Geographical and temporal patterns of cyanobacterial assemblages in the Danube Delta lake complexes

Abstract

Danube Delta shallow lakes experience cyanobacteria blooms that can negatively affect the aquatic ecosystem. Although there are several studies on Danube Delta cyanobacteria, little is known about their spatial-temporal patterns and the potential predictive role they can offer.We therefore analyzed the distribution of cyanobacteria in 19 lakes belonging to three lake complexes, and tested whether their seasonal dynamics are in line with the predictions of the PEG model. Furthermore, we investigated to which extent cyanobacteria diversity and abundance were related to lake hydrogeomorphological characteristics such as: surface, water level, connectivity, water retention, flood risk, transparency. Although lakes had different seasonal cyanobacterial assemblages, the biovolume and genus richness had a geographical pattern, decreasing from south-east (lakes forming the fluvial delta) towards north-west (lakes forming the maritime delta). Cyanobacteria biovolume reflected largely the PEG model peaking in summer (the fluvial delta) and autumn ( the maritime delta). Genus richness followed the same pattern. Cyanobacteria distribution was predicted by various abiotic (e.g. risk of flooding, connectivity) and biotic factors (e.g. submersed macrophytes, phytoplankton diversity, peat deposits). Our study contributes to the understanding of cyanobacteria diversity and distribution in shallow interconnected lakes by revealing the complexity of predictors for geographical and seasonal patterns.

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“…Since in DDBR is highlighted a strong seasonality regarding the phytoplankton distribution [38], this tendency was also found in the eDNA quantity extracted from our samples during spring, summer, and autumn (Figure 6). As expected, higher values were registered both in July and September since, according to our data, we found cyanobacteria mass development in some lakes as well in September or even in May [21]. The DNA concentration highlighted this tendency as well, making the XS method a very reliable one for this kind of study.…”

“…We sampled a number of lakes (Figure 1 and Table 1) belonging to all four lake complexes (LCs) of DDBR, namely: Șontea-Furtuna (LC 1), Isac-Gorgova (LC 2), Matița-Merhei (LC 3), forming the fluvial delta, and Roșu-Puiu (LC 4) part of the maritime delta [25]. The heterogeneity of the LCs also can be highlighted by the water level, which can be a surrogate for the hydrological regime [26], and it was shown to influence not only the water volume sampled but as well the ecological state of the lakes [21]. Water level can be considered, at the same time, a driver of the cyanobacteria community distribution in shallow lakes [21].…”

“…The heterogeneity of the LCs also can be highlighted by the water level, which can be a surrogate for the hydrological regime [26], and it was shown to influence not only the water volume sampled but as well the ecological state of the lakes [21]. Water level can be considered, at the same time, a driver of the cyanobacteria community distribution in shallow lakes [21]. The water level in the DDBR lakes usually increases from West to East, following the water flow direction of the Sulina main branch.…”

“…Particularly, benthic forms can produce protective sheaths or mucilage [20] that harden the analysis since they interfere with DNA extraction. For the Danube Delta we reported plenty of filamentous but also colony-forming species [21]. Most of them have enormous quantities of mucopolysaccharides that make DNA extraction very challenging [22].…”