Abstract
Seagrasses are ecosystem engineers-providing nursery ground and refuge for fish and invertebrates, sediment stabilization, and regulate the nitrogen (N) cycle. Seagrasses worldwide are in decline, bringing about a shift in N dynamics in shallow coastal systems (Waycott et al. 2009). The decline of seagrasses is driven largely by the chronic increase in anthropogenic N (Nixon 1995), which triggers a state change where shallow coastal systems become dominated by epiphytes (Bulthuis & Woelkerling 1983) and algae (McGlathery 2001). This study used a land use-based N loading model to calculate the inputs of N to the Virginia coastal bays-a shallow coastal system characterized by a low human population density, low water nutrient concentrations, and the most successful seagrass (Zostera marina [L.]) restoration in the world. In addition, this study measured N fluxes across a restoration chronosequence to quantify the effects of restored seagrass on the N cycle. Using the N loading model, we determined that agricultural fertilizer was the dominant terrestrial N source, though deposition of rainfall to the surface of the bays was the largest overall source of N. On average, Virginia coastal bays received 7.2 x 10 4 kg N y -1 or 2.11 g N m -2 y -1 , markedly lower than most other shallow coastal waterways in temperate systems (Boynton et al. 1995, McGlathery 2007). Nitrogen fixation (N 2 fixation) rates were significantly higher in vegetated sediments-compared to bare sediment-and decreased with depth. Furthermore, the older seagrass meadow (8 years old; seeded in 2001) fixed significantly more N 2 than the younger meadow (3 years old; seeded in 2006) and bare sediment. N 2 fixation rates in the older meadow and bare sediment were comparable to other Z. marina and bare sediment systems, respectively. Denitrification (N 2 loss), however, was the dominant process. Denitrification rates increased as seagrass meadows aged, and were correlated to increases in sediment organic matter. The N removal capacity of bare sediment was half of the total N loading rate into the bays (1 g N m -2 y -1 ), and vegetated sediments removed 3.9–5.8 g N m -2 y -1 , supporting the concept of seagrass beds as a "nutrient sponge".
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