The effect of turbulent mixing on the growth and distribution of phytoplankton in turbid waters

"1994".

Project Number: The effect of turbulent mixing on the growth and distribution of phytoplankton in turbid waters - M/03/5084, SB/1/22; LWRRDC Project 90/31, MDR1.

MDFRC item.

9 pages.

The availability of light for algal growth is a complex function of the incident irradiance. light penetration and the depth to which water turbulence mixes the algal cell s. Major impediments to modelling and understanding the role of light in controlling the growth of algae are estimating mixing depth and predicting the growth responses of algae to a changing light climate. The aims of this project were to examine how the mixing potential within a turbid water body varies in time and space, to identify the causes of this variation, to identify more precisely the mixing depth, and to describe the impact of mixing on light availability and algal growth. Experiments were carried out at Rushy Billabong which is a small , turbid oxbow lake situated on the floodplain of the Murray River near Albury. The billabong was instrumented with a weather station and thermistor chains to obtain the detailed physical measurements required to quantify and model the influence of meteorological and radiative forcing on the mixing regime. Information from these dev ices was continuously recorded. Weekly measurements were taken to provide background information on bio logical and chemical variables, supported at various times by intensive 24 hour sampling regimes. These sampling protocols provided information on the three-dimensional distribution of algae and heat over a range of time period s. Analysis of the heat budget for the billabong indicated that it was dominated by solar radiation and by evaporation. The rate of evaporation could not be accurately modelled using currently available evaporation formulae derived for large water bodies and a new evaporation model was developed to account for the higher than expected evaporation rate from small water bodies. The billabong typically stratified during a summer day and mixed overnight as a result of penetrative convection. Several mixing models were applied to the billabong to explain the temperature structure and to quantify the intensity of turbulent mixing. The Princeton Ocean Model a three dimensional circulation/stratification model appears to be capable of simulating the diurnal cycle of stratification in the billabong and is being further modified for modelling algal distributions. Nutrients did not appear to be limiting during the growing season and net algal growth was a function of light availability. Changes in algal biomass were linearly correlated with changes in light availability for most of the period. Assuming a chlorophyll specific absorption coefficient of 0.01 m2 mgchla -1 ,the results suggested that light was limiting below values of ca. 0.07 mole quanta mgchla-1 day-1 comparing favourably with values of ca. 0.05 mole quanta mgchla-1 day-1 obtained previously in Mt Bold Reservoir, South Australia. A more complex numerical model was also developed to simulate how the affect of mixing on algal growth might be modified by algal flotation rates, particularly under conditions where surface scums can form.