MARSH VERTICAL ACCRETION IN A SOUTHERN CALIFORNIA ESTUARY, USA

Vertical accretion was measured between October 1992 and March 1994 in low and high saltmarsh zones in the north arm of Tijuana estuary from feldspar market horizons and soil corings. Accretion in the Spartina foliosa low marsh (2-8.5 cm) was related almost entirely to episodic s torm-induced river flows between January and March 1993, with daily ti dal flooding contributing little or no sediment during the subsequent 12-month period of no river flow. Accretion in the Salicornia subtermi nalis high marsh was low (similar to 1-2 mm) throughout the 17-month m easuring period. High water levels in the salt marsh associated with t he storm flows were enhanced in early January 1993 by the monthly extr eme high sea level, when the low and high marshes were flooded about 0 .5 m above normal high tide levels. Storm flows in January-March 1993 mobilized about 5 million tonnes of sediment, of which the low salt ma rsh trapped an estimated 31 941 tonnes, including 971 tonnes of carbon and 77 tonnes of nitrogen. Sediment trapping by the salt marsh during episodic winter floods plays an important role in the long-term maint enance of productivity of Tijuana estuary through nutrient retention a nd maintenance of marsh surface elevation. The potential exists, howev er, for predicted accelerated rates of sea-level rise to out-pace mars h surface elevation gain during extended periods of drought (i.e. low sediment inputs) which are not uncommon for this arid region.