Global warming has been shown to strongly influence inland water systems, producing noticeable increases in water temperatures. Rising temperatures, especially when combined with widespread nutrient pollution, directly favour the growth of toxic cyanobacteria. Climate changes have also altered natural water level fluctuations increasing the probability of extreme events as dry periods followed by heavy rains. The massive appearance of Dolichospermum lemmermannii ( = planktonic Anabaena), a toxic species absent from the pelagic zone of the subalpine oligotrophic Lake Maggiore before 2005, could be a consequence of the unusual fluctuations of lake level in recent years. We hypothesized that these fluctuations may favour the cyanobacterium as result of nutrient pulses from the biofilms formed in the littoral zone when the lake level is high. To help verify this, we exposed artificial substrates in the lake, and evaluated their nutrient enrichment and release after desiccation, together with measurements of fluctuations in lake level, precipitation and D.lemmermannii population. The highest percentage of P release and the lowest C∶P molar ratio of released nutrients coincided with the summer appearance of the D.lemmermannii bloom. The P pulse indicates that fluctuations in level counteract nutrient limitation in this lake and it is suggested that this may apply more widely to other oligotrophic lakes. In view of the predicted increase in water level fluctuations due to climate change, it is important to try to minimize such fluctuations in order to mitigate the occurrence of cyanobacterial blooms.

In a deep, subalpine holo-oligomictic lake, the relative abundance of Archaea and Crenarchaeota, but not that of Bacteria, increases significantly with depth and varies seasonally. Cell-specific prokaryotic productivity is homogeneous along the water column. The concept of active Archaea observed in the deep ocean can therefore be extended to a deep oxic lake.The abundance, activity, and community composition of epilimnetic and hypolimnetic prokaryotes have been less thoroughly investigated in deep lakes than in oceans. Strong evidence that the depth gradient plays a role in modulating the balance between the domains of Bacteria and Archaea has been found in various marine systems (8,12,13,20). It has been shown that the percentage of Bacteria in the deep marine hypolimnion decreases (up to 5,000 m) while, conversely, the percentage of Archaea increases. The percentage of Crenarchaeota is also higher in the mesopelagic zone than in surface waters (10).Although Archaea have been found in a variety of freshwater habitats (3), little has thus far been published on differentiating between Bacteria, Archaea, and Crenarchaeota in the hypolimnion of deep lakes. An exception is a study of the high-altitude ultraoligotrophic Crater Lake (21, 22), where group I marine Crenarchaeota were observed in deep-water populations (22). This study and another study of various lakes from three continents (9) are based on summer sampling, making it impossible to ascertain the effects of temporal variability on the vertical distribution of Archaea and Crenarchaeota, as has been done for marine systems and shallow lakes (for examples, see references 8 and 11).Our primary objective was to follow variations in the relative abundance of Bacteria, Archaea, and Crenarchaeota found in the hypolimnetic waters of a deep holo-oligomictic lake with a permanent oxic hypolimnion and compare them with those in the epilimnetic assemblages. We used the catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) technique and compared the data thus obtained with prokaryotic productivity.Environmental characteristics and samplings. Lake Maggiore is a large, deep, subalpine lake (surface, 212 km 2 ; maximum depth, 372 m) in Northern Italy included in the LTER network (http://www.ise.cnr.it/lter). The lake has recovered from an eutrophic state (15) and is now oligomesotrophic (2, 14). A particular hydrodynamic feature of Lake Maggiore is that the full winter overturn occurs only at the end of a particularly cold and windy period (1).Temperature profiles (IDRONAUT OS316 multiparameter probe) indicated that stratification began in June (Fig. 1). The oxygen profiles showed that the water column was in a uniform oxic condition. Samples were taken in February, June, August, and October 2007 at 3 m, 10 m, 200 m, and 350 m, immediately fixed, and stored at Ϫ4°C (1 month).Prokaryote community composition. CARD-FISH analyses for Archaea, Bacteria, and Crenarchaeota were performed in two replicates according to Pernthaler et al. (17) and Teira e...

We analysed the long-term dynamics of hypolimnetic and epilimnetic bacterial abundances and organic carbon concentrations, both dissolved (DOC) and particulate (POC), in the deep holo-oligomictic Lake Maggiore, included in the Southern Alpine Lakes Long-Term Ecological Research (LTER) site. During the 28 years of investigation, bacterial abundance and POC concentrations did not decrease with declining phosphorus concentrations, while DOC concentrations showed a pronounced decrease in the epi-and hypolimnion. We used the annual mean total lake heat content and total annual precipitation as climate-related variables, and in-lake total phosphorus as a proxy for trophic state. The model (forward stepwise regression, FSR) showed that reduced anthropogenic pressure was more significant than climate change in driving the trend in DOC concentrations. Bacterial dynamics in the hypolimnion mirrored the fluctuations observed in the epilimnion, but average cell abundance was three times lower. The FSR model indicates that bacterial number variability was dependent on POC in the epilimnion and DOC in the hypolimnion. In the hypolimnion, cell biovolumes for rod and coccal morphotypes were significantly larger than in the epilimnion.

Microgels such as transparent exopolymer particles (TEP) can be as important in freshwater as they are in the ocean because they constitute microenvironments of unique physical, chemical, and biological properties. Previous studies have shown that TEP concentrations can significantly contribute to the organic carbon pool in both marine and freshwater. Despite the relevance of TEP as a microbial substrate in aggregate formation, their fate within the hypolimnion has not been studied in deep lakes. This work is the first extensive analysis on the TEP space-time distribution and their role in the carbon cycle of Lake Maggiore, a subalpine deep lake where the presence of TEP became macroscopically evident in recent times. Our study demonstrated that in oligotrophic Lake Maggiore, characterized by low total organic carbon (TOC) concentration (summer max ~1.8 mg L −1), TEP can reach 36% of TOC in the photic layer while its weighted average in the hypolimnion below 100 m depth is 1.7%. We therefore hypothesised that most TEP is recycled in the upper layers, and that its contribution as carbon burial in the sediment may be lower than expected. TEP concentration exhibited a clear seasonal variability mirroring that of the autotrophic plankton. The models explain TEP variability as a function of chlorophyll a, a proxy for phytoplankton biomass, and a weaker effect of picocyanobacteria. TOC, but not TEP, influenced the abundance of bacterial aggregates, leaving open the role of bacteria and phytoplankton association in TEP formation.

Settling flux and velocity of seston, Particulate Organic Carbon (POC) and chlorophyll a were measured at three depths during 8 seven-day exposure periods in Lago di Mergozzo (Northern Italy). Sedimentation rates of seston varied from 250 to 1200 mg me2 d-' with a prevalence of the inorganic fraction (130-900mgm-2d-1) over the organic (160-320mgm-2d-'). The percentage of organic fraction inside the traps was always lower than outside. The comparison of preserved and unpreserved traps showed no significant difference in both organic matter content and bacterial numbers. We inferred from this result that bacterial activity in the traps did not cause a measurable POC loss during the seven day exposures. Therefore, the higher settling velocity of the inorganic particles was responsible for the higher percentage of this fraction in the traps. The settling velocity of sestonic particles increased, during the stratification period, with increasing depth and reached a maximum value of 2.5 m d-I.