Orr Lake has been subject to ongoing health monitoring since 2002 through the Ministry of Environment and Climate Change’s (MOECC) Lake Partner Program. Under this program a dedicated lake resident collects total phosphorus samples and measures water clarity using a Secchi disk to document changes in lake health over time. The Nottawasaga Valley Conservation Authority has been measuring winter dissolved oxygen (DO) profiles in Orr Lake since 2010. A late-winter oxygen profile can be viewed as a barometer of winter length, lake nutrient status and potential stress on lake ecosystems.
Total phosphorus (TP) is an important nutrient in water as it is often the “limiting” factor controlling the growth of plants and algae. Phosphorus levels in water can occur naturally in very low through to very high concentrations, though low to moderate levels are more common. Problematic levels of phosphorus occur when these natural levels are enriched by human inputs which may lead to excessive plant and algae growth. This excessive growth can cause problems such as restricted recreational uses and aesthetic concerns, as well as oxygen depletion resulting in fish kills.
Orr Lake is a mesotrophic lake suggesting its nutrient levels would naturally be elevated when compared to the nutrient-poor (oligotrophic) lakes of the Canadian Shield. Phosphorus concentrations in a densely populated lake like Orr Lake will increase from septic systems, lawn maintenance (fertilizers, pesticides), and soaps that drain into the lake. Phosphorus levels can be reduced by plant uptake and binding to lake sediments - maintaining natural shorelines is a key tool to offset potential phosphorus impacts. The MOECC Provincial Water Quality Objective for phosphorus in lakes is 0.02 mg/L. Orr Lake has consistently been below the standard. 2014 results show the first occurrence where phosphorus levels exceed the Provincial standard (0.024 mg/L in one sample). Further monitoring is needed in order to determine if this elevated level is part of a trend or merely a one-time occurrence.
Secchi depth is measured with a Secchi disk, a circular plate with a rope through the middle which is divided into quarters bearing black and white coloured sections. The disk is lowered into water until it is no long visible and a measurement is taken at this point. High readings indicate that the water is relatively clear and not turbid, whereas lower readings indicate turbid or coloured water. In general, turbidity and the presence of colour (algae) are indicative of high lake productivity and clearer water is indicative of lower lake productivity. The Secchi disk reading for 2014 was not provided by the MOECC.
Dissolved oxygen is referred to as the amount of free oxygen dissolved in the water. It is required in water to allow fish, insects and other aquatic life to breath and survive. Tolerant warm water game fish can live comfortably in waters with DO concentrations above 3 mg/L, becoming stressed and perishing at levels below 3 mg/L. During the summer dissolved oxygen is created by aquatic plants releasing oxygen into the water using photosynthesis, as well as wave-action which physically mixes atmospheric oxygen into water. As the fall and winter approach, these plants die, start to decompose and deplete the dissolved oxygen levels in the water column. Persistent ice cover during the winter prevents the addition of dissolved oxygen. Monitoring late-winter oxygen levels over time is important to identify potential stresses to the lake ecosystem and associated long-term trends.
The NVCA has been measuring late-winter DO since 2010 on Orr Lake. Despite it being one of the coldest winters on record, 2014 results mirror previous years with an oxygen sag starting around 2m depth, stressful conditions begin around 2.25m and near anoxia deeper than 2.5m. The NVCA did not anticipate any significant fish-kill this spring.
For more info, contact Ian Ockenden of the NVCA by phone at (705) 424-1479 x234 or email email@example.com.