The influence of environmental factors and nutrient concentrations in tissues of the seaweed Ahnfeltia tobuchiensis (Rhodophyta: Ahnfeltiales) on the primary production and dark respiration of its population

Complex investigations of the influence of environmental factors, viz., the temperature, photosynthetically active radiation (PAR), ambient seawater concentrations of ammonium (NH₄), and orthophosphate (PO₄), as well as the contents of organic carbon (C), nitrogen, phosphorus, and a-chlorophyll (Ch) on the rate of photosynthesis (P_n) and dark respiration (R_d) in the tissues of the unattached red seaweed Ahnfeltia tobuchiensis (Rhodophyta: Ahnfeltiales) population, were performed in the summers of 2000 and 2008 in Izmeny Bay (Kunashir Island) under in situ conditions. The dependence of photosynthesis on PAR intensity (P-I dependence) is described by the equation of a hyperbolic tangent. The population of A. tobuchiensis forms a layer up to 50 cm thick with an area of 23.3 km₂ and a biomass of 125 000 tons. The P_n rate of seaweed population during daylight hours varies within a wide range, with an average of 1.04 mg O₂ O₂/(g _{dry weight} h) and largely depends on PAR intensity and availability (r = 0.70–0.98). The maximum photosynthesis rate (P_max) is substantially defined by the ambient concentration of NH₄ (r ² = 0.91, p < 0.01). The rate of R_d during the night is on average 0.1 mg O₂/(g _{dry weight} h) and mainly depends on the content of Ch in seaweed tissues (r ² = 0.83, p < 0.01), which, in its turn, is regulated by the ambient concentration of PO₄ (r ² = 0.86, p < 0.01). With average biomass values of 5.4 kg/m² or 1.8 kg_{dry weight}/m², the net primary production (P_n) of seaweed population is estimated to be on average 22.5 g O₂/(m² day) or 8.4 g C/(m² 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 A. tobuchiensis population are attained due to the highly efficient use of weak light and a low light-saturation level of photosynthesis, compared to other seaweeds.