MEPS 254:69-80 (2003)  -  doi:10.3354/meps254069

ABSTRACT: The effects of Z. japonica on dissolved nutrient fluxes between the water column and the benthos in Yaquina Bay, Oregon, USA were examined. Nutrient fluxes in Zostera japonica habitats were compared to those in adjacent unvegetated sediment in warm and cool seasons, and in daylight and darkness. In daylight, Z. japonica habitats were net sinks for nitrate (NO₃), ammonium (NH₄) and dissolved reactive phosphorus (DRP) in the warm season, and for NO₃ and DRP in the cool season. At night in the warm season, Z. japonica habitats were net sinks for NO₃ and NH₄. Unvegetated sediments were net sources of NH₄ to the water column in daylight in both seasons, and net sinks for NO₃ at night in the warm season. On a diel basis, Z. japonica habitats habitats were net sinks for NO₃ and NH₄, and unvegetated sediments were net sources of NH₄. Porewater NO₃ and NH₄ concentrations in unvegetated sediments were twice those in Z. japonica habitats. Nutrient fluxes in abutting monospecific patches of Z. japonica and the native seagrass Z. marina were also compared. Both species were sinks for NO₃, NH₄ and DRP. NO₃ influx rates per unit above-ground biomass were higher in Z. japonica habitats, but areal rates of NO₃ influx were higher in Z. marina habitats, reflecting the latterŒs higher biomass. The data demonstrate that Z. japonica invasions alter water column-benthos nutrient fluxes. These alterations may in turn affect pelagic primary production. At current biomass levels, Z. japonica is estimated to remove 50 to 60 mole dissolved inorganic nitrogen (DIN) h^-1 and 0.2 to 2.2 mole DRP h^-1 from the Yaquina Bay water column, and continued expansion of Z. japonica in the estuary could lead to substantial reductions in nutrient availability.

KEY WORDS: Estuary · Invasion · Nonindigenous species · Nutrient flux · NE Pacific · Porewater · Seagrass · Sediment · Zostera japonica