Production ecology of Ruppia maritima L. s.l. and Halodule wrightii Aschers, in two subtropical estuaries

Abstract

Seasonal growth and production dynamics of Ruppia maritima L. s.l. were compared over a 3-year period in two south Texas estuaries characterized by different salinity and N regimes as a result of freshwater inputs. Measurements of shoot production in the Guadalupe Estuary (0–25‰ salinity) and the Nueces Estuary (32–38‰ salinity) revealed no major differences in the magnitude of growth, but the plant populations differed in the seasonality of growth, the time of flowering, and the persistence of an over-wintering population. During the period of rapid shoot development, from March to August, leaf elongation rates usually ranged from 2 to 4 mm·day −1 (0.04–0.08 mg dry wt·mg shoot −1·day −1), although peak growth rates of up to 8 mm·day −1 were also recorded. In both estuaries, R. maritima was a strict opportunist, yearly colonizing bare areas and completing its entire growth cycle in 4 months. Overwintering populations of R. maritima existed at the lower nutrient and higher salinity site in Nueces Estuary but not at the high nutrient, low salinity site in Guadalupe Estuary. Halodule wrightii Aschers, was absent from Guadalupe Estuary, but in the Nueces Estuary, H. wrightii maintained large and persistent overwintering populations characterized by sustained year-round growth and larger below-ground root and rhizome fractions of total biomass (50–85%) compared to R. maritima (20–70%). The high nutrient regimes associated with large inputs of freshwater in the Guadalupe Estuary appear to have little beneficial effect on the growth dynamics of R. maritima. Instead, significant reductions in underwater light may be most important for growth. Fouling by algal epiphytes, higher riverine inflow, and local physiographic differences in wave exposure appear to be the primary factors regulating light levels and thus the proliferation and year-round persistence of R. maritima.