Strong site effects dictate nutrient patterns in a Mediterranean floodplain

Abstract

Nutrient patterns in wetlands are usually studied at a single sampling station and not for more than two or three hydrological years. This makes environmental assessment difficult, particularly when spatial heterogeneity and wetland responses depend upon water availability, as often occurs in Mediterranean wetlands. Furthermore, the spatial scale of environmental control of wetland dynamics is usually hypothesized to change as a result of water connectedness, so the higher the rate of water renewal, the larger the spatial scale of environmental control. To test this idea, we carried out a study of monthly nutrient loadings, organic carbon, total nitrogen, and phosphorus in a Spanish hypertrophic floodplain (Tablas de Daimiel National Park) for six years at three sites to ascertain the significance of both the spatial heterogeneity and the long-term responses of nutrient patterns to water availability. Our study demonstrated that strong site effects prevailed on nutrient chemistry in this wetland, irrespective of the degree of wetland connectivity, since there were very few significant correlations among different sites for a single nutrient and no significant correlations with loadings. Causes for this behavior might be that water inputs to this Mediterranean wetland were highly variable and leveling ecological effects were dampened by site processes. Spatially, an increasing gradient of total organic carbon and nitrogen occurred toward the outlet, but total phosphorus peaked in the middle of the wetland. Such a decoupling was also shown by PCA analysis where three different factors explained 69% of overall variability of nutrient patterns: flooding, both organic carbon and total nitrogen content, and total phosphorus. No temporal trend in nutrient content was observed. No simultaneous seasonal patterns were obvious in nutrient data from the same site or for the same nutrient at different sites. Periodicity analyses did not show overlapping of annual rhythms. That lack of patterns could be evidence of the major importance of site processes. This wetland behaved as a sink for nutrients, a fact also suggested by the increasing carbon and phosphorus content in sediments of all sites over time. Sediment nitrogen dynamics, however, might experience differential among-site processes resulting in no trend in accumulation in some areas. These results challenge current views on the spatial scale of environmental control on wetland performance and suggest that the role of water availability in ecological connectivity may be more complex than previously suspected.