Assessing temporal and spatial variability of phytoplankton composition in a large reservoir in the Brazilian northeastern region under intense drought conditions

Abstract

The present study was carried out in Castanhão Reservoir, a large aquatic system in the Brazilian semi-arid region that serves multiples uses as water drinking supply and intensive fish-cage aquaculture site. In order to understand the effects of environmental conditions on the spatial and temporal variability of the phytoplankton functional groups (FG) and the main 'characterizing taxa', sub-superficial water samples were collected from March 2012 to August 2013, a period distinguished by the continuous drop in reservoir volume due to rainfall shortage. Eighteen functional groups and 102 total phytoplankton taxa were found in the Castanhão reservoir during the study. No significant differences were observed relative to spatial variation of total phytoplankton composition throughout the reservoir (PERMANOVA, P>0.05). On the other hand, according to cluster analysis results, three temporal phases have been identified (Similarity Profile, P<0.05), based on 102 phytoplankton taxa. The 'characterizing taxa' was found using the Similarity Percentage procedure (cut-off 90%), being thus defined as those taxa that contributed the most to the similarity within each temporal phase. Nineteen 'characterizing taxa' described the Castanhão reservoir, with predominance of those typical of mixing and turbidity conditions. Cyanobacteria dominated through the three temporal phases. According to the redundancy analysis, nutrient availability and water transparency were found to influence the phytoplankton temporal dynamics. The phase I (rainy season) was most represented by Planktolyngbya minor/Pl. limnetica (FG=S1), which reached best performance under strongly decreased phosphate-P concentrations and low water transparency. In phase II (dry season), Pseudanabaena sp.1 (FG=?) outcompeted other cyanobacteria probably due the increase in water transparency and decrease in ammonium-N. Finally, in phase III (rainy season) the decrease of water transparency triggered a recovery of shade-adapted cyanobacteria, but at this time mostly represented by Pseudanabaena limnetica (FG=S1). Phase III was also distinctive from the other ones by the highest Nitrate-N and phosphate-P concentrations related to thermocline disruption, which favored an increase in total phytoplankton biomass recorded by the augment of green algae density (FGs=X1, J and F). We concluded that the temporal dynamics of phytoplankton composition was associated to environmental changes in Castanhão Reservoir from 2012 to 2013, which were driven by seasonal climate variation from region (rainy/dry seasons), as well as, by the reduction in reservoir volume that resulted in the disruption of the thermocline, water mixing and an increase in inorganic P and N.

Abiotic factors driving cyanobacterial biomass and composition under perennial bloom conditions in tropical latitudes

“…The biomass of Planktolyngbya, a shade-adapted filamentous cyanobacterium (Fabbro & Duivenvoorden, 2000), was related to higher concentrations of suspended solids (VSS and TSS) and shallower depth, dominating year-round in the meso-eutrophic Tabatinga lake. In agreement with previous reports, Planktolyngbya has been associated with turbid environments, with mixed layers and relatively lower values of phosphate in Brazilian semiarid lakes (Pinto and Becker 2015;Barroso et al 2018). Meanwhile, Aphanocapsa, a coccoid cyanobacterium typically found in eutrophic environments (Padisák et al, 2009), was detected under mesotrophic conditions and associated to lower suspended solids and deeper water depth (Zmax).…”

Limnology of the largest multi-use artificial reservoir in NE Brazil: The Castanhão Reservoir, Ceará State

“…This is facilitated by the dynamic of aquafeed pellets, which floats long enough to be transported prior to sedimentation, as demonstrated by Molisani et al (2015). This process explains why the water column, even surrounding fish farms, are relatively oligotrophic as demonstrated in many previous surveys (Molisani et al 2010, 2013, Barroso et al 2018.…”

“…The phytoplankton communities of the Castanhão Reservoir included six identified taxonomic classes (Cyanophyceae, Chlorophyceae, B a c i l l a r i o p h y c e a e , Z y g n e m a p h y c e a e , Coscinodiscophyceae and Xanthophyceae) (Silva 2015), distributed in seven functional groups (MP, S1, SN, P, D, X1 and F) (Barroso et al 2018), which responded to limnological conditions and also serve as indicators of trophic state. In March 2012, for example, cyanobacteria typical of turbid environments dominated, with Planktolyngbya minor/limnetica (S1), Pseudanabaena limnetica (S1) and Pseudanabaena catenata (MP) as major species.…”

“…On the other hand, August 2012 and January 2013 were sampling campaigns characterized by Pseudanabaena/Romeria sp., Pseudanabaena biceps, Planktolyngbya limnetica (S1) and diatoms from functional groups D and P, well adapted to mixing conditions and a mesotrophic state. In August 2013, Cyanophyceae adapted to turbid waters of the S1 group and fast growing Chlorophyceae of functional groups X1 and F dominated the phytoplankton community (Silva 2015, Barroso et al 2018). The dominance of these cyanobacteria from functional group S1 during most of the 2012-2013 period was an indicative of increasing water mixing.…”

“…The dominance of these cyanobacteria from functional group S1 during most of the 2012-2013 period was an indicative of increasing water mixing. Stronger mixing conditions during drought years related to lower reservoir volume, was indicated by the replacement of small diatoms (MP and C) by large diatoms belonging to functional groups D and P (Barroso et al 2018). The increased concentration of organic detritus at the bottom of the water column and the subsequent decomposition of this material consume large amounts of oxygen and induce strong oxygen deficits and nutrient accumulation in the hypolimnion.…”