Suspended solids concentrations were measured at routine 2-3 week intervals and on additional windy days for at least one year in each of seven shallow (mean depth < 2 m) South Island, New Zealand lakes . Surface wave characteristics were estimated from water depths and local meteorological data using a shallow-water wave forecasting model for fetch-limited waves . Bottom shear stresses were computed from surface wave characteristics for the sampling stations and for a hypothetical lake-average station . The calculated shear stresses were, on average, much better predictors of suspended solids concentrations than alternative models using two different functions of wind speed, wave height 2 /depth or wavelength/depth . A combination of the sample station and lake average shear stresses provided slightly better predictions than the sample station values alone, suggesting that currents also contribute significantly to the concentration at a given point . Regressions of suspended solids on the combined function had r 2 values ranging from 0.47-0.73 in the seven lakes . The slopes of these regressions were negatively related to the settling velocity of the lowest quartile of the sediment, and to macrophyte biomass, in multiple regression (r 2 = 0 .94, p < 0.01) .

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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 (Zm < 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.

1. The persistence of effects of sediment resuspension on chlorophyll a, phytoplankton production and plant nutrients was examined by artificially resuspending sediment in enclosures in three shallow lakes and monitoring concentrations for 4–8 days. Realism was assessed by relating initial suspended solids concentrations to those observed during natural wind events. 2. Positive effects on the phytoplankton, persisting for at least 4–6 days were detected in eight of the nine experiments, and similar effects on nutrient concentrations were also common, although suspended solids had normally returned to control levels within 24 h 3. The phytoplankton during the periods of persistence was normally dominated by planktonic rather than benthic/meroplanktonic genera. 4. Several of the positive responses appeared to be associated with relief of nitrogen deficiency in the algae. 5. Persistent effects from previous resuspension events may cause baseline concentrations of phytoplankton and nutrients in shallow lakes to be overestimated and the effects of resuspension on phytoplankton and nutrients to be underestimated

The effects of an introduced willow tree species (Salix fragilis) on the densities of macroinvertebrates were examined in two Central Otago, New Zealand, streams during 1991. Significantly lower invertebrate densities and biomass were observed in willow-lined sections of the streams than in nearby open sections in summer, autumn, and winter. This result was observed in riffles and pools, for most dominant species and nearly all functional feeding groups. The effect was not associated with differences in the amount of fine paniculate organic matter (< 1 mm), stone surface organic layer biomass, or chlorophyll a concentration, which were similar it open and willow-shaded sites. Amounts of coarse paniculate organic matter (> 5 mm) were significantly higher in willow-shaded riffles, but this did not result in increased abundance or biomass of shredders. Willow trees reduced incident stream illumination by as much as 80%, but did not appear to influence water chemistry between open and willow-shaded sites. The decreased invertebrate densities probably result from a decrease in average substrate size and/or a lowering of food production through shading effects. M93063

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Seasonal variations in the species composition, abundance, biomass, and vertical distribution of zooplankton in two eutrophic, warm-monomictic lakes are described for the period December 1969 to February 1972. In Lake Hayes, Boeckella dilatata was the dominant crustacean and Ceriodaphnia dubia was subdominant. Lake Johnson was dominated by Ceriodaphnia and Bosmina meridionalis and Boeckella were subdominant. Daphnia were common in spring in Lake Hayes, but very rare in Lake Johnson. Chydorus and Asplanchna appeared in summer in both lakes. Zooplankton were slightly more abundant in both lakes in 1971 than in 1970; the annual mean was 1.7 times higher in Lake Johnson than in Lake Hayes in both years. Biomass was highest in spring. In Lake Hayes it ranged from 0.7 to 4.6 g.m -2 dry weight; in Lake Johnson it fluctuated in 1971 from 0.3 g.m -2 in March to 7.4 g.m -2 in November. Animals occurred at all depths during holomixis, but were nearer the surface during stratification. Vertical distribution was often bimodal, with a peak in the metalimnion and a second, smaller one in the epilimnion.In Lake Johnson a stratum of anoxic water developed in the metalimnion each summer. In 1971 Boeckella were confined to the oxygenated hypolimnetic water below this zone, and almost disappeared from the lake as the hypolimnion became anoxic in late summer. In 1970, when they were largely confined to the epilimnion, the population increased over the same period. Ceriodaphnia had a similar vertical distribution to Boeckella in 1971, but did not decline in numbers, perhaps because of reproduction by the small number of animals remaining in the epilimnion. It is postulated that an increase in the cyclopoid population of the oxygenated hypolimnion was the result of recruitment, either from the benthic region or by migration from the epilimnion through the anoxic layer.Chydorus has appeared in the plankton of Lake Hayes since 1953, and cyclopoids have become more abundant. These changes are consistent with other evidence that the lake is becoming more eutrophic.