Seasonal Trends in the Head Capsule Length and Body Length/Weight Relationships of Two Amphipod Species

Abstract

[Amphipod body lengths and weights are important variables in life history, biomass and production determinations. Length and weight are often estimated from head capsule length-body length or weight relationships based on a single field collection. Possible seasonal variation in these relationships is rarely considered. We used seasonal data for two amphipod species (Gammarus lacustris and Diporeia hoyi) to test the hypothesis that head capsule length-body length/weight relationships remain seasonally constant. Slopes of monthly relationships were significantly different (ANCOVA). However, the differences in slope and intercept were trivial, and all monthly regressions yielded similar predictive results. Regressions based on pooled seasonal samples underestimated measured biomass by 5%, while predicted lengths varied between 0 to ± 2 mm from measured lengths. We conclude that head capsule length-body length/weight relationships for these amphipods are seasonally robust, and may be estimated from single collections provided all size classes are obtained. If all size classes can not be obtained from one sample, relationships should be based on pooled samples., Amphipod body lengths and weights are important variables in life history, biomass and production determinations. Length and weight are often estimated from head capsule length-body length or weight relationships based on a single field collection. Possible seasonal variation in these relationships is rarely considered. We used seasonal data for two amphipod species (Gammarus lacustris and Diporeia hoyi) to test the hypothesis that head capsule length-body length/weight relationships remain seasonally constant. Slopes of monthly relationships were significantly different (ANCOVA). However, the differences in slope and intercept were trivial, and all monthly regressions yielded similar predictive results. Regressions based on pooled seasonal samples underestimated measured biomass by 5%, while predicted lengths varied between 0 to ± 2 mm from measured lengths. We conclude that head capsule length-body length/weight relationships for these amphipods are seasonally robust, and may be estimated from single collections provided all size classes are obtained. If all size classes can not be obtained from one sample, relationships should be based on pooled samples.]