The concentrations of N, P and C in the above-ground biomass of 14 dominant macrophyte species (including Chara globularis and Nitella translucens) in seven shallow lakes of NW Spain were measured by Fernandez-Alaez et al. (1999) that found significant differences for the three nutrients among the species and among the groups of macrophytes. The charophytes showed the lowest P (0.053% dry weight) and C (35.24% dry weight) content. Also, only the charophytes exhibited a strong association between N and P (r = 0.734, p < 0.0001), reflecting an important biochemical connection in these species.

Phosphorus was established as a limiting factor of all the macrophytes (N:P = 35:1), especially charophytes, in which it was below the critical minimum. Siong et al. (2006) used sequential P fractionation to study the nutrient speciation in three submersed macrophytes species, Chara fibrosa, Najas marina and Vallisneria gigantea, and the implications for P nutrient cycling in the Myall Lake, New South Wales, Australia. The mean TP of both Najas marina and Vallisneria gigantea was significantly higher than that of Chara fibrosa, even when the comparison made was based on the ash-free dry weight (AFDW). However, P co­precipitation with calcite (CaCO3) induced during intense periods of photosynthesis occurs in hard water lakes, and this indirect mechanism of reducing P bioavailability in the water column may have been underestimated in assessing Chara beds acting as nutrient sink in shallow lakes. According to their results, besides the indirect mechanism above, P in the water column was also directly co-precipitated with encrusted calcite along the charophyte intermodal cell, and such a calcification should be regarded as a positive feedback in stabilizing Chara dominance in lakes. Siong & Asaeda (2009) studied the effect of Mg on the charophyte calcite encrustation, and assessed whether charophytes growing on the non­calcareous sediments of the Myall Lake could function as an effective nutrient sink for P in a

similar manner to charophytes growing on the calcareous sediments of freshwater calcium — rich hard water systems. According to the last authors, calcification of Chara fibrosa was significantly inhibited by Mg in the water column and, consequently, reduced the formation of Ca-bound P that has a potential sink for P. However, a large percentage of non­bioavailable forms of P in the lake sediments suggested that P sink was through burial of dead organic matter and subsequent mineralization process.

The inorganic phosphorus concentration was not yet significantly related to the charophyte biomass. Palma-Silva et al. (2002) observed that the charophyte community (Chara angolensis and Chara fibrosa) sometimes occupied the entire benthic region in the Imboacica coastal lagoon in Brazil, and presented a large variation in C:N:P ratio. Results of their investigation (samples taken in March, April, May, July and October 1997) indicated that the charophytes fast growth may have absorbed a great amount of the nutrients entering the lagoon. Values of nutrient concentrations in the charophytes biomass were, according to those authors, within the expected range for the group, with the most eutrophic sampling station in the lake showing the highest N and P values. C:N:P ratios presented high values, and the biomass values were higher in the less eutrophic areas. The biomass reached maximum values of between 400 and 600 g DW m-2, and the C:N:P ratio varied from 51:7:1 to 1603:87:1, indicating that the two Chara species may grow in a wide range of nutrient concentration. The same authors concluded that the charophyte community would be responsible by the nutrient decrease in the water column and keeping the water clear after drawdowns (Palma — Silva et al. 2002).

Several authors concluded that the nutrient kinetics favor the phytoplankton growth over Chara, thus assuming a P-limited condition. Therefore, although nutrient concentration may influence the charophyte phenology and abundance, light appeared to be a stronger regulator in the Okeechobee Lake. Schwarz & Hawes (1997) also observed the influence of the water transparency on the variation of the charophyte biomass in the Coleridge Lake, New Zealand. In the latter lake, total algal biomass did not surpass 180 g DW m-2 between 5 and 10 m depth. Pereyra-Ramos (1981) worked with seven charophyte species collected from Polish lakes and observed an increase of their fresh dry weight during the summer (July): Chara rudis 2.07 kg m-2, Chara vulgaris 1.61 kg m-2, Chara contraria 0.54 kg m-2, Chara fragilis 0.39 kg m-2, Chara jubata 0.37 kg m-2, Chara tomentosa 0.28 kg m-2 and Nitellopsis obtusa 0.24 kg m-2. Together, the charophytes represented 53% of the total submersed macrophytes biomass, 28% of Elodea sp. and 8% of Ceratophyllum demersum, two submersed macrophytes. According to Howard-Williams et al. (1995), Chara corallina biomass in deep (average 90 m depth) New Zealand lakes ranged around 300 g DW m-2. Bakker et al. (2010) registered a strong decline of the Chara sp. biomass under the nutrient enriched condition of Lake Loenderveen, Norway. Similar situation was already detected by Blindow et al. (1993) and van de Bund & van Donk (2004) for other water bodies.