Dynamics of algal production and ephemeropteran grazing of periphyton in a tropical stream

Abstract

We estimated the net primary productivity of periphyton algae and rate of macroinvertebrate grazing in a third-order stream in coastal Atlantic forest, Brazil. Net primary productivity was estimated from the growth curve of periphyton chlorophyll, based on the relationship between algal density and algal production, with baetid ephemeropteran grazers excluded by highintensity electric shocks. We fit a logistic model and cite the maximum productivity 44.7mgCm � 2 d � 1 . We manipulated grazing pressure by excluding the shrimp Macrobrachium olfersi by low-intensity electrical exclusion—the shrimp was known to have a negative effect on ephemeropteran grazing and to cause a trophic cascade on periphyton. Thus, rate of grazing was estimated at the normal level of grazing and with grazing enhanced by the exclusion of shrimps. Normal level of grazing reduced periphyton algal stock (6mg chlorophyll m � 2 ) by slightly less than 50%, enhanced grazing by slightly more than 50%. This implied that grazing removed algae at approximately the maximum net productivity—that is 19%d � 1 and24%d � 1 ofalgalstockinnormalandenhancedgrazingrespectively,equivalentto 44.1mgC m � 2 d � 1 in both cases. The observed net primary productivity obtained by direct measurementmatchesthatexpectedfrompredictiveequationsbasedonchlorophylllevels,but isatthelowerpartoftherangeobservedinworldwideandtropicalsurveys.Itmatchesresultsof recentstudiesinthetropicswhichusedopen-channeldielchangesinoxygentocalculategross primary productivity. The method appears reliable and is potentially applicable to situations in which other methods are constrained by low productivity and other technical difficulties.