Hallow and D Ep Lak S: Determining Successful Mana·-Ge·M.Ent Options

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.

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