Nutrient Addition Rates Research Articles

Effects of chaetognath predation and nutrient enrichment on enclosed estuarine copepod communities.

The effects of predation by the chaetognath Sagitta hispida and nutrient enrichment on estuarine copepod community structure were studied by experimental manipulations in large enclosures. Chaetognath abundance and nutrient addition rates were manipulated in a factorial design. Predation by chaetognaths resulted in a significant decline in the relative abundance of Acartia tonsa and an increase in the relative abundance of Oithona colcarva. However, these effects were evident only at chaetognath densities far higher than observed in natural populations. Nutrient enrichment resulted in a decline in relative abundance of Paracalanus crassirostris, and, in the absence of chaetognath predation, an apparent increase in the relative abundance of A. tonsa. The effects of chaetognath predation were independent of enrichment level, apparently because of the absence of effects of enrichment on total copepod and chaetognath densities.

The English–Wabigoon River System: V. Mercury and Selenium Bioaccumulation as a Function of Aquatic Primary Productivity

An experiment was carried out in four 100-m3 in situ enclosures to determine the effect of primary production rate on mercury and selenium concentrations of biota and to test the possibility of ameliorating mercury pollution problems by increasing ecosystem primary productivity. Two enclosures were controls. Primary productivity in a third enclosure was increased fourfold by addition of NaNO3 and NaH2PO4. This stimulation was not sufficient to change pH although the growth rate of fish was enhanced. In this enclosure, mercury concentrations in pearl dace (Semotilus margarita) whole body and muscle samples increased two- and five-fold, respectively, exceeding the dilution of mercury by the enhanced growth rates. In the fourth enclosure, at the highest rate of nutrient addition, primary productivity was increased ninefold, pH was elevated from about 7.9 to 9.2, and the growth rate of fish was stimulated. In this case, the elevation of pH retarded the rate of mercury bioaccumulation in comparison with the enclosure of intermediate productivity. Based on these results, stimulation of primary productivity is not recommended as a mercury ameliorating procedure. Stimulation of primary productivity resulted in a general decrease in the concentration of selenium in the aquatic food chain probably resulting from dilution of selenium by enhanced growth rates of fish and other biota.

Relative addition rate and external concentration; Driving variables used in plant nutrition research

Abstract An increasing literature accounting for various types of experiments indicates that far lower external nutrient concentrations are required by plants than is usually thought to be the case. It is concluded that the ion uptake capacity of the roots, as described by the carrier concept, is high compared to that required for maintenance of the internal concentration. Serious errors in experimental conclusions are associated with insufficient and constant nutrient addition rates. The main errors are caused by non‐steady states of the plants both with regard to the internal nutrient concentrations and the relative growth rate.A dynamic concept has been proposed for direct use as the treatment variable within the range of sub‐optimum nutrition. The nutritional factor is expressed as a flow, the relative nutrient addition rate in laboratory studies and the nutrient flux density in the field. The experimental use of the relative addition rate has led to steady‐state nutrient status and relative growth rate and the interpretation of plant responses which differ fundamentally from accepted views. Thus, for instance, deficiency symptoms disappear, as in natural conditions, when the internal nitrogen concentration is stable, independent of level. The nutrition/growth relationships are very different from those observed when external concentration is varied. The regression line of relative growth rate on relative addition rate passes near to the origin at an angle close to 45 to the axes, which implies that the obtained relative growth rate approximates closely the treatment variable. A striking example of observed differences is the positive effect on nitrogen fixation exerted by high relative nitrogen addition rates compared to the well‐known negative effect of increasing external nitrogen concentration.The application of fertilizer on the basis of the nutrient flux density concept provides the possibility of supplying fertilizers corresponding to the consumption potential of the vegetation and to the natural flux density resulting from mineralization in the soil. Nitrogen utilization is high under such conditions and the resulting feedback of nutrition on the mineralization rate suggests that there will be a long‐term increase in fertility.

Growth and nutrition of three Salix clones in low conductivity solutions

The ability of Salix aquatica Smith, S. fragilis L. and S. vlminalis L. to grow in low conductivity solutions was investigated. The effects of four different relative nutrient addition rates on the N, K, P, Ca, and Mg contents of the plants, on concentrations in the media, and on dry matter distribution between roots, stems, and leaves, were measured. No significant and persistent increase of concentration of any element resulting from the daily nutrient additions was measured in the media. Practically all nutrients were consumed by the plants at the same rate and in about the same proportions at which they were supplied. Linear regressions were found between any two of the three factors: relative rate of nutrient supply, nitrogen status, and relative growth rate. High nutrient addition rates favoured leaf development while low nutrient addition rates stimulated root growth. It is concluded that the decisive factor governing the mineral uptake rate in Salix is the rate of nutrient supply and that these clones are well suited for short rotation energy forestry in so far as their ability to absorb the mineral elements efficiently is concerned.