A mechanism to account for macrophyte decline in progressively eutrophicated freshwaters

http://faculty.umb.edu/anamarija.frankic/eeos476/Class%20Materials/Habitats/burkholder2007.pdf

Seagrasses and eutrophication

Data on nutrient contents of 27 seagrass species at 30 locations were compiled from the literature. Mean (f SE) concentrations of carbon, nitrogen and phosphorus in seagrass leaves were 33.6 2 0.31, 1.92 f 0.05, and 0.23 2 0.011 % dry wt, respectively. The median C:N:P ratio was 474 :24: 1, which represents a C:P ratio more than 4 times, and a N:P ratio more than 1.5 times that of oceanic seston. These ratios are, however, less than those previously reported for marine macrophytes (550 : 30 : 1) by Atkinson & Smith (1984). Nitrogen and phosphorus variability within species was large, but carbon contents exhibited little variability. Accordingly, carbon:nutrient (N and P) ratios were inversely related to changes in nutrient content, and the rate of change in C:N and C:P ratios with increasing nitrogen or phosphorus content in plant tissues should shift from high to small as nutrient supply meets the plant's demands. The median nitrogen and phosphorus contents reported here (1.8 % N and 0.20 % P as % DW) correctly discriminated between seagrass stands that did or did not respond to nutrient enrichment, thus offering a useful reference for comparisons of seagrass nutrient contents.

/pdf/seagrass-nutrient-content-2i0zz3brui.pdf

Seagrass nutrient content

Published data on mean annual epilimnetic total N (TN) and P (TP) were analyzed to find how TN : TP varies with lake trophic status. TN : TP is high in oligotrophic lakes and very low in eutrophic lakes, declining in a curvilinear fashion with increased TP. Comparison of this trend with published N : P in lake nutrient sources suggests that TN : TP reflects the source of nutrients: the ratio is high in oligotrophic lakes because they receive their N and P from natural, undisturbed watersheds which export much less P than N; mesotrophic and eutrophic lakes receive various mixtures of nutrient sources that have lower average N : P, and very eutrophic lakes have N : P that correspond very nearly to the N : P of sewage. Two inflection points were identified in the TN : TP relationship (-20 and - 100 wg TP liter-l) the first probably reflecting the large difference between TN : TP in nutrient export from undisturbed terrestrial ecosystems and that of meso- and eutrophic sources such as urban and pasture land runoff and sewage, and the second probably reflecting increased rates of denitrification in eutrophic lakes. Analysis of published manipulation experiments shows that N limitation is not only significantly more frequent in lakes of low ambient TN: TP (TN: TP mass ratio 5 14) but is also significantly more frequent in lakes with TP > 30 wg liter’.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1992.37.5.0936

The nitrogen : phosphorus relationship in lakes

Herbivory on freshwater macrophytes

A tissue analysis technique was used to evaluate nitrogen and phosphorus supplies in natural waters for the growth of angiosperm aquatic plants. Tissue content of nitrogen and phosphorus was employed as an index of element availability in lakes from which plants were collected. This required establishment in the laboratory of the critical level for each element, that is, the minimum tissue content associated with maximum growth.

To establish critical levels, a system was developed for culturing algae-free plants in a synthetic nutrient medium. The critical nitrogen content for the several species studied was approximately 1.3%; the comparable phosphorus value was 0.13%.

The nitrogen and phosphorus contents of 13 species of aquatic plants collected during the summer from nine lakes were compared with the critical concentrations. In nine samples obtained mostly during periods of heaviest plant growth, the phosphorus content was at or below the critical level; in no sample was the nitrogen content less than the critical concentration. The results indicated that in all but one of the lakes, phosphorus supply was more likely to limit higher aquatic plant growth than was nitrogen.

The primary importance of this study is as an initial step in the development of a satisfactory technique for evaluating nutrient supplies in natural waters.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1966.11.4.0529

G. C. Gerloff

Papers

Tissue analysis as a measure of nutrient availability for the growth of angiosperm aquatic plants1