Wave-induced shear stresses, plant nutrients and chlorophyll in seven shallow lakes

1. Sediment resuspension dynamics were investigated in relation to changes in water column nutrients (TP, TN, PO₄-P, NO₃-N and NH₄-N), chlorophyll a and phaeopigment in seven shallow ( Z _m < 1.5 m) lakes in South Island, New Zealand, ranging in area from 0.1 to 180 km².
2. Benthic shear stress, calculated from wind speed, effective fetch and depth, was a considerably better predictor of nutrient and pigment concentrations than wind speed.
3. For TP, TN, chlorophyll a and phaeopigment, sixteen of the possible twenty-eight linear correlations with benthic shear stress were significant at P < 0.05, with 16–87% of the variation being explained by shear stress.
4. Wind decreased the ratios of TN : TP, with ratios exponentially approaching those of the sediments as shear stress increased in four of the lakes.
5. Relationships of dissolved inorganic nutrients to shear stress were considerably weaker than those for total nutrients and showed no consistent trend over the seven lakes.
6. Estimated annual mean TP inclusive of resuspension was over four times higher than that derived from measured calm samples in two lakes.
7. The number of nutrient and pigment parameters that were significantly correlated with shear stress and the strengths of the relationships varied widely from lake to lake. We could establish no simple relationships between these effects and any single characteristic of the lake, sediment, or water.
8. A function is developed to predict the rate of entrainment of TN and TP in response to an applied shear stress, where the independent variables are sediment nutrient content and particle size, and the macrophyte density in the lake.