Iron and sulfur are very important in aquatic systems by controlling anoxic phosphorus release from sediments. This is generally well known, but the details are more complicated - as always.

I'm sory, but I not studu this components. I study stable isotopic

When you say sulfate concentrations are "very high," what sort of values are you seeing?

Brian, we are seeing annual mean concentrations surface water sulfate concentrations of ~50 mg/L at inflow locations, interior  marsh concentrations range from below detection to ~20 mg/L.

The link below has a section of discussing sulfate trends.

http://my.sfwmd.gov/portal/page/portal/pg_grp_sfwmd_sfer/portlet_prevreport/2014_sfer/v1/chapters/v1_ch3b.pdf

When you say "iron is low" is it the dissolved iron as Fe(+2) or iron total? or the sediment abundance of iron?  the iron (Fe+2) can be low due to the fact that you have sulfate reduction and the iron scavenges by H2S.

In general, in marine and marginal marine environments iron and sulfur (as well as any complex coupling between them) are very important.  

Here's another paper that you might find interesting and we are currently working on similar things in salt marshes:

http://www.pnas.org/content/111/40/E4139.abstract

The southern Everglades system is pretty low in total dissolved iron (Fe) in surface water, very little work has been done to parse out Fe+2 and Fe+3. However some of the soils/sediment samples are relatively high in Fe which could be explained by H2S scavenging Fe. I never really thought about H2S and Fe interaction. Thanks for that nugget of intrigue...it seems like something to explore further. Also thanks for the paper. 

low iron in surface water is more likely due to oxidation of iron with oxygen. 

In the subsurface, Fe(2+) can interact with H2S. if you can smell H2S it should be good indication for low dissolve iron(+2) concentration as this reaction is fast. (And by the smell of H2S pretty much indicate that H2S is in excess)