Ecological Archives M085-013-A1

Franz Hölker, Michael J. Vanni, Jan J. Kuiper, Christof Meile, Hans-Peter Grossart, Peter Stief, Rita Adrian, Andreas Lorke, Olaf Dellwig, Andreas Brand, Michael Hupfer, Wolf M. Mooij, Gunnar Nützmann, and Jörg Lewandowski. 2015. Tube-dwelling invertibrates: tiny ecosystem engineers have large effects in lake ecosystems. Ecological Monographs 85:333–351. http://dx.doi.org/10.1890/14-1160.1

Appendix A. Detailed description of PCLake.

A1 Model description

PCLake is a dynamical ecosystem model to study the effects of eutrophication in non-stratifying shallow lakes, which is applied in both a scientific and a management setting (See Janse 2005 for a full model description). Various interactions and feedback mechanisms represented in modeled ecosystem together entail a highly non-linear ecosystem response to rising nutrient loadings: a regime shift between a clear macrophytes dominated state, and a turbid phytoplankton dominated state (Janse 1997). As relevant processes are explicitly modeled, dynamical ecosystem models such as PCLake are well suited for studying feedback loops that are imposed or altered by chironomids, and assessing their importance for self-stabilizing mechanisms within the framework of alternative stable states (Fig. 3).

A2 Bifurcation analysis

The ‘critical’ nutrient loading, at which the system undergoes a sudden regime shift, is used as a measure of ecosystem resilience. To analyze the sensitivity of the lake to nutrient loading, the model is run for 25 years for various phosphorus loadings, increasing from 0.1 to 6 mg P·m^-2·d^-1 in steps of 0.2 mg, for both clear and turbid initial conditions. The nitrogen loading is consistently kept 10 times the P loading to maintain phosphorus limitation (cf. Janse et al. 2008). Summer half year average Chlorophyll-concentrations in the last year of the run is used to evaluate the ecosystem state. The model is numerically solved by the Runge-Kutte Cash-Karp method.

A3 Scenarios

The aim of the first modeling exercise was to obtain insight in how the current implementation of benthic invertebrates affects ecosystem functioning, in the context of alternative stable states. Two scenarios were used to provide contrasts to the default situation (Table A1). Firstly the functionality of the benthos was abolished by setting the death rate of benthos to 100%. Secondly, the density of benthos was increased by enhancing their performance. Bifurcation analysis was used to explore their effect on ecosystem functioning, along with manual inspection of individual process rates.

Table A1. Parameter settings for three scenario.

For the second modeling exercise a filter-feeding benthos group was added to the food web of PCLake. Equations and parameter settings for chironomids growth were broadly adapted from the existing zoobenthos and zooplankton modules (Janse 2005, Table A2). Assimilation is modeled as function of food availability, temperature and a logistic density dependence correction. The filtering mechanism is analogue to that of zooplankton. Chironomids feed on phytoplankton and detritus in the pelagic zone. A preference factor denotes what fraction of the amount of the particular food component present in the filtered water is actually ingested by the animal, the remainder is rejected. Respiration, nutrient excretion and mortality are modeled as first order processes. Chironomids are assumed to have a higher nutrient to biomass ratio compared to their food. A ‘reference’ nutrient to biomass ratio (the ratio they need for their functioning and which try to maintain) is pursued by assimilating nutrients with a great efficiency than carbon whereby nutrient assimilation efficiencies are made dependent on the (variable) nutrient to dry weight ratios of the food, a relatively low phosphorus excretion (nutrients are retained in the body), and increased respiration (extra utilization of carbohydrates) when the P or N content become too low. Excreted nutrients return to the water column while egested matter (detritus and nutrients) are assumed to be retained in the sediment layer. Benthivorous fish exert top-down pressure on chironomid abundance via predation. Estimations of the initial population density and the carrying capacity are based on Beattie (1982) (Table A2).

We used bifurcation analysis to explore the effect of a filter-feeding benthos group on ecosystem functioning. The initial biomass of the filter-feeding benthos group was collected from the ordinary benthos group. All other settings of PCLake were kept default.

Table A2. Overview of the settings of the chironomid extension of PCLake.

Literature Cited

Beattie, D. M. 1982. Distribution and production of the larval chironomids populations in Tjeukemeer. Hydrobiologia 95:287–306.

Janse, J. H. 1997. A model of nutrient dynamics in shallow lakes in relation to multiple stable states. Hydrobiologia 342:1–8.

Janse, J. H. 2005. Model studies on the eutrophication of shallow lakes and ditches. Ph.D. Thesis, Wageningen University, The Netherlands.

Janse, J. H, L. N. De Senerpont Domis, M. Scheffer, L. Lijklema, L.Van Liere, M. Klinge, and W. M. Mooij 2008. Critical phosphorus loading of different types of shallow lakes and the consequences for management estimated with the ecosystem model PCLake. Limnologica 38:203–219.

Janse, J. H., M. Scheffer, L. Lijklema, L. Van Liere, J. S. Sloot, and W. M. Mooij 2010. Estimating the critical phosphorus loading of shallow lakes with the ecosystem model PCLake: Sensitivity, calibration and uncertainty. Ecological Modelling 221:654–665.

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