The photosynthetic and growth rate responses of two freshwater angiosperms in lakes of different trophic status: responses to light and dissolved inorganic carbon

SUMMARY 1. Two rooted freshwater macrophytes (Lagarosiphon major (Ridley) Moss; Myriophyllum triphyllum Orchard) were grown in two lakes of differing trophic status on sediments collected from an eutrophic lake. In the two experiments (summer and winter) in the oligotrophic lake (Lake Taupo, New Zealand), the relative growth rates of both species were approximately double those recorded in the experiments in the eutrophic take (Lake Rotorua, New Zealand). These growth responses occurred even though the light and temperature regimes were similar at the experimental sites in both lakes and the concentrations of inorganic nutrients were higher in the eutrophic lake water. 2. Dissolved inorganic carbon (DIC) analysis of both lake waters indicated that the oligotrophic water had approximately 10‐fold more DIC than the eutrophic water. This corresponded, at natural pH, to a 2‐fold higher concentration of free‐CO2 in the oligotrophic lake water (49 μM) than in the eutrophic lake water (23 μM). Photosynthetic experiments demonstrated that the maximum rates of photosynthesis were 2.46 and 2.25‐fold higher in L. major and M. triphyllum, respectively, when the shoots were incubated in Lake Taupo water at Lake Taupo levels of free‐CO2 relative to Lake Rotorua water at Lake Rotorua levels of free‐CO2. 3. It is concluded that these differences in photosynthetic rates would lead approximately to a 2‐fold difference in growth rate, thus explaining the growth responses observed in the field experiments.