Changes in submerged macrophyte abundance altered diatom and chironomid assemblages in a shallow lake

Submerged macrophyte abundance strongly influences aquatic ecosystems. Because of a lack of monitoring data, however, the long-term dynamics of such aquatic plants are poorly understood. Increasingly, paleolimnologists use changes in subfossil algae and invertebrates to infer past submerged macrophyte dynamics and assess how human activities have altered this important primary producer component of aquatic ecosystems. We evaluated the sensitivity of subfossil diatom and chironomid assemblages to historically documented changes in macrophyte abundance in Chenango Lake, New York, USA, where macrophyte cover has been monitored since 1978. We also tested the ability of a semi-quantitative diatom-based macrophyte-abundance inference model to detect the pronounced macrophyte decline that was observed between 1993 and 2001. Diatoms responded to the recent loss of macrophytes, with a decline in the relative abundance of macrophyte-associated taxa. Estimates of macrophyte abundance fluctuated according to the diatom-based inference model. Chironomid changes were coherent with the diatom-inferred macrophyte zones. The largest shifts in subfossil assemblages occurred before the start of the monitoring record and coincided with construction of a ~4.3-m-high dam on the lake, which substantially expanded the littoral habitat. Even in heavily managed systems, large reductions in macrophyte abundance can be detected with paleolimnological approaches.