Interrelations between the vertical distribution of Daphnia and chlorophyll a in two large limnetic enclosures

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Abstract
This paper summarises the major factors influencing the daytime vertical distribution of Daphnia hyalina var. laciutris (Sars) in two large experimental enclosures (Lund Tubes). In both tubes, the depth at which the Daphnia aggregated during the day was closely related to sub-surface ir-radiance. On a few occasions aggregations of Daphnia were found in ‘dark’ water associated with a deep chlorophyll maximum. On many occasions, however, the animals' light response per se ensured their aggregation at depths of maximum phytoplankton abundance. The most important factor influencing the dispersion of animals in the water column was the steepness of the light gradient. In turbid water vertical aggregations were well defined, whereas in clear water the animals were widely dispersed around their depth of maximum abundance. Daphnia also tended to disperse in the water column when their population density was high or when food was scarce. A simple model based on surface irradiance, water turbidity and population density explained the basic pattern of vertical distribution throughout the season. The implications of these findings are discussed in relation to the effects of zooplankton grazing and speculations on the adaptive significance of depth regulation.

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