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

LEON, JOSE G.; BEAMUD, S.G.; TEMPORETTI, P. F. ; ATENCIO, A.G.; DIAZ, M.M. ; PEDROZO, F.L.

International Review of Hydrobiology

Wiley

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

Reservoir trophic state is controlled by light and nutrient availability, as well as by hydraulicmanagement and stratification pattern. In arid zone reservoirs, the inflow and outflowdischarges have extreme seasonal variations which produce well-defined cycles of filling anddraining. Moreover, since stratification often occurs, epilimnion and hypolimnion renewal ratesmay vary producing different environmental conditions throughout the water column. Thesevariation patterns may affect phytoplankton growth at both temporal and spatial scales. For twohydrological years, we analyzed the influence of light climate, nutrients, residence time (Tw),and stratification on phytoplankton biomass (as chlorophyll-a [Chl-a]) in an irrigation reservoir(276 hm3) in the arid central Andes in Argentina (33°S). The reservoir was turbid (ZSecchi: 1.4 m)with relatively high levels of dissolved nitrogen and phosphorus (60 mgP L 1 and 560 mgN L 1,respectively). Stratification occurred from mid-spring to late-summer, when hypolimneticoxygen and pH decreased whereas dissolved nutrients increased. The reservoir wasmesotrophic (Chl-a: 4.6?10.8 mg L 1) and showed two semi-annual peaks of Chl-a (summerand winter). Depending on the water column circulation, Chl-a was directly related to Tw for60 > Tw > 200 and inversely to ammonia, during mixing and stratification periods, respectively.Phytoplankton development was strongly influenced by the seasonal variations of inflow/outflow and stratification. Chl-a peaked in summer, when inflow plunged into the hypolimnion ofthe stratified reservoir and in the warm and mixed epilimnion algae proliferated until nutrientsdepletion. Alternatively, the winter Chl-a maximum is likely to be produced by the higherconcentration of nutrients combined with the turbulent mixing in the water column which acts asa mechanism that helps to overcome light-limitation. Since stratification modifies the verticaldistribution of Tw,theuse of a single annualvalue ofthis hydraulic parameter to assess its effecton the reservoir trophic state results inadequate. This paper constitutes the first description ofhydrologic effects on Chl-a dynamics in a reservoir in arid central western Argentina.