Eutrophication and Trophic State
Gertrud Niirnberg
Whv Does Lake Water why is there such a variation? Before I states, may be used to clear lakes. summarize some aspects why water To tum a subjective value (a "good" lQualitvJ Differ from quality differs from lake to lake, water lake) into an objective appraisal, lake to lake:» quality has to be defined and explained. limnologists typically use three or four lake characteristics to describe water ow many lakes do you know? A Definition: ''Water Quality" quality, and not just clarity. Most of Do you have a favorite lake these variables reflect conditions of the H (Figure 1)? Do you know a Laymen often use water clarity and upper water layers (epilimnion); one lake that you'd transparency to describe water quality. reflects conditions in the bottom layer rather not know However, water quality is not a (hypolimnion). All these characteristics (Figure 2)? quantitative variable per se. are based on summer conditions, since Chances are Sociological studies found that people lake quality often deteriorates when that you know who are used to relatively clean, clear, temperature and light increase so that quite a few and swimmable lakes have much higher algae growth rates are at maximum, and lakes that are expectations about the transparency of a also, because that is when humans all different. "good" lake, than people living in typically are in close contact with lakes. Some invite regions with few clear lakes or few you for a dip, natural lakes in general. This becomes Transparency some entice particularly obvious when comparing This is the water quality variable you to get the lakes across the continent or even people usually note first. It is mainly bass boat out, some are just gorgeous to between continents: south-central North determined with a Secchi disk and can look at-with the mountains behind and Americans, central Europeans, and be measured by any person with a boat, the meadows in front, or with the South Africans are used to green lakes; string, and a weighted disk. Secchi disk skyscrapers in the back, but tranquility northern Europeans (Scandinavians), as transparency reflects algal biomass as around. So everybody knows by well as northern Canadians and people well as water color and varies between 0 experience that lakes are different, but from some northern and mountainous and several meters (Figure 3). mEt-
Figure 1. Grandview Lake on the Canadian Shield, the author's favorite lake, where she lives, swims, skates, and slds. Figure 2. A highly productive, less-desirable lake.
Wlllllll • Sprlng2IJIJ1 29 'II* Water Quantv Table I. Trophic state categories based on summer epilimnetic water quality (Nlimberg 1996). pollutants can be washed into the lake Oligotrophic Mesotrophic Eutrophic HYpe reutrophic when the slope is steep or the ground is relatively impervious. Generally, almost Total Phosphorus ().lg/L) < 10 10-30 31- 100 > 100 - all rain that fa!ls."on a parking lot gets Total Nitrogen ().lg/L) < 350 350-650 651- 1,200 < 1,200 flushed into a nearby stream or lake, Chlorophyll ().lg/L) < 3.5 3.5-9 9.1-25 >25 unless storm water ponds and other holding structures are used. Similarly, Secchi Disk transparency (m) >4 2-4 1-2.1 < 1 runoff from adjacent fields and lawns is Anoxic Factor (d!yr) 0-20 20-40 40-60 > 61 flushed into the lake, including soil particles and fertilizer, especially if lake's deviation from an idealized cone nutrients and pollutants quickly through fields are tilled vertically on a slope. shape, i.e., the ratio of mean depth the system with limited time for Development around remote lakes divided by the square root of the lake assimilation. On the other hand, without sewage-collecting structures surface area. A small morphometric flushing prevents a self-cleaning used to fertilize many North American ratio means weak stratification and process of settling pollutants and lakes. However, advances in septic polymixis, a larger ratio implies nutrients to the bottom sediments. system technology keep the effect of summer stratification and in extreme Detailed models are available that shoreline development to a minimum, cases, meromixis (Osgood 1988). This describe the influence of hydrology especially if combined with sound ratio was found repeatedly to influence combined with morphometry on the fate development strategies (setbacks from water quality relationships. For of entering nutrients (Ntirnberg 1998, shoreline, natural shorelines). example, lakes with high ratios have a 2001). Since the location of a lake within tendency to have high Anoxic Factors its natural surroundings is so important for a given nutrient level (Ntirnberg Chemistry and Climate to its water quality, another lake 1995), which in turn can mean In addition to the chemical classification system was developed prolonged self-fertilization. differences between hard versus soft based on the eco-region principle water lakes, lake color is a useful (Omernik et al. 1991). This concept Water Flow (Hydrology) indicator of natural organic acids realizes that the trophic state of lakes Another important influence on (humic and fulvic acids). Again, these varies between geological regions. A individual lakes is related to the characteristics are heavily influenced by quarter of the lakes with the best water hydrology, in particular, the water the catchment geology. Once, stained quality per region is used to represent movement through it, also called the lakes were believed to comprise a "natural" or "background" water quality "flushing rate." The faster the flushing completely different trophic state class, with only minimal anthropogenic rate, the more the lake resembles a river. "dystrophic" or low productivity. influences. Corresponding water quality Reservoirs in particular often have high However, more recent research indicates thresholds have been listed for many flushing rates. The influence of flushing that stained lakes behave quite similar states and can be used for target-setting. rates on water quality is complicated. to clear lakes, with indications that Despite the strong dependence of On one hand, flushing helps moving biomass and productivity (especially water quality on its location within the when considering the whole water watershed or catchment basin, water column, including heterotrophic quality has to be influenced by production by bacteria) is even individual lake characteristics as well, increased (Ntimberg and Shaw 1998). because you can have very "clean" lakes There also is a tendency for colored located beside lakes that are nutrient lakes to have higher nutrient enriched with murky water and algal concentrations and hence trophic state blooms. (Figure 11, significant positive regressions of nutrients on color, about Lake Shape (Morphometry) 30% of the variance explained). Probably the most obvious Climate has always influenced lake ,o? difference among lakes lies in their water quality as well. The ratio of Mean Depth (m) shapes (morphometry). In general, it evaporation versus precipitation dictates appears that shallow lakes tend to have Figure 10. Summer epilimnetic total whether a lake will become more _ higher [email protected]. than phosphorus-eoncentratiOJ:L versus mean___ ,,,..... ,.- conceritrated with time, and more salty deeper lakes (Figure 10). More depth, where mean depth can be calculated and eutrophic. Many prairie and desert generally, lakes can be shallow and as lake volume over lake surface area. lakes, such as the Great Salt Lake, UT, large, shallow and small, deep..and Shallower Z!Lke_~tend to_l;}qve h!g_her are known as--saline-lakes. Recently, phosphorus concentrations than deeper large, and deep and small. A good changes in color were observed and measure of these combinations is the lakes (crosses), except for many Central European lakes (filled circles). traced back to climate changes. There is
32 5iJtln!120111 • WtZINf and hydrology. As these relationships __ . 1998. Prediction of annual and are predictable and quantifiable, they seasonal phosphorus concentrations in 100 can be used to forecast lake water stratified and polymictic lakes. Limnology s quality and its impact on future and Oceanography 43: 1544-1552. ;£ __ . 1996. Trophic state of clear and 0 scenarios (for example, the ill!Pact of 0 colored, soft- and hardwater lakes with () increasect development; for a more 10 special consideration of nutrients. anoxia. detailed description, see Niimberg and phytoplankton and fish. Lake and LaZerte 2001). In this way, water Reservoir Management 12: 432-447. quality can be modeled-for individual __ . 1995. Anoxic factor, a quantitative dl<:) lakes and specific conditions even if we measure of anoxia and fish species Phosphorus ( 'f.'9/L) only know some basic information, e.g., richness in Central Ontario lakes. the location of a lake on a map. The Transactions of the American Fisheries Figure 11. Average lake color and map would give us the general Society 124: 677-686. epilimnetic phosphorus concentrations. Omernik, J.M., C.M. Rohm, R.A. Lillie and watershed characteristics (including N. Mesner. 1991. Usefulness of Natural approximate geochemistry and usage of Regions for Lake Management: Analysis more research investigating the effect of the watershed sub-areas), the lake's size of Variation among Lakes in Northwestern warmer and dryer weather on lake and shape, and the climate. All these Wisconsin, USA. Environmental levels, organic acid concentration, and variables could then be used in Management 15: 281-293. Osgood, R.A. 1988. Lake mixes and internal trophic state. empirical models to make predictions about the water quality, just as Frank phosphorus dynamics. Archive Hydrobiologia 113: 629-638. In-Lake Usage Rigler demanded. Naturally, how a lake is treated, can Gertrud Nii.rnberg is the NAIMS Director Further Reading influence lake water quality. Misuse of Region XI, eastern Canada. She is head of Freshwater Research, a limnological i.e., overcrowding by boaters and Burns, N. M., J.C. Rutherford and J.S. company focussing on eutrophication related swimmers, dumps and spills from Clayton. 1999. A monitoring and research and modeling of lakes and vessels or shoreline structures--can classification system for New Zealand reservoirs. She has published comparative increase nutrient concentrations and lakes and reservoirs. Lake and Reservoir research and empirical lake models on negatively effect water clarity. On the Management 15: 255-271. phosphorus, iron, and anoxia and on lake Carlson, R.E. 1977. A trophic state index for other hand, in-lake management management techniques in numerous lakes. Limnology & Oceanography 22: treatments such as chemical scientific journals and has been an associate 363-369. precipitation or withdrawal of nutrient editor to the NAIMS journal, Lake and Ntirnberg, G.K. and B.D. LaZerte. 2001. rich hypolimnetic water can positively Reservoir Management, since 1996. You can Predicting Water Quality, Chapter 5 in reach Gertrud at:
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