For the first time, photosynthesis of Myriogramme mangini (Gain) Skottsberg, one of the dominant red macroalgae in the sublittoral of Signy Island (South Orkney Islands), was studied in situ under natural irradiance levels in specially developed incubation chambers. M. mangini is adapted to low irradiance levels. Water transparency varied over the year. A maximum attenuation coefficient (kd) of 0.328 m−1 was reached in January, and the water was clearest in September with a kd value of 0.079 m−1. Classification of the water type on a Jerlov scale, gave water type 4 in January and II in May and June. The mean kd value over the year was 0.158 m−1 and the water was classified as Jerlov's water type 1. The euphotic depths (Zeu) for 1%, 0.1% and 0.01% surface irradiance levels were 29.1 m, 43.7 m and 58.3 m, respectively. Photosynthetic characteristics were determined, with the oxygen production rates and irradiance levels measured in situ, using P-I curves. The initial saturation irradiance (Ik) varied significantly from 18.0 ± 1.5 μmol m−2 s−1 in autumn to 10.5 ± 1.8 μmol m−2 s−1 in early spring. Mean photosynthetic capacity (Pg max) ranged from 57.2 ± 1.3 μmol O2 g−1DW h−1 to 63.1 ± 1.6 μmol O2 g−1DW h−1. The photosynthetic efficiency (α) was 3.2 ± 0.2 μmol O2 g−1DW h−1 (μmol m−2 s−1)−1 in autumn and 6.0 ± 1.0 μmol O2 g−1DW h−1 (μmol m−2 s−1)−1 in early spring. Compensation irradiance (Ic) was low and ranged from 2.5 to 2.8 μmol m−2 s−1. Estimates of annual net production rates were 128.1 and 0.6 mmol O2 g−1DW y−1 at 5 m depth and 20 m depth, respectively. A maximum depth of occurrence of M. mangini was predicted at 22.9 m (2.7% of the surface irradiance).

The relationships between leaf structure, nitrogen concentration and CO2 assimilation rate (A) were studied for 14 grass species grown in the laboratory under non-limiting nutrient conditions. Structural features included leaf thickness and density, and the proportion of leaf volume occupied by different types of tissue (mesophyll, epidermis, vessels and sclerenchyma). Relationships were assessed for data expressed per unit leaf area and fresh mass. The latter was found to be closely related to leaf volume, which allowed us to use A per unit leaf fresh mass (Afm) as a surrogate of A per unit leaf volume. Assimilation rate per unit leaf area (Aa) was positively correlated with leaf thickness and with the amount of mesophyll per unit leaf area; the relationship with leaf nitrogen content per unit area was only marginally significant. Afm was negatively correlated with leaf thickness and positively with fresh mass-based leaf organic nitrogen concentration. A multiple regression involving these two variables explained 81% of the variance in Afm. The value of Afm was also significantly related to the proportion of mesophyll in the leaf volume, but surprisingly the correlation was negative. This was because thin leaves with high Afm and nitrogen concentration had proportionally more mechanically supportive tissues than thick ones; as a consequence, they also had a lower proportion of mesophyll. These data suggest that, in addition to leaf nitrogen, leaf thickness has a strong impact on CO2 assimilation rate for the grass species studied.

Suppose that A1, A2, · ··, An are disjoint subarcs of a rectifiable arc A ⊆ Rk. The location and length of each subarc are assumed to be continuous random variables. An expression for the expected fraction of A covered by subarcs each of length greater than τ ≧ 0 is obtained and applied to two conceptually distinct problems associated with light penetration of an absorbing medium.