Dissolved Oxygen
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Introduction:
All living animals need to breathe oxygen to survive, including aquatic animals like fish. Oxygen also plays a key role in aerobic digestion and decomposition, and therefore is essential for ecosystem health and balance through the recycling of nutrients from dead organisms back into the environment.
Dissolved oxygen is a measurement of the concentration of free oxygen in the water column. This free oxygen comes from organisms, aquatic plants and algae through photosynthesis, and can also be transferred from the air at the surface of the water. When the ecosystem is in balance, there is enough oxygen being produced or transferred into the water to support both decomposition and respiration. However, when there is excess organic waste from sources such as sewage and algal blooms, the oxygen demand can start to outweigh the supply. Decomposers (eg. bacteria, worms, etc) remove more oxygen from the water faster than normal so that the larger volume of waste can be digested, meaning that there is a decrease in the amount of oxygen available in the water column for fish and other animals to breathe. If dissolved oxygen gets too low, there could be an increase in the deaths of fish and other animals, producing even more organic waste and adding to the already elevated decomposition demand.
The amount of available oxygen in the water column is also affected by several abiotic factors, including temperature, salinity, pressure and depth, wind, wave action, and flow. The result is that the amount of dissolved oxygen and the capacity for it will vary between areas and types of water bodies in the Ottawa River Watershed. Seasons can also have an impact as cold water can hold more oxygen than warm water, therefore dissolved oxygen can decrease in the summer when the weather is warmer. Furthermore, smaller bodies of water are more susceptible to temperature changes and flow, which makes dissolved oxygen more variable in tributaries than lakes. This limits the ranges for some species, including fish, that have lower tolerances for changes in habitat conditions. Due to its close connection with temperature, dissolved oxygen is an indicator of watershed ecosystem health that is of particular interest in the context of climate change.
What do we know about dissolved oxygen data in the watershed?
Available dissolved oxygen data for the mainstem of the Ottawa River is from October, 1966 to the present. The horizontal dotted lines represent the minimum dissolved oxygen threshold for freshwater species (black: cold water species – early life stages; dark grey: cold water species – other life stages; light grey: warm water species). Data points below each line indicate DO levels that are too low to sustain species adapted to that habitat type.
With many dissolved oxygen measurements falling below the minimum threshold for early life stages of cold-water-adapted species, there raises concerns that those species could be at risk. However, a more in-depth analysis of the timing of life cycles and if they correlate with when the low-dissolved oxygen measurements occur are needed to be confident that of what this data indicates.
A major challenge for understanding watershed health is that data is limited and sporadic. Dissolved oxygen data is only available at 5 locations along the main stem of the Ottawa River. Although data is available as far back as the 1960s for some of those locations, there are many multi-year gaps, and most locations only reported dissolved oxygen once per month or less.
Further data:
Want to dive further into the data on Dissolved Oxygen in the watershed? Take a look at our dashboard for this indicator.
Relationships to other indicators:
Dissolved Oxygen
Dissolved oxygen (DO) is the concentration of free oxygen in the water column and comes from aquatic plants and algae through photosynthesis, and can also be transferred from the air at the surface of the water. DO is essential for all aquatic life and for the decomposition of organic waste. The amount of available oxygen in the water column is also affected by several abiotic factors including temperature, salinity, pressure and depth, wind, flow, and wave action.
Algal Blooms
Algae are a group of single-celled and multicellular plant-like organisms. Algal growth contributes and supports the ecosystem in much the same way that plants do, through the production of oxygen and as a potential food source for smaller animals. However, when environmental conditions shift from their normal ranges, this can promote algal blooms, a rapid increase in population size of algae.
Water Temperature
With global temperatures on the rise, water temperature (particularly annual maximum temperatures) will be a crucially important indicator of watershed health moving forward. Water temperature is strongly correlated with a number of other important indicators of ecological status, and can often help to explain, give context to, or even predict changes in habitat quality.
Chlorophyll-a
Chlorophyll-a is used in oxygenic photosynthesis and is the predominant form of chlorophyll in green plants and algae. Measuring the presence of chlorophyll-a in an aquatic environment allows for a better understanding of the density and prevalence of algae which can have an impact on the concentration of dissolved oxygen or likeliness of algal blooms.
Total Phosphorus
Phosphorus is a fundamental building block of life. It has key structural roles in genetic material (eg. DNA) and cellular membranes, and is an active component of cellular energy production, storage, and transfer, meaning many necessary biochemical processes that all living organisms rely on (such as respiration, and nutrient absorption) can only happen if phosphorus is available.
Riparian Connectivity
Natural, vegetated riparian areas, the unique, transitional space between aquatic and terrestrial ecosystems, provide numerous benefits including nearshore habitat, filtering runoff, moderating local temperatures, and stabilizing shorelines. Measuring riparian connectivity can provide important information on the health of a watershed and a better understanding of the resilience to threats these areas have compared to shorelines that have been developed.
Combined Sewer Overflows
Everytime there is a Combined Sewer Overflow, sewage, along with stormwater is released into a waterbody introducing pollutants, pathogens and excess organic waste. Untreated or inadequately treated sewage poses health risks for people who use the water for recreational activities, such as swimming, paddling, etc. It also impacts aquatic ecosystems as the presence of organic matter, and its decomposition, decreases the amount of dissolved oxygen available.
Flow
How much water or volume of water present at a given time and how quickly it moves along the length of the river determines flow. Flow can provide context to a number of other indicators and as an important measure to factor when examining water chemistry or understanding impact of pollutants (for example, the same concentration of pollution will impact a river with low flow differently than one with high flow). Comparing flow within the same reach can help identify different trends and, when affected by barriers, can impact aquatic biota.
Change in Land Use
Land use is a classification for all the various ways an area of land be used and includes agriculture, urban, roads, natural and harvested forests, wetlands and protected areas. Changes in land use have implications for both the quantity and quality of water introduced to the river through runoff and can help analyse changes in other indicators in proximity to where land use changes have occurred.
Benthic Invertebrates
Benthic invertebrates are the organisms found in soils, rocks and organic materials that make up stream, river, and lake beds. Pollution tolerance varies greatly between different species, so the relative abundances of pollution-sensitive and pollution-tolerant benthic invertebrate species is a good indicator of water quality and ecosystem health.
Invasive Species
Invasive species are non-native species that have been introduced to a habitat and are able to outcompete native species. Invasive species disrupt the established ecosystem which can result in a decrease in habitat quality and health.
Fish Richness
Fish richness is a measure of how many different species of fish are present in an ecosystem or habitat. Generally speaking, higher species richness is better, but some confounding factors include habitat type and the presence of invasive species (which can increase richness while threatening ecosystem health).
Water Mercury
Mercury is highly toxic, but naturally occurring concentrations of it are bound by plants and sediment and removed from the water column. However, human activity introduces excess mercury, resulting in free mercury in the water column. Water mercury is therefore a useful indication of human impact on the watershed.
Ice On/Off
Ice on is the date when a body of water is fully covered by ice, and ice off is the date when the ice disappears from the water surface. These measurements provide information about several environmental conditions, such as air temperature, water temperature, and flow.
Further resources:
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