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Limnology – the Basics Lake & Watershed Group Primer Limnology – The Basics Frank M. Wilhelm ur LakeLine editor invited me to occur in our lakes (I’ll primarily focus on favorite lake change. Cultural write a readily accessible lakes here) that make them function. eutrophication is the acceleration of this Ointroduction to “limnology” for this Considered as a whole, lakes are living process shortening it to tens of years so issue of LakeLine that includes ecosystems. that ecosystem changes become “everything” – basically to distill volumes noticeable during a human lifetime. It is of textbooks into an article – a formidable Nutrients – includes a wide variety of the result of human activities that increase task. Reflecting on how to tackle this elements necessary to fuel life. In the case the rate of sediment and nutrient transport “small” task, I have chosen to focus on of freshwater, nitrogen (N) and to our aquatic ecosystems. what I would consider most important to phosphorus (P) are typically the nutrients know as background if I were a member in shortest supply – this is termed a Trophic state – is a classification system of the public attending a seminar or limiting nutrient – and hence our devised by limnologists based on the technical presentation regarding some overwhelming focus on them because productivity (amount of plant growth) in a aspect concerning my local lake. I will adding just a little can have large effects lake. While trophic state is a continuum start with some basic definitions, then (Schindler 1971). Realize that N and P are (Figure 1), you will encounter the move from the formation of lakes to only two of a plethora of nutrients. For following terms – Oligotrophic (low physical, chemical, and biological example, silica may limit the growth of nutrients, low algal biomass, high processes. Hopefully, you’ll make it to the diatoms (small algae) in some lakes transparency); Eutrophic (high nutrients, end, and emerge with an appreciation for seasonally. high algal biomass, low transparency); the beauty and complexity that is the and Mesotrophic (intermediate aquatic realm – something that has Eutrophication – is the presence of excess conditions). Trophic state can be assigned captivated me since chasing salamanders nutrients that stimulate high plant based on the concentration of nutrients in and tadpoles in creeks and quarry ponds biomass; it comes in two flavors – natural the water column, the amount of plant during my childhood. I consider myself and cultural. The former is the typical biomass, or the depth to which light fortunate to have lakes and reservoirs as process for all waterbodies as they evolve penetrates (Carlson 1977; Carlson and my “office” and to be able to work on to solid land over geologic time periods. Simpson 1996). them regularly. However, it comes with As soon as a lake is created, it’s on a the burdensome insight that we are rapidly death march to solid land – stick around Secchi disk – is a 0.2-m diameter approaching a reckoning: an expanding for 10,000-plus years and you’ll see your weighted disk with opposing white and human population that needs access to clean water and a finite quantity of such water available on earth. Thus, it is imperative that each of us becomes informed and takes appropriate actions to protect this life-giving resource. Thanks for joining me, let’s get started. Limnology – is the study of inland water; it includes some waters more saline than the ocean, ponds, streams, rivers, to lakes and reservoirs – large and small. Basically, if the aquatic system is inland or drains to the ocean, it is encompassed in limnology. So, what do limnologists Figure 1. Trophic state boundaries from oligotrophy to eutrophy for total phosphorus, study? There is a myriad of physical, chlorophyll a, and Secchi depth based on Carlson 1977, and Carlson and Simpson chemical, and biological processes that 1996. 6 Summer 2020 / NALMS • LAKELINE black quadrants that limnologists use to changes with scale of interest. For weathered and materials are easily measure the depth to which light that example, the catchment of Lake Ontario is transported to a receiving water body; plants use to photosynthesize penetrates composed of many small catchments of calcium and magnesium are chief among into a lake. Because light needs to reflect individual lakes, whereas most of us on dissolved elements. In contrast, “hard” off the disk for you to be able to see it, the smaller lakes look to the high points rocks such as granites and feldspar resist actual depth to which useable light around our own lake and use those to weathering and result in little transport of penetrates is approximately two times the draw in the boundary of the land that materials to lakes. Secchi depth. NALMS is the coordinator drains to the lake. Lakes represent the An accurate determination of the of the Secchi Dip-in program which drain of the landscape where water constituents in the water requires a encourages citizens to measure the Secchi accumulates. It serves us well to laboratory analysis. A rough generaliza- depth on their lake during the month of remember this, as the health of a lake will tion is that hard water lakes tend to be July each year and enter the data into an be reflective of what is happening in its more productive than soft water lakes. online database. You can find more catchment. It’s easy to add something to a Another generalization is that certain information here: https://www.nalms.org/ lake but much harder, if not impossible to organisms such as those requiring calcium secchidipin/ and I encourage you to remove it again. for incorporation into body parts like participate. shells will not occur in soft water lakes Lake formation because of inadequate underlying base Algae – are small plants that are Lakes can be classified by their basin chemistry. Hence some lakes are thought suspended in the water column and for type and the way in which they were to be less susceptible to hosting which one usually requires a microscope created. Many lakes result from populations of the highly invasive zebra to see them properly. When light is catastrophic geologic events, be it shifts in and quagga mussels (e.g., Karatayev et al. present, they photosynthesize turning light faults, volcanism, or the wandering of 2015; Mellina and Rasmussen 1994). energy into chemical energy in the form glaciers (think Great Lakes of North Base chemistry is also important in of sugar. They form the basis of the food America, African Rift Valley lakes, or the determining a lake’s resistance to change chain, similar to grass on land. lakes in the Sierra Nevada Mountains) under assaults of human inputs such as that create divots in the landscape which acid precipitation – soft water lakes such Macrophytes – are large plants that are fill with water. For example, consider one as those in the Adirondack Mountains. are easily seen with the naked eye, like lily of my favorite types – kettle lakes – aptly highly susceptible (Driscoll and Newton pads and cattails. Similar to algae, they named for their near vertical-sided basin 1985) compared to hardwater lakes also photosynthesize, and are usually shapes that are like a kettle. Each is elsewhere. It should also be noted that found around the margin of lakes which is formed upon burial of a sizeable chunk of depending on the aquatic life present and called the “littoral zone.” The littoral zone ice that broke off from a retreating glacier. their density, they can seasonally is defined by the depth to which When the ice melts, it leaves behind a influence chemistry as well (e.g., Lehman macrophytes grow along the bottom of kettle-shaped depression. Interestingly, 1980). While not readily apparent to us, water bodies. quaking bogs often occur atop kettle lakes water chemistry varies widely in different after vegetation from the sides has gown lakes across the landscape which has Zooplankton – are small animals that inward in a floating mat to entirely cover consequences for what we find in them. typically graze algae and other small bits the top of the kettle lake. Other examples and pieces in the water column. They in of lake types include reservoirs created by Temperature and lake stratification turn are food for larger organisms such as dams – debris, landslides, beavers, and Perhaps one of the most striking fish. It’s interesting to note that even the humans, and oxbow lakes formed from features in moderately deep to deep lakes most voracious fish predators such as pike the pinching off of part of a meander river is the occurrence of temperature or bass feed on small zooplankton when during a flood event. How was your stratification, where a layer of warm, they first start life. Zooplankton density favorite lake formed? low-density water (termed the epilimnion) changes seasonally, with high densities in is atop a layer of cold, high-density water early to mid-summer, and low densities Water chemistry (termed the hypolimnion) throughout the over winter. Just as interesting as how a lake’s summer (Figure 2; 3A). This is called basin is formed, is the geologic setting in “direct stratification” and is the result of Microinvertebrates – are easily seen which it occurs as this predominantly differential heating of the water column. denizens such as dragon-, May-, or controls the water chemistry in the lake. A It has important consequences for the Damselflies, or amphipods (scuds) or mnemonic I give to my students is that biotic communities in lakes. Because the shrimp. Beware, some such as giant water “Hard Rock equals Soft Water, and Soft epilimnion is in constant contact with the bugs can deliver a memorable bite and Rock equals Hard Water.” Soap or atmosphere gas exchange is good; it also regularly capture and consume small fish! shampoo in the shower at the lake cabin contains the algae that produce oxygen will quickly reveal your geologic setting.
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