Production ecology of Ruppia maritima L. s.l. and Halodule wrightii Aschers. and the biomass of associated macrophytes in two south Texas estuaries : final report to the Texas Water Development Board

Seasonal growth and production dynamics of Ruppia maritima L. s.l. were compared over a three year period in two south Texas estuaries that were characterized by different salinity and nitrogen regimes as a result of freshwater inputs. Continuous measurements of shoot production in the higher saline Nueces Estuary (32 - 38 %₀) and the lower saline Guadalupe Estuary (0 - 25 %₀) revealed no major differences in the magnitude of growth, but the plant populations differed significantly in the seasonality of growth, the time of flowering and the persistance of an overwintering population. During the period of rapid shoot development, from March to August, growth rates usually ranged from 2-4 mm day⁻¹ (0.04 to 0.08 mg dry wt mg shoot⁻¹ day⁻¹), although peak growth rates of up to 8 mm day⁻¹ were also recorded. At the high nutrient site in Guadalupe Estuary, a combination of heavy fouling by epiphytic macroalgae and wave exposure prevented the establishment of an overwintering population compared to the less exposed higher saline site in Nueces Estuary. As a result, R maritima was a strict opportunist, with a monocarpic reproductive pattern, yearly colonizing bare areas and completing its entire growth cycle in four months. Halodule wrightii Aschers. was absent from Guadalupe Estuary, but simultaneous measurements of growth in H wrightii in the Nueces Estuary revealed a nearly opposite strategy which included the presence of large overwintering populations, with a significant proportion of the plant's total biomass contributed by its roots and rhizomes. For R maritima these results indicate that large changes in salinity and nutrient regimes, as caused by increased freshwater inflow, have little direct effect on the growth of this euryhaline species, but could, under extreme conditions, produce an environment that prohibits its establishment as a result of fouling by epiphytic algae.