setting color axis limits in ggplot2

Question

I am trying to make a heat-map of chlorophyll fluorescence vs depth and time. I have things working pretty ok, but I'm trying to improve my colour contrast. I generate my heatmap with the following code.

ggplot(subset(ctdamotInt2, variable == 'fluorescence'), aes(time, depth)) +
geom_tile(aes(fill = log10(value))) + scale_y_reverse(limits = c(110, 0)) +
scale_x_time(limits = c(min(subset(ctdamot, variable == 'nh4')$time) - 2 * 60^2, max(subset(ctdamot, variable == 'nh4')$time) + 2* 60^2)) +
geom_point(data = samplesCTD, aes( x = time, y = depth)) +
scale_fill_gradient2(low = "blue", mid = "white", high = "green")

Generally I am finding that the dark green colours essentially never get utilized and so my heatmap ends up looking washed out and doesn't do a great job of communicating where chlorophyll fluorescence is greatist If I were working in matlab, I would get around this by setting

 caxis([-1 0.4])

which would set all values above 0.4 to the maximum green value. You wouldn't be able to tell the relative difference of the really high values, but you'd at least be able to get a better idea about the relative differences of the intermediate values that make up most of the plot. Any suggestions on how I can have a larger proportion of this plot be green? I suppose I could manually rescale the input values, but would rather not if there is a better way.

Edit: At the request of Mike H

dput(head(ctdamotInt2,100))

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    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,  1L, 1L, 1L,
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,  1L, 1L, 1L, 1L,
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    1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("temperature",  "salinity",
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    )), .Names = c("variable", "depth", "time", "value"), row.names = c(NA,  100L), class = "data.frame")