Effects of naturally senescing aquatic macrophytes on nutrient chemistry and chlorophyll a of surrounding waters1

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Abstract
Field experiments were performed to demonstrate the effects of senescing stands of Myriophyllum spicatum (Eurasian water milfoil) on water chemistry and chlorophyll a in a large soft‐water Indiana reservoir. Replicate enclosures were used to isolate water columns over monospecific stands of M. spicatum until annual dieback began. Systems were sampled routinely for 119 days before, during, and after fall dieback. Decomposing macrophytes supplied significant inputs of nitrogen and phosphorus to surrounding waters. Nitrogen inputs were sharply pulsed and never accounted for >2.2% of annual allochthonous inputs. Phosphorus inputs from senescing plants equaled up to 18% of the annual (1978) total phosphorus loading for the reservoir. Annual dieback occurred in late summer and early autumn when conditions were suitable for phytoplankton response. Phytoplankton biomass (Chl a) showed significant increases in response to senescing macrophytes.

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