Somatic, respiratory, and photosynthetic responses of the seagrass Halodule wrightii to light reduction in Tampa Bay, Florida including a whole plant carbon budget

Treatment plots (1.5 m(2)) within a shallow, monospecific seagrass bed of Halodule wrightii (Aschers.) located off Mullet Key (Fort DeSoto Park), Pinellas County, Florida, were subjected to in situ light reductions of 43, 60, and 86% from September 1994 to early March 1995 and compared with control plots for changes in morphology and photosynthesis vs. irradiance (PE) response. Winter die-back caused a 50% reduction in biomass and shoot density in control plots between September and February/March. A further reduction in total and non-photosynthetic biomass of 50% compared to control occurred as a result of treatment effect regardless of the amount of light reduction. However, reduction in photosynthetic biomass was related to the amount of light reduction (r(2) = 0.89). There was an inverse relationship between light reduction and shoot number (r(2) = 0.98). Leaves on shoots within the 43% light reduction plots were longer than leaves on shoots in either control or other light reduction treatment plots after the second month of the experiment. This is probably a morphological response to light reduction, as reported for other species. Respiration and PE responses (alpha, P-MAX, I-C, and I-K) and leaf chlorophyll content in H. wrightii were variable and did not exhibit trends related to light reduction. The I-CPLANT values ranged from 30 to 100 mu Em(-2)s(-1), and averaged 39 to 62 mu Em(-2)s(-1). This corresponds to approximately 4.5-7% of surface irradiance (ST) in September and February/March, respectively, in Tampa Bay. Carbon budget calculations, using the H-SAT model and accounting for water column attenuation and shade in the treatments, indicate net areal productivity by H. wrightii in control plots was 0.25 g C m(-2)d(-1), and in 43% light reduction plots was 0.025 g C m(-2)d(-1), in September. Other light reduction treatments were at a carbon deficit in September, and all treatments and controls were at a carbon deficit in February/March. These deficits explain the loss of biomass as a result of winter die-back and with a > 60% reduction in light. The discrepancy between the morphological changes that occurred in H. wrightii as a result of a 43% light reduction and a calculated positive carbon budget suggest that other sources of light attenuation (epiphytes, surface scattering, etc.) must be accounted for, or it may reflect carbon budget precision or assumptions.