ANOVA Output

I have a question about my R-output when I run a three-way ANOVA.  I just plugged in the
interaction term into the formula and presto!  ANOVA!  But I noticed that if I change
the order of the formula (or interaction term), I get slightly different ANOVA outputs.
 I've pasted the output at the bottom of this message.  I didn't think that this should
happen, so I would appreciate if anyone had any feedback on this problem.

Thanks in advance, Kat.

ANOVA <- aov(Chlorophyll.a~Treatment*SamplingPeriod*Site)
summary(ANOVA)

Df  Sum Sq Mean Sq F value    Pr(>F)    
Treatment                       3   356.5   118.8  4.2878  0.005276 ** 
SamplingPeriod                  3   374.7   124.9  4.5069  0.003911 ** 
Site                            1  1016.5  1016.5 36.6791 2.629e-09 ***
Treatment:SamplingPeriod        9   467.6    52.0  1.8747  0.053284 .  
Treatment:Site                  3   167.8    55.9  2.0176  0.110424    
SamplingPeriod:Site             3  1670.2   556.7 20.0884 2.383e-12 ***
Treatment:SamplingPeriod:Site   9   277.2    30.8  1.1115  0.352455    
Residuals                     534 14799.5    27.7                      
---
Signif. codes:  0 ?***? 0.001 ?**? 0.01 ?*? 0.05 ?.? 0.1 ? ? 1

ANOVA <- aov(Chlorophyll.a~SamplingPeriod*Treatment*Site)
summary(ANOVA)

Df  Sum Sq Mean Sq F value    Pr(>F)    
SamplingPeriod                  3   369.5   123.2  4.4437  0.004264 ** 
Treatment                       3   361.8   120.6  4.3510  0.004840 ** 
Site                            1  1016.5  1016.5 36.6791 2.629e-09 ***
SamplingPeriod:Treatment        9   467.6    52.0  1.8747  0.053284 .  
SamplingPeriod:Site             3  1662.0   554.0 19.9894 2.718e-12 ***
Treatment:Site                  3   176.0    58.7  2.1166  0.097111 .  
SamplingPeriod:Treatment:Site   9   277.2    30.8  1.1115  0.352455    
Residuals                     534 14799.5    27.7                      
---
Signif. codes:  0 ?***? 0.001 ?**? 0.01 ?*? 0.05 ?.? 0.1 ? ? 1

ANOVA <- aov(Chlorophyll.a~Site*SamplingPeriod*Treatment)
summary(ANOVA)

Df  Sum Sq Mean Sq F value    Pr(>F)    
Site                            1  1008.9  1008.9 36.4050 2.998e-09 ***
SamplingPeriod                  3   374.1   124.7  4.4990  0.003953 ** 
Treatment                       3   364.8   121.6  4.3871  0.004607 ** 
Site:SamplingPeriod             3  1654.8   551.6 19.9026 3.050e-12 ***
Site:Treatment                  3   172.6    57.5  2.0761  0.102364    
SamplingPeriod:Treatment        9   478.2    53.1  1.9172  0.047282 *  
Site:SamplingPeriod:Treatment   9   277.2    30.8  1.1115  0.352455    
Residuals                     534 14799.5    27.7                      
---
Signif. codes:  0 ?***? 0.001 ?**? 0.01 ?*? 0.05 ?.? 0.1 ? ? 1

Katrina W. Chu, H.B.Sc.
M.Sc. Candidate
Canadian Rivers Institute
University of New Brunswick
10 Bailey Drive
Fredericton, NB, E3B 6E1
E-mail: katrina.chu at unb.ca