Photosynthetic Production and Biomass of the Subtropical Seagrass Halodule wrightii along an Estuarine Gradient

Abstract

Seasonal patterns of aboveground and belowground biomass, leaf chlorophyll (chl) content, and in situ differences in photosynthetic parameters were examined in the shoal grass Halodule wrightii along an estuarine gradient in the western Gulf of Mexico. Continuous measurements of biomass were collected over a 5-yr period (1989–1994) with respect to several abiotic factors in three estuarine systems that were characterized by significant differences in salinity and ambient dissolved inorganic nitrogen (DIN; NO2−+NO3−) regimes that ranged from 5–25‰ (0–80 μM DIN) in the Guadalupe estuary to 35–55‰ (0–9 μM DIN) in the upper Laguna Madre, Photosynthesis versus irradiance (P vs. I) parameters, measured from December 1989 to April 1991, showed no significant differences among the three sites, and there were no significant differences in leaf chlorophyll content and chl a:b ratios among sites over the entire 5-yr period. Saturation irradiance in Halodule wrightii is estimated at 319 μmoles photons m−2 s−1 based on measurements collected at the three sites over a 2-yr period. No strong seasonal variations were observed in total plant biomass, but root:shoot ratios (RSR) showed a clear pattern of maximum RSR values in winter and minimum values in summer. There were no significant differences in RSR among sites, and no consistent correlations could be established between plant parameters and sediment porewater NH4+, salinity, or temperature. Sediment porewater NH4+ values generally ranged from 50 μM to 400 μM (average 130–150 μM) but could not be correlated with significant differences in sediment composition between the sites. The high productivity of Halodule wrightii under a variety of light, nutrient, and salinity conditions explains its ubiquitous distribution and opportunistic strategy as a colonizing species. However, the persistence of a dense algal bloom in Laguna Madre coincident with low DIN levels (<5 μM) contradicts previously accepted relationships on nutrient stimulation of algal growth, and provides strong evidence that water quality parameters for estuarine seagrasses are decidedly estuarine-specific. Consequently, a knowledge of the long-term history of estuarine systems is critical to habitat managers, who are required to establish minimum water quality criteria for the protection of submerged aquatic vegetation in estuarine systems. *** DIRECT SUPPORT *** A01BY074 00028