In the temperate region of the World, aquatic macrophytes show very sharp annual variation, with a growth season of their aerial biomass during the spring and summer, and another season of the underground biomass and detritus accumulation during the fall and winter (Esteves & Camargo 1986). In the tropical region, however, deterministic of the aquatic macrophytes biomass seasonality are the rainy and dry periods (Esteves 2011). Very little was done, however, up to now regarding the charophytes biomass seasonality in the tropics. Perhaps the only contribution in this regard is the work by Carneiro et al. (1994), who studied the extensive Chara hornemanii beds prospering in the Piratininga Lagoon, State of Rio de Janeiro, southeast Brazil at the depth from 0.30 cm to 1 m, and realized that N and P inputs, low water turnover and low water column depth favored growth of phytoplankton, macroalgae and aquatic macrophytes, including charophytes. The same authors also observed a very clear seasonal behavior of the charophyte population that stared during the winter and lasted until the beginning of summer, when the alga covered about 60% of the lagoon sediments. During the summer, the alga biomass reached 500 mg m-2 (Carneiro et al. 1994).

Using aerial photographs and field work in brackish water lagoons of Aland Island, Finland, Berglund et al. (2003) observed seasonal and interannual growth, distribution and biomass variation of some charophyte species. According to the last authors, filamentous green algae contributed with 45-70% of the total biomass studied, charophytes with 25-40% and vascular plants with 3-18%. The biomass peak was reached in July and August, and the average biomass was negatively correlated with the charophytes exposition to direct sun light, i. e. the charophyte coverage was greater when their exposition to solar radiation was low, being highly affected by the presence of filamentous algae.

Seasonal changes in the biomass of a monospecific community (Chara globularis) and of several communities with high charophyte coverage (Chara globularis-Myriophyllum alterniflorum, Chara globularis-Potamogeton gramineus and Nitella translucens-Potamogeton natans) were studied monthly, from May 1996 to June 1997, by Fernandez-Alaez et al. (2002) in three shallow lakes in northwest Spain. Weather and hydrological regime strongly influenced the seasonal biomass patterns and the between-the-year differences in the biomass of the macrophytes. The Chara globularis community biomass showed a bimodal pattern, with maximum in mid-July (128 g DW m-2) and late autumn (165 g DW m-2). Chara globularis overwintered as a green plant and during the subsequent growth period characterized by high temperature and low rainfall reached a maximum of 305 g DW m-2 in June 1997. The highest biomass of Chara globularis in the Chara-Myriophyllum community was reached in July (Lake Sentiz 160 g m-2, Lake Redos 204 g m-2), while the minimum (Lake Sentiz 10 g m-2; Lake Redos 3 g m-2) was recorded in February or March. Myriophyllum alterniflorum (average biomass 95 g m-2) was a better competitor than Chara globularis in Redos lake and appeared to be favored by the early beginning of the growing season in 1997 and by the later increase in the water level. Nitella translucens biomass (average 64 g m-2) showed a high stability during the entire study period, but lacked a well-defined seasonal pattern. Potamogeton natans had a marked maximum biomass in August (426 g m-2). Although the stability of the Potamogeton natans population was low, shading did not have a significant influence on the development of Nitella translucens biomass.

Torn et al. (2006) measured the seasonal dynamics of the biomass, elongation growth and primary production rate of Chara tomentosa in Rame Bay, NE Baltic Sea, a shallow and semi — enclosed sea inlet on the western coast of the Estonian mainland, during the vegetation

period of 2002. Their measurements showed extremely high plant heights (up to 1.42 m) and biomass values (5.2 kg (w. w.) m-2) indicating the importance of the charophyte for the aquatic ecosystem. Torn et al. (2006) observed that the apical part of the plants grew more intensively from early spring to midsummer, whereas that of the subapical one was very low during the entire study period. The plant’s net primary production rate peaked in July (43.4 mgO g(d. w.)-1 24 h-1), remarkably lower rates being measured in May and September. The elongation growth and primary production were not correlated with the water nutrient concentrations and temperature. As the active growth of Chara tomentosa takes place during a relative short period at the beginning of summer, the amount of available solar radiation and the temperature levels during this sensitive time may have had a significant effect on the community in the same year (Figure 2).

Seasonal growth of Chara globularis var. virgata caused a regular summer depletion of Ca2+ and HCO3- by associated CaCO3 deposition, and a more extreme and unusual depletion of K+ was followed over three years (1985-1987) by Talling & Parker (2002) in a shallow upland lake (Malham Tarn) in northern England. Chemical analysis of the Chara globularis var. virgata biomass and of the underlying sediments indicated a large benthic nutrient stock that far surpassed that represented by the phytoplankton. Growth in the Chara globularis var. virgata biomass and the magnitude of the water-borne inputs influenced removals of Ca2+, K+ and inorganic N. According to Talling & Parker (2002), several features of Malham Tarn are suggestive in relation to the general case of phytoplankton-phytobenthos interaction and possible long-term change. So, the low P concentrations in the open water are probably linked to the fairly low phytoplankton abundance and influenced by the dense benthic Chara globularis var. virgata with a major capacity for P uptake. Also, the additional Chara globularis var. virgata capacity for K+ uptake led to a major seasonal reduction of concentration in the lake water and outflow, of a magnitude rarely if ever recorded elsewhere. The annual growth of Chara globularis var. virgata seemed to involve further translocation of N, P and K from stocks in the sediments.