Organic matter sources through annual and decadal timescales in a polymictic reservoir.

Phytoplankton productivity and allochthonous organic matter loading fluctuate through various timescales in response to changing environmental conditions. Organic matter compositional shifts can influence ecosystem structure and function by altering energy and nutrient flow pathways and turnover times. Organic matter carbon and nitrogen elemental and isotopic ratios were analyzed from sediment traps and a sediment core to determine organic matter provenance through annual and decadal timescales and to describe sediment transport mechanisms influencing organic matter delivery. Carbon to nitrogen ratios (C:N) indicated that allochthonous sources contributed much organic matter both annually and throughout the reservoir’s life. Annually, C:N and carbon isotope ratios (δ¹³C) suggested a shift from primarily allochthonous organic matter in winter towards primarily autochthonous organic matter thereafter. Nitrogen isotope ratios (δ¹⁵N) indirectly recorded this seasonal organic matter source shift through nitrate utilization degree by phytoplankton. Decadally, C:N, δ¹³C, and δ¹⁵N identified a shift from relatively large allochthonous organic matter contributions early towards increasing autochthonous organic matter contributions as the reservoir aged. Carbon, nitrogen, and phosphorus sediment concentrations recorded increasing phytoplankton productivity through time concurrent with phosphorus enrichment. δ¹³C and δ¹⁵N also suggested enhanced phytoplankton productivity with reservoir age. Urban growth and dairy operation intensification were possible sources of elevated external phosphorus loading leading to eutrophication. Phytoplankton productivity – δ¹³C relationships opposed those advanced from stratified natural lakes possibly resulting from continuous water column mixing. However, variable isotopic compositions of dissolved inorganic nutrients may have influenced phytoplankton isotopic composition. Isotope ratio mixing models suggested that river plume sedimentation, sediment resuspension, and horizontal advection influenced organic matter delivery with individual mechanisms being more important seasonally. Linear regression models identified river discharge and wind-induced mixing as dominate factors influencing secondary sediment transport in the riverine and lacustrine regions respectively. Wind-induced mixing entrained deep-water advective river sediments into the photic zone rather than resuspending surface sediments. These findings suggest that allochthonous sources contribute much potential energy and nutrients annually and decadally in this reservoir and secondary sediment transport mechanisms influence organic matter delivery and potentially bioavailability.