I review recently published research (1970–89) on freshwater periphyton, with emphasis on epilithon and epiphyton Brushing syringe-samplers are recommended for sampling epilithon, due to their Sow

A Critical Review of Recent Freshwater Periphyton Field Methods

Qualitative and quantitative measures of the Aufwuchs (scum flora) on artificial substrates in situ were used to evaluate the effects of grazing by freshwater pulmonate snails in a shallow pond in southeastern Michigan. Grazer densities of ∼ 216 snails/m2 marked reduced standing crop so that after 45 days grazed substrata had 6.46 mg dry weight, 604 µg C and 4.18 µg chlorophyll a as compared to controls with 30.62 mg dry weight, 3699 µg C and 6.29 µg chlorophyll a, all on a per dm2 basis.

Effects of grazing on the quantity and quality of freshwater Aufwuchs

(1)

From May to December 1971 at six sites in upstate New York, samples of freshwater Aufwuchs (the largely algal “scum” flora) were collected on sets of exposed microscope slides. Successive standing crop biomass estimates are presented as analyses of total organic carbon (by wet oxidation) and total nitrogen (by micro-Dumas). Nutritional quality of the Aufwuchs can be assessed in carbon to nitrogen (C:N) ratios, and these are correlated with substantive data from concurrent studies on the growth and fecundity of second trophic level snails (Laevapex and Lymnaea).




(2)

Peak values of carbon biomass for the six sites range from 1.1 mgC/dm2 to 4.2 mgC/dm2, and mean C:N ratios ranged from 3.7:1 to 10.1:1 (corresponding to 29% protein). Higher snail growth rates (computed as increase in mgC/100 snails/30 days) and greater fecundities correlate, beyond certain minimal levels of carbon biomass, to lower C:N ratios in the corresponding Aufwuchs samples.




(3)

In discussing these nutritional aspects of Aufwuchs production, it is emphasized that freshwater macrophytes have C:N ratios well above 17:1, and are not much fed upon. In smaller bodies of fresh water, the most important sector of primary production is that of the Aufwuchs, and its nutritional quality in terms of lower C:N ratios is of unique significance to the bioenergetics of the second trophic level invertebrates.

Variation in aufwuchs at six freshwater habitats in terms of carbon biomass and of carbon: Nitrogen ratio

A series of three overlapping sets of slides were exposed in riffle and pool habitats in a fourth order river in the Upper Peninsula of Michigan. Each set was exposed for 6–8 weeks and overlapped the preceeding set by 2 weeks. Diatom cell densities and community structure were determined after daily or weekly exposure periods for each set. The first set was placed on August 4, 1982, and the third set removed on October 16, 1982. Species diversity and evenness peaked quickly during colonization. Both indices decreased as the length of exposure increased. Early colonizing diatom species that occasionally accounted for large proportions of the total diatom community were soon replaced by other diatom species that tended to persist through time. Major dominant species were well established by day 28. Severe net cell losses (up to 17% of the total density) were recorded after only an 8–9 day exposure in both pools and riffles. Pool slides showed greater cell densities during the first few day's exposure than did slides exposed in riffle zones. After this brief conditioning period, however, the riffle slides showed more rapid cell growth and/or accumulation rates. Mean cell densities were similar between pool and riffle slides after 6–8 week exposures. Seasonal changes appeared to strongly influence diatom species succession. Seasonal changes in water velocity, temperature, or light may have the same effect as the more dramatic flood events which reset periphyton to earlier successional stages, resulting in increased major changes in species composition of the periphyton diatom community.

Diatom colonization dynamics in a lotic system

Periphyton and plankton samples were collected at four littoral stations in a relatively shallow, eutrophic lake (Elk Lake, B.C., Canada) over a six month period from August 1967 to January 1968. The most abundant planktonic diatoms demonstrated a pattern of seasonal succession typical of temperate lakes, and all were present in the periphyton. This pattern was identical at all stations:Fragilaria crotonensis was dominant from August to October,Asterionella formosa in November and December, andMelosira italica (plusM. varians) in January.F. virescens, although never dominant, peaked in October. Periphyton communities were dominated byAchnanthes minutissima, Cocconeis placentula, F. crolonensis andF. virescens. Considerable station variation in successional patterns occurred over different exposure periods; station differences were least in those samples immersed for monthly intervals, and greatest in those immersed for a cumulative four-month period. Interaction between the phytoplankton and periphyton was illustrated by the occurrence of species common to both habitats. A decrease in cell numbers and percent abundance of these species in planktonic populations coincided with their increase in the periphyton, a relationship which appeared dependent on the breakdown of thermal stratification in November. For example, following turnover,F. crotonensis andA. formosa settled out of the plankton and correspondingly increased in percent abundance in the periphyton. This interdependence was less evident in the four-month samples, whereA. minutissima andC. placentula dominated throughout the entire period and appeared to out-compete the more typical planktonic components for diminishing substrate area. Species interaction or competition was accentuated as exposure duration and periphyton total cell standing crops increased and species diversity decreased, and appeared to account in part, for station differences in successional patterns.

A. P. Austin

Papers

A Method of Collecting Periphyton in Lentic Habitats with Procedures for Subsequent Sample Preparation and Quantitative Assessment