For many years now there has been a focus on determining the causes of nuisance algal blooms in Lake of the Woods. We know, based on a volume of scientific evidence, that too much phosphorus is the culprit, and this fact steers the question towards how much phosphorus is too much phosphorus?

In order to answer the question, we need to determine:

  1. How is phosphorus getting into the lake? and then,
  2. What are the effects of that phosphorus on the algal community?

apples orangesPhosphorus studies for Lake of the Woods: Environment and Climate Change Canada and Minnesota's Total Maximum Daily Load StudyThis is a difficult task, especially for a large international water body like Lake of the Woods.  To get going we need to develop a phosphorus (P) budget (and several attempts have been made at this over the years).  That will tell us how the P is entering the lake and how this affects in-lake P concentrations. If we can do this, we will have a handle on the first question and so the focus of this article will be a look at several different P budgets from different sources.  We will need to leave that second question till later.  It’s a bit of a Duesenberg.

Here (Table 1) we compare the components of three P budgets that have been developed for Lake of the Woods.  The earliest attempt by Hargan et al. (2011) provided an initial budget that, while missing some crucial inputs, compares surprisingly well with later more rigorous attempts.  Two recent budgets include: Minnesota’s Total Maximum Daily Load (TMDL) for the south basin of Lake of the Woods and Environment and Climate Change Canada’s (ECCC) effort to link phosphorus inputs to in-lake conditions throughout Lake of the Woods.  It is important to note that all three of these budgets recognise that the lion’s share of both water and nutrients that enter Lake of the Woods do so via the Rainy River

OK so let’s get to this business of apples and oranges. If we want to compare the components of the three budgets outlined in the table, the first thing to do is look at what data are being used. If we look at the top of the table, we see that the TMDL study uses data from 2005 to 2014 and focuses on the south basin of Lake of the Woods. The ECCC budget uses data from 2016 to 2018 and incorporates the entire lake. Hargan used data before 2011 and described loads to the whole lake. Right off the bat we have apples and oranges. There’s more. If we look at the waste load estimates and glance up at the dates it seems likely that the TMDL loads includes the Abitibi pulp and paper mill in Fort Frances that was active during their data collection range (about 40 MT). For the ECCC budget the mill had shut down in 2014, prior to their study. It would have been active for Hargan’s attempt. It is easy to see why there may be differences in the estimates and there are similar problems with comparing line items stitched throughout the entire table. Use of the term tributaries has clearly different meanings in each of the three budgets. ECCC and Hargan may be comparing similar sources but the TMDL budget has a larger tributary number which contains Rainy River loads. Some of the largest differences are with internal loads (a large component) which was not included in Hargan’s budget. So, if we really wanted to get away from apples and oranges we would need to add or subtract loads from the idle Abitibi mill, add internal loads, compare similar areas, and so forth. That is not going to happen here.

Table 1 – Comparing three Phosphorus budgets for Lake of the Woods.Table PLoadComp

There is one final consideration. In these budgets, if most of the sources are considered and if the data are representative of average conditions, then the total loads should be similar. In this case since most of the load is from the Rainy River, we can cock our heads sideways and look at the total loads. We see that the TMDL and ECCC loads are similar. Hargan is lower but does not include an internal load.

At this point it is important to understand that both the TMDL study and the ECCC project have determined total loads and go forward to indicate reductions in P that are required to improve conditions in the lake. Hargan did not do that. Since reductions will occur primarily in the Rainy River we come to a spot where apples may be closer to apples. The TMDL study recommends an 17.3% reduction and the ECCC study says that a 20% reduction will fulfil most of their goals in the lake. Close. Of course, one study pertains to the south portion of the lake and the other encompasses the entire lake...sigh.

The good news here is that for the first time we have studies that recommend what must be done to ameliorate harmful algal blooms in Lake of the Woods. We can now start talking about these numbers. The next steps to meet reductions will be challenging but we have come a long way towards addressing these algal blooms. Stay tuned.