Short-term dynamics of submerged aquatic vegetation diversity and abundance in deep lakes.

Question: What are the short-term trends of submerged aquatic vegetation (SAV) in deep lakes? Is it possible to measure significant changes in SAV distribution in deep lakes characterized by different trophic states and stable water physico-chemical conditions using standard procedures over a 3-yr period (2010–2013), as defined by the Water Framework Directive?. Location: We collected data from four deep lakes belonging to the Italian volcanic lake system (central Italy), which is one of the most important reserves of freshwater in Mediterranean Italy as well as one of the main hotspots of stonewort diversity in the Mediterranean area. Methods: Macrophytes were sampled along five transects in each lake in 2010 and 2013. Data on aquatic plant diversity, composition and abundance were collected from each 1-m depth intervals down to the maximum colonization depth. Simultaneously, the water quality and climate were assessed by standard procedures. Data were analysed with PCA, ANOVA, PROTEST and rank abundance curves. Results: The sampling revealed a significant rearrangement of SAV. Increased species diversity in the first meters of depth (0–5 m) was due to the migration of mid-depth species to lower depths. A reduction in the maximum growing depth of macrophytes was observed within the deepest stonewort communities (below 5 m). No significant changes emerged in water quality or climate. Conclusions: Over 3 yr, SAV displayed significant changes that appear to be independent of summer nutrient conditions and climate drivers. In particular, vascular and stonewort species underwent a distinct spatial rearrangement according to specific behaviours. The present data point to the need for more rigorous analyses of the intra-annual dynamics of macrophytes in deep lakes. An influence of the significant increase in spring precipitation (which is in keeping with local climate variability) recorded in central Italy in the last decade cannot be ruled out a priori, nor can the possibility of stochastic fluctuations be excluded. All these considerations further highlight the critical role of aquatic plants as bioindicators