The interaction between UV radiation and mineral availability has not been investigated in bryophytes, despite the important peculiarities of their mineral nutrition. The question under investigation was whether an improvement in phosphorus (P) availability, and the consequent increase in tissue P concentration, modified the responses to enhanced UV radiation in two aquatic bryophytes (the liverwort Jungermannia exsertifolia subsp. cordifolia and the moss Fontinalis antipyretica) under laboratory conditions. These responses were evaluated in terms of photosynthetic pigment composition, chlorophyll fluorescence, photosynthesis and respiration rates and the accumulation of protecting UV-absorbing compounds (both the commonly used bulk UV-absorbance of methanol extracts in both species and the concentrations of five hydroxycinnamic acid derivatives in the liverwort). The culture period (36 days) affected significantly most variables of both species. However, no variable in either species showed significant differences between the P-supplemented and non-supplemented samples at the end of the experiment, except the vitality index OD₄₃₀/OD₄₁₀ in the liverwort. This was probably due to the fact that the bryophytes studied have low nutritional requirements, and thus increasing tissue P may have only increased luxury consumption, without any modification in the responses to enhanced UV radiation.