The influence of environmental factors and nutrient contentrations in tissues of the seaweed <i>Ahnfeltia tobuchiensis</i> (Rhodophyta: Ahnfeltiales) on the primary production and dark respiration of its population
Complex investigations of the influence of environmental factors - temperature, photosynthetically active radiation (PAR), ambient seawater concentrations of ammonium (NH<sub>4</sub>) and orthophosphate (РО<sub>4</sub>), as well as contents of organic carbon (C), nitrogen, phosphorus, and chlorophyll <i>a </i>(Ch) in seaweed tissues on the rate of photosynthesis (P<sub>n</sub>) and dark respiration (R<sub>d</sub>) of the unattached red seaweed <i>Ahnfeltia tobuchiensis</i> (Rhodophyta: Ahnfeltiales) population were performed in summer 2000 and 2008 in Izmena Bay (Kunashir Island) under <i>in situ</i> conditions<i>.</i> The dependence of photosynthesis on PAR intensity (P-I dependence) is described by the equation of a hyperbolic tangent. The population of <i>A. tobuchiensis</i> forms a layer of up to 50 cm thick with an area of 23.3 km² and a biomass of 125 000 tons. The P<sub>n</sub> rate of seaweed population during daylight hours varies within a wide range with an average of 1.04 mg О<sub>2</sub>/(g<sub>dry wt</sub> h) and largely depends on PAR intensity and availability (r = 0.70-0.98). The maximum photosynthesis rate (P<sub>max</sub>) is substantially defined by the ambient concentration of NH<sub>4</sub> (r<sup>2 </sup>= 0.91, p < 0.01). The rate of R<sub>d</sub> during the night is on average 0.1 mg О<sub>2</sub>/(g<sub>dry wt</sub> h) and mainly depends on the content of Ch in seaweed tissues (r<sup>2 </sup>= 0.83, p < 0.01), which, in its turn, is regulated by the ambient concentration of PO<sub>4</sub> (r<sup>2 </sup>= 0.86, p < 0.01). With average biomass values of 5.4 kg/m<sup>2</sup> or 1.8 kg<sub>dry wt</sub>/m<sup>2</sup>, the net primary production (P<sub>n</sub>) of seaweed population is estimated to be on average 22.5 g О<sub>2</sub>/(m<sup>2</sup> day) or 8.4 g C/(m<sup>2</sup> day). Based on these indices, the investigated population is one of the most productive ecosystems of the World Ocean. It is supposed that such indices of the <i>А. tobuchiensis</i> population are attained due to the highly efficient use of weak light and a low light saturation level of photosynthesis, compared to other seaweeds.