Eutrophication of freshwater is an increasing problem globally as human populations, animal husbandry and agriculture intensify, and is associated with many deleterious consequences, including algal blooms, invasion of opportunistic species and, ultimately, changes in ecological functioning. These issues within Lake Winnipeg, MB, Canada, have led to research to develop bioindicators of nutrients within streams in the Red River, the largest nutrient contributing river to the lake. The research presented herein examines the use of the aquatic plant Sagittaria cuneata (Sheldon) as a bioindicator of phosphorus (P) and nitrogen (N). S. cuneata is an abundant species in rivers and streams in Manitoba, and has plastic emergent-leaf morphology and easily identifiable features. Plasticity within the genus has been linked to several factors, such as P concentration and water depth; therefore, S. cuneata was proposed as a potential bioindicator. Two studies were conducted to assess the relationship of leaf morphology with N and P: a controlled experiment where nutrients were added to either sediment or water, and a field study wherein S. cuneata leaves were collected in streams with varying water and sediment chemistry. Experimental results showed that plants used sediment nutrients to increase biomass, and that leaf morphology varied more during the growing season in sediment-enriched compared to water-enriched plants, which showed little variability of leaf morphology. Field studies indicated leaf morphology was variable at all scales of replication, and many variables contributed to leaf size, including sediment and water chemistry. Together, these results indicated that leaf morphology was affected by nutrient availability, but also complex interactions among other environmental variables. Thus, S. cuneata is not currently a reliable indicator of nutrient and future studies should further pursue the variability of leaf morphology.
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