Reduction in photosynthetic efficiency of Cladophora glomerata, induced by overlying canopies of Lemna spp.

The duckweeds Lemna minor L. and L. minuscula Herter reduced PSII quantum efficiency (F'(q)/F'(m)) of the filamentous green alga Cladophora glomerata Kutzing by up to 42% over seven days when floating above mats of C. glomerata in containers. Dissolved oxygen (DO) increased by 23% at 30degreesC in containers with C. glomerata over controls. But when the water surface in the containers was covered with Lemna spp. floating above C. glomerata, DO was 83% lower at 30degreesC over seven days than in control samples with no duckweed or alga. Dissolved oxygen was lower beneath a thick mat (1 cm) of either Lemna spp. covering the surface than under a thin layer (single-frond canopy). PAM fluorimetry showed that maximum PSII efficiency (F-v/F-m) of C. glomerata in containers was reduced under a canopy of L. minor by 17% over seven days, and under L. minuscula by 22%. F'(q)/F'(m) of C. glomerata in containers exposed to 51 mumol m(-2)s(-1) PPFD decreased under a canopy of L. minor by 16% over seven days, and under L. minuscula by 19% compared to controls. When light response curves were compared, F'(q)/F'(m) was significantly reduced under canopies of L. minor at the highest temperatures tested (28degreesC and 30degreesC). L. minor significantly reduced relative electron transport rate (rel. ETR) of the controls by up to 71% at 30degreesC. Relative electron transport rate did not reach light saturation point (E-sat) except at 28degrees and 30degreesC under mats of L. minor. Whereas the highest rate of production (rel. ETRmax) and E-sat increased with temperature in controls, under a canopy of Lemna, decreases were observed. It is suggested that, during periods of high summer temperature and irradiance, shading inhibits oxygenic photosynthesis in mats of C. glomerata beneath canopies of Lemna spp. This results in less oxygen being produced by the C. glomerata (oxygen produced by Lemna spp. is not released into the water), and this may further inhibit the C. glomerata by limiting oxygen-dependent electron transport and/or photo respiration. This feedback loop could lead to the eventual senescence of the C. glomerata. The combination of low oxygen, high temperature and stressed filamentous algae, particularly in slow or standing water, may help to explain sudden collapses in DO concentration, with detrimental effects on water quality downstream. (C) 2002 Elsevier Science Ltd. All rights reserved.