Two Trophic Upsurges With Differing Responses During the Filling of a Mid-Continental Reservoir (Mozingo Studies III)

Abstract

Mozingo Lake was sampled monthly for 4 y following its impoundment in order to further understand trophic upsurges and to refine the classic model of upsurges. Upsurge 1 occurred in Month 8, when lake area increased by 184% and inundated only floodplain with comparable coverages of forest, cropland, and grassland (30%–33% each). At this time nitrate increased dramatically and was related to both precipitation and lake area increase, likely due to ammonia-based fertilizers from the flooded cropland. Phytoplankton biovolume remained low, however, likely due to the abundance of Daphnia and other zooplankton grazers. After recovery, the lake expanded dramatically again in Month 21 (Upsurge 2); lake area increased by 36%, but this expansion flooded land with less floodplain (23%) and with different proportions of land use (45% grassland, 31% forest, 14% cropland). Upsurge 2 experienced initial increases in nitrate and zooplankton, but these were minor compared to Upsurge 1, and phytoplankton remained low. These initial responses were followed by substantial increases in phosphate, which related to lake area increase; cyanobacteria biovolume expanded, but zooplankton abundance declined. Therefore the two upsurges were driven by different factors: nitrate in Upsurge 1, but phosphate in Upsurge 2. These drivers, in turn, may reflect differences in the newly-flooded land, including differences in land use and in landscape position (i.e., slope). In addition to seasonal succession, the phytoplankton of Mozingo Lake exhibited longer-term primary succession that related first to nitrate, then to grazing, then to multiple factors, suggesting increases in food web complexity. In contrast to the classic model of upsurges, the biotic responses in Mozingo Lake were quite transitory, and the nature of the upsurges varied with the characteristics of the newly-inundated land. Therefore, it appears that trophic upsurge is more variable than the classic model suggests.