R. Sánchez-Andrés

Freshwater wetland eutrophication

The traditional perception of wetlands as nutrient sinks has led them to be used as wastewater disposal areas for a long time, resulting in a severe alteration of the structure and function by eutrophication. Nutrient loading is usually linked to hydrological alterations which encompass shifts in vegetation patterns and nutrient cycling. The eutrophication process in wetlands accelerates primary productivity and increases net accumulation of organic matter and nutrients but also enhances organic matter decomposition, microbial activity, and soluble nutrients in sediments. Internal loading becomes the main nutrient source to the wetland, even in the years of low external inputs, controlling the nutrient dynamics. Since soil phosphorus microbial biomass responds positively to phosphorus enrichment in wetlands, mineralized phosphorus in wetland soils appears as the most responsive microbial indicator to nutrient enrichment in wetlands. Therefore, phosphorus internal loading is the critical factor in regulating eutrophication status of wetlands. N2O and N2 emissions by wetlands can be enhanced in the future as nitrate availability in wetlands continues to be high due to increased pollution. The biological symptoms of wetlands degraded by eutrophication differ little from those observed in shallow lakes, these changes being consistent with predictions made by alternative state theory. The turbid state shows phytoplankton dominance and elevated biomass of planktivorous and benthivorous fish. Zooplanktivorous fish contribute to eutrophication chiefly via food web-mediated effects. Benthivorous fish increase the nutrient availability to phytoplankton chiefly by transferring sediment-bound nutrients to the water column during bottom foraging. Little of the methodology found to be useful in shallow lake restoration has been applied to wetland management. Whereas catchment nutrient management programs may be insufficient because of substantial storage of nutrients in wetland sediments, recent studies indicate that biomanipulation of fish standing stocks could contribute significantly to wetland eutrophication abatement. Therefore, biomanipulation, when appropriately timed in accordance with low water levels, combined with sediment dredging and other interventions, may be a low cost–high benefit tool for wetland eutrophication abatement.

Biogeochemical Indicators of Nutrient Enrichments in Wetlands: The Microbial Response as a Sensitive Indicator of Wetland Eutrophication

In wetlands it is still usual to use the same indicators of eutrophication which were developed to study the effects of nutrient enrichment in lakes; however, since hydroecology and biogeochemistry of wetlands is significantly different from lakes, monitoring of these indicators does not allow a good diagnosis of the changes undergone by the wetland ecosystem under nutrient enrichment scenarios. Microbial activities and their respective community responses have been considered as a measure of ecosystem stability and an indicator of ecosystem perturbation through changes on functional properties associated with nutrient cycling. As in most aquatic ecosystems, the addition of a limiting nutrient to wetland ecosystems promotes primary productivity and stimulates microbial processes. As nutrient loading increase, biogeochemical processes in wetlands are altered, changing their concentrations in water and soil, and therefore, nutrient fluxes and cycling. Nutrient enrichment induces changes in soil physicochemical and microbiological characteristics that may then serve as indicators of nutrient enrichment. In this review, a set of microbial community measurements known to be sensitive to nutrient enrichment in aquatic systems, such as extracellular enzyme activities, respiratory activities, microbial biomass C, N, and P, and microbially mediated N and P turnover rates have been used to characterize physiological response of the microbial community to wetland eutrophication. Some indicators as metabolic efficiency and phosphatase activity clearly reflect the main shifts on wetland ecosystem processes induced by nutrient enrichment and may be considered better than those that are currently used to assess the effects of eutrophication. Moreover, the combined use of different ecophysiological measurements such as extracellular enzymatic ratios and microbial biomass under resource allocation models and ecological stoichiometry demonstrates that ecophysiological measures are sensitive indicators of wetland eutrophication. Further studies are needed refining this approach to get the complex biogeochemical variability of the different wetland types, and to move from a site-based heuristic model to a holistic approach, describing eutrophication patterns in wetlands.