Proximate composition of periphyton communities growing in experimental streams
Date
1989
Authors
Ridley-Thomas, Christopher Iain
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Abstract
The proximate composition of periphyton communities grown under diverse conditions was analyzed to determine the variability in the potential nutritional value of the community to higher trophic levels. Ash-free protein, lipid and polysaccharide and ash weight were measured. The analytical methodology employed involved two new modifications. Firstly, the Hach method of nitrogen analysis was modified for periphyton communities and used to estimate protein. The method consistently gave greater nitrogen recoveries than traditional Kjeldahl analyses when used on rather indigestible samples. It is recommended that the applicability of traditional Kjeldahl methods to periphyton analysis be carefully examined. Secondly, numerous methods of ash determination were examined and found to give highly variable results. To improve consistency it is recommended that ashing be carried out for 4h at 45o 0 c with 250mg samples.
Seasonal variation in periphyton proximate composition was noted in all components analysed. Ash-free protein and polysaccharide were highest in summer and autumn, as was production. Lipid was highest in winter and spring, when the relative dominance of diatoms was at a maximum. Ash weight was also highest in winter as a result of the dominance of diatoms and increased levels of inorganic detritus in the water due to higher run-off.
The effects of current velocity on periphyton proximate composition were examined during autumn; protein was significantly lower in streams with a current velocity of 7cm.s-l than at velocities of 14cm.s-l and 28cm.s-1. Lipid levels were significantly higher at a current velocity of 28cm.s-1 . It is postulated that the increased supply of nutrients that results from increases in current velocity stimulates production levels and these in turn lead to increased protein levels. No significant current velocity effect was noted on proximate composition values during winter. Periphyton communities tend to be light and temperature limited at this time of year so the increased supply of nutrients at higher currents is of little consequence.
Reduction in light levels imposed during summer resulted in increased biomass and decreased temporal heterogeneity in communities. Addition of nitrogen, phosphorus, or nitrogen and phosphorus also led to increases in biomass. Proximate composition was unchanged in streams with added nitrogen; streams with added phosphorus showed a transient increase in polysaccharide levels. Streams with added nitrogen and phosphorus had higher protein levels. Ash-free protein levels were positively correlated (r=0.9416, O.Ol>p>0.005) with biomass accrual rates.
Chironomid grazer densities substantially altered periphyton proximate composition values at all times of the year apart from winter. However, they did not alter observed relationships between biomass accrual and ash-free protein, or the range of seasonal variation. It is concluded from this that the measured variability in periphyton proximate composition is due to changes within the primary producer component of the community. It is suggested that the observed changes in proximate composition have a significant effect on the growth and survival of higher trophic level organisms.