Lakes are ‘a little world within itself, a microcosm within which all the elemental forces are at work and the play of life goes on in full, but on so small a scale as to bring it easily within the mental grasp’ (Forbes, 1887). A microcosm is an isolated system, independent from the wider environment. This thinking has persisted to this day in some fields (Arlinghaus et al., 2008).
The study of limnology has diversified over time, encompassing new branches of science. The 20th century saw metrology, chemistry and hydraulics brought into the field (Strom, 1929; Arlinghaus et al., 2008). Many of the views that were once held are changing. The notion that lakes are driven by small scale local processes is being challenged by larger scale changes such as global fluctuations in climate. Systems science is showing that lakes are conceptually more complex than once thought. The nature of how we approach lakes has changed. Is a microcosm approach still relevant?
As a System
The Lake as a Microcosm (Forbes, 1887) indicated that within a given lake, organisms interact systematically. Lake processes could be broken down into constituent parts and pairs of interactions. Through conceptual models these could then be used to understand the lake as a whole. This approach continues to be used (Lawton, 1995; Daehler & Strong, 1996; Jessup et al., 2004), but has attracted opponents (Chorley & Kennedy, 1971; Carpenter, 1996; Srivastava & Vellend, 2005).
The traditional approach would have viewed simple relationships between cause and effect. Phosphorous was identified to be the determining factor in the cause of eutrophication. To reverse the effect one simply had to reduce the amount of nutrients entering the system and it would recover. This method, however, has been found to be woefully inadequate (Lane & Lau, 2001). The complex nature can mean that even with the removal of phosphates, eutrophication can still be a problem. Nutrients had been found to have formed as sediment on lake beds. This sought to leech into the waters by internal loading. Viewed in isolation phosphate removal seemed the perfect solution but ignored the possibilities of nutrient interactions with the whole of the system.
Study of interactions through predation, competition and herbivory and carbon and nutrient cycling to higher trophic levels for example show that the lake is a complex system of multi-scale processes from microbial to larger scale seiche dynamics. A change in any individual aspect of the lake can have implications elsewhere to the effect that the system can change its identity completely and potentially irreversibly (Holling, 1973).
The nature of such changes is defined by a number or equilibrium states. Some of these are transient and others are more permanent. By applying external pressures onto a key component or set of components, a new system identity can be achieved as a result of the complex, multi-scale interactions (Walker & Salt, 2006). This understanding has spawned a new branch of ecology; resilience theory, of which one aspect is complex system interactions and multiple stable states.
Compartmentalising the lake ecosystem was seen as a suitable approach to an overall understanding of the system. It allows the identification of key threats to individual components. For example if the pH of a lake was to fall below 4 then damage to Salmonids, Bream, Goldfish and Carp is a strong possibility (Alabaster & Lloyd, 1982). Thus an approach would be taken to keep pH above 4 and may result in action being taken to raise it. One method would be to add CaCO3. But in doing so the lake may have crossed a threshold; a point in which the system can no longer absorb the changes that are occurring and the system will shift. So much CaCO3 can be added that it can be taken up by plants. It enters the food web and can poison potential food sources.
The aforementioned examples seek to show that it is impossible to address the concerns of once component of the lake without impacting on others or even the entire system itself. Processes at one scale are not immune to changes at another.
Processes and Effects
The microcosm concept implies that whilst the processes that occur in lakes are interconnected and heavily reliant upon one another over various scales, they are overall on a vastly smaller scale compared to systems outside of the lake. Further to this it is deemed that few, if any; connections to systems outside of the lake spatially and temporally exist. As a result any outside perturbations should have minimal immediate impact on lake functions.
Increasingly it is being noted that large scale processes outside of the immediate system are determining the outcomes and processes of individual lake systems. This is particularly the case in the field of meteorology.
Lakes were seen as individual entities, all subject to the same physical laws but each having a distinct local external meteorological forcing. To understand the lake processes the local lake weather had to be studied. The difference between climate and weather, however, is merely scale. Any regional meteorological data was often not given any attention to if it was not at a high enough spatial and temporal resolution as to classify it as weather as opposed to climate data (Livingstone, 2008).
In the short term lakes have been shown to respond to regional fluctuations in air temperatures, even on a temporal scale of days (Livingstone & Padisak, 2007). The isolated nature of lakes implies that any outside perturbations would be minimal, if not non-existent, and would not be felt for a significant period of time if they were.
In the longer term, and on a larger scale, the effects of climatic changes will be felt in lake ecosystems all over the world. Average global temperatures are rising and will affect processes in lake system over a multitude of scales. Increased temperatures have been predicted to result in significant long term warming in the epilimnion and metalimnion, with possibly cooling of the hypolimnion (Peters et al., 2002). Global climate changes are considered to be ‘outside’ of the local system. To test lake responses to climate change, under a microcosm approach, would require breaking down the system to component parts. By altering a set of variables, under controlled conditions, a response could be observed. The issue is that global changes cannot be effectively modelled to give results relating to locations on such a small spatial scale (Raisanen, 2006). With lakes being bound by very individual location specific characteristics, it makes it difficult to infer the effect of global changes in climate.
Further to this there are the effects that occur more regionally. River basin and catchment hydrology may undergo change. The direct effect of increased solar radiation on lake temperatures can be measured in a controlled setting. Changes that may occur as a result of flooding and droughts cannot be as precisely measured as they are beyond the lakes immediate boundaries. Thus they would be beyond the scope of a microcosm based approach. Replicating the effects of a flooded catchment in a laboratory setting is a challenging process at best.
Whilst a microcosm approach is not applicable in some circumstances as prescribed previously, that is not to say that they are not useful and do not play a significant role in limnology today. A microcosm approach can be used to successfully test the impact of a variety of perturbations and influence on a lake system (Luckinbill, 1973; Tilman, 1977; Dykhuizen & Davies, 1980).
As noted earlier, one problem that occurs is in dealing with the complexity of the system. A more systematic approach can be problematic in that the complexity of the system is not determined by nature but by the researcher (Lawton, 1995). However this can be an advantage in some aspects of investigation (Drake et al., 1996).
A common area of study is the assemblages of communities after exposure to disturbance (Jiang & Patel, 2008) such as temperature (McKee et al., 2003) and in food web theory.
The testing of food web theory is challenging. Simplification of the relationships between food chains into trophic categories is difficult due to the nature of their interactions (Power, 1992). A systematic approach can address these issues. All components of the food chain should be defined, before any real world observations are made, based on current best known theory. Through comparison of the controlled study and the natural patterns, a more thorough understanding can be achieved.
With regards to a temporal scale, it has been argued that the use of a systematic approach is favourable over more field based approaches. It has been noted that microcosm studies, whilst in real time are much shorter than field experiments, produced the equivalent organism related results over a much shorter period of time (Ives et al., 1996). The characteristics of such an approach mean that nature can be ‘sped up’. Experiments can be undertaken without time becoming the strong limiting factor that it is in the natural environment. The success of the experiments, in terms of mimicking the natural world depends greatly on the ability of the study to reproduce real life interactions and system dynamics. In the past a microcosm approach has not always achieved this.
One aspect of microcosm studies that has been very successful relates to the food web and the associated energy flows. The idea that trophic consumers and producers on one level are inexorably linked with those on other levels has formed the basis of much discussion in modern limnology.
Thinking has moved from linear relationships of cause to effect to the more complex and integrated concept and quantifications of energy flows (Fellows et al., 1998).
It was initially theorised that populations of a given species are limited by the availabilities of resources, competition and predation. The overriding factor determining the success of a species was therefore its position in the food chain.
The cascading trophic concept was born out of the initial attempts at food chain theory. The idea is that changes a change in predator numbers will reduce herbivore abundance and thus impact positively on any plant species through a reduction in consumers. Changes at the top will ‘cascade’ down the system to the primary producers (Bronmark & Hansson, 2005).
Further to this is the notion of the Bottom-up: Top-down approach. It theorises that the maximum attainable biomass is determined by nutrient availability. It also takes into account the notion that higher trophic levels are influenced by top-down interactions such as a reduction in fish predators. Lower trophic levels are considered to be affected primarily be bottom-down processes. The influences of these actions weaken as you move down and up the trophic levels, respectively.
The complex nature of the system can be highlighted by the concept that in lakes of particularly high nitrogen concentrations the fish populations will have at best minimum governing effect on the algal abundance (Bronmark & Hansson, 2005). Thus even though fish should have a top-down influence on the algal biomass it is in fact a bottom-up process that is the controlling factor. This brings in to focus the limits of a microcosm approach.
Further complexity is highlighted in the concept of multiple stable states. As previously mentioned the current state of a system can shift by applying change to entities within.
In lakes this may encompass fish removal, increased nutrient loading and trophic changes. The microcosm approach dictates that the interconnectedness of the system will result in shocks moving throughout and to all other components.
Each system is a product of the components and interactions that exist within it. All components are linked, whether directly or otherwise any alteration in the properties of one will most likely have an impact on those immediately connected and others by proxy.
Changes in the systems properties are gradual as it takes time for any alterations to filter through from one component to the next. At a point the system would have undergone so much change that it is subjected to an identity change. At this point it is said to have passed a threshold.
Shallow lake ecosystems have two dominant states. The first is a turbid, algal bloom dominated system. The second is a clear, submersed vegetation dominated state. The identity of the states is a product of the different physical, chemical, and biological conditions for the biota living in these aquatic systems (Carpenter et al., 2002). They therefore are integral in understanding the dynamics of trophic interactions in the system (Beisner et al., 2003).
Without appreciating the connectedness of the system it is impossible to truly understand the nature of the system itself. A change in the characteristics of any part of the system can create knock on affects which have the ability to change the very identity of the system.
The idea of a lake as a microcosm, an interconnected and complex system has persisted from its initial conception with Forbes seminal lecture. The microcosm approach to limnology is both relevant and obsolete. The approach is still used as it allows a valuable insight into the complexity of lakes. Through experimentation a modelling and subsequent comparison to natural systems, key understandings can be derived. For observing changes over long periods of time this is a particularly good study approach.
However with increasing awareness of changes on greater scales, such as climatic changes, it is realised that the microcosm analogy does not hold true. Changes above and beyond the level of the lake system are having increasing influence on its functions and identity. Lakes cannot simply absorb these perturbations and continue functioning in the same way.
The microcosm has stood the test of time; it is still being used as a successful approach at lake system level. However when it comes to studying the effects of change on a much greater scale it appears to be cast aside as it is unable to allow effective study and investigation.
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