Lake Data Statistical Analysis Report Part II – Dr. Lorin Hatch

Lake Data Statistical Analysis Report Part II – Dr. Lorin Hatch

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Phase 1 of the statistical analysis, basically
we’re collecting, the district’s been collecting data since the late 60s and more intensely
1997 and I worked here in the 04-06, ramped it up a little bit more so we’re more data
rich at the district. So, as I mentioned, I’ll do a little recap
of that first work that was done last year sort of the phase 1 just asking some general
questions, long-term trends, doing a formal statistical analysis of lake monitoring data. Phase 2 I also had some objectives pertaining
to the things that came out of phase 1 and some recommendations for potential further
investigation. As you are all well aware, in Lake Minnetonka
there are two dozen sampling stations that are sampled biweekly and there are 11 lakes
that have long-term data that are draining into Lake Minnetonka. Or at least within the watershed. Phase 1 study objectives were to assess the
number of monitoring stations needed to assess the health of Lake Minnetonka. It is quite an effort to get boats out there
especially in April when the weather is not as great and cover all those locations in
a reasonable amount of time and if some of those bays that are adjacent to each other have
similar water quality statistics on an annual basis or monthly basis, it might suggest that
you might not need to go to one of those bays all the time. Instead of monthly, maybe every other year
or something like that. And that was the point of that study. The next was assessing the sampling frequency
to assess the health and sort of the health defined by the MPCA where you’re looking at
the June July August September values for nutrients, total phosphorus, response variable
chlorophyll-A is proxy for algal and then secchi disk transparency. So I’ll get into that a little more and then
perform trend analysis just very on an annual basis for all of these water bodies. Lake Minnetonka and watershed lakes. So in that first objective, again do adjacent
bays have similar water quality? If so, maybe you can monitor them in a different
frequency. So that exercise looked at sort of clusters
of all the bays that might have an influence so we came up with the six areas, you can
see southwest lakes, Halsted’s, Cook’s, down by Smithtown, central lakes you’re familiar
with these. So anyway, it was just a way to look at these,
there are similar water quality on reasonably adjacent bays. So to cut to the chase here the results were
finding that especially in the southwest part of the lake, in here, there are a number of
bays that have similar water quality issues [indistinct speaking] Smithtown and Phelps
were a lot like West Upper…and a number of others as well. [indistinct speaking] Similar to Maxwell’s. For the second one, it was really do you need
to sample once a month or twice a month assess the lake health. And I mentioned the MPCA approach to that. And I don’t know if this is surprising or
not, but it turned out that there weren’t statistical differences between say sampling
once a month, 4 times a year, versus 8 times a year. And some of you recall it really didn’t matter
if you sample in the beginning of the month or the end of the month. So statistically there was really no difference
at least for getting more information out of it. Whether you want to, that’s just one metric,
one reason to reduce sampling versus reasons to keep sampling like to measure than important
event. Statistically anyway we’re showing that you
really only need to sample once a month. Then the trend analysis this is fairly simple. Just looking at water quality improving, degrading,
or remaining the same. We looked at just sort of the raw data as
it was sampled and then we did some weighting, weighted the variables, adjusting them for
annual runoff, average annual precipitation during the open water season, and then average
air temperature. And looking at some of the stations have quite
a bit of data and there were about six bays that actually had seven years of good data
in the early 80s and then a lot of data recently. So, just a simple comparison to see how those
bays were doing. And as a result for the upper watershed lakes,
some lakes were improving in water quality, some did not show a trend, and only Lake Virginia
was showing significant degradation in water quality. Again perspective, total phosphorus, chlorophyll,
and clarity. For Lake MInnetonka really didn’t see much
of a trend, again we had 26 bays, we saw some improving some degrading, but standard deviations
were similar in magnitude to averages so it makes it really difficult to statistically
show significant changes, to show a trend, there’s just a lot of variation in a given
year. Chlorophyll and you’ll see this in table form
later. The number of bays had increasing chlorophyll
and again you can see a lot of those in the west part of Lake Minnetonka. And we noticed that some of the bays had very
low chlorophyll and they only had to pump up a few parts per billion to show a significant
difference so those may not be as much of a concern. For phosphorus you saw some surface water
phosphorus was improving, which is nice. And you can see that Halsted Bay decreasing
from 1997 to 2012. Phosphorus increasing in Peavey which is a
funny lake. Not really a bay, it’s just a lake-let tucked
into the north shore and finally eutrophic state index is an index that considers secchi,
chlorophyll, and total phosphorus, any changes that were found in that were what we found
in either the chlorophyll or total phosphorus. Finally again I mentioned several bays had
quite a bit of improvement. Both in some of the cleaner bays had improved
secchi, chlorophyll between those two time periods and some of the more eutrophic bays,
like Jennings showed an improvement. Again there’s a long ways to go on Jennings
and those euthrophic bays, but it was good to see. So, that’s kind of the end of where we left
it off. One of the questions was for the lakes for
the bays or lakes that had a trend was there a certain month that really contributed to
that improvement or decrease in water quality. Point being that maybe you could potentially
target what sort of mitigation you might apply to a lake or bay. You’re getting increased phosphorus in June
that would suggest that there are things in the watershed that you could do to prevent
that from getting in there, but if it’s later in the summer, August, September that might
suggest that you might need to focus on the lake treatments things like that. And there’s also this dilemma where you saw
decreasing total phosphorus concentrations over time but the chlorophyll concentrations
were going up in some of the bays which is counter intuitive because you’d think more
nutrients equals more algae but the evidence was really and the analysis didn’t really
bear that out, that’s one of the reasons phase 2 was kicked off was to really look at the
variables on a monthly time scale for these surface water monitoring stations and also
look at the deep water samples in these water bodies. Total phosphorus and then the soluble reactors
to phosphorus on a monthly and annual time scale. And finally the last two objectives were really
keying in on the big three that have always been kind of troublesome, Halsted’s Bay, Jennings
Bay and Stubbs Bay. Always have that poor water quality for one
reason or another to sort of look at the relationships between phosphorus and chlorphyll and secchi
depth. Put it under a microscope a little more and
then also a quick look at can we relate the stream loadings of phosphorus to the phosphorus
concentrations in those bays. Look at that, so this, apologize for the dark
lines there, but this is the trend analysis for the surface data and I pointed out there
was raw data and then there was this weighted data. What you are seeing here is if any of those
approaches had showed a significant statistically significant increase, any of those four methods,
I put it up there. So it’s really kind of difficult to see on
a large number of trends except certainly Piersons was doing quite well in all the parameters,
you can see the blue bars going across. Here’s the lake we were grouping it by parameters,
so secchi disk depth July through September and then the annual down here we were grouping
it by time period, June and it is difficult to see any general trends here, but you can
see something on the annual basis, but not necessarily see trend changing on a monthly
basis but there are exceptions but basically it didn’t come out very anything you know
really jumped out in this analysis. For the Lake Minnetonka bays this is just
the parameters total phosphorus annual basis in those western southwestern bays this is
Jennings this is eastern part of lower lake etc smattering of the rest Layfette and the
thing that kind of pops out is that there is a lot of poor water quality or at least
decreasing water quality in chlorophyll in a lot of the bays particularly in the western
bays and here what you’re seeing is improvements light blue for total phosphorus. We’re sort of seeing that issue not only on
the annual basis but also on a monthly basis as well. And if you look at by month, June July August
September, a lot of the degradation in the chlorophyll August and September is really
when we are seeing the degradation in the chlorophyll. Now for the sort of the second task was looking
at the bottom water quality. Bottoms of these lakes seeing in improvements
in Gleason Lake, Long Lake seeing more of a degradation in the phosphorus in the bottom. But in general you are seeing a lot of significant
improvement in the phosphorus in the bottom waters, which is good. Now, if we look at the similar data for the
Lake Minnetonka bays by parameter, this is the total phosphorus, this is the sizable
fraction, in general where you are seeing the decreasing water quality, increases in
phosphorus in the bottom waters over time is Cook’s Bay Peavey Lake and St. Alban’s
Lower Lake South, but there are a number that showing quite a bit of improvement, Halsted,
Smithtown, Forest Lake, Lafayette, etc. So those are encouraging signs. By time period we really didn’t see, if you
saw in general decreasing water quality for a given parameter, it typically happened in
most of the year during open water season, say Cook’s Bay saw decreasing water quality,
Peavey Lake etc and total phosphorus decreased. Alright, I don’t want to go into the details
here, this is where we shifted gears and looked at the Halsted system, the Jennings and the
Stubb’s Bay system. Just looking from 1997-2012 data sort of busting
up and getting those averages for secchi chlorophyll phosphorus and just briefly seeing the highest
clarity in June is the blue diamonds and then the rest of the year it’s kind of settles
in not much difference between months or years. For chlorophyll, the blue diamonds are the
lowest in June which coincides with clarity in June but the rest of the year it’s kind
of a mess but you do see this trend since about 2007 where it scatters and increases
quite a bit over time levels. Total phosphorus, you’re really not seeing
a whole lot of variation. Bottom trends of phosphorus, not really seeing
a whole lot of variation there. Now, again I didn’t want to go into Halsted,
Jennings, and Stubb’s, but they are all kind of similar. Total phosphorus and chlorophyll you might
expect increased algae in the relationship you’re really not seeing anything fall out
there. It’s not a varied relationship there on an
annual basis or by month. For Jennings you kind of see, you can see
a relationship along there, there is some significant relationships there again in a
different system. And Stubb’s you’re really not seeing a good
relationship between the nutrients and biological response. However, chlorophyll and secchi disk transparency
for the three bays had quite a good relationship. Now look at the orange, that’s sort of what
we’ve looked at all along on an annual basis you see a slight trend but if you incorporate
the high water quality in June and stretch those axies out you can really see that there’s
a good relationship between the chlorophyll and transparency. And this is just another way of looking at
it, these are significant probability values in a nova test saying that for Halsted, Jennings,
and Stubb no matter what the month, what the annual, there is a pretty good relationship
between chlorophyll and secchi. However, between nutrient and phosphorus chlorophyll
you’re not seeing that. Another way of looking at it is looking at
all the bays on an annual basis to sort of compare phosphorus and chlorophyll and secchi
what you are seeing is not very many significant relationships with phosphorus and chlorophyll
but a lot of good relationships with the chlorophyll and secchi, whether this is a systematic phenomenon
in the lake, we can’t really see at this point. Now the upper watershed lakes, those 11 lakes,
we were looking at that and there are a few more good relationships ties in between nutrients,
most of them had a great relationship between chlorophyll and transparency. One of the things that really kind of stood
out to me was that when I started looking at these relationships, Halsted, Jennings,
and Stubbs, we put a date on these points. And looking at late 1990s to 2005 and then
2006-2012, there seems to be kind of shift going on. What that means, or if it’s real, I’m not
sure. Basically it’s saying prior to 2005 you could
have a high level of chlorophyll excuse me phosphorus and you’d get a certain level of
chlorophyll. After that time, 2006 to present you have
decreasing phosphorus but you have higher chlorophyll. Very strange, you know you’d think you’d get
less nutrients for the algae so what’s going on? Again these are pretty high levels relatively
high levels of nutrients, anyway around 100 parts per billion in there a little less than
Stubb’s but what that means, I kind of left it at that. And finally looking at just the load of phosphorus
coming into Halsted, Jenning’s, and Stubb’s from their respective creeks. Again, you’re not seeing a really great relationship
between phosphorus and coming from the creek the load and what’s in the bay. Halsted’s pretty, Six Mile’s pretty flat,
there may be a relationship between the load from Painter’s Creek to Jenning’s Bay especially
the summer average and Stubb’s bay you’re not really seeing great relationship there. Again this is kind of a cursory analysis,
you know, it’s nice to show that relationship between what you’re doing in the watershed
can have an impact on what’s going on in the bay or the lake and this attempt was really
showing that except for maybe Jennings. Kind of gives justification for what you are
doing in the watershed. [indistinct question from audience] Hmmm,
there might be, yeah, I’m. Part of it might be the trend, you got over
a year residence [indistinct] Yeah [indistinct] There might be some staggering over a year
year and a half have you looked at the trends to figure out the residence time, Yeah, agreed,
there could be that and other reasons as well is there some lag time happening in here. I was hoping for something to pop out really
nicely but Joe is pointing out there is so much else going on. So in concluding that monthly averages were
not closely aligned to the annual averages of water quality variables and for the surface
water and for the bottom phosphorus if you found a trend on the annual scale there was
at least one month showing a similar trend. Looking at the Jennings and Stubb’s Bay again
chlorophyll and secchi were pretty correlated or were highly correlated with one another. There was a connection between phosphorus
and chlorophyll that was not totally apparent, was not as strong as the chlorophyll. Again there is sort of a shift in the phosphorus
and chlorophyll relationship around 2005. And again, in the approach I took there wasn’t
a strong relationship between phosphorus loading in the creeks or bay surface, very simplistic
approach that I took. Future directions, again, these are things
that were brought up, provide a stronger connection between phosphorus loading in the bay, phosphorus,
residence time, turnover, there’s stratification, there’s a lot of stuff going on and that might
be of interest for further investigation. And also one of the primary drivers of chlorophyll
in those bays again what is driving them it isn’t evident that phosphorus is, it’s a component
of it but perhaps there was a shift that took place in the mid-2000s due to different species
of phytoplankton, quality and quantity of zooplankton, tagging to zebra mussels and
then using advanced statistical technique because there’s a wealth of data out there
there is some interaction with. Internal loading, nitrogen, water temperature,
stratification, and other plankton measures. So anyway, that’s kind of where I left off
and…a little dry but anyway. I’ll take any questions if you have any. In the future there might be analysis of stream
water quality data in a year or two. [indistinct audience question] No that wasn’t
part of the task [audience talking] Yeah, yeah that certainly be part of it, doing fisheries
studies on at least those three bays [audience talking] Yeah, I just remember Halsted’s there’s
so much tannins in the water the vegetation floating islands in Mud Lake the cattails
and other vegetation, of course it’s the outlet of a major wetland system, receiving waters
from that whole Six Mile Creek system maybe phosphorus did go down maybe it opened a window
and increased clarity slightly but I don’t know it’s the secchi measurements are so variable
over the years that there might be a need for light probes to really characterize the
lake depth [indistinct audience question] Yeah I just found it odd that the phosphorus
is showing kind of shift but you are getting more chlorophyll. That’s why I was wondering if there was a
shift unfortunately intense data hasn’t been collected over that late 90s to present [indistinct
audience question] Yeah the detention basin for Six Mile as well. I wonder if the I think there has been a lot
of notches dug between bays a couple years ago, that’s my gut feeling. I’m just wondering if there was a strong connection
between Halsted and Stubbs and Maxwell Bay. What was the connection before people started
going in and cutting deeper swaths, I don’t know. Some really old old map. Some of those connections are real, some of
those I just wonder Stubb’s Bay is a bit of land teased goes up into that there’s no way
to get through so I just wonder if things changed or if things were different a long
time ago. Halsted was more or less a terminal point
and then lake levels got really high in the rest of the lake. [indistinct audience question]
It was kind of hit and miss. And if there was a real relationship or not
it was kind of hard to tell and you’re looking at an annual average for the entire upper
basin and you’re weighting the factors by so nothing really popped out as a strong driver. Yeah, Stubbs, Jennings, and Halsted could
have been a real mystery for a long time you know or they’ve been a focus of mitigation
measures trying to understand before they took out the maple plain treatment plant it
was really bad over there and the late 60s Wayzata Bay had the same water quality that
Stubb’s Bay has today and much worse so you took away that huge point that went offline
86 so I’d like to know the answer about what went on anyway. Thanks [clapping]

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