Changes in element concentrations in aquatic bryophytes over an annual cycle

The concentrations of seven elements (N, P, K, Ca, Mg, Fe and Na) were measured every two weeks during an annual cycle, in four species of submerged aquatic bryophytes: two mosses (Fontinalis antipyretica and F. squamosa), a foliose liverwort (Jungermannia exsertifolia subsp. cordifolia) and a thalloid liverwort (Pellia endiviifolia). The hypothesis under consideration was whether their elemental composition over the year showed a seasonal pattern and, if so, whether those changes depended on internal or environmental factors. The elements showing most frequently an annual cycle were N, P, Na and Fe, and the most common temporal trend was that with the lowest concentrations appearing in spring and the highest ones in autumn. Our data suggest that the seasonal cycles depended on the interaction of both internal and environmental factors: 1) the annual cycle of vegetative growth in aquatic bryophytes, which causes a concomitant cycle of dilution/concentration of elements within the plant tissues; 2) the annual changes in sclerophylly in some species, such as J. cordifolia; and 3) the temporal variation in the physical and chemical features of the stream of origin. However, only scattered correlations were found between the concentrations of a given element in the bryophytes and the concentration of that element in the surrounding water. Thus, bryophytes do not behave as mere absorbers of elements, but more complex systems, when the temporal variation in their element composition is considered.