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Continuous year-round measurements of photosynthetically active radiation (PAR) were collected in relation to leaf elongation and plant biomass in the shoal-grass, Halodule wrightii Aschers., within three different estuarine systems on the south Texas coast (Laguna Madre: May 1989 to September 1993; Corpus Christi Bay: February 1990 to September 1993; San Antonio Bay; May 1990 to April 1991). Large differences in water transparency at all three sites masked seasonal variations in surface insolation as reflected in average diffuse attenuation coefficient ( k) values ranging from 0.7 to 2.9 and differences in the maximum depth penetration of H. wrightii, which varied from 0.6 to about 1.3 m. The continuous presence of a chrysophyte (brown tide) algal bloom in Laguna Madre since 1990 led to significant decreases in spring leaf elongation rates and a nearly 50% decline in below-ground biomass, which was reflected in root:shoot ratio (RSR) values that declined from 5.4 in 1989 to 2.3 in 1992. Increased turbidity and lower light levels in San Antonio Bay also corresponded with diminished plant biomass and the subsequent loss of plants; at both locations, the annual quantum flux ranged from 2200 to 2400 mol myr, or about 18% of surface irradiance (SI). In contrast, H. wrightii populations growing at ca.1.2 m depths and characterized by high RSR values (≥4.0) were exposed to 5100 to 5700 mol myr, or about 41 to 46% SI. Under these conditions, plants were exposed to daily saturating levels of PAR ( H) of 3 to 8 h during the spring/summer period of maximum growth, compared to an average of 2 h in Laguna Madre (after 1990) and San Antonio Bay based on field-derived measurements of photosynthetic parameters. Leaf elongation in H. wrightii exhibited a clear circannual rhythm at all sites, regardless of underwater light levels and therefore was not a sensitive indicator of light stress. Instead, chronic long-term reductions in underwater PAR were most strongly reflected in total plant biomass. The higher light demand (18% SI) for H. wrightii in relation to many other seagrasses (11% SI; Duarte 1991) may be related to its higher photosynthetic light requirement, but may also reflect the different methods used to evaluate the minimum light requirements of seagrasses. In estuarine and coastal waters, which are characterized by large and unpredictable variations in water transparency, continuous measurements of in situ PAR are invaluable in assessing the growth and photosynthetic response of seagrasses to variations in underwater irradiance. [ABSTRACT FROM AUTHOR]