Introduction.  Many lakes in the Tacoma area (and worldwide) experience hazardous algal blooms (HAB) that have in recent years become both more frequent and more severe.  The main cause of this problem is increased anthropogenic loading of nutrients - in particular phosphorus (P).  However, the source(s) of the nutrients and the response of the system varies greatly from one lake to next and thus, when it comes to designing a treatment plan, each lake must be studied and evaluated separately.  Because the annual cost of treatment can easily be >$100,000, understanding "how your lake works" is critical.  The purpose of this webpage is to summarize background information that can serve as a guide to evaluating HAB treatment options.
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Where Do Algae Acquire Nutrients?  Algal growth requires both nutrients and sunlight, and therefore occurs most vigorously in the photic zone.  Nutrients that are concentrated in the bottom waters will reach the photic zone when a lake turns over (typically in late Fall in the PNW) or where/when cyanobacteria migrate to deeper waters, which they can do on a daily cycle during blooms in late summer or fall.  This ability to regulate their buoyancy gives cyanobacteria an advantage when a lake is also crowded with green algae.  Some cyanobacteria can also take up nutrients directly from the sediment when they are in a dormant state.  During this “luxury uptake” process the cyanobacteria take in more nutrients than needed at the time, and they can utilize this excess at a later stage. 
 

Causes of High SRP in the Hypolimnion.  In lakes that develop thermal stratification it is common to find elevated levels of SRP (soluble reactive, or dissolved, P) in the bottom waters.  At least three processes can cause this:  (1) release of P from the sediment, (2) decomposition of recently sedimented organic matter that has accumulated on or near the bottom, and (3) pooling of SRP-rich groundwater that is colder that the lake’s surface waters and has traveled from its entry points to deeper portions of the lake.  It is essential to identify where the SRP-rich water is coming from before deciding on a HAB mitigation plan.

 

Stability of Iron and Manganese Oxides/Hydroxides.  In nature Fe and Mn are highly effective at binding and sequestering P and NO3 (and many trace metals): they are one of nature's main ways of controlling a lake's nutrient levels as long as they are in their oxidized state.  Conversely, when Fe and Mn are in reduced state they will dissolve and release any bound elements.  When it comes to choosing a HAB mitigation strategy for a lake a crucial question is whether conditions in the water column, particularly in the bottom waters, are oxidizing enough to stabilize ferric iron.  Bottom water chemical analyses can help to answer this question as described in this document.  It was written specifically to address concerns about the use of zero valent iron (ZVI) but the basic principles it covers are applicable to any situation where redox state is a concern.  A final point to emphasize is that dissolved oxygen measurements are not useful or reliable for evaluating the oxidation state of iron in bottom waters: ferric iron can be dominant even if DO is zero.     . 

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