Print PageAlternative stable states in shallow lakes and ponds are commonly observed in response to eutrophication, one of the greatest threats to aquatic ecosystems worldwide. Models of alternative stable states use turbidity as a key variable in the ecosystem shift, and turbidity is controlled by a combination of nutrient levels and species composition. In general, increases in nutrients and the absence of vegetation are understood to increase turbidity. However, recent studies have also identified bioturbation as another cause. It is not well known whether each of these mechanisms of turbidity (nutrients, vegetation, and bioturbation) have similar effects in ecosystems. While many studies document eutrophication in shallow lakes and ponds, gaps exist in understanding how temporary ponds (vernal pools) respond to these changes in turbidity. California vernal pools are seasonal wetlands, which support an aquatic and terrestrial community during an annual cycle of inundation and desiccation. We conducted a series of nutrient addition and species removal (vegetation and zooplankton) experiments in mesocosms to test whether the origins of turbidity have different effects on water physicochemistry and community patterns. Mesocosms were lined with vernal pool soil and variables during the aquatic (physicochemistry, biotia) and terrestrial (plant composition) phases were measured in response to treatments.
Results/Conclusions
Vernal pool responses to turbidity were highly variable, depending on its origin (nutrient addition, vegetation removal, or bioturbation). Nutrient addition and vegetation removal both increased turbidity directly related to algal densities. However, bioturbation by tadpole shrimp (Lepidurus packardi) produced increased turbidity through sediment disturbance unrelated to algal densities. Further, water chemistry variables responded differently in each of the turbidity origins. Consistent changes occurred with macrophytes and zooplankton as they covaried with turbidity. Macrophyte cover and richness decreased with turbidity because of light occlusion. In contrast, zooplankton density and richness increased as turbidity increased in response to increased nutrients, vegetation removal, and bioturbation. Zooplankton responses were likely the result of several mechanisms, including resource availability and increased cyst exposure. California vernal pools are the focus of much conservation because the habitat is greatly reduced and support high levels of endemism. The distinction among these effects will greatly enhance our ability to effectively manage this ecosystem. Further, approaches to alternative stable states in shallow lakes and ponds should consider the sources of turbidity since this can have variable effects in the ecosystem.
