Stream animals hang on when surface water dries up

Posted on December 16, 2014

Under drought conditions, the survival of freshwater species depends on the availability of refuge habitats. The bottom substrate of a stream is a potential refuge, but little is known about its use by stream invertebrates after surface water has disappeared. To help fill this information gap, researchers sampled three intermittent streams on the east coast of North Island, New Zealand, over a two-year period. The streams had beds of pebbles and cobble. During winter baseflow conditions, the streams were up to 14 cm deep and 1.1 m wide, but were reduced to disconnected pools in January and February, before flow resumed again in mid-late March. Benthic macroinvertebrates were collected from riffles during flow periods and compared with those found in “dry” riffle sediment in mid- February, when surface water was absent and near- surface moisture content was 6-14% by mass. When sediment was rewetted, active members of most of the invertebrate species found in local flowing streams were recorded, in most cases within a few hours. Copepods, ostracods, snails and beetle larvae were particularly common. When sediment samples were incubated at different temperatures, the major invertebrate groups were less abundant in warm (22-25oC) conditions than at cooler temperatures. It seems that shallow stream bed sediments can be a key dry-season refuge for invertebrates in intermittent streams. And because sediment temperatures were much lower in the beds of streams running through dense forest than through open pasture, riparian vegetation may protect the refuge potential of sediment habitat by moderating summer streambed temperatures.

Reference: Storey, R.G. & Quinn, J.M. 2013. Survival of aquatic invertebrates in dry bed sediments of intermittent streams: temperature tolerances and implications for
riparian management. Freshwater Science 32(1), 250–266.