hallow and D ep Lak s: Determining Successful Mana·-ge·m.ent Options
G. Dennis Cooke, Paola Lombardo~ an([Christina Brant
hallow lakes are far more Table 1. Characteristics of shallow and deep lakes abundant than deep lakes, and more people are concerned about Characteristic Shallow Deep their quality, management, 1. Likely Size of Drainage Area to Lake Area High Lower and 2. Responsiveness to Diversion of External P Loading Lower Higher rehabilitation. 3. Polymictic Often Rarely Knowledge 4. Benthic-Pelagic Coupling High Low about them has not developed as 5.1ntemal Loading Impact on Photic Zone High Low rapidly as 6. Impact of Benthivorous Fish on Nutientstrurbidity High Low knowledge of 7. Fish Biomass Per Unit Volume Higher Lower Cooke (and grandkid) deep lakes,-but 8. Fish Predation on Zooplankton High Lower progress now is 9. Nutrient Control of Algal Biomass Lower Higher being made. 10. Responsiveness to Strong Biomanipulation Higher Lower Much of this 11. Chance of Turbid State with Plant Removal Higher Lower work is 12. Probability of Fish Winterkill Higher Lower European, for example the 13. % AreaNolume Available for Rooted Plants High Low works of Moss, 14. Impact of Birds/Snails on Lake Metabolism High Lower Hosper, Scheffer, 15. Chance ofMacrophyte-free Clear Water Low Higher Meijer, Van Donk, Jeppesen, Sondergaard, Lombardo Persson, deep areas behind a dam, has a large concentrations often are the result of Benndorf, surface area relative to mean depth, and external loading. Shallow lakes may Hansson, and does not have summer-long thermal differ greatly from deep lakes in their Bronmark, as stratification. Many lakes that meet responses to this action. well as North these criteria are impoundments of American (e.g., small streams, which may have a large Nutrient Dynamics Havens, drainage area relative to lake area. Bachmann, Because stream nutrient levels are often Water column nutrient Hoyer, Canfield, high it may be very difficult and/or concentrations in shallow lakes are the Carpenter, and expensive to lower nutrient product of both external loading Schelske). The concentrations in these small (nutrients from land runoff, direct purpose of this impoundments. Some shallow lakes precipitation and other sources outside Brant report is to may be part of a former, usually large, the lake) and internal loading. Internal provide some basic ideas about shallow wetland complex. phosphorus loading is the release of lake ecology, especially about the The first step in conventional lake phosphorus from storage in lake differences between shallow and deep management is the reduction of external sediments to the water column. Because lakes, and to usethis information to nutrient-loadll!fto lower the ---~---- - . ~oW-lakes-frequently are mixed from describe some shallow lake concentration of nutrients (phosphorus, top to bottom, internal loading can be as management ideas. Table 1 is a nitrogen, etc.) in the water column and significant as external loading in - summary of thesercomparisons-. thereby redace-ilie abundaaee of algae. increasing water column phosphorus A shallow lake has an average This may be sound advice, but it is concentration. During drier summer depth of about 3 meters (10 feet) or less, based on our understanding of deep months, it is the dominant nutrient excluding small "deep holes" or small lakes where open water phosphorus source. Deep lakes may have lower surface often-for example, a shallow or (grass carp) are very destructive to water nutrient concentrations after "polymictic" lake (frequent rooted plants. We have carefully diversion or control of external nutrient circulation)-can be estimated by the documented a turbid, algae-dominated sources because nutrients released from ratio of lake mean depth to lake area by shallow lake in northeastern Ohio where sediments in deep, oxygen-free water using the Osgood Index (01). The Index grass carp and common carp prevent the mostly remain there, and because estimates the probability of complete lake from becoming clear and materials that have settled (sedimented) mixing during summer months. Lakes dominated by macrophytes. to the lake bottom also remain there. with an OI of 3-4 or less are polymictic In deep lakes, fish disturbance of External nutrient diversion and! or and may be strongly affected by internal shallow water sediments is limited to control therefore can produce a lower loading, regardless of the amount of the near shore area. concentration in surface waters of deep external loading. lakes and possibly fewer algae. Algal Dominance in Shallow Lakes Shallow lakes may continue to have high concentrations after treatment or The Osgood Index = mean depth High pH (9 or above) promotes diversion of external loading because (in meters) divided by the square phosphorus release from iron complexes 2 continued internal loading affects the root oflake area (in km ) or O.I. = and sediments, even in the presence of entire water column. These lakes zdA0 abundant dissolved oxygen. High pH is therefore are resistant to long-term caused by intense photosynthesis, change in nutrient concentrations and perhaps during an algae bloom or in may continue to have algal blooms and Shallow, nutrient-rich lakes appear areas of dense rooted plant growth. turbid water after a significant and to exist in one of two highly stable Shallow lakes are more susceptible than expensive nutrient diversion. conditions-either turbid and deep lakes to this process because water A major mechanism of internal dominated by algae or clear and volume is lower and photosynthesis phosphorus loading involves thermal dominated by rooted plants occurs throughout the water column, stratification, which prevents water (macrophytes or "weeds"). Many lake and even on the lake sediments if the column mixing. In productive lakes, users view either of these conditions as water is clear. Many shallow lakes do there may be loss of dissolved oxygen highly undesirable and expend large not have the chemical buffering capacity in deep water through microbial amounts of effort and!or money to of deep lakes so pH changes occur respiration, and the subsequent release obtain a clear, weed-free lake. In faster and over a wide range. This of phosphorus from iron complexes in addition to internal nutrient loading, process can be significant in the mud. In deep lakes, the water there are aspects of the ecology of maintaining the algae-dominated lake. column remains thermally stratified all sb,allow lakes that make them unlikely Algae may be found throughout the summer so that released phosphorus to remain clear and weed-free for any water column in a mixing lake, but only cannot reach the upper, lighted waters to significant period of time. in the surface water of a deep stratified stimulate algae growth except by lake (unless light reaches the lake's diffusion or by certain climatic events. Role ofFish bottom or the algae are in a "resting But in shallow lakes, phosphorus stage" in lake sediments). But, released from lake sediments influences Fish may play a very significant sometimes the quantity of algae can be the entire water column because shallow role in maintaining turbid, algae-rich very low despite high nutrient lakes usually destratify and mix several water in shallow lakes, and can prevent concentrations, producing clear water times per summer. For example, shallow them from becoming clear water conditions. This is caused by lakes may stratify on warm, calm days, systems. The entire water column is microscopic animals called zooplankton followed by rapid oxygen depletion in habitable by fish in shallow lakes, their that graze on algae. One group of deeper water and a significant biomass per unit volume is greater, and zooplankton, the genus Daphnia, is phosphorus release. Cooler, windier their impact on lake metabolism may be particularly significant in reducing algal weather that follows causes complete much greater than in deep lakes. biomass through grazing. mixing and introduction of the released Benthivorous fish (bottom-dwellers), Zooplankton-eating fish called phosphorus to the entire lake. This can like common carp and bullheads, mix "planktivores" dominate many shallow occur rapidly in shallow lakes because bottom sediments and nutrients into the and deep lakes. Examples include water volume is low, leading to more water column, and may do so over the young-of-the-year of most species, plus rapid heating and cooling, and to rapid entire area of a shallow lake when it is juvenile or stunted bluegill, European loss of dissolved oxygen. Algal blooms mixing and dissolved oxygen is carp, crappie, other sunfish, perch, and
may follow each stratification! abundant. They also ex~rete larg.e -.C · ··· gimard shad:-The presence of densL destratification event. quantities of urine and fecal matter populations of planktivores will mean . The probability that a lake will (grass carp eliminate half of their daily blooms of algae because zooplankton - ----remain stratified all summer- for food intake), further increasing the grazing. control is low or absent. This is example, a deep or "dirnictic" lake available nutrient levels in the lake's an important mechanism that maintains (circulates twice a year)-or mix water column. Common carp and amurs the algae-dominated, turbid water condition even when nutrient 111*
WllUIIE • Sonng 2lJIJ1 43 ~~~• Shallow and Deep lakes all related to the intense coupling of Macrophyte Dominance sediments and water, and to the roles of in Shallow Lakes concentrations have declined through fish. In these lakes, internal nutrient nutrient diversion or from treatment of loading, partly influenced by fish Shallow lakes differ from deep the internal loading process with alum. activities, can maintain high lakes in another significant way. In The amount of algae in deep lakes concentrations and an algae-rich water mimy shallow lakes, rooted plants thrive is greatly influenced by zooplankton column. If zooplankton that graze algae across most or all of the lake area. grazing, especially when nutrient are abundant, a condition most likely to whereas in deep lakes rooted plants are diversion has produced a significant occur when planktivorous fish are rare, usually fimited to water less than ten change in an essential nutrient then the water column will be clear, feet deep because sufficient light does concentration. But in shallow lakes, macrophytes will grow, and sediment not penetrate beyond that depth. In there is a more complex array of factors, erosion will be less likely. some deep lakes, the area of shallow water is so small that macrophytes are never abundant. Dense macrophyte growths in shallow lakes help stabilize shoreline and bottom sediments. Because plants themselves are a type of habitat, biodiversity in shallow lakes can be high. Benthivorous and planktivorous fish may be less abundant, relative to piscivorous fish such as largemouth bass, and algae eating zooplankton thrive and maintain clear water. In addition, macrophytes appear to produce compounds that inhibit algae growth. The combination of abundant plants, few or no benthivorous and planktivorous fish, and more predatory fish and zooplankton, results in a shallow lake with few algae and clearer water. This 0 condition can be very stable, just like the turbid, algae-dominated condition, even when nutrient concentrations are high. When rooted plants are absent, whether from light limitation in turbid, algae-rich lakes, or from some management procedure such as a grass carp application, there will be shoreline and lake bottom erosion from the action of boats and wind. Such erosion Figure 1. Main characteristics of deep (top) and shallow lakes (center and bottom). In deep increases turbidity and internal loading lakes, the bulk of nutrient-rich sediments remains in the deepest portion of the lake. Nutrient and helps to maintain the turbid state recycling is limited in the upper water layers, and macrophytes and benthivores are limited even when external loading is less. to a small area in the shallow littoral. Clear water is maintained by piscivores (which could spawn in the vegetated littoral), which keep zooplanktivorousfish density low. Zooplankton (dark areas) limit the biomass of suspended algae (light areas). In shallow lakes, water Management Considerations mixes throughout the water column, and nutrients are easily entrained from the sediments (internal loading). If enough submerged vegetation is present, sediment resuspension by Shallow lakes are susceptible to wind or benthivores is limited. Plants support abundant piscivores which control strong biomanipulations (fish additions planktivore and benthivore abundance, and zooplankton thrive keeping suspended algae low. or removals that affect the entire aquatic Water is clear and plant and animal diversity is high. When enough nutrients or silt enter a food web). Turbid, plant-free lakes may ~
44 SlltlllY21JOl "mtlJNl consume all the oxygen beneath winter Washington lakes. Clear water and experiments, which mimic effects of lake ice. Since little oxygen is produced macrophyte dominance should continue snail-eating fish, produced rich growths by photosynthesis beneath snow in an alum-treated shallow lake if the of algae on plant leaves and a decline in covered lake ice and since oxygen planktivorous and benthivorous fish plant growth rate. Snails therefore replenishment from the atmosphere density is low. Alum treatments of deep appear to be another buffer that can't occur through lake ice, oxygen lakes will control sediment phosphorus maintains the clear-water:macrophyte concentrations may decline to lethal release, but may have little impact on dominated condition. levels for some fish. surface water algal blooms unless the What about herbicides and The types and abundances of fish lake has significant phosphorus algicides as shallow lake management appear to be the keys to carrying out a transport from deep to shallow waters. tools? Both types of chemicals can act biological manipulation of a shallow Waterfowl play a major role in as switches to induce the turbid algal lake. Adding grass carp is a powerful shallow lake and pond ecology. Some bloom state. Some herbicides are highly manipulation and almost always birds are plant eaters and can prevent effective in eliminating submersed produces a switch from a macrophyte the establishment of rooted vegetation plants for the season. Some of them dominated lake to a more turbid and following fish removaL Bird exclosures have low toxicity to non-target algae-dominated state, assuming enough may have to be constructed over beds of organisms, making them ideal tools for grass carp are added. Removal of new plants until plants are established this purpose. The turbid state with algae common carp, grass carp, bullheads, and spreading. will appear if nutrient levels in the and sunfish, followed by re-stocking treated lake are high and if with largemouth bass, bluegills, and planktivorous fish are abundant. A lake channel catfish may induce a shift to Much new knowledge free of submergent plants may be highly clear water, but such manipulations are desirable for recreation. Most fish difficult, costly, and require about how lakes function biologists have concluded, however, that maintenance. Bass fishing in lakes of has been obtained from a sustainable game fishing lake (e.g., this type can be very good, although the studies of shallow lakes. largemouth bass fishery) requires 20-40 use of minnows for bait should be percent rooted plant coverage. Partial banned because this is a good way to re lake treatment with certain herbicides, introduce planktivores to the lake. Swim or the use of a harvester, may be the Clearer water may provide an ideal areas in the macrophyte-dominated best option. In most cases, the algicide situation for fish-eating wading birds, clear water state can be maintained with used is copper sulfate. It is only briefly which in turn may shift fish fauna in the a harvester or judicious use of an effective when applied in nutrient-rich lake. More than one grass carp stocking herbicide. lakes, and very small amounts eliminate to a clear, plant-dominated lake has Rough fish removal and re-stocking algae-eating zooplankton just like been foiled by Great Blue Herons. with predatory fish and bluegills could planktivorous fish can do, thereby Canada geese and mallard ducks are fail in nutrient-rich polymictic lakes acting as a switch for a change to the because dissolved oxygen may fall to capable of greatly altering lake water turbid state, or acting as a stabilizing low levels during temporary quality. Geese, for example, may mechanism for already turbid lakes. stratification or during ice and snow defecate up to 90 times per day in Copper use in shallow lakes can cover, eliminating the game fish and winter, adding nutrients to the lake. produce a "chemical dependence" when Geese appear to be less attracted to thus predator control of small, natural grazing controls have been zooplanktivorous fish. Therefore, an lakes where land vegetation extends to eliminated. attempt to switch a lake from turbid to the shoreline (lakescaping), versus lakes Nutrient-rich shallow lake and pond clear may have to be accompanied by where land along the shore has been users and owners are faced with a the installation of artificial circulation to cleared for monoculture lawns to the choice of clear water, dominated by lakeshore. Birds are also attracted to maintain adequate dissolved oxygen. submersed plants, turbid water Artificial circulation also could reduce lakes where residents provide food. dominated by algae, or continuous and phosphorus release from oxygen-free Manipulation of a shallow lake to expensive mechanical-chemical lake sediments. produce clear water therefore may treatments to maintain a highly An alum application is an involve land management to discourage unnatural and unstable state of few nuisance waterfowl, and lakescaping to additional or alternative means of algae and macrophytes. The key to reduce inflows of lawn fertilizers and controlling internal loading in switching from one to the other, at a pesticides. polymictic lakes. This technique is given nutrient level, is in the types and We have tested Bronmark and higbJs-:-,~ffective in shallow lakes._When abundances of fish and.:possibly of __ Weisner's rdea thafsnails can be____ -- properly used, it is not toxic to lake snails. When nutrient concentrations are significant in maintaining the clear biota, including rooted plants. Welch lower, which may occur from control of water state. Snails graze on the biofilm and his colleagues from the. University external and/or internaljoading, a of afgae, which can cover submerged of Washington have clearly switch to a stable clear water state is demonstrated this in shallow plant leaves and sharply reduce light more likely. m.- availability to them. Snail exclusion
WllliNE • $prJng2001 45 '"* Shallow and Deep lakes lake users and owners can take steps to Society. He received the OLMS promote, maintain, or switch lake state Distinguished Service Award in 1989, the A shallow lake or pond that has low between clear water, macrophyte NALMS Technical Excellence Award in 1990, and the University's Distinguished amounts of algae and submersed plants dominated or the turbid, algae Teaching Award in 2000. He can be reached is uncommon, unexpected, and dominated states depending upon their by e-mail at:
Europeans, Japanese, and others view the clear water, macrophyte dominated shallow lake as highly desirable. They are horrified by our willingness to destroy habitat and biodiversity in our quest for a mostly illusory goal of clear, macrophyte-free shallow lakes. An alternative is to accept A. w. RESEARCH reality and to carry out plant removal at swim and dock areas only, and to work towards maintaining clear water. If there LABORATORIES, INC. are deep, macrophyte-free areas, paths to them can be provided. A most Specializing in: important way to rehabilitate lake biodiversity and to reduce nutrient Aerial Lakeshore Analysis loading is to "lakescape" all shoreline Identifies sources of nutrients for every residential properties. This means, at a property on a shoreline; minimum, planting a two- to three Environmental Assessment meter wide buffer zone along the Overflights lakeshore. This should eliminate geese, Documents potential regulatory stop lawn fertilizer runoff, sharply violations and environmental concerns; reduce lawn care costs, and provide habitat for the other lake shore/littoral Lake Testing and Strategic zone residents. Management Plans; Much new knowledge about how Environmental Responsibility Plans lakes function has been obtained from Managing impacts from Golf Courses studies of shallow lakes. We know that or other shoreline developments. nearly all of them will be biologically _pro4JL~tive, but that the type oL_= -,_<\~V;fusEARCHiJA:BORATORIEs, INc., are locat€00t dominant organisms will vary, depending upon the presence and 16326Airport Road, Brainerd, MN 56401, strengths of buffering mechanisms such_ (218) 829-7974, or on the web at: as fish planktivory, internal loading, or
46 SIJIInU2IJIJJ • UOO:liNt