Nutrient limited growth of the phytoplankton assemblage in two Texas reservoirs was studied by a combination of nutrient addition experiments and statistical modeling. Dilution bioassays were run to ascertain the qualitative and quantitative patterns in nutrient limitation. Algal growth was frequently and strongly nutrient limited, particularly when temperature was > 22°C. By itself, N was more often stimulatory than P, though strong additional enhancement of growth by P and trace nutrients was often detected. Monod growth kinetics indicated that half-saturation constants for N limited growth for the entire algal assemblage were in the range 20-200 μg N/L, relatively high compared to literature values, and increased with increasing temperature. Maximal growth was also an increasing function of temperature. A single temperature-dependent model was fit to the growth dynamics for all experiments showing N-limitation. The model μ = 0.0256.T([DIN]/66.0 + [DIN]) where μ is specific growth rate (d-1), T is temperature (°C) and [DIN] is dissolved inorganic N (μmol/L) fit the experimental results reasonably well (r2 = 0.82). However, only a modest predictive power for growth in the controls (our best estimate of growth in situ) was achieved (r2 = 0.26). Thus, even with unusually detailed, site-specific fitting of model parameters, accurately modeling algal growth in natural ecosystems can remain a challenge.
Bibliographical noteFunding Information:
We are grateful to Mark Ernst for advice and assistance and to the Tarrant County Water Control District for funding. Judy Robinson, Caroll Duhon, and Clayton Napier provided expert technical assistance. Additional funding was provided by NSF grants 9119781 and 9421925 to RWS, and grant 9418096 to JPG. The Max Planck Institut für Limnologie provided support for writing the manuscript.