The importance of spatial heterogeneity in near-bed currents and algal resource patchiness in regulating the movement patterns, foraging activities, and distribution of the mobile stream insect, Agapetus boulderensis Milne (Glossosomatidae: Trichoptera) was investigated with field observational studies and field experiments. Near-bed current velocities were characterized for topographically-complex substrata concurrently with spatial patterns of algal density and taxonomic composition. Activities of self-marked mobile larvae were examined in relationship to mapped current and algal resources. Near-bed currents alone influenced the distribution and local abundance of individuals in a size-dependent fashion - fifth (final) instar larvae occurred across a velocity range of < 5 to 50 cm s-1, while smaller third and fourth instar larvae were found only at currents ranging from < 5 to 30 cm s-1. Agapetus movement was also constrained by local current. Direction of movement on stone surfaces was significantly into or lateral to oncoming flow. Rate of movement was inversely related to local current speed and positively related both to temperature and to travel in the upstream direction. Agapetus movement and foraging were also related to algal density and taxonomic composition. Movement rate was inversely correlated with estimated diatom density but positively correlated with densities of green and blue-green algae for larvae on one stone. Gut analysis revealed a significant positive preference by Agapetus for diatoms, particularly Cocconeis placentula, which was overrepresented in the gut relative to other diatoms or to blue-green algae. Together, patterns of local current and of algal distribution and composition were important descriptors of movement and foraging activity of this mobile grazer under naturally heterogeneous field conditions. A field experiment under controlled conditions of current and algal density showed both factors were independent and non-interactive determinants of grazer movement rates. This study supports previous research showing the importance of algal patchiness in mediating grazer foraging activity and distribution, but it further indicates that patterns of resource use by grazers may be constrained by heterogeneous local currents, independent of the effects of food distribution and abundance.
