Friends or foes? The ecological relationships between micro- and macroalgae in low-order tropical streams

Streambeds are abundantly colonized by benthic algal assemblages. Environmental factors are the most assessed to find out predictable correlations between algal presence and habitat conditions, but biological interactions can also be important determinant of species composition in stream algae communities. Although there is plenty of information about algal ecology, the mechanisms leading their spatial distribution remain insufficiently clarified. Herein, we contribute to a better understanding regarding this latter issue, carrying out an experimental study to assess possible interactions between micro- and macroalgae during a successional process (75 days) in nine streams, using artificial substrates. The experimental analyses revealed the following main results: (i) streams with higher abundance of macroalgae also registered higher microalgal densities; (ii) the relative interaction index showed values greater than zero for all sampling dates, suggesting positive interaction between micro- and macroalgae during colonization process; and (iii) the multiple partial regression analyses (redundancy, pRDA) showed that environmental variables and microalgae density together explained 25% of macroalgal variation. These results indicate that interactions between stream micro- and macroalgae may contribute to shape the community structure during the successional process on the streambed. As observed previously, microalgae are early successional settlers and macroalgae are late successors, but both community components, besides their inherent competition, seem to cooperate in order to keep their coexistence. Based on this perspective, some typical ecological concepts related to the stream macroalgae, specially from tropical regions, may be expanded. For instance, studies have reported that the highest richness and abundance of lotic macroalgae in tropical/subtropical streams are determined basically by variation in climatic and environmental factors. Our results showed that, in addition to climatic and environmental factors, the increase in richness and abundance of stream macroalgae may also be affected by biotic interactions. In this context, our results suggest that the higher abundance of microalgae on the streambed seems to allow a better development of the benthic algal community, reaching to the later successional stages, in which the relative occurrence of larger and more complex algae (i.e., macroalgae) become more evident.