Resource dynamics and seasonal changes in competitive interactions among three cladoceran species

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Abstract
A new approach to measuring zooplankton feeding success on natural seston assemblages enabled us to test laboratory studies in the field without being confined to radiotracers and their associated problems. We compared changes in the nutritional status of three species of cladoceran - Daphnia rosea Sars, D. middendorffiana Fischer and Holopedium gibberum Zaddach - exposing them to different natural seston compositions. A modified version of the lipid -ovary index was applied in time-series experiments at Castle Lake, California, throughout the summer of 1982. We used the same index in competition experiments which were designed to detect shifts in competitive interactions among all of the above species as a consequence of changes in the resource base. The time-series experiments with various seston compositions indicated that temporal and vertical distribution patterns of grazers were strongly affected by the availability of suitable food. Daphnia rosea and H. gibberum had no noticeable effect on each other's growth in early and midsummer, whereas the decline of the D. rosea population appeared to be accelerated by H. gibberum later in the season. Daphnia middendorffiana, spatially separated from the other two species, was unable to survive on food preferred by D. rosea and H. gibberum, whereas the latter two cladocerans were negatively affected by D. middendorffiana when placed in water containing hypolimnetic seston. Our results suggest that resource abundance and composition has a much larger influence on some zooplankton communities than previously acknowledged.

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