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Submerged aquatic vegetation (SAV) is an important biological, chemical, and physical component of many spring-fed lotic ecosystems, influencing water column chemistry, stabilizing sediments, and providing habitat. Excessive levels of NOx-N in groundwater discharged from many springs, generally attributed to fertilizer application and wastewater, are often implicated in the loss of SAV and propagation of algae. Nitrogen enrichment is presumed to impact ecosystem integrity and primary producer community structure as algal proliferation results in macrophyte shading. However, the paradigm of nitrogen effects on SAV often takes precedence over other potential causes of SAV decline, and the factors exerting direct and indirect control on SAV growth in Florida springs are poorly understood. Potential controls include water column and pore water chemistry, sediment texture, flow velocity, human disturbance, algal abundance, and light regime. Long-term data quantifying the relationship between SAV productivity and environmental variables is inadequate, impeding the ability to recognize the predominant physical, chemical, and biological factors that drive ecosystem changes. An in situ assessment of SAV growth under a natural gradient of ambient conditions over an annual cycle will elucidate the role of nitrogen availability and a multitude of environmental conditions in SAV growth. Understanding the environmental controls on primary producer community structure and function is integral to effectively managing and restoring spring-fed river ecosystems. In particular, it remains unclear whether management efforts to reduce nitrogen concentrations are sufficient to restore SAV communities. To address this question, monthly dynamics of SAV shoot elongation and aboveground productivity are assessed in Silver River and Alexander Springs Creek, two spring-fed rivers in north-central Florida with similar physical attributes, but strongly contrasting nitrate concentrations. Growth attributes are evaluated within and between sites across natural gradients in environmental drivers. The study focuses on growth attributes of the two dominant SAV species, Sagittaria kurziana (tapegrass) and Vallisneria americana (eelgrass) using a modified leaf clipping technique, and tests the hypothesis that SAV growth is primarily controlled by physical attributes such as light, flow velocity, and sediment texture, rather than chemical and biological conditions. Preliminary length and width measurements of intact shoots reveal differences in SAV morphology and growth form. Maximum water velocity is positively correlated to shoot length-to-width ratio for both species, suggesting augmented energetic investment in longer and narrower leaf blades at high water velocity sites to reduce resistance and drag. Shoot length also increased with high canopy cover (low light), which reveals maximizing light capture under stressed conditions. Light stress causes SAV to redirect resources away from other vital processes (rosette production, total biomass, reproduction) in order to implement shade-adaptation mechanisms (canopy formation and leaf elongation). A decrease in growth rate is observed for both species with increasing distance from the headspring. Potential causal mechanisms include elevated downstream turbidity resulting in diminished light attenuation or growth inhibition due to toxicity or other undetermined factors, thus requiring further investigation. |