Photosynthetic characteristics of seagrasses (Cymodocea serrulata, Thalassia hemprichii and Zostera capricornia) in a low-light environment, with a comparison of leaf-marking and lacunal-gas measurements of productivity

Abstract

We describe the photosynthetic characteristics of three seagrasses and the relationship between their production and natural light intensities (photosynthesis-irradiance response, i.e. PI curves). Seagrass production (gross production minus shoot respiration) was measured in the field by the rate of gas release from the lacuna1 space of whole seagrass shoots and compared with net leaf production. Field work was carried out on the seagrasses Cymodocea serrulata (R. Br.) Aschers, and Magnus, Thalassia hemprichii (Ehrenb.) Aschers., and Zostera capricornia Aschers. in the turbid, warm waters of Cairns Harbour, Queensland, Australia.

The photosynthetic efficiencies (the initial slope of the PI curves) of all of the seagrass species were 10 times greater than any previously measured for the same species in higher-light environments. The high compensating light intensities (80-92 µE m^-2 s^-1) showed that the plants have high respiration rates that were probably due to the high water temperatures (29-33ºC) of the harbour. The seagrasses responded to small increases of light at low light intensities by rapidly reaching saturating light intensities, and the maximum rates of production were between 0.4 and 0.6 mg C h^-1 shoot^-1. The average period of exposure to saturating light intensity was 2 h day^-1. One-quarter of the gross production was lost to plant respiration. The net productivity and respiration of all three seagrasses was calculated from this photoperiod.

Net leaf production in situ compared well with the seagrass production estimates that were measured with the lacunal-gas technique. Most of the production appeared to be allocated to the above-ground tissue, a feature consistent with seagrasses growing in low-light and terrigenous sediments.