Population dynamics and production of the pelagic amphipod Hyalella montezuma in a thermally constant system

SUMMARY. 1. The population dynamics and annual production of the multivoltine. pelagic amphipod Hyalella montezuma were studied over a 3‐year period in the thermally constant environment of Montezuma Well. Arizona. 2. H. montezuma showed two maxima which coincided with spring and autumn phytoplankton maxima. Juveniles comprised over 85% of the population in the pelagic zone compared to 37% in the littoral vegetation and there were significantly more females in the littoral vegetation. It appears that juvenile and adult H. montezuma show different habitat preferences. 3. Total annual mean production for H. montezuma calculated by the size frequency method and adjusted for multiple generations, was 357 kg ha−1 yr−1, which is higher than single‐species production estimates reported for most zoobenthic amphipods and multivoltine planktonic crustaceans. Average energy production for H. montezuma was 4640 kJ ha−1 yr−1 in the pelagic zone and 1072 kJ ha−1 yr−1 in the littoral vegetation. 4. Average cohort P/B̄ ratios for H. montezuma were higher in the pelagic zone (5.5) than in the litttoral vegetation (3.7). Juveniles had higher cohort PB̄ ratios than adults in the pelagic zone, while the reverse relationship was true in the littoral vegetation. We propose that different size‐selective predators may contribute to the differences in PB̄ ratios for juveniles and adults in these two habitats.