Toxic Cyanobacterial blooms from “the pulse of P from the shore drying and rewetting cannot be ignored, particularly in an oligotrophic lake.” Click here to read the full study or an excerpt below.
Bloom drivers, a recent study showed , depend on the cyanobacterial taxon and on the lake trophic state; in mesotrophic lakes, temperature is a better predictor of Nostocales blooms. In Lake Garda, where the trophic status has increased to mesotrophic conditions, with P concentrations almost doubled in 40 years , warming temperatures and an exogenous introduction of cyanobacteria are likely at the origin of D. lemmermannii appearance. Similarly, climate change induced warming could explain the colonization of the eutrophic lakes Como and Iseo. Conversely, in low nutrient conditions exogenous nutrient loading seems to be the most effective driver of the intensification of high biomass cyanobacterial bloom . Therefore, even if the 32-year data set of continuous epilimnetic temperature records in Lake Maggiore confirms the significant trend toward warming, the P loading source remained an open question. In fact, in Lake Maggiore no obvious nonpoint source nutrient pollution can be found. Furthermore, the lake is deep and, in the last thirty years, the mixing depth never exceeded 200 m , therefore a P recirculation from the sediment as a cause of the cyanobacteria bloom is unlikely.
Having ruled out P resuspension from the deep sediments, we tested the hypothesis of a release from the littoral zone. Our data set of daily average lake levels and monthly average temperature from 2005 to 2011 showed that the blooms occurred at summer temperatures and in particular, focusing on the summer months, a synchrony between the blooms and episodes of water level fluctuations emerged. A connection between water level fluctuations and the release of nutrients from the littoral zone had already been hypothesized , , , but proved hard to quantify and understand.
Through our experiments with artificial substrates, we demonstrate a variation of the C∶P molar ratio during the two years of study. In both years, the peak of D. lemmermannii (23 August 2010 and 30 June 2011) occurred after a rapid increase of lake level following a period of drought, as was observed since 2005. The data from the artificial substrates indicate that bloom formation corresponded to the release of material with a low C∶P molar ratio and a high percentage of phosphorus released from the shore. These conditions are explained by the release of previously accumulated phosphate and the higher mineralization of organic phosphorus by activation of phosphatase  that follows drought and rewetting events. Thus, desiccation and rewetting lead to increased availability of P in the littoral zone, potentially triggering cyanobacteria growth also in oligotrophic systems. Even though other abiotic factors like high temperature, CO2 and underwater light conditions influence cyanobacterial blooms, here we show that lake level fluctuations have a crucial role in D. lemmermannii colonization of oligotrophic deep lakes, like Lake Maggiore. We are aware that also biotic interactions can have a role in cyanobacterial bloom. In particular, the submerged macrophytes can be light-limited by the increased water level and consistently the cyanobacterial bloom favoured. Likely, this interaction can only be of local importance in this deep lake, whose steep littoral slope supports a low submerged macrophyte biomass.
The amount of nutrients present on the biofilm formed on the artificial substrates fluctuated along the year in accordance with the seasonal pattern of autochthonous particulate organic matter production, reflecting the population successions occurring in the lake. We concomitantly observed variations of the C∶P and C∶N molar ratio, which indicate a change in the quality of the biofilm on the substrates. The maximum amount of carbon (2206 kg) occurs in spring and that of phosphorus (9.7 kg) in late summer. Considering that 10 kg of elemental phosphorus correspond approximately to 80 kg of monocalcium phosphate, a common fertilizer, this quantity of P amounts to a noteworthy nonpoint nutrient source from the shore, particularly in an oligotrophic lake. Moreover, our estimate of P on the substrates does not include the organic phosphorus present in the microorganisms that could be, successively, either directly used by cyanobacteria through phosphatase activity, or mineralized by microorganisms .
The maximum release of nutrients was 145 kg of C and 18.1 kg of P, in 1 month, on lake perimeter, with 1 m lake level fluctuation. Even if the absolute amount released was higher for C than for P, the relative release was 5% for C and 77% for P (two years average). The percentage of P released fluctuated more than C in the two years, and reached the maximum in coincidence with the cyanobacteria bloom. The contribution of the P released from the shore was 53 mg P m−2 in 2010 and 20 mg P m−2 in 2011. These estimates are not far from the allochthonous P input as areal contribution from Lake Maggiore rivers which are 22 and 26 mg P m−2 y−1 in 2010 and 2011, respectively . Nevertheless, it must be emphasized that river input in the lake is a point-source and the nutrient load quickly sinks in the water column due to temperature differential between lake and colder river waters. On the other hand, the contribution from the shore is a non-point-source, which remains near the surface layers.
Over the last decade, we assisted to drought-induced decreases in lake-level, frequently followed by heavy precipitation due to extreme meteorological events. In light of this, the pulse of P from the shore drying and rewetting cannot be ignored, particularly in an oligotrophic lake.
In conclusion, our experiments show the important role of nutrient release from the drying and rewetting of lake shores in the expansion of cyanobacteria in oligotrophic lakes. Fluctuations in water levels seem to be increasing as an adverse effect of climate change, suggesting that nutrient release from the littoral zone will gain a growing impact on freshwater ecosystems . Nevertheless, changes in water levels also offer a different ground for water management interventions. In fact, most central European lakes have been largely regulated since long time and water levels are thus often amenable to external control . For this reason, highlighting the mechanisms that relate water level fluctuations, nutrient pulses and cyanobacterial blooms promises a better handle on possible measures to employ in limiting the spread of CHABs and thus preserve fundamental freshwater ecosystems and resources.