r - setting color axis limits in ggplot2 -

i trying make heat-map of chlorophyll fluorescence vs depth , time. have things working pretty ok, i'm trying improve colour contrast. generate heatmap 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 finding dark green colours never utilized , heatmap ends looking washed out , doesn't great job of communicating chlorophyll fluorescence greatist if working in matlab, around setting

 caxis([-1 0.4]) 

which set values above 0.4 maximum green value. wouldn't able tell relative difference of high values, you'd @ least able better idea relative differences of intermediate values make of plot. suggestions on how can have larger proportion of plot green? suppose manually rescale input values, rather not if there better way.

edit: @ request of mike h

dput(head(ctdamotint2,100))      structure(list(variable = structure(c(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, 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, 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), .label = c("temperature",  "salinity",     "fluorescence", "oxygen", "nh4"), class = "factor"),          depth = c(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, 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, 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), time = structure(c(1482764087,      1482767687, 1482771287, 1482774887, 1482778487, 1482782087,      1482785687, 1482789287, 1482792887, 1482796487, 1482800087,      1482803687, 1482807287, 1482810887, 1482814487, 1482818087,      1482821687, 1482825287, 1482828887, 1482832487, 1482836087,      1482839687, 1482843287, 1482846887, 1482850487, 1482854087,      1482857687, 1482861287, 1482864887, 1482868487, 1482872087,      1482875687, 1482879287, 1482882887, 1482886487, 1482890087,      1482893687, 1482897287, 1482900887, 1482904487, 1482908087,      1482911687, 1482915287, 1482918887, 1482922487, 1482926087,      1482929687, 1482933287, 1482936887, 1482940487, 1482944087,      1482947687, 1482951287, 1482954887, 1482958487, 1482962087,      1482965687, 1482969287, 1482972887, 1482976487, 1482980087,      1482983687, 1482987287, 1482990887, 1482994487, 1482998087,      1483001687, 1483005287, 1483008887, 1483012487, 1483016087,      1483019687, 1483023287, 1483026887, 1483030487, 1483034087,      1483037687, 1483041287, 1483044887, 1483048487, 1483052087,      1483055687, 1483059287, 1483062887, 1483066487, 1483070087,      1483073687, 1483077287, 1483080887, 1483084487, 1483088087,      1483091687, 1483095287, 1483098887, 1483102487, 1483106087,      1483109687, 1483113287, 1483116887, 1483120487), class = c("posixct",      "posixt")), value = c(27.3483, 27.3483, 27.3483, 27.3483,      27.4404348314607, 27.5325696629213, 27.624704494382, 27.7168393258427,      27.8089741573034, 27.901108988764, 27.9932438202247, 28.0853786516854,      28.1006709677419, 28.1151870967742, 28.1297032258065, 28.1602961677656,      28.3392342471866, 28.5181723266075, 28.6971104060285, 28.8760484854494,      29.0549865648704, 29.1744078768732, 29.2330425521923, 29.2916772275114,      29.3503119028306, 29.4089465781497, 29.4675812534688, 29.5262159287879,      29.5233725024786, 29.5198033650201, 29.5162342275617, 29.5126650901032,      29.5090959526448, 29.5055268151863, 29.5019576777279, 29.4983885402694,      29.494819402811, 29.4392079391567, 29.3230472306014, 29.2068865220461,      29.0907258134908, 28.9745651049355, 28.8584043963802, 28.7422436878249,      28.6260829792696, 28.5099222707143, 28.5396702257581, 28.6045126836247,      28.6693551414913, 28.734197599358, 28.7990400572246, 28.8638825150912,      28.9287249729579, 28.9935674308245, 29.0584098886912, 29.1232523465578,      29.1880948044244, 29.2529372622911, 29.3177797201577, 29.3826221780244,      29.447464635891, 29.5123070937576, 29.4047436790674, 29.2746548739928,      29.1445660689182, 29.0144772638436, 28.8843884587691, 28.7542996536945,      28.6242108486199, 28.4941220435453, 28.4440444629526, 28.4161338799902,      28.3882232970279, 28.3603127140655, 28.3324021311032, 28.3044915481409,      28.2765809651785, 28.2486703822162, 28.2207597992539, 28.1928492162915,      28.1649386333292, 28.1370280503668, 28.1091174674045, 28.0812068844422,      28.0532963014798, 28.0253857185175, 27.9974751355552, 27.9695645525928,      27.9416539696305, 27.9137433866682, 27.8858328037058, 27.8579222207435,      27.8300116377811, 27.8021010548188, 27.7741904718565, 27.7462798888941,      27.7183693059318, 27.6904587229695, 27.6625481400071, 27.6346375570448     )), .names = c("variable", "depth", "time", "value"), row.names = c(na,  100l), class = "data.frame")