IN-SITU PHOTOSYNTHESIS IN THE SEAGRASS HALODULE-WRIGHTII IN A HYPERSALINE SUBTROPICAL LAGOON

Photosynthesis versus irradiance (P vs I) parameters in the shoalgrass Halodule wrightii Ascherson were calculated from measurements of oxygen evolution collected in situ, using entire plants in Laguna Madre, Texas, USA. Eleven experimental incubations were performed from May 1989 to April 1991 using pulsed oxygen electrodes that collected data continuously under natural daylight conditions. For comparison with in situ measurements of photosynthesis, P vs I parameters were calculated from laboratory measurements on blade segments incubated in a small volume chamber. For field plants, average saturation irradiance (I(k)) was 319 mumol m-2 s-1, photosynthetic capacity (P(max)) was 374 mumol O2 g-1 dry wt (dw) h-1, and relative quantum efficiency (alpha) generally ranged from 0.5 to 1.6 mumol O2 g-1 dw h-1 (mumol photons m-2 s-1)-1. Whole plant respiration was 70 mumol g-1 dw (leaf) h-1, and compensation irradiance (I(cp)) was ca 85 mumol m-2 s-1. Chlorophyll a concentrations averaged 12.8 mg g-1 dw, and the mean chl a: chl b ratio over the 2 yr period was 2.2. For blade segments incubated in the laboratory under similar temperatures to field plants and at corresponding chlorophyll concentrations, P(max) was not significantly different but alpha was significantly higher, ranging from 3.4 to 5.3 mumol O2 g-1 dw h-1 (mumol photons m-2 s-1)-1. The higher alpha values resulted in significantly lower estimates for I(k) (mean of 101 mumol m-2 s-1). The higher alpha in laboratory plants was largely related to the perpendicular orientation of leaf tissue to a directed light field, which is not reflective of natural conditions for H. wrightii. The significant difference in I(k) values calculated from incubations performed in the laboratory and field has profound effects on model calculations of H(sat), the duration of irradiance-saturated photosynthesis, and thus predictions of the minimum light requirements required to sustain growth in H. wrightii. Consequently, the application of laboratory-derived I(k) values for H. wrightii would result in overestimates of its maximum depth limits and rates of areal primary production.