SOURCES, ACCUMULATION AND TIDAL EXCHANGE OF NITROGEN, PHOSPHORUS AND ORGANIC CARBON IN TRANSPLANTED ESTUARINE MARSHES (BIOGEOCHEMISTRY)

The contribution of transplanted marshes to estuarine biogeochemical cycles was investigated by comparing sources and pools of organic matter in natural and transplanted marshes and by measuring nutrient exchange between transplanted marshes and estuarine waters.; Accumulation of N, P and organic C in natural and transplanted marsh soils (top 30 cm) was related to marsh age and hydroperiod. The young (1-15 years old) transplanted marshes contained the smallest soil nutrient pools while the irregularly flooded natural marshes had the largest amounts of N, P and organic C. Rates of organic C accumulation (185-190 kmol ha('-1) yr('-1)) were similar in a natural and a transplanted marsh at one location and were equivalent to 31-32% of the total net primary production at that particular site.; Transplanted marshes were sources of organic C for estuarine waters and sinks for total N and P. Based on weighted nutrient concentrations, the two transplanted marshes exported dissolved organic C and N and imported particulate N, ammonium, dissolved organic P and phosphate.; The stable isotope composition of natural and transplanted marsh soils were similar and suggested that emergent vegetation was the principal source of soil organic matter. The (delta)(' 15)N of marsh soils fell within the range of emergent macrophytes (+1 to +4('o)/oo) while soil d ('13)C ranged from -18 to -26('o)/oo. The (delta) ('13)C (-25('o)/oo) and (delta)(' 15)N (+5('o)/oo) of estuarine detritus indicated mixing between terrestrial material and phytoplankton.; Transplanted marshes are young communities which lack the large soil organic matter reservoirs and nutrient recycling capabilities characteristic of many natural marshes. While the high productivity of natural marshes is sustained in part by nutrient recycling between the vegetation and soil, transplanted marshes must rely on external sources of N and P (ammonium and phosphate from estuarine waters) during establishment. As transplanted marshes age, biogeochemical attributes such as organic matter accumulation and nutrient transformation will likely become comparable to natural marshes.