Physiological Responses to Temperature and Irradiance in Spirogyra (Zygnematales, Charophyceae)1

Spirogyra Link (1820) is an anabranched filamentous green alga that forms free‐floating mats in shallow waters. It occurs widely in static waters such as ponds and ditches, sheltered littoral areas of lakes, and stow‐flowing streams. Field observations of its seasonal distribution suggest that the 70‐μm‐wide filament form of Spirogyra should have a cool temperature and high irradiance optimum for net photosynthesis. Measurements of net photosynthesis and respiration were marie at 58 combinations of tight and temperature in a controlled environment facility. Optimum conditions were 25°C and 1500 μmol photons m−2 s−1, at which net photosynthesis averaged 75.7 mg O2 gdm−1 h−1. Net photosynthesis was positive at temperatures from 5° to 35°C at most irradiances except at combinations of extremely low irradiances and high temperatures (7 and 23 μmol photons m−2 s−1 at 30°C and 7, 23, and 35 μmol photons m−2 s−1 at 35°C). Respiration rates increased with both temperature and prior irradiance. Light‐enhanced respiration rates were significantly greater than dark respiration rates following irradiances of 750 μmol photons m−2 s−1 or greater. Polynomials were fitted to the data to generate response surfaces; such response surfaces can be used to represent net photosynthesis and respiration in ecological models. The data indicate that the alga can tolerate the cool water and high irradiances characteristic of early spring but cannot maintain positive net photosynthesis under conditions of high temperature and low light (e.g. when exposed to self‐shading).