Abundance and Seasonality of Meiofauna, Including Harpacticoid Copepod Species, Associated with Stems of the Salt-Marsh Cord Grass, Spartina alterniflora

Abstract

Ten Spartina alterniflora plants were sampled monthly in a Louisiana estuary to determine the abundance and species composition of stem-dwelling meiofauna and small macrofauna. Most organisms were associated with epiphytic algae found relatively high on standing stems; one harpacticoid copepod, Leptocaris brevicornis, was associated with vascular tissue. Only 15% of the stem fauna was found within 6 cm of the sediment surface. Highest abundance of total meiofauna (>8,000 individuals per 100 cm2 stem surface) occurred in July. The overall seasonal average was 1,563 individuals per 100 cm2 (about 800 per stem). Nematodes (24% of the total) and harpacticoids (adults and copepodites 19%, and nauplii 15%), were abundant and omnipresent taxa. Rotifers (30%) were limited to the summer months, but were extremely abundant when present, 5,037 individuals per 100 cm2 in July. Mites were common (10%) while several groups, for example, amphipods, isopods, polychaetes, and insect larvae, were rare. The stem harpacticoid assemblage was not diverse; four sediment-dwelling and three species reported only from stems were recorded. Overall, abundance was low from December to May, and high from June through November. Winter and spring minima may have resulted from several factors. Estuarine water levels in the Gulf of Mexico are lowered by as much as 25 cm in the winter, and stems likely were desiccated. Highest rhizomatic growth occurs in the spring, and the resulting reduced epiphyte populations may have influenced meiofauna. The density of stem meiofauna above the sediment surface averaged about 225 individuals per 10 cm2 sediment surface, but frequently exceeded that in the surrounding sediments.