Apply gradient mask *after* animation in rotating pattern

Hope this is not a wrong place to ask this...
 

 I have these "cells" rotating in a circular tube and am wanting them to change color or fade in and out as they circle through the halves of the tube.  E.g. they get "brighter" as they go through the right half of the tube and "paler" as they go through the left half.  So I have constructed as shown below.  But the gradient mask seems to apply when the svg is initially rendered, not after each step of animation, so I get the brighter and paler halves just rotating with the flow.  Is there a different approach that might achieve my intent?  

Thanks!  
John
 

 code:
<svg xmlns="http://www.w3.org/2000/svg"
	xmlns:xlink="http://www.w3.org/1999/xlink">

<defs>
    <g id="ovals3" fill="inherit" stroke-width=".5" stroke="#006" > 
      <ellipse cx="8" cy="4" rx="2" ry="1"/>
      <ellipse cx="4" cy="1.5" rx="2" ry="1"/>
      <ellipse cx="2.5" cy="5.5" rx="2" ry="1"/>
    </g>
    <pattern id="Flow2" patternUnits="userSpaceOnUse" width="11" height="7.5" fill="#800080" >
        <use xlink:href="#ovals3" />
    </pattern>
    <path id="toweroutline" d="M 135 110
        a 45 45, 0, 1, 0, 1 0
        m 0 16
        a 30 30, 0, 1, 1, -1, 0
        "
        stroke="black" fill="none"  
     />
</defs>


<linearGradient id="gradient1">
	<stop offset="0.0" stop-color="black"/>
	<stop offset="1" stop-color="white"/>
</linearGradient>

<mask id="Ma">
<rect id="maskrect" x="90" y="100" width="100" height="200" stroke="none" fill="url(#gradient1)"/>
</mask>

<clipPath id="towercp" ><use xlink:href="#toweroutline"/></clipPath>

<g id="tower" clip-path="url(#towercp)" >
  <use xlink:href="#toweroutline"/>
  <circle id="flow2" cx="135" cy="158" r="50" fill="url(#Flow2)" opacity="1" mask="url(#Ma)">
     <animateTransform attributeName="transform" type="rotate" dur="10"
       from="0,135,158" to="360,135,158" repeatCount="indefinite" />
  </circle>
</g>

</svg>



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