent of irradiance and temperature, and the C:N ratio ranged from 5 to 8 with a slight tendency to lower values at low irradiance. Cell volume was not influenced by either temperature or irradiance. |
