  We study the in-plane anisotropy of the thermoelectric power and electrical
resistivity on detwinned single crystals of isovalent substituted
EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_2$. Compared to the resistivity anisotropy the
thermopower anisotropy is more pronounced and clearly visible already at
temperatures much above the structural and magnetic phase transitions. Most
remarkably, the thermopower anisotropy changes sign below the structural
transition. This is associated with the interplay of two contributions due to
anisotropic scattering and orbital polarization, which dominate at high- and
low-temperatures, respectively.
