Effects of deposited sediment on patch selection by two grazing stream invertebrates

Patch selection for cobbles covered by different food types, with and without deposited sediment, by two common New Zealand stream invertebrates, the snail Potamopyrgus antipodarum (Hydrobiidae) and the mayfly Deleatidium sp. (Leptophlebiidae), was quantified. Each taxa was exposed to cobbles covered by (1) filamentous green algae (FGA) or diatoms, and (2) diatoms or heterotrophic biofilms. Two cobbles of each food type were used in each trial, one of which was contaminated by deposited sediment. All cobbles were embedded in plaster in small basins to prevent animals hiding under them, and a known number of animals placed into each basin. The location of each animal was recorded over time, with some observations being made during darkness to see whether this influenced animal movement. More Potamopyrgus were found on cobbles covered by FGA than cobbles covered by diatoms. Sediment-contaminated cobbles covered with FGA also supported more snails than uncontaminated cobbles covered with diatoms. More Potamopyrgus were found on cobbles covered by diatoms than biofilms, and sediment reduced their preference for both these foods. More Deleatidium nymphs were found on diatom-covered cobbles than FGA-covered cobbles. Sediment reduced the preference of Deleatidium for diatoms, but more animals were found on cobbles with sediment-contaminated diatoms than uncontaminated FGA. Patch selection was similar between cobbles covered by either diatoms or biofilms, but sediment reduced the preference of both these foods to Deleatidium. More Deleatidium were recorded on bare plaster during darkness, suggesting they were more mobile at night and searching for new habitats. These results help explain the common occurrence of Potamopyrgus in streams draining developed catchments where FGA blooms and deposited sediment are common, and of Deleatidium in streams draining less developed catchments where FGA and deposited sediment are uncommon.[PUBLICATION ABSTRACT]