2015 Mississippi Lake State of the Lake Report

By Melissa Dakers, Co-Chair, Environment Committee, Water Quality Steward
Edited by Robert Betcher

Table 1: Lake Trophic Status

Table 1: Lake Trophic Status

 

Phosphorus Levels Increase but Remain Within Historical Range

Water sampling, carried out in 2015 by the Mississippi Lakes Association (MLA) in co-operation with the Mississippi Valley Conservation Authority (MVCA), showed a significant increase in average lake phosphorus levels compared to 2014. The higher average is due to the high levels found in samples collected in the fall, during the reported blue-green algae bloom. Although the levels have increased compared to 2014, they still fall within the range of values over the past few years.

Water Quality Sampling

Water quality information on Mississippi Lake has been gathered under a variety of programs since 1968, primarily to examine the trophic status of the lake (the amount of biomass present in the lake, see Table 1). Mississippi Lake, being shallow and having a broad surface area, is subject to excessive aquatic vegetation and algae growth and was considered to be eutrophic in the late 1960’s through much of the 1970’s. Water sampling includes measurement of water clarity, which is primarily affected by the amount of suspended algae, using a Secchi Disc. The concentration of active chlorophyll (chlorophyll a), as a measure of the amount of 

photosynthesizing plants (algae and phytoplankton) in the water is also assessed, along with total phosphorus, since elevatedphosphorus concentrations are a major factor in promoting plant growth and algae blooms. Sampling programs have also included other water quality parameters such as dissolved oxygen, pH and temperature which will not be discussed in this article, other than to say that dissolved oxygen measurements have and continue to show life-supporting oxygen concentrations throughout the water profile. Table 1 indicates how the measurement of water clarity, chlorophyll a and total phosphorus concentrations can be used to assess the trophic status of the lake.

In 2015, MLA volunteers once again were out on the lake collecting samples, as we have each year for the past ten years. In co-operation with the MVCA, as part of the Watershed Watch program, we collected samples three times during the open water part of the year: at deep water locations near Burnt Island and Pretties Island to monitor the trophic status of the lake; just below the Innisville rapids (termed the Inlet site) to measure water quality coming into the lake; and 100m upstream of the Highway 7 bridge (termed the Outlet site) to examine the water quality leaving the lake. The MLA added these last two sites to the sampling program in 2008. At each of these sites, samples are collected through the euphotic zone (the upper lake level where sunlight remains sufficient for photosynthesis by plants – the top sample results in Table 2), and 1 meter above the bottom of the lake.

Figure 1 – Total phosphorus results from euphotic zone (depth at which sunlight can penetrate) samples collected as part of the Watershed Watch Program.  The PWQO line is the provincial water quality objective. 

 

Table 2:  Water Quality Results for 2015

The MLA provides staffing and funding for water sample collection, while MVCA funds the laboratory analysis costs as part of our joint commitment to collect frequent and regular water quality information. This allows us and our partners, to understand annual and longer-term water quality variations in the lake and how these may impact aquatic vegetation and algae growth, fish, waterfowl and other species, as well as our enjoyment of the lake.

Results

The results of the 2015 MLA/MVCA sampling program which relate to the trophic status of the lake (Burnt Island and Pretties Island sites) are provided in Table 2. In general, although there is variability in trophic classification based on the results from individual parameters, Mississippi Lake would be classified as a mesotrophic lake in 2015, similar to observations over the past several decades. Please refer to Table 1 for definitions of the tropic status while reading the discussions below.

 

In 2015, Secchi Disc readings on Mississippi Lake remained stable; readings averaged 2.8 m in both 2014 and 2015 at the two deep lake locations, indicating little variation in water clarity. These values fall on the lower end of what we have observed over the past 14 years. The average for the lake of 2.8 metres would indicate a eutrophic status based solely on water clarity. The average chlorophyll a concentration was 1.2 µg/L in 2015, a decline from 2014 when the average was 5.0 µg/L, and is in the lower range measured in the past 14 years. This indicates a reduced amount of algae and phytoplankton in the lake. The 2015 average reading places the lake in the mesotrophic status, based on the ranges set out in Table 1.

Phosphorus is generally considered to be the main contributor to eutrophication. The average total phosphorus reading in the euphotic zone increased in 2015, in comparison to results from 2014, but only one of the samples registered a total phosphorus value exceeding the provincial objective of 20 µg/L (indicating a eutrophic status). This sample was taken in the fall, during the reported blue-green algae bloom. The 2015 average value was 17.7 µg/L, a significant increase from the average value of 12.2 µg/L found in 2014, although the phosphorus concentration in 2014 was at the lower end of historical observations.

In Figure 1, we present the historic results for annual total phosphorus concentrations in the euphotic zone at each of the Watershed Watch deep water sampling locations. The trophic zone classifications are indicated by the background colours on the graph. At each station, we see that the 2015 averages are near the medium to higher range of readings, over the period of record. Continued sampling on the lake will provide data that can be used in any future studies of lake water quality.

Summary

Taken together, the water quality results for 2015 categorize our lake within the mesotrophic status as set out in Table 1. This is consistent with the lake’s status, observed over the past few decades of monitoring, suggesting that the water quality of the lake has remained relatively stable. While we have seen multi-year and year-to-year trends and variations in water quality parameters in the past, there is only a poor understanding of why these changes have occurred. We currently have little ability to predict how the lake will respond to future potential drivers such as climate change, increasing development within the watershed, or zebra mussel population fluctuations to name a few. The Mississippi Valley Conservation Authority is currently undertaking a number of interesting studies, which may provide some answers, or at least provide a sufficiently improved understanding to better phrase the questions.

In the meantime, it is important to continue to monitor the water quality in the lake to provide the information needed by current and future studies, and to individually be responsible stewards of the lake by limiting our nutrient footprint. Use phosphate-free soaps and detergents, minimize your use of lawn fertilizers, maintain a healthy shoreline, keep your septic system in good working order and properly deal with your grey water. If everyone does their part, Mississippi Lake will remain healthy and productive, enjoyed by our residents and visitor for many generations to come.

 

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