| Research conducted for this dissertation focused on phytoplankton and zooplankton ecology in Pyramid Lake, Nevada with emphasis on ecophysiology and bloom dynamics of the diazotrophic cyanobacterium Nodularia spumigena. This research was a key component to improving the understanding of internal nutrient cycling in N-deficient Pyramid Lake since N. spumigena provides 20-90% of the lake's annual nitrogen input via nitrogen fixation. Objectives of this study were to: (1) compare present and historical phytoplankton assemblages and determine impacts of N. spumigena on species composition, (2) analyze the trophic relationship between phytoplankton and zooplankton and examine the influence of N. spumigena on the zooplankton community, (3) quantify spatiotemporal nitrogen fixation and estimate annual N loading, and (4) identify physiochemical factors controlling N. spumigena blooms.; Current and historical phytoplankton species assemblages were similar; however, comparison between assemblages following the massive 1992 and small 1993 N. spumigena blooms showed that community composition was influenced by the magnitude of N loading from nitrogen fixation. Regression models provided the first evidence that N also influences zooplankton biomass in Pyramid Lake and were consistent with the observed correlation between elevated N and high zooplankton biomass during the year after the massive 1992 bloom. Examination of 1986-1993 zooplankton data showed an apparent cycle in zooplankton species diversity consistent with switches between bottom up control by N. spumigena N input and top down grazing control.; N loading from nitrogen fixation during 1990, 1991, 1992 and 1993 was estimated to be 258, 158, 3000, and 102 Mg, respectively. In spite of the system's P-richness, bioassay results coupled with very low ambient P during the 1992 bloom suggested that P availability might be important during large blooms. However, field studies indicated that physical factors, particularly wind stress, controlled the onset and magnitude of N. spumigena blooms. Small blooms occurred during low wind stress periods in otherwise windy years and were not accompanied by dramatic changes in ambient nutrients. In contrast, very low wind conditions during 1992 promoted the early onset and massive growth of the bloom; a combination of P depletion and increased wind stress caused its collapse. Rather than being controlled by nutrients alone, the onset, accretion and collapse of blooms was determined primarily by physical forcing. |
