Monday, May 2, 2011

Manganese as a Recorder of Hypoxia

I haven't been doing much writing about otoliths lately (at least not here), despite the goal of this blog being to write about otoliths.  In fact, recently I have been writing more about invasive species, which is a topic I absolutely hate!

Really the only excuse I can come up with for not writing about otoliths is laziness.  I wanted to write about otoliths, I really did, I just needed some inspiration...which came in the form of some current events.  My interest was peaked when I read the title; "Research: Fish Can Detect And Record Marine 'Dead Zones'".  Fish you say?  Record you say? All the key words were there, this definitely had to be about otoliths...and it was...inspiration at last!  

So I dug deeper and found the paper the article was reporting on, by Limburg et al. (2011) entitled Tracking Baltic hypoxia and cod migration over millennia with natural tags.

The paper focuses on hypoxia and declines in cod stocks in the Baltic Sea.  Along with over fishing, habitat degradation is hypothesized to have led to declines of cod in the Baltic Sea.  The Baltic Sea experiences natural and anthropogenic caused hypoxia events (depleted oxygen), which is thought to have a negative impact on fish.

During hypoxic events manganese is released from the sediment into the water.  Once in the water it is free to be incorporated into fish otoliths.  The researchers used the concentrations of Mn in cod otoliths, from the Neolithic Age, the Iron Age, the 1980's, the early 1990's and the late 1990's, to examine hypoxia events in the Baltic Sea.  It's pretty awesome in and of itself that otoliths from the Neolithic and Iron Ages can be found, but to analyze them and get meaningful data is pretty mind blowing, and can certainly be used when investigating changes in the environment throughout time.

The pattern of Mn in the otoliths followed what one would expect.  The Neolithic otoliths had low Mn, and otoliths from the late 1990's had the highest concentrations of Mn.  The increasing Mn throughout time indicates that hypoxc events are more frequent now than in the past, likely due to anthropogenic impacts.

For me, the most interesting finding was that elevated Mn was found in areas of the otolith relating to early years of life, particularly the first year.  Using Ba/Sr ratios (used to differentiate freshwater and salt water) in  these otoliths, it was hypothesized these fish utilize low salinity areas with riverine inputs as nursery grounds.  These areas may be exposed to low oxygen conditions, thus having higher Mn concentrations.  

So what does this tell us?

If the modern fish were captured alive (which I'm assuming they were), it could be concluded that young fish are able to cope with hypoxic events.  It is possible there is some advantage gained by young fish being able to tolerate hypoxic conditions.  Young cod may enter hypoxic waters to avoid predation or avoid competition for food resources (I'm pretty sure this concept has been demonstrated with common snook, but I was unable to locate a reference).   It would be interesting to examine growth rates and condition of juvenile cod exposed to different oxygen levels, in order to investigate differences in habitat quality.  However, the researchers did note that Mn in the water can linger after hypoxic events, so Mn in the otolith does not necessarily mean the fish was directly exposed to hypoxia.   

Using Mn in otoliths to examine and record hypoxia is interesting to say the least, and this paper provides a specific example of the record keeping ability of otoliths.  I haven't really noticed any trends with Mn in my own research yet, but I think as I look at data from later in the summer (August-October) it may start to show up in water samples and otoliths because of low oxygen levels in the water during this time period.

Limburg KE, Olson C, Walther Y, Dale D, Slomp CP, & Høie H (2011). PNAS Plus: Tracking Baltic hypoxia and cod migration over millennia with natural tags. Proceedings of the National Academy of Sciences of the United States of America PMID: 21518871

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