Ecological goal functions and plankton communities in lakes

Abstract

Descriptive studies have been the core of plankton ecology for many decades and will surely be fruitful in the years to come. However, the results that they provide are usually very difficult to adapt for predictive purposes. Other approaches will have to be taken into account, in the same way that the use of chlorophyll a has been used as an analogue in eutrophication studies. Ecological goal functions are the most recent candidates to fulfil the need for such compounded variables in plankton research. These relate some high-order ecological functions (energy flow, information, food web interactions) to ecosystem performance and can be considered as indices of ecological self-organization. Our study applies the concept of goal functions, which are functions that can describe the direction of ecosystem development, to lake plankton. Thirteen gravel pit lakes in Central Spain, without planktivorous fish and differing in trophic status, have been selected; their plankton communities, along with abiotic factors, have been studied during late-season mixing and mid-stratification. Size-spectral features (slope and intercept of the normalized biomass size spectrum), emergy (the quality of energy that increases since it is capable of controlling and causing work in the pelagic environment that would otherwise not be possible), exergy (the actual free energy of plankton biomass, including the free energy stored in information) and size-based connectance of food webs have been calculated on plankton data. Both emergy and exergy changed in response to mixing and stratification, emergy clearly increasing with water temperature. Exergy related to trophic status, whereas emergy only showed such a relationship in late mixing. The intercept of the size spectrum could be a good predictor of both goal functions during stratification. Connectance did not vary statistically between thermal periods. The hypothesis that connectance peaks at intermediate levels of trophic gradient no longer proved sustainable. Also, our data and those of other authors reveal that, despite earlier suggestions, no relationship exists between the slope of the size spectrum and trophic status. Some evidence suggests that goal functions, and hence self-organization, explain more than half of plankton biomass variability in lakes in summer. Exergy can be useful as a synthetic variable in plankton seasonal and long-term changes, emergy being a good proxy of pelagic ecosystem health, overall productivity and the food web. Thus, the application of goal functions along with size spectra to plankton field studies appears to be promising.