Nutrient dynamics and eutrophication patterns in a semi-arid wetland:: The effects of fluctuating hydrology

A three-year study was carried out in Las Tablas de Daimiel National Park, a Spanish semi-arid wetland, to describe eutrophication patterns and the responses of the main primary producers in contrasting inundation years. Total and dissolved nitrogen and phosphorus, phytoplankton chlorophyll-a (monthly) and emergent macrophyte cover (yearly) were measured together with water levels. Water balances were taken to assess phosphorus and nitrogen annual nutrient budgets. The wetland could be termed hypertrophic. Nutrients displayed distinct seasonality because nitrogen peaked both in winter and summer, whereas phosphorus did so in late spring and early summer. A reduction in both total nitrogen and phosphorus in humid years due to higher water levels was also observed. An important spatial heterogeneity in nutrient content occurred throughout the study, part of which could be ascribed to hydrological fluctuations since the input site showed higher mean values in rainy years. Mass balances suggested that sedimentation played a key role in nutrient dynamics, with macrophyte leaching and waterfowl additions being minor sources. Internal loading was much higher than external loading in this wetland, thereby being responsible for higher sedimentation rates. Phytoplankton chlorophyll-a displayed seasonality, with peaks in late summer, when macrophyte activity was decreasing. No relationship between either chlorophyll-a and nutrients or hydrological conditions could be ascertained for most sampling stations and years of study. However, the water level-dependent, emergent macrophyte sawgrass (Cladium mariscus) presented a statistically significant relationship with both nitrogen and phosphorus. Our results demonstrate that macrophytes must be taken into account in wetland eutrophication models. A conceptual model comparing the responses of primary producers to eutrophication in lakes and wetlands is outlined.