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 (k(d)) of 0.328 m(-1) was reached in January, and the water was dearest in September with a k(d) 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 k(d) value over the year was 0.158 m(-1) and the water was classified as Jerlov's water type 1. The euphotic depths (Z(eu)) 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 (I-k) varied significantly from 18.0 +/- 1.5 mu mol m(-2) s(-1) in autumn to 10.5 +/- 1.8 mu mol m(-2) s(-1) in early spring. Mean photosynthetic capacity (P-g max) ranged from 57.2 +/- 1.3 mu mol O-2 g(-1)DW h(-1) to 63.1 +/- 1.6 mu mol O-2 g(-1)DW h(-1). The photosynthetic efficiency (alpha) was 3.2 +/- 0.2 mu mol O-2 g(-1)DW h(-1) (mu mol m(-2) s(-1))(-1) in autumn and 6.0 +/- 1.0 mu mol O-2 g(-1)DW h(-1) (mu mol m(-2) s(-1))(-1) in early spring. Compensation irradiance (I-c) was low and ranged from 2.5 to 2.8 mu mol m(-2) s(-1). Estimates of annual net production rates were 128.1 and 0.6 mmol O-2 g(-1)DW 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). [KEYWORDS: annual production; irradiance; k(d); light penetration; macroalgae; photosynthesis; Rhodophyta King-george-island; long-term culture; south-shetland; sea ice; light; respiration; temperature; seasonality; phaeophyta; daylengths]
Original languageEnglish
JournalAntarctic Science
Journal publication date1996

ID: 313066