Chapter Twenty-Four

Ecology, eutrophication and the safeguarding of lakes

24.1 Brian Moss who sadly recently passed away was Emeritus professor of Botany at Liverpool University and one of the most influential freshwater ecologists in Europe. His comprehensive work – Lakes, Loughs and Lochs gives a thorough insight into lake ecology. Here he introduces the idea of a lake ecosystem being like a play.

“A glance at a theatre programme, or the list of people that rolls endlessly on the screen at the close of a film, shows that the true cast is much greater than the short list of leading actors, the director and producer. There are scores of scene makers, shifters and painters, wardrobe mistresses and seamstresses, electricians and best boys, the producers of props, the manages for stage, the makers-up and hairdressers, those who wrangle the animals, even the drivers cooks and caterers. So it is with the cast of the evolutionary play that is staged in the ecological theatres of lakes and every other natural system. The most obvious natural history might come in the leap of the salmon or the breeding of the great crested grebe, the lunge of a heron or even the twilight migrations of a small crustacean, but none of these could happen were it not for armies of microorganisms.

So it is with the entire planet. A litre of ocean water or that of a not particularly productive lake will contain about a million algae and protozoa 100 millioin bacteria and up to 100 billion viruses. The biosphere could continue indefinitely without any larger plants and animals, even (especially) ourselves, but a biosphere lacking microorganisms could not exist. A natural history of lakes needs to start with the microbial scene-shifters, before it looks at the names in neon lights. The bonus of doing that comes in an insight into the natural history of most of Earth’s story, the nearly 4 billion years before multicellular organisms began to evolve around the Cambrian period half a billion years ago, in the last 10 per cent of Earth’s time.”

24.2 The first few chapters of this outline of limnology followed Brian Moss’s advice and looked at the evolution of these “scene-shifters” before considering the evolution of the “names in neon lights”. But now it is time to turn to the drama they are engaged in every day in Lake Annecy.

24.3 The diagram above gives a brief outline of the food web in a lake, beginning with the primary production of organic material by Moss’s scene shifters, the bacteria and algae. This in turn is grazed upon by the bigger characters the Rotifers, Copepods and Diatoms. The smaller fish feed on these characters and in turn become prey for the bigger fish. The waste matter produced by the fish, together with the dead Crustacean in turn is broken down by the bacteria and once again recycled through the food web.

24.4 Brian Moss puts some practical numbers on this general outline, when he describes productivity in a typical Scottish loch [with some addition explanation by me in brackets].

“The overwintering population of fish in a modestly deep, middlingly productive lake of 30 m average depth might be about one fish in every 150 cubic metres of water.   Were the lake 100 hectares in area [one square kilometre] a typical Scottish Loch, there would be 200,000 fish. Despite the ambitions of anglers to catch big fish, on average each fish will weigh only around 10-20 grams, so the total fish biomass in the loch would be around 3 tonnes.   The mass of water by comparison would be 30 million tonnes and that of the phytoplankton about 15 tonnes. Three tonnes might seem comparatively large [at 20% of the biomass of phytoplankton], because the conversion of energy through food webs generally conveys only about 10 per cent at each step and there are likely to be two or three steps (algae to zooplankton to zooplanktivorous fish to piscivorous fish) from algae to fish. [So fish would be expected to have a productivity of 10%*10%*10% or around 1/1000 that of algae, not 1/4.] But when the turnover is considered, the numbers look much more reasonable. Algae will replace their biomass in the face of grazing and other losses about 30 times per year, fish, if their length of life is on average around four years, about 0.25 times per year. The relative productivity will then be 450 tonnes per year for the phytoplankton and 0.75 tonnes per year for fish [i.e. fish productivity would be around 1/600 that of algae.]

24.5 Whilst the above is a general description applicable to a typical lake, if we are talking about the ‘economy’ of a lake, it begs the question are some lakes more efficient than others ? That is, are some lakes more productive of bigger fish as a percentage of total organic matter. Do some lakes have big fish swimming in clear water while others have only smaller fish swimming in cloudy, greenish water? Such a simplistic classification of lakes has been hotly debated by limnologists and it is worth considering this debate before attempting to answer the question.

24.6 Terminology was developed early in the 20^th century to describe the nutrient (trophic) basis for production, ie the process of making organic matter in the lake. The concentration of nutrients in the water is the primary criterion of richness. Lakes rich in nutrients were called “eutrophic” (well fed), those poor in nutrients were called “oligotrophic” (poorly fed). An intermediate condition, mesotrophic, has been established more recently. [Edmonson p 68]. But Edmonson goes on to point out that such simplistic classifications of lakes can be misleading. Brian Moss makes the same point more pointedly by quoting the Danish limnologist C Wesenberg-Lund on the question of lake classification.

‘Naumann has tried to press nature into a series of highly artificial schemata which are unquestionably very valuable for all those scientists whose time is just as scanty as his own, whereas from a purely scientific point of view, as far as I can see, they have very little value’ He was tart but right.

24.7 Nevertheless, one way of looking at the economy of a lake is as a production factory for large fish. The production of a lake is best seen on the basis of unit area rather than volume, simply because light enters through the surface and, irrespective of how much water there is below, the amount of energy available to drive the whole production process remains the same. But different lakes have different productivity, some being more efficient than others in converting total biomass into fish. A comparison of Lake Leman and Lake Annecy illustrates the point. Lake Leman is classified as mesotrophic which means it is relatively rich in nutrients.

24.8 The following diagram gives an simplified illustration of this idea of productivity. The idea is crucial to understanding the workings of Lake Annecy. Although described as such for marketing purposes lake Annecy is not ‘pure’ in the sense that a bottle of mineral water is pure – ie devoid of organic life. But it is a relatively efficient lake in that it allows the larger (noble) fish to flourish, and produces relatively more large fish per volume of organic matter than other lakes, for instance the neighbouring Lake Bourget. And this is why the lake has clear water and noble fish.

24.9 These ecological concepts such as food chains, population regulation, and productivity were first developed in the 1700s, through the published works of microscopist Antoni van Leeuwenhoek (1632–1723) and botanist Richard Bradley (1688?–1732). Biogeographer Alexander von Humboldt (1769–1859) was an early pioneer in ecological thinking and was among the first to recognize ecological gradients, where species are replaced or altered in form along environmental gradients, such as a cline forming along a rise in elevation. Humboldt drew inspiration from Isaac Newton as he developed a form of "terrestrial physics". In Newtonian fashion, he brought a scientific exactitude for measurement into natural history and even alluded to concepts that are the foundation of a modern ecological law on species-to-area relationships. Natural historians, such as Humboldt, James Hutton, and Jean-Baptiste Lamarck (among others) laid the foundations of the modern ecological sciences.

24.10 The term "ecology" (from the Greek, house + study) is of a more recent origin and was first coined by the German biologist Ernst Haeckel in his book Generelle Morphologie der Organismen (1866). Haeckel was a zoologist, writer, and later in life a professor of comparative anatomy, as well as being a fine artist whose original depictions of the microscopic organisms whose importance Brian Moss so emphasises, awoke a generation to the existence of these marvellous creatures (see Art Nouveau)

24.11 Who actually founded the science of ecology is much debated. One candidate would Carl Linnaeus who principles on the economy of nature influenced Charles Darwin, who adopted Linnaeus' phrase in ‘The Origin of Species’. Linnaeus was the first to frame the balance of nature as a testable hypothesis. Haeckel, who admired Darwin's work, defined ecology in reference to the economy of nature, which has led some to question whether ecology and the economy of nature are synonymous.

24.12 From Aristotle until Darwin, the natural world was predominantly considered static and unchanging. Prior to The Origin of Species, there was little appreciation or understanding of the dynamic and reciprocal relations between organisms, their adaptations, and the environment. (An exception is the 1789 publication Natural History of Selborne by Gilbert White (1720–1793), considered by some to be one of the earliest texts on ecology.) Evolutionary theory changed the way that researchers approached the ecological sciences.

24.13 And so the ecology of Lake Annecy has become one of the most studied, over the longest period of time of any lake in the world (along with its ‘twin’ Lake Washington) and each year INRA produces a detailed report summarising the drama which has unfolded that year. (See INRA)

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