Long-term trends in water quality and their impact on macroinvertebrate assemblages in eutrophic lowland rivers.

Long-term trends in water quality in eutrophic lowland rivers in eastern England were investigated and their impact on macroinvertebrate assemblages studied. Dissolved oxygen (DO) declined significantly in eight rivers in Essex and Suffolk over 40 years to 1998. Chloride concentrations significantly increased during the same period in most rivers. Total oxidized nitrogen and soluble reactive phosphorus increased until the 1980s, then began to decline. Biotic scores (Lincoln Quality Index) generally increased over 14 years to 1998 and there were significant positive relationships between biotic scores and several nutrients. Invertebrate families and environmental variables sampled over the eight rivers in a dry year (1997) and a wet year (1998) were subjected to multivariate analysis. River stretches were grouped according to substrate requirements of indicator invertebrates. In the dry year, those river stretches behind mills or immediately downstream of sewage treatment works (STW) were grouped. In the wet year, there was only one separate group, comprising sites downstream of STWs. Nutrients, DO and low flows have a much greater influence on water quality, and hence invertebrate assemblages, during drought years than during wet years.

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 Kützing by up to 42% over seven days when floating above mats of C. glomerata in containers. Dissolved oxygen (DO) increased by 23% at 30 degrees C 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 30 degrees C 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 (Fv/Fm) 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 micromol 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 (28 degrees C and 30 degrees C). L. minor significantly reduced relative electron transport rate (rel. ETR) of the controls by up to 71% at 30 degrees C. Relative electron transport rate did not reach light saturation point (Esat) except at 28 degrees and 30 degrees C under mats of L. minor. Whereas the highest rate of production (rel. ETRmax) and Esat 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 Lenma 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 photorespiration. 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.