Long-term relationship between phosphorus inputs and wetland phosphorus concentrations in a northern everglades marsh

Assessments of long-term relationships between changes in nutrient inputs a nd wetland nutrient concentrations can be complicated by fluctuations in ot her environmental factors as well as by problems typical of long-term monit oring data. Consequently, statistical analysis of these types of data sets requires careful consideration of environmental covariates, potential biase s in the monitoring design, and irregularities caused by changes in field s ampling protocols. We evaluated the relationship between anthropogenic phos phorus (P) inputs and water-column total P (TP) concentrations in a norther n Everglades marsh by statistically analyzing available data collected from several sampling programs over the past 20 years (1978-1997). Canal inputs of agricultural runoff contribute most of the P to the marsh and have prod uced a zone of enrichment within the marsh during the past few decades. Reg ression analyses showed that both canal and marsh TP concentrations increas ed during the 1980s and then decreased in the 1990s. However, the statistic al relationship between canal P inputs and marsh TP, while significant, gen erally was weak except for marsh locations adjacent to the canal. Stronger relationships existed between marsh TP and hydrologic parameters such as ma rsh water depth, which is controlled by changes in weather patterns and mar sh management. In particular, dry conditions during the 1980s may have cont ributed to observed increases in marsh P concentrations and the movement of a P 'front' further into the marsh. Higher rainfall and water depths and a gricultural best management programs initiated during the 1990s have been a ssociated with reduced P concentrations in canal waters entering the marsh. While it is anticipated that this reduction eventually will result in lowe r marsh TP concentrations, this effect is not yet evident, possibly due to internal loading of P from enriched marsh soils. Our findings illustrate so me of the environmental factors that can complicate attempts to develop emp irical relationships between P inputs and wetland P concentrations and to u se such relationships to forecast changes in marsh concentrations based on past monitoring data alone.