Department of Physics, Chemistry and Biology (IFM)

Invertebrate grazing effects on epiphyton biomass was calculated as a function of 3 different parameters of measurement namely; actual food consumption (F), food consumption at varying densities (FC) and weight-specific consumption (WSC); (see appendix I for definitive calculations of each parameter). However as expected, feeding experiments would only present a limited opportunity in terms of the amount of potential foods available to test organism, as compared to a variety of food choices available to them in their natural environments, but these experiments targeted how well would the invertebrate species used in this study respond to the abundance of epiphyton biomass in their respective natural environments. To this end, analysis of the 3 parameters used to measure the grazing effects of Lymnaea columella; Asellus aquaticus and Gammarus pulex, indicated significant (P<0.0001) grazing differences of all three species on epiphyton biomass (Table 1).

Both the high and low densities of Lymnaea columella; Asellus aquaticus and Gammarus pulex, responded strongly to the epiphyton food source as could be seen from the significant main effects (Figures 1a & b). Actual food consumption (F) in relation to each species was analyzed with a one-way analysis of variance (ANOVA). At high density, there was a reliable main grazing effect on epiphyton biomass in form of the actual food consumed by each species with Means of 1.4786 g individual¯¹; 0.7826 g individual¯¹; and 0.7410 g individual¯¹; for Lymnaea columella; Asellus aquaticus and Gammarus pulex respectively; F=16.108; P<0.0001. Similarly, at low density, same reliable grazing effects was shown by test species with Means of 1.7400 g individual¯¹; 0.5210 g individual¯¹; and 0.5000 g individual¯¹; for Lymnaea columella; Asellus aquaticus and Gammarus pulex; F=31. 360; P<0.0001 (Table 1).

The general pattern was similar for the densities tested as in actual food consumption (F) (Figures 2 a & b) where the two densities (high and low) of Lymnaea columella, Asellus aquaticus and Gammarus pulex showed strong grazing effects on epiphyton biomass. Food consumption among species was higher with increase in species density (Table 1). At high density population (5 individuals per replicate), one-way analysis of variance (ANOVA) test detected a significant grazing effects on epiphyton biomass in terms of the collective amount of food consumed by each species with Means of 0.3550 g individual¯¹; 0.0280 g individual¯¹; and 0.322 g individual¯¹; for Lymnaea columella, Asellus aquaticus and Gammarus pulex; F=43.880; P<0.0001. At low density (3 individuals per replicate), similar main grazing effects was demonstrated by all three species with Means of 0.3418 g individual¯¹; 0.0138 g individual¯¹ and 0.0146 g individual¯¹;   for Lymnaea columella, Asellus aquaticus and Gammarus pulex; F=95.564; P<0.0001 (Table 1).

The response of Lymnaea columella, Asellus aquaticus and Gammarus pulex to ingestion rates of epiphyton biomass was dependent on body size and this interaction varied from species to species (Figures 3 a & b). Weight-specific consumption (WSC) among the species showed significant effects on epiphyton biomass with increase in species density as in the cases of Asellus aquaticus and Gammarus pulex (Table 1). Mean values of 1.5782 g individual¯¹; 5.5758 g individual¯¹; and 4.3116 g individual¯¹;   were recorded from a one-way analysis of variance (ANOVA) test at high density for Lymnaea columella, Asellus aquaticus and Gammarus pulex respectively; F=15.607; P<0.0001. At low density, Lymnaea columella seem to be more effective compared to its grazing effect at high density as there was increased significant effect of grazing on epiphyton biomass where Means of 2.2824 g individual¯¹;   was recorded for Lymnaea columella; 5.0806 g individual¯¹;   for Asellus aquaticus and 4.7946 g individual¯¹; for Gammarus pulex. The effects were significant at F=4.469 and P<0.035 at low density species populations (Table 1).

The tendency of many aquatic invertebrate species to exhibit an opportunistic or generalists feeding habits by diversifying their food sources was significantly supported by observations from this study (Figures 4 a & b). Consumption rate (CR) was analyzed with a one-way analysis of variance (ANOVA). At single species treatment, the consumption rate (CR) of invertebrate species feeding on dead leaf   alternative food source were significant with Means of 0.0039 g individual¯¹; 0.0099 g individual¯¹; and 0.0039 g   individual¯¹; for Lymnaea columella; Asellus aquaticus and Gammarus pulex; F=25.468; P<0.0001. Similarly, at combined species (two species) treatment, there was a significant effect of feeding on dead leaf discs exposed to invertebrate species with Means of 0.0006 g individual¯¹; 0.0003 g individual¯¹; and 0.0001 g individual¯¹;   recorded for combinations of Lymnaea and Asellus; Lymnaea and Gammarus as well as Asellus and Gammarus respectively; F=5383; P<0.007