Influence of groundwater chemistry on hyporheic invertebrate assemblages is revealed by fine-scale sampling

Relationships between hydrochemistry and invertebrate assemblages were assessed at fine spatial scales within the hyporheic zone of an upland stream. Eighteen stacked colonisation samplers were placed in the streambed and sampled regularly over an 8-month period. Each stack consisted of three individual sampling chambers, positioned at mean depths of 15, 30 and 45 cm below the bed surface. Each chamber had an internal volume of 600 cm3 and was filled with sediment in the 2 - 8 mm (b-axis) size range. Water was extracted from each chamber at approximately fortnightly intervals, to characterise its hydrochemistry (dissolved oxygen concentration and alkalinity) and infer the presence of groundwater. Six randomly selected stacks were removed in each of three seasons (winter, spring and summer), with invertebrates present inside then identified and enumerated. Despite marked inter-sample differences in hydrochemistry and the variable presence of groundwater within the hyporheic zone, sampling locations were characterised by broadly similar invertebrate assemblages. Generalised linear mixed models indicated that inter-sample differences in assemblages were primarily related to depth beneath the streambed rather than local hydrochemical conditions, indicating only a limited effect of groundwater discharge on hyporheic communities in this stream. Overall, the chambers revealed finer spatial scale variation than methods used in previous published studies and allowed environmental and ecological characterisation at comparable spatial scales. Future studies using such chambers should seek to increase the number of measured covariates, to better understand the controls on invertebrate assemblages.