EFFECTS OF FLOWING WATER ON NITROGEN‐ AND PHOSPHORUS‐LIMITED PHOTOSYNTHESIS AND OPTIMUM N:P RATIOS BY SPIROGYRA FLUVIATILIS (CHAROPHYCEAE)1,2

Abstract

ABSTRACTThe effects of flowing water on net photosynthesis, dark respiration, specific growth rate, and optimum N:P ratios by Spirogyra fluviatilis Hilse were assessed. The alga was cultivated under nitrogen or phosphorus limitation in laboratory streams at three flow velocities: 3, 12, and 30 cm·s−1. The Droop equation adequately described respiration and photosynthesis (PSnet) as a function of N or P cell quota (QN or Qp). The data show that for N‐ or P‐limited Spirogyra fluviatilis, flowing water is physiologically costly. Generally, flowing water had little effect on respiration rates; however, the proportion of gross photosynthesis devoted to dark respiration did increase with flow velocity. For photosynthesis, the minimum N and P cell quotas increased with velocity, and the theoretical PSnet maxima for N and P both appeared greatest at 12 cm·s−1. The Droop models showed that for any given QN or Qp, PSnet, was reduced by the 30‐cm·s−1 treatment. Consistent with this finding, independent estimates of specific growth rates for P‐limited S. fluviatilis in the laboratory streams were inversely related to flow velocity when ambient PO4−3 was undetectable. However, growth was not diminished at the fastest velocity when PO4−3 was available for uptake. Thus, the increase in cellular phosphorus demand can be offset by flow‐enhanced P uptake when conditions permit; otherwise, growth will be impaired.The optimum N:P ratios for S. fluviatilis at 3, 12, and 30 cm·s−1 were 50, 58, and 52 by atoms, respectively, when calculated for PSnet= 0. The optimum ratios were inversely related to PSnet and decreased to approximately 20 when PSnet was near maximum. The potential for flowing water to mediate nutrient partitioning among lotic algae by altering growth rates and optimum nutrient ratios is discussed.