Shade effects on stream periphyton and invertebrates: an experiment in streamside channels

The effects of shade on periphyton and invertebrates were investigated by comparisons in 12 replicate channels beside a pasture stream where shade cloth reduced the photosynthetically available radiation (PAR) by 0, 60, 90, and 98%. Periphytic productivity decreased with increasing shade. Periphyton chlorophyll a was consistently low (< 30 mg m(-2)) under 98 and 90% shade and usually low at 60% shade, whereas blooms were common during summer in the unshaded channels. More periphytic taxa occurred in the open than shaded channels and the relative concentration of the photo-protective pigment beta-carotene decreased with increasing shade. Nitrate uptake rates by periphyton, measured in separate chamber studies, decreased progressively as shade increased from 60% through 90% to 98%, and were more strongly correlated with gross primary productivity than periphyton chlorophyll a and particulate carbon levels. Total invertebrate and chironomid densities declined significantly as shade increased from 60 to 90% and invertebrate taxa richness declined markedly between 90 and 98% shade. Most collector-browsing invertebrates (other than chironomids) had similar densities under 0-90% shade and only declined at 98%. This suggests a weak coupling of these invertebrates with local periphyton production and also suggests that energy derived from up-stream sources may be their most important food in these pastoral streams.