Microalgal biovolume is commonly calculated to assess the relative abundance (as biomass or carbon) of co‐occurring algae varying in shape and/or size. However, a standardized set of equations for biovolume calculations from microscopically measured linear dimensions that includes the entire range of microalgal shapes is not available yet. In comparison with automated methods, the use of microscopical measurements allows high taxonomic resolution, up to the species level, and has fewer sources of error. We present a set of geometric shapes and mathematical equations for calculating biovolumes of >850 pelagic and benthic marine and freshwater microalgal genera. The equations are designed to minimize the effort of microscopic measurement. The similarities and differences between our proposal for standardization and previously published proposals are discussed and recommendations for quality standards given.

In this study we examined the role of nutrient cycling and herbivory in regulating stream periphyton communities. Population, community, and ecosystem—level properties were studied in laboratory stream channels that had nutrient inputs reduced compared to channels where ambient nutrient levels were maintained. We reduced nutrient inputs in four of eight channels by recirculating 90% of the flow, whereas the other four channels received once—through flow of spring water. We examined the interaction between herbivory and nutrients by varying the number of snails (Elimia clavaeformis) among streams with different nutrient input (circulation) regimes. Reduction in nutrient input viar recirculation resulted in lower concentrations of nutrients in the water but did not result in significant differences in biomass, carbon fixation, or algal taxonomic composition. However, herbivory had large effects on these characteristics by reducing biomass and areal rates of carbon fixation and simplifying periphyton taxonomy and physiognomy. Lower rates of nutrient input significantly affected characteristics associated with nutrient cycling. Streams with reduced nutrient inputs had lower periphyton nutrient contents, higher ratios of total : net uptake of P from water, and higher rates of phosphatase activity than streams with ambient nutrient inputs. However, the effects of reduced nutrient input on cycling characteristics were reduced or eliminated by intense herbivory. Our results indicate that nutrient cycling can increase in response to reductions in nutrient input once sufficient biomass or detritus accumulates to facilitate cycling, thereby allowing biomass accrual and carbon fixation rates to be maintained at levels achieved at higher nutrient inputs. In streams with low nutrient input, cycling can meet a large fraction of the nutrient demand of the periphyton. Because our experimental design involved a reduction from ambient nutrient levels rather than the usual enrichment techniques (e.g., additions to water, nutrient—diffusing substrates) in aquatic ecosystems, we have been able to observe how biological structure and processes are influenced by external nutrient inputs at the low natural levels of most undisturbed streams.

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low‐nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 106 and 2 × 106 cells‐cm−2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop. Synedra ulna (Nitz.) Ehr. and Achnanthes minutissima Kütz. were codominant during early stages of community development in both habitats, but succession to an A. minutissima‐dominated community was much faster in NP‐enriched than in control conditions. Species dominating early stages tended to immigrate quickly, whereas species that increased in relative abundance during community development had either fast growth rates or fast immigration and average growth rates. Decreases in growth rates indicated resource supply became limiting during community development in control and enriched channels. Density‐dependent competition was indicated because nutrient concentrations in the water column and light did not decrease during the 30‐d study. Species autecologies were defined by effects of nutrient enrichment, shading, and community development on species growth rates. Differing autecologies of early and late succession species indicated that competition for nutrients was more important than competition for light. Species autecologies also indicated ecological strategies. The species most stimulated by nutrient enrichment were least able to maintain growth rates as algal abundances on substrata increased. In addition, these species that best sustained their growth rates during succession tended to have the highest immigration rates, indicating that drift and immigration may have been an important mechanism of persistence for some populations when resources become limiting within thick benthic mats.

1991. Interactive effects sf nutrient reduction and herbivory on biomass, taxonomic structure, and P uptake in lotic periphyton communities. Can. 1. Fish. Aquat. Sci. 48: 1951-1 959. Four treatments were imposed on eight laboratory streams in a factorial design to examine the roles of nutrient reduction and kerbivory on periphytsn communities. Treatments included two flow regimes (once-through flow or 90% recirculated water) and two levels of grazer density (1 0001 or OI.m-2, using the snail Elimia clavaeforrnis). Periphyton biomass was significantly greater in streams without snails than in those with them, but water supply had no overall significant effect on biomass, even though inorganic P and N concentrations were significantly lower in recirculated than in once-through streams. Areal-specific P uptake rates (measured with 3"P) were significantly greater on two dates in ns-snail streams compared with snail streams, presumably because of the greater biomass levels in the former systems. Differences in biomass-specific P uptake rates were not significantly affected by either grazer density or water supply. Relative abundances of most algal species were unaffected by the water supply treatment, although percent biovolume of two Epithernia species was greater in no-snail, recirculated than in no-snail, once-through streams. Grazing activity dramatically reduced the percent biovolurne of species with upright growth forms, resulting in dominance by species with prostrate growth forms.Quatre traitements snt 6te appliques 2 huit cours d'eau en laboratoire seloo un plan d1exp6rience factoriel afin d'etudier le r6le de la reduction des elements nutritifs et des herbivores sur des communautes de periphyton. bes traitements comprenaient deux regimes de debit (circulation unique ou recirculation de I'eau 2 90 %) et deux niveaux de densite des brouteurs (1 000 ou O~r n -~; organisme utilise : Edirnia cbavateformis). La biomasse du Nriphyton etait significativernent superieure dans les cours d'eau d6pourvus d'escargots 2 ce qu'elle etait dans ies autres, mais, dans I'enserrrble, J'alimentation en eau n'avait aucun effet significatif sur la biornasse meme si la concentration en P et N inorganique etait significativement moindre dans I'eau recircaalee que dans I'eau non recirculee. Le taux d'absorption de P par unit6 de surface (mesure au moyen du "P) a ate significativement superieur 2 deux dates dans les cours dleau sans escargot 3 comparer aux cours d'eau avec escargots; on peut penser que cela est aatribuable A la plus grande biornasse trouvke dans les premiers circuits. Les diff6rences dans le taux d'absorption du P en fonction de la biornasse n'ont pas 6te significativement modifikes par la densit6 des brquteurs ni B e mode d'alimentation en eau. L'abondance relative de la plupart des especes algaires n'a pas etk rnodifike par le mode d'alimentation en eau meme si le volume de matiere organique, en pourcentage, de deux especes dPEpithernia ktait supkrieur dans les cours d'eau recirculee et sans escargot que dans les...