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EFFECTS OF THE INVASIVE, NONINDIGENOUS SEAGRASS ZOSTERA JAPONICA ON NUTRIENT FLUXES BETWEEN THE WATER COLUMN AND BENTHOS IN A NE PACIFIC ESTUARY
Larned, S T. EFFECTS OF THE INVASIVE, NONINDIGENOUS SEAGRASS ZOSTERA JAPONICA ON NUTRIENT FLUXES BETWEEN THE WATER COLUMN AND BENTHOS IN A NE PACIFIC ESTUARY. MARINE ECOLOGY PROGRESS SERIES. Inter-Research, Luhe, Germany, 254:69-80, (2003).
Since its introduction in the early to mid-20th century, the Asian seagrass Zostera japonica has become established in marine and mesohaline portions of many estuaries in the Pacific Northwest. Z. japonica forms dense patches from 0.3-2.4m above mean lower low water, a zone that often lacks vegetation in uninvaded estuaries. Native seagrasses are major sources and sinks for dissolved nutrients, and the establishment of a nonindigenous seagrass in previously unvegetated habitat may alter nutrient source-sink patterns. I used benthic chambers to examine the effects of Z. japonica on dissolved nutrient fluxes between the water column and the benthos in Yaquina Bay, Oregon. Nutrient fluxes in Z. 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 in daylight in both seasons, and net sinks for NO3 at night in the warm season. On an estuary-wide basis, Z. japonica was conservatively estimated to remove 50-60 moles DIN h-1 and 0,2-2 moles DRP-1 from the water column. NO3 fluxes in Z. japonica habitats were positively related to above-ground seagrass biomass, but not to below-ground biomass, suggesting that Z. japonica leaves and epiphytes controlled NO3 influx. Porewater NO3 and NH4 concentrations in unvegetated sediments were twice as high as in Z. japonica habitats. Depleted porewater DIN in the Z. japonica root zone may have reduced DIN efflux. I also compared nutrient fluxes in abutting monospecific patches of Z. japonica and the native seagrass Z. marina. 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 higher biomass of Z. marina. The data reported here suggest that Z. japonica invasions can alter water column-benthos nutrient fluxes, and these alterations may in turn affect benthic and pelagic primary production.