. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. o O control • fertilized 40 80 120 Snail density (snails/cage) 160 200 e) df MS Fertilizer Density Pert. X Density 1 * 4 ** 4 Figure 1. (a-c) Time courses of nitrate (at, ammonium (b). and phosphate (c) concentrations in pore water in control and fertilised cages, (d) Change in chlorophyll a as a function of snail density. Data points are calculated from regressions of change in chlorophyll over time for individual cages, (e) Two-wav ANOVA of the results of chlorophyll response to the fertiliz
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. o O control • fertilized 40 80 120 Snail density (snails/cage) 160 200 e) df MS Fertilizer Density Pert. X Density 1 * 4 ** 4 Figure 1. (a-c) Time courses of nitrate (at, ammonium (b). and phosphate (c) concentrations in pore water in control and fertilised cages, (d) Change in chlorophyll a as a function of snail density. Data points are calculated from regressions of change in chlorophyll over time for individual cages, (e) Two-wav ANOVA of the results of chlorophyll response to the fertilizer and density treatments. * indicates significance at P = ,' ** indicates significance at P = fertilized and control treatments (Fig. Id). A two-way ANOVA showed that both snail densities and nutrient concentrations had significant effects on chlorophyll a concentrations (Fig. le). In terms of the effect of herbivore density, we interpret the results of Fig. Id to suggest that 1) lower snail densities allowed increases in microphytobenthic biomass (note the position of the dashed horizontal line showing no change), and 1) snail densities exceeding the ambient of 97 snails per cage reduced microphyte biomass. These results suggest that mud snails can control abun- dance of their food, which means, perhaps, that field densities are poised at a level that does not deplete the food supply. In terms of the effect of nutrient supply, the addition of nutrients significantly increased microphytobenthic biomass (Fig. le). In spite of the trends suggested by Fig. Id. the response of micro- phyte biomass to fertilization was not significantly larger at lower snail densities (Fig. le). The results were insufficient to allow us to discern the possible interaction between grazing pressure and nutrient supply, in part because there were only two levels of the nutrients examined, and because even in the fertilized sediments concentrations were relatively low. The results of this e
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
