Sink or Swim: Growth Dynamics and Zooplankton Hydromechanics

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Abstract
Adjusting and maintaining a vertical position is of acknowledged importance to zooplankton, but the development of the energetically significant values of weight in water (equal to the force the animal must exert to maintain position), weight per unit of body mass, and sinking speeds with animal growth does not appear to have been studied. A framework for developing these quantities over time is presented, and the interpretation of the functional relation is greatly facilitated by developing the equations in nondimensional form. The results indicate that a zooplankter''s weight in water, weight per mass, and sinking speed are likely to change nonlinearly with animal growth, may change quite rapidly at certain stages of development, and may have maxima or minima. The changes could have significant energetic implications for zooplankton, and the study of body density does not reveal these changes. The development of weight in water and the development of sinking speed are frequently altered and offset by developmental and biochemical adaptations such as swim bladders, lipid accumulation, and ion regulation. Stress from suboptimal conditions will alter a zooplankter''s hydromechanics and may further compound any debilitating effects.