Stratification and residence time as factors controlling the seasonal variation and the vertical distribution of chlorophyll-a in a subtropical irrigation reservoir

LEÓN JOSÉ; BEAMUD SG; TEMPORETTI P; ATENCIO A; DIAZ M; PEDROZO F

INTERNATIONAL REVIEW OF HYDROBIOLOGY

WILEY-V C H VERLAG GMBH

Lugar: Weinheim; Año: 2016 vol. 101 p. 1 - 12

1434-2944

Reservoir trophic state is controlled by light and nutrient availability, as well as byhydraulic management and stratification pattern. In arid zone reservoirs, theinflow and outflow discharges have extreme seasonal variations which producewell-defined cycles of filling and draining. Moreover, since stratificationoften occurs, epilimnion and hypolimnion renewal rates may vary producingdifferent environmental conditions throughout the water column. These variationpatterns may affect phytoplankton growth at both temporal and spatial scales.For two hydrological years, we analyzed the influence of light climate,nutrients, residence time (Tw), and stratification on phytoplankton biomass (aschlorophyll-a [Chl-a]) in an irrigation reservoir (276 hm3) in the arid centralAndes in Argentina (33°S). The reservoir was turbid (ZSecchi: 1.4 m) withrelatively high levels of dissolved nitrogen and phosphorus (60 mgP L1 and 560 mgNL1,respectively). Stratification occurred from mid-spring to late-summer, whenhypolimnetic oxygen and pH decreased whereas dissolved nutrients increased. Thereservoir was mesotrophic (Chl-a: 4.6?10.8mgL1) and showed two semi-annualpeaks of Chl-a (summer and winter). Depending on the water column circulation,Chl-a was directly related to Tw for 60>Tw>200 and inversely to ammonia,during mixing and stratification periods, respectively. Phytoplanktondevelopment was strongly influenced by the seasonal variations of inflow/ outflowand stratification. Chl-a peaked in summer, when inflow plunged into thehypolimnion of the stratified reservoir and in the warm and mixed epilimnionalgae proliferated until nutrients depletion. Alternatively, the winter Chl-amaximum is likely to be produced by the higher concentration of nutrientscombined with the turbulent mixing in the water column which acts as amechanism that helps to overcome light-limitation. Since stratificationmodifies the vertical distribution of Tw, the use of a single annual value ofthis hydraulic parameter to assess its effect on the reservoir trophic stateresults inadequate. This paper constitutes the first description of hydrologiceffects on Chl-a dynamics in a reservoir in arid central western Argentina.