Influence of light and nutrients in controlling thedynamics of filamentous bloom-formingcyanobacteria in a highly turbid shallow lake

GRACIELA SALERNO; RICARDO ECHENIQUE; AGUILERA ANABELLA; LUIS AUBRIOT

Florianópolis

Congreso; 17th International Conference on Harmful Algae; 2016

High cyanobacterial biomass imposes extreme environmental conditions by selfshading and fluctuating nutrient availability. The aim of the study was todetermine the role of light and nutrients in the dominance and succession ofbloom-forming cyanobacteria in a highly turbid hypertrophic shallow lake (BuenosAires province, Argentina). A complementary approach was used: high frequencyof sampling (fortnightly, July 2012 to May 2014) and experiments with co-culturesof filamentous cyanobacteria isolated from the water body (Planktothrixagardhii and Raphidiopsis mediterranea). Laboratory assays were performedunder two light intensities (40 and 80 µmol photon m-2 s-1) and two phosphateconcentrations (5 and 200 µM K2HPO4). In the lake, P. agardhii dominated duringthe whole sampled period, followed by Nostocales species ( R.mediterranea , Aphanizomenon gracile and Sphaerospermopsisaphanizomenoides)which biomass was positively influenced by temperature andlight availability (summer seasons). Contrary to predictions, low N:P ratios (~ 5)did not lead to the dominance of Nostocales. On the contrary, the massive growthof P.agardhii induced extremely low light conditions that led to the collapse of thisgroup in early autumn. This collapse is thought to have represented a pool of Nand P that supported the continuous growth of P. agardhii. In co-cultureexperiments, R. mediterranea growth was strongly promoted under 80 µmolphoton m-2 s-1 whereas P.agardhii did not tolerate high light conditions. Bothspecies equally tolerated P deficient conditions. Together, these results highlightthe importance of light availability and self-shadingin structuring thephytoplankton dominated by filamentous cyanobacteria in extreme turbidenvironments.