Ecologists have hypothesized that planktivorous fish have greater effects on the plankton and water quality of oligotrophic lakes than eutrophic lakes . We tested this hypothesis in a tank-mesocosm experiment of factorial design in which five biomass levels of filter-feeding omnivorous gizzard shad (Dorosoma cepedianum) were cross-classified with two levels of lake trophic state achieved by filling tank-mesocosms with water and plankton transported by truck from two lakes with different trophic states . The presence of gizzard shad significantly increased total phosphorus, primary productivity, chlorophyll, and particulate phosphorus (PP) 2-20 and 20-200 Am and significantly decreased Secchi depth, cladocerans, copepods and PP > 200 Am . The effects of gizzard shad on chlorophyll, Secchi depth, cladocerans, copepods and PP 2-20 and >200 Am were dependent on lake trophic state and most intense in the eutrophic lake system . This experiment suggests that filter-feeding omnivorous fish interact synergistically with trophic state so that the limnological effects of omnivorous fish become more intense with increased eutrophication .

We quantified the effects sf planktivore biomass and planktivore type in an experimental messcosm study sf factorial design in which five levels of fish biomass (0-75 @m3) were cross-classified with two plantivore types: filter-feeding gizzard shad (Dorossrna cepedianurn) and visual-feeding bluegill (Lepomis macrochirus). As fish biomass increased, cladocerans, cyclopoids, particulate phosphorus (PP) >200 pm, and chirsnomids declined; conversely, rotifers, primary productivity, chlorophyll a, turbidity, unicellular flagellates, colonial and unicellular green algae, pennate diatoms, total phosphorus, and 20-200 and 12-20 p m PP were enhanced. In the presence of gizzard shad, as compared with bluegill, cyclspoids, turbidity, unicellular green algae, pennate diatoms, >200 pm PP, and chironomid tubes were higher whereas colonial green algae and c0.2 pm PP were lower. Fish biomass operated independently of planktivore type for most variables, except copepods, colonial green algae, turbidity, and 20-200 krn PP. Although gizzard shad and bluegill have different trophic cascade pathways, fish biomass was more important than planktivore type as a regulator of plankton communities and water quality. planktivore biomass md plankton biomass appear to be nonlin-

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Aeroponics, a soil‐less plant culture system in which fresh nutrient solutions are intermittently or continuously misted on to plant roots, is capable of sustaining plant growth for extended periods of time while maintaining a constantly refreshed nutrient solution. Although used relatively extensively in commercial installations and in root physiology research, use of aeroponics in nutrient studies is rare. The object of this study was to examine whether nutrient uptake rates could be calculated for aeroponic systems by difference using measurements of concentrations and volumes of input and efflux solutions. Data were collected from an experiment with cranberry plants (Vaccinium macrocarpon Ait. cv. Stevens) cultured aeroponically with nutrient solutions containing various concentrations of ammonium‐N and isotopically labelled nitrate‐N. Validation of the calculated uptake rates was sought by: (1) evaluating charge balance of the solutions and total ion uptake (including proton efflux) and (2) comparison with N‐isotope measurements. Charge balance and proton efflux calculations required use of chemical modelling of the solutions to determine speciation of dissolved phosphate and acid‐neutralizing capacity (ANC). The results show that charge balance requirements were acceptably satisfied for individual solution analyses and for total ion uptake when proton efflux was included. Relative rates of nitrate/ammonium uptake determined by difference were in agreement with those determined by isotopic techniques. Additional information was easily obtained from this experimental technique, including evidence of diurnal variation in nutrient uptake, correlation between ammonium uptake and proton efflux, and the relationship between ion concentration and uptake. Use of aeroponic systems for non‐destructive measurement of water and ion uptake rates for numerous other species and nutrients appears promising.

We examined the effects of gizzard shad Dorosoma cepedianum and threadfin shad Dorosoma petenense on zooplankton abundance and on reproduction by white crappie Pomoxis annularis in a March–July 1986 Texas pond experiment of factorial design. We used four treatment combinations: no shad, gizzard shad, threadfin shad, and gizzard shad + threadfin shad. Densities of cyclopoid copepodids and Daphnia sp. were suppressed in the presence of threadfin shad but not gizzard shad. The presence of gizzard shad or threadfin shad was associated with decreased total number and biomass of young‐of‐year white crappies, a shift in white crappie size distributions toward larger individuals, and increased mean weight of white crappies in some size‐classes. The biomass of young‐of‐year gizzard shad decreased in the presence of threadfin shad. Our study suggests that gizzard shad and threadfin shad may decrease the density and biomass of young‐of‐year white crappies, but increase their mean size.