Dynamics of Lake Michigan Natural Phytoplankton Communities in Continuous Cultures Along a Si:P Loading Gradient

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Abstract
The hypothesis that phytoplankton species assort themselves along resource ratio gradients according to their relative competitive abilities for the potentially limiting resources was tested using natural communities from Lake Michigan. Algae were grown in six continuous cultures for 46 d on a gradient consisting of four silicon to phosphorus (Si:P) loading ratios: 313:1 (two cultures), 71:1, 4.6:1, and 0.9:1 (two cultures). Diatoms were the superior competitors for P and dominated the three high Si: P ratio cultures at steady state. Green algae dominated the three low Si: P ratio cultures. The two cultures at Si: P = 313 were very similar and were dominated by the diatom Synedra filiformis. Stable coexistence was demonstrated at Si: P = 71, where S. filiformis and Diatoma elongatum co-occurred. The three low Si: P ratio cultures were dominated by a green unicell. Six common species, including the three dominant species, were isolated and tested for their ability to grow under limitation by Si and P. Trade-offs in competitive ability predicted from continuous culture results were confirmed. Species were ranked in competitive ability for Si as follows: the green unicell and the green alga Monoraphidium contortion (with no Si requirements) were better than D. elongatum and Asterionella formosa, which were better than S. filiformis and Fragilaria crotonensis. The ranking for P was exactly the opposite. Results illustrate that phytoplankton natural community continuous cultures provide a method for evaluating potential changes in community structure resulting from long-term changes in loading ratios of resources.