Cold fronts induce changes in phycoperiphyton structure in a shallow lake: Wind forces drive algae succession and nutrient availability

We evaluated wind as the main structuring force in driving phycoperiphyton structure, community composition and succession in a warm, polymictic shallow lake in southern Brazil. Mangueira Lake is continuously mixed due to its exposure to wind, and during cold fronts the wind changes from the dominant NE direction to a SSW direction. Our question was: could changes in wind forces induce phycoperiphyton succession and determine population structure? To answer this question, we studied the phycoperiphyton successional response to a change in wind forces under three different situations: on clean macrophyte leaves in an open site (Open) and in an enclosed site protected from the wind (Enclosure), and the Natural community growing on uncleaned macrophyte leaves in an open site. Cold fronts improved nutrient availability and changed the algal community. The phycoperiphyton natural community was dominated by Epithemia spp bound in gross masses of green filaments during cold fronts. However, the wind direction typically changes when cold fronts are over (NE-E) and wind blowing from the off-shore direction dislodged the attached algae biomass and pushed it toward to lake shore, inducing community changes. The macrophyte bank exhibited rapid colonization and acted as a refuge for phycoperiphyton, providing habitat heterogeneity, whereas the enclosure acted as a buffer against wind forces, delaying the succession derived from settlement of loosely adhered algae. Furthermore, in the enclosure, the succession only started after a strong disturbance (rain and wind > 10 m s(-1)) in which lake water flooded the mesocosm inducing colonization. Phycoperiphyton showed resilience and recovered rapidly after the disturbance, when the rain supplied inocula and wind favored colonization with growth forms that take advantage of local conditions, depending of wind dynamics.