Nutrient addition effects and the role of vegetation in California vernal pools

Increased nutrient deposition from excessive fertilizer application, manure production, and other industrial discharges into aquatic environments is altering the structure and function of freshwater ecosystems worldwide. Eutrophication is characterized by the input of excessive nutrients, which can cause a shift from a relatively clear, macrophyte-dominated condition (oligotrophic) to a turbid, algae-dominated condition at the highest nutrient levels (eutrophic). It has been extensively demonstrated to occur in continuously flooded aquatic systems, but is poorly understood in seasonal wetlands. This study investigated the effects of nutrient addition and the role of vegetation in the manifestation of these effects in California vernal pools – a highly threatened seasonal wetland ecosystem susceptible to eutrophication from nutrient-laden storm water runoff and excrement from grazing animals. Applying predictions observed in continuously aquatic systems and recent nutrient-addition experiments in vernal pools, it was hypothesized that the response of vernal pools to nutrient enrichment will be consistent with eutrophication observed in continuously aquatic systems (e.g., increased algae, turbidity, conductivity, decreased dissolved oxygen) and that the presence of vegetation will mitigate the effects of eutrophication, with beneficial effects to invertebrate populations. The experimental design consisted of five nutrient (phosphorous) treatments replicated three times and placed in a vegetation or vegetation-removal treatment. Turbidity, dissolved oxygen, algae concentration, invertebrate density, invertebrate species richness, pH, and conductivity were measured biweekly over six weeks. Treatment effects were determined using factorial multivariate analysis of variance with Fisher’s Least Significant Different post-hoc tests conducted for significant effects of nutrient addition treatments. Correlations between dependent variables were estimated using Spearman rank correlation coefficients. Vernal pool mesocosms did not exhibit a eutrophic response consistent with continuously aquatic systems when nutrients were added, but this may be attributable to the timing of the study. Nutrient addition did increase invertebrate species richness. The presence of vegetation reduced algal densities and thus lowered turbidity and also had positive effects on invertebrate species richness. This study demonstrated that the presence of vegetation mitigated some adverse effects of eutrophication and informed the ongoing debate regarding the benefit of grazing as a management tool in vernal pool ecosystems.