Sediment pool and plant content as indicators of nitrogen regimes in Portuguese estuaries

Abstract

Excess nutrient loading is a major environmental problem that greatly affects the community structure and ecosystem function of rivers and estuaries. Human activities have greatly intensified the delivery of nutrients to aquatic environments causing eutrophication in many European waters. To evaluate whether different nitrogen regimes affect the N contents of sediments and macrophytes, two Portuguese estuaries with different intensities of land use in their watersheds — Mondego and Mira — were studied. We measured N concentrations in water, plant aboveground biomass and leaf N and C concentrations in Spartina maritima, Halimione portulacoides, and Zostera noltii; N and C concentrations were also measured both in the bare sediment and in sediment directly beneath plant canopies. The different intensity of land use practices did not consistently result in differences among nitrogen-related variables in Mondego and Mira estuaries. Concentrations of DIN in water were rather high compared to most other estuaries elsewhere. There was no difference in nitrate concentrations in the water column, but ammonium and dissolved organic N concentrations were higher in Mondego than in Mira. Aboveground biomass of H. portulacoides and Z. noltii was greater in Mondego, but not that of S. maritima. %N in the sediments beneath S. maritima and H. portulacoides and in the leaves of H. portulacoides was larger in Mondego than in Mira, but these effects were not observed for other variables and were of small magnitude. The links between N in plants and in ambient water and sediment were also hard to interpret. % N in leaves was positively related to N content of sediments and to nitrate concentration in the water column, but was inversely related to ammonium concentration in water. We conclude that Mondego and Mira estuaries are quite well-enriched in nitrogen, and that it is not evident that they differ in the level of nitrogen enrichment. Because marsh plants in Mondego and Mira are likely N-saturated, it is unlikely to think that nitrogen concentrations or contents in water, plants, or sediment can well serve as sentinels for nitrogen enrichment, as internal pools and processes are either full or N-replete.