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The freshwater environment
Published in Andrew Farmer, Managing environmental pollution, 2002
The growth of freshwater plants is primarily limited either by lack of light (in deep or turbid water) or by nutrient supply. Many plants are adapted to low nutrient conditions. However, if the nutrient limitation is overcome by the addition of pollutants, then the more aggressive species will respond and can come to dominate the freshwater ecosystem. The following impacts are likely to occur: The first impact of nutrient enrichment to freshwaters is a bloom of phytoplankton. These microscopic plants are able to multiply very rapidly if all their nutrient needs are met, and water bodies can very rapidly turn into a green ‘soup’. Alternatively, the blooms may appear as a ‘scum’ on the water, particularly around the edge of lakes. Such blooms reduce the light available to higher plants in the lake or river. In many instances, the bloom is temporary, as the zooplankton populations that graze on the phytoplankton also increase in numbers. However, continual excessive nutrient addition can maintain a bloom. In Europe and North America such blooms often occur in spring and autumn, and this reflects the rainfall patterns, when run-off from agricultural land containing nutrients occurs and temperatures are still adequate to promote algal growth. The appearance of a bloom is a good indication that a problem exists, and its source should be investigated and rectified.Excessive growth of aggressive higher plants. Submerged species, such as Hydrilla verticilata in Florida or Myriophyllum spicatum and Potamogeton pectinatus in Europe, directly outcompete other species within the water body. Equally damaging are floating species, e.g. Lemna species in temperate regions or Eichornia crassipes in tropical ecosystems. These form a complete blanket on the water surface and so deprive submerged species of light. The reduction in the growth of submerged plants will reduce the production of oxygen in the water bodies. Oxygen-deprived waters (anoxic waters) are unable to support fish.Excessive growth of filamentous algae and epiphytes. Epiphytes grow on the leaf surfaces of other plants and the growth of these aquatic algae can prevent light reaching the plants on which they grow. This is a particularly serious problem when very nutrient-poor lakes (oligotrophic lakes) are polluted, as the plants characteristic of these lakes are often slow-growing and are easily smothered. Filamentous algae can form extensive mats within a water body, and can smother all the other plants that it contains. Blue-green algae (more closely related to bacteria than other algae) may form these mats. These can be a particular problem, as they also release toxins that can lead to fish kills and may even pose a threat to humans and their domestic animals while swimming in the waters that contain these toxins. Even though these blue-green mats may not cover a lake, their presence in sufficient quantities will lead to warnings being issued about bathing in such lakes.
Water quality ramifications of temporary drawdown of Oregon reservoirs to facilitate juvenile Chinook salmon passage
Published in Lake and Reservoir Management, 2021
S. K. Hamilton, C. A. Murphy, S. L. Johnson, A. Pollock
The timing of drawdown will involve trade-offs among multiple factors that include the depth and chemistry of reservoir waters, life cycles of sensitive or highly valued animals, reservoir and downstream water uses, and potential for shifts in the downstream thermograph. Depending on the design of the dam and the existence of thermal and chemical stratification, water that is released from deeper than normal depths can be lower in temperature and DO, and may differ in chemistry, from the normal outflow. In addition to oxygen stress, anoxic waters can contain toxic levels of sulfide or ammonia that could directly harm downstream aquatic organisms. Vertical profiles of water quality in the reservoir will reveal large volumes of thermally and chemically distinct water, but as was evidently the case in Fall Creek Reservoir, water of distinct chemistry may exist near the sediment–water interface, and may be difficult to discern in profiles. If a reservoir is drained in a season when the water column is well mixed (or early in the stratification period), the risks of discharging water with low DO or otherwise undesirable chemical content are lower.
Intense variability of dissolved oxygen and temperature in the internal swash zone of Hamilton Harbour, Lake Ontario
Published in Inland Waters, 2021
Bryan Flood, Mathew Wells, Jonathan D. Midwood, Jill Brooks, Yulong Kuai, Jingzhi Li
Wind energy imparted to stratified lakes energises internal seiches and can result in upwelling of the cold, and often hypoxic, hypolimnion (Mortimer 1952, Spigel and Imberger 1980, Imberger and Hamblin 1982). Basin geometry and bathymetry are important parameters that influence the spatial heterogeneity of internal seiches and the internal swash zone, the area along the bed over which the thermocline flows back and forth (Fig. 1; Shintani et al. 2010, Cossu et al. 2017). Over time scales of hours, internal seiches lead to changes in water currents, temperatures, and oxygen levels that are most pronounced in the benthic zone where the thermocline intersects the lakebed (Coman and Wells 2012a). Because the oxycline often coincides with the thermocline in eutrophic lakes, the internal swash zone experiences fluctuating physical properties, being subjected in turn to warm, well-oxygenated epilimnion waters and cold, often poorly oxygenated hypolimnetic waters. If upwelling is strong enough, the internal swash zone can overlap the ecologically important littoral zone, resulting in high variability of temperature (T) and dissolved oxygen (DO), altering the availability of viable fish habitat. Fishes respond to changes in T and DO in their environment by moving to locations that better match their thermal and DO preferences (Levy et al. 1991). In extreme cases, the full upwelling of anoxic water deprives fish of any viable habitat, resulting in widespread fish kills (e.g., in Lake Erie; Rao et al. 2014). Understanding how basin morphometry affects T and DO variability is therefore an integral part of understanding fish habitat. Previous studies have typically measured T and DO at low spatial and/or temporal resolution and extrapolated horizontally throughout the lake, precluding them from observing and studying the response of fish to thermocline motion due to internal wave activity (e.g., Sellers et al. 1998, Plumb and Blanchfield 2009). Here, we present high spatial and temporal resolution records around the large basin of Hamilton Harbour situated at the western end of Lake Ontario, Canada. We show how the basin morphometry influences the spatial changes in fluctuations of DO and T within the dynamic internal swash zone.