October Update on Water Quality at Highland Lake

posted Dec 20, 2018, 1:26 PM by Joseph Bickard

October Update on Water Quality at Highland Lake
Dr. Karen Wilson and Rosie Hartzler

You may be wondering, What was the result of the testing this summer? Some of you may have heard that the bloom did not occur to the extent that it has occurred in the past. What is the verdict on Highland Lake? This report is not a set of conclusions, but an update on what we know at this point.

It was a summer of extensive and in-depth testing.

Karen Wilson (USM), Jeff Dennis, (DEP), Linda Bacon (DEP), and Keith Williams (HLA) collaborated on the development and implementation of a testing program that included 2 interns plus a cadre of volunteers from the HLA.

The goal of this summer’s testing program was to identify the cause (s) of the bloom.

Three hypotheses were explored:
1. Is Phosphorus a major contributor to the bloom?
2. Is there something occurring in the Food Web dynamics that is driving the bloom?
3. Is climate change contributing to the bloom?
4. Or is it some combination of all 3?

This update will focus on a discussion of the following items:

1. How the testing was conducted.

2. What has been discovered so far and what it means:

        a. Secchi Disk readings
        b. How was the bloom different this year?
        c. Dissolved Oxygen and Temperature readings and what clues they might provide in figuring out why the bloom                     occurs in HL.
        d. What about the impact of the alewives?
        e. Low Lake Levels
        f. Green cotton candy like substance appearing in shallow water – metaphyton in HL and what it means
        g. Phosphorus – what is going on?
        h. Toxicity (or not) of cyanobacteria
        i. Bathymetry (re. calculation of depths in HL)

1. How the testing was conducted:
Description of the full sampling program and why it was devised the way that it was



Picocyanobacteria (Pcy) Bloom Toxicity

Assess potential toxicity of the Pcy bloom

Limnology: Secchi depth/transparency

Defines "bloom“

Limnology: dissolved oxygen/ temperature

Quantifies water column structure (temp) and biological activity (DO) by depth

Total Phosphorus (TP)

To evaluate the role of TP in controlling & supporting bloom:  understanding how and when TP moves vertically in the water column, how mass of TP changes over time and how these relate to onset and crash of the Pcy bloom

Algal pigments (chlorophyll, phycocyanin &, if possible, phycoerythrin)

Epilimnetic core allows comparison to previous years; other depths help track movement of algal community within and above the metalimnion

Water column Aluminum, Iron & TP interactions

Rule out possibility that Al and/or Fe are involved with accumulation of P in epilimnion

Pcy taxonomy: eDNA

Characterize the taxonomic composition of the picocyanobacteria

Food web interactions: eDNA

Measure change over time in DNA signals of smaller organisms in the food web


Species composition and size distribution

Adult alewife migration

Size and timing of the spawning alewife run affects the size and growth of the YOY alewife population and, indirectly, the impact on the trophic web.

Juvenile alewife growth, abundance & habitat use

Characterize the relationship of juvenile alewife to other food web parts, measure timing of population and individual growth, changes in prey items over time, and lake habitat use

Shallow sediment Aluminum, Iron & Phosphorus content

Assess the potential for phosphorus release from sediments that come in contact with the metalimnetic dissolved oxygen minima


More accurately track changes in phosphorus mass over time and estimate the extent of sediment in contact with the metalimnetic DO minima.

2. What has been discovered so far:

a. Secchi Disk readings

One question researchers wanted to answer: Was the water clarity the same at all points on Highland Lake? Following a “Secchi Blitz” where a group of HLA volunteers took daily readings at 13  designated sites on HL from mid-July through the end of August, accumulating over 340 Secchi disk readings, it was confirmed that water clarity was uniform all over Highland Lake in 2018. 

Why does this matter?

One reason is that when the bloom occurred during the summers of 2014 – 2017, researchers wondered if the intensity of the bloom (when water clarity was obstructed by the presence of the picocyanobacteria) was different in the deep areas of the lake from what was occurring in shallower parts of the lake. 

Our 2018 data suggest that the bloom is uniformly distributed. Another question researchers wanted to know was how well secchi disk depth readings (a measure of water clarity) track chlorophyll measurements in the lake. Chlorophyll is the pigment used by algae cells to photosynthesize so we use it as a measure of the number of algae in the lake. 

Turns out water clarity measurements and chlorophyll measurements were strongly related in 2018, strengthening our assumption that it is algae (in this case picocyanobacteria) that is reducing water quality. 

See chart below of how Secchi Disk Readings correlate with Chlorophyll readings go to link for this chart .

See plot below of nearly 400 Secchi readings that were made on Highland Lake this year, going through the end of September.  Submitted by Keith Williams

b. How was the bloom different this year? 

 Why didn’t the water get as cloudy as it did in 2017?

Obviously the scientists were surprised by this, as were everyone else. There are a lot of questions about what did happen. Did the bloom occur, but possibly at a deeper level in the lake? In other words, water clarity did drop to a level of about 3 meters,  and then almost immediately began to improve.

c. Dissolved Oxygen and Temperature readings and what clues they might provide.

Karen Wilson and Jeff Dennis think the unusual pattern of the lake’s dissolved oxygen and temperature profiles might provide some explanations as more samples are analyzed this fall. 

Highland Lake, like most other lakes in Maine, experiences stratification, which basically means the separation of lakes into three layers: Epilimnion: the well-mixed warm top layer, Metalimnion (or thermocline): the middle layer, where temperature changes sometime dramatically from top to bottom, and the Hypolimnion: the bottom layer, isolated from the top of the lake. 

Many more productive lakes experience a loss of oxygen in the hypolimnion, but in Highland Lake, the lowest oxygen levels are actually in the metalimnion, and recover somewhat in the hypolimnion. 

Depth-specific measurements of nutrients, chlorophyll, and environmental DNA (all still being analyzed) should help determine if that oxygen minima in the metalimnion is related to the picocyanobacteria bloom. Stay tuned!

d. What about the impact of the alewives?

Alewives - Volunteers observing incoming alewife at the top of the fish ladder, count a record number of adult alewives (estimated at 65,000) migrating into HL this spring to spawn. 

One hypothesis was that alewife eat the zooplankton that eat the algae in the lake – more alewife might result in more algae (causing the bloom of years past). 

We now have three years with alewife numbers around 40,000 and blooms, one year with alewife numbers around 6,000 and a bloom, and now one year with record high numbers and no bloom.

As of September 27th, juvenile alewife were still in the lake, although there are fewer of them and those that remain have grown considerably since this summer. Most if not all should leave as lake levels rise and flows down Mill Brook make their migration to the estuary easier.

Zooplankton, eaten by larval and juvenile fish of all species, were sampled bi-weekly or weekly throughout the summer. Zooplankton are the primary food of juvenile alewives. 

Zooplankton samples have not yet been counted, but one interesting result from this summer is that there appears to be more zooplankton in the water column at night than during the day. In many lakes, larger zooplankton typically spend the daylight hours in the dark bottom waters of the lake, and then migrate to the surface waters to feed at night to avoid being eaten by fish during the day – and this appears to be happening in Highland Lake. Water samples will be analyzed for environmental DNA to help identify zooplankton in the lake.

e. Low Lake Levels

This summer was unusually hot and humid with a record number of days where the dew point reached 70. Rainfall during June and July was low, yet rainfall in August was above average. Usually more rain leads to more runoff which leads to increased levels of phosphorus in the lake leading to algal blooms.

There is no evidence yet to indicate that lower than usual lake levels contributed to the fact that we did not have the typical bloom.

The Highland Lake Association is very concerned about the reported lower than usual lake levels this summer. We are committed to figuring out to best way to regulate lake levels, in the effort to allow enough water flow to enable the alewives to efficiently leave the lake, but also to maintain water levels sufficient for home owners to dock boats safely.

f. Green cotton candy like substance appearing in shallow water – metaphyton in HL and what it means

Metaphyton, a type of algae that looks like cotton candy showed up intermittently in the shallower areas of the lake during July and August. "This is nothing new", reports Keith Williams. "There are multiple types of metaphyton and they come and go in Highland Lake".

Picture of metaphyton taken at a dock on the west side of the lake at the south end of the lake in mid-September.

g. Phosphorus – what is going on?

What did the data tell us about phosphorus levels in HL? Results of phosphorus sampling revealed some very interesting data.

This chart shows during the 2018 season, Phosphorus readings spiked during and after the worst secchi readings. However, this year we learned that phosphorus levels appear to be highest in the metalimnion – the same area of the lake with low oxygen levels. Again, we think this is connected to the bloom, but waiting for more data to make further conclusions.

Levels of Phosphorus, averaging above 10 ppb in Highland Lake have created concern among residents. These data led the Highland Lake Association to initiate two ordinance changes in the Town of Windham: (1) Any development must demonstrate that it will mitigate potential phosphorus at a level of 0.020 lb / acre / year. (2) A developer is no longer allowed to “pay to pollute “ – in other words, a developer used to be able to pay a stormwater compensation fee in lieu of mitigating potential phosphorus that might be generated by a development.

h. Toxicity (or not) of cyanobacteria

Another concern that folks have is whether the picocyanobacteria is toxic. Samples of the cyanobacteria have been sent to a specialty lab at UNH and the results are pending

 i. Bathymetry (re-calculation of depths in HL)

The Bathymetry project was completed. This project focused on re-calculating the depths of the water in all sections of Highland Lake. We were able to complete this project due to the generosity of Lakes Environmental Association who allowed the HLA to borrow the necessary equipment.

See map below:


1. After one season of the most intensive and comprehensive water quality sampling program ever at Highland Lake it is important to remember that one year does not a trend make. However, considerable progress was made understanding how Highland Lake “works,” putting us far ahead of where we were last year, and allowing us to fine tune our hypotheses and efforts.

2. A more complete picture of what occurred in Highland Lake this summer will be forthcoming – and this is going to test your patience – but given the way that labs process samples, these results should be available in November or December.

3. The Highland Lake Leadership Team is discussing the possibility of holding a Highland Lake Public Forum in early 2019. Stay tuned.

4. For more information, contact Rosie Hartzler, President Highland Lake Association. Rosie.works.maine@gmail.com