Diel and vertical variability of seston food quality and quantity in a small subalpine oligomesotrophic lake

Vertical profiles of seston food qualify and quantity) were measured in subalpine Castle Lake for particulate carbon, chlorophyll a fatty acids and phosphorus in addition to abiotic parameters including water temperature and dissolved oxygen levels. Field and laboratory incubation experiments were employed to manipulate Daphnia rosea growth environments. Sestonic eicosapentaenoic acid (EPA) content was much lower, whereas carbon to phosphorus (C.-P) ratios were much higher in the epilimnion compared to the deep-water chlorophyll maximum (16 20 in). In a temperature-controlled incubation, Daphnia grew faster when fed seston from the deep-water chlorophyll maximum. In in situ cage incubations, D. rosea grew fastest in the epilimmon. Daphnia rosea in a migrating treatment exhibited intermediate growth rates between the ones for epilimnion and hypolimnion. A projection of D. rosea growth rates by growth models without regard to water temperature showed much higher growth potentials in the hypolimmon. However, with inclusion of water temperature, epilimnetic water always gave higher potential growth rates. In this lake, warmer temperatures of epilimnetic water override the higher food qualify and quantity provided by the deep-water chlorophyll maximum.