Over the past few years algal blooms have
been occurring in Lake Utopia. In order to better understand the cause(s)
of the blooms, the Department of Environment had commissioned five
studies. A brief overview of these studies is presented below.
Based on what is presently known, the overall approach to correcting
the cause(s) of algal blooms is to allow for improved flushing of
water in the Lake and to reduce the overall amount of excess nutrient
inputs to the Lake.
The bathymetric (lake depth) survey shows Lake
Utopia is made up of two basins, one in the north end of the Lake
and one in the south end. There is a relatively shallow ridge near
the middle of the Lake that could impede the complete mixing of
waters between the two basins.
Based on a low lake water level of 16.7 meters above sea level,
Lake Utopia has a maximum depth of approximately 30 meters in the
northern basin of the lake, and approximately half this depth in
much of the southern basin. The average depth of the lake is 11.2
meters below the low water level.
The primary objective of this study was to assess the inflow and
outflow of the canal which is the only outflow from the Lake. Periodically
the flow of water through the canal reverses whereby water from
the Magaguadavic River flows into Lake Utopia. Based on information
related to both inflow and outflow, a further objective was to estimate
the yearly load of nutrients entering Lake Utopia through the canal.
Based on the calculated inflows through the canal, phosphorus loads
from the Magaguadavic River would be in the order of 600 kilograms
per year, and phosphorus exports from the lake would of a similar
magnitude. This implies that the lake was not able to flush any
excess phosphorus in the November 2007 to October 2008 period.
There are various estimates of the average long term flushing rate
of Lake Utopia and although the absolute numbers vary, it is accepted
that the Lake flushing rate is relatively low.
The purpose of this survey was to assess the
developed areas around Lake Utopia and check for signs of nutrient
input from possible malfunctioning septic systems or other point
sources. Water samples were collected and analyzed for nutrients
and bacteria, which are two possible components of malfunctioning
septic systems. The survey around the shore and beach areas did
not identify sources of septic system effluents that would provide
a significant source of nutrients to the lake.
Based on the number of dwellings around the Lake Utopia shoreline,
it is estimated that if all septic systems were releasing effluent
to the Lake, the input of phosphorus from residential development
could range from 80 to 180 kilograms per year.
Considering the potential future development around Lake Utopia
and the fact that in the future, existing septic systems will get
older, it is prudent to continue to work with local residents to
ensure the contribution of nutrients to Lake Utopia is as low as
possible.
The purpose of this survey was to check for
possible excess nutrients in bottom sediments around the effluent
pipe in the northeastern area of the Lake. Bottom sediment samples
were collected 25 meters apart around the effluent pipe and analyzed
for redox potential, sulfides, total organic carbon, total nitrogen,
total phosphorus, zinc, copper and aluminum.
There is woody debris and logs at the lake bottom in the area of
the effluent pipe and the majority of samples collected contained
a high percentage of this woody debris. Because of the influence
of the woody debris in the sediment samples, and the overall small
area that was covered by the survey, it is not conclusive if there
is a large scale buildup of nutrients in the bottom sediments. However,
several samples contained relatively high phosphorus that may indicate
that nutrients have accumulated in the bottom sediments.
The overall objective of this study was to review
all available data and all previous studies and provide expert advice
as to the cause(s) of algal blooms in Lake Utopia. The study identifies
several potential concerns:
- The low flushing capacity of the lake that reduces
the ability to flush any excess nutrients (regardless of source)
from the Lake;
- It is possible that phosphorus in the sediments
at the bottom of the lake are contributing to the sustainability
of the algal blooms. The phosphorus contained within the sediments
may have accumulated over the last twenty years. A likely source
of the phosphorus is the hatchery effluent which in the past had
considerably higher concentrations of phosphorus than the current
effluent.
The next steps will be to work with industry
to continue to improve effluent quality, and to improve flushing
of the lake. We will continue to work with local residents to communicate
best practices of living near lakes. Funding from sources such as
the Environmental Trust Fund will be used to help with future work.
Additional projects such as the plume (mixing zone) study will be
carried out, and the Department of Environment will continue the
monitoring within the lake.
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