Lead: Marc Schallenberg
Not all lakes with the same nutrient availability exhibit the same trophic state. Some lakes are more resilient to eutrophication and, therefore, degrade more gradually in response to increasing nutrient loading. Previous studies have shown that at least 37 shallow lakes in New Zealand have undergone rapid changes in trophic state as they crossed catchment land use (%pasture) thresholds (Schallenberg & Sorrell 2009). The presence of the invasive macrophyte, Egeria densa, and some pest fish species were associated with the lakes that had "flipped".
In this critical step, an analysis of a 43-lake shallow lake dataset showed that lakes that tended to have low algal biomass in relation to the proportion of their catchment that was in pasture (i.e. the lakes that tended to be resilient) were those with higher macrophyte biomass, lower sediment resuspension and shorter water residence times. In contrast, lakes that tended to be more vulnerable were those with the opposite characteristics and those that contained goldfish and/or perch. Thus, in shallow lakes, the presence of these exotic fish tended to be correlated with unusually high algal biomass.
We also analysed factors related to resilience and vulnerability to eutrophication in a dataset of 23 deep lakes. In this dataset, only the smaller lakes tended to be degraded (large, deep lakes had close-to-pristine water quality). Nevertheless, our analysis showed that the presence of perch (and exotic fish species in general) in some of the lakes tended to result in higher algal biomass for a given proportion of pasture in the lake's catchment.
Resilience to eutrophication comes about due to the presence of stabilising feedback mechanisms. Usually these have a biological (aquatic plants, grazers) or chemical (oxygen) mechanism. These stabilising feedback mechanisms inhibit the proliferation of algae, but only up to a threshold of nutrient loading, beyond which the nutrient availability and algal productivity (and other related factors) will overwhelm the stabilising feedbacks.
The behaviour of trophic state variables over time can give an indication of whether lakes are stable (resilient) or vulnerable to change. We analysed a number of long-term regional council lake datasets to derive patterns of responses in chlorophyll a over time to give guidance on how to identify lakes that are showing instability, a tendency to flip. The patterns of change in algal biomass differ remarkably among lakes but four fundamental patterns were derived. One pattern reflects strong seasonality in algal production. In such lakes, the analysis of annual means or medians is less likely to highlight low resilience and flipping behaviour than the analysis of seasonal algal biomass. Thus, evidence from careful analysis of time series data can illustrate resilience.
Link to CS1.3.2 (Foodweb biomanipulation techniques to enhance lake restoration) because biomanipulation aims to reduce resilience to lake recovery from eutrophication. Link to CS1.3.1 (Managing sediment nutrient legacies) because legacies can increase resilience to lake recovery from eutrophication. Link to CS1.3.4 (Prioritising lakes and interventions for management actions) because understanding resilience is important in selecting management actions and predicting restoration success. Thus, information will be shared amongst these critical steps.
Schallenberg M, Kelly D, Verburg P, Hamilton D. (in prep) Mechanisms underpinning lake resilience to eutrophication and oligotrophication. Freshwater Biology.
Woolway RI, Carrea L, Merchant CJ, Dokulil MT, de Eyto E, DeGasperi CL, Korhonen J, Marszelewski W, May L, Paterson AM, Rimmer A, Rusak JA, Schladow SG, Schmid M, Shimaraeva SV, Silow EA, Timofeyev MA, Verburg P, Watanabe S, Weyhenmeyer GA. 2018. Lake surface temperature. [in "State of the Climate in 2017"]. In press, Bulletin of the American Meteorological Society.
Woolway I, Verburg P, Lenters J, Merchant C, Hamilton D, Brookes J, de Eyto E, Kelly S, Healey N, Hook S, Laas A, Pierson D, Rusak J, Kuha J, Karjalainen J, Kallio K, Lepisto A, Jones I. 2018. Geographic and temporal variations in turbulent heat loss from lakes: a global analysis across 45 lakes. Limnology and Oceanography. doi: https://doi.org/10.1002/lno.10950.
Li M, Xiao M, Zhang P, Hamilton DP. 2018. Morphospecies-dependent disaggregation of colonies of the cyanobacterium Microcystis under high turbulent mixing. Water Research, 141, 340-348.
Woolway R I, Verburg P, Merchant CJ, Lenters JD, Hamilton DP, Brookes J, Kelly S, Hook S, Laas A, Pierson D, Rimmer A, Rusak JA, Jones ID. 2017. Latitude and lake size are important predictors of over-lake atmospheric stability. Geophysical Research Letters, 44, 8875-8883.
Leach TH, Beisner BE, Carey CC, Pernica PM, Rose KC, Huot Y, Brentrup JA, Domaizon I, Grossart H-P, Ibelings BW, Jacquet S, Kelly PT, Rusak JA, Stockwell JD, Straile D, Verburg P. 2017. Patterns and drivers of deep chlorophyll maxima structure in 100 lakes: the relative importance of light and thermal stratification. Limnology and Oceanography, 63, 628-646.
Novis P, Schallenberg M, Saulnier-Talbot E, Kilroy C. 2017. The diatom Lindavia intermedia identified as the producer of nuisance pelagic mucilage in lakes. New Zealand Journal of Botany, 55, 479-495.
Weaver AM, Schallenberg M, Burns CW. 2017. Weather conditions and soil properties mediate the effects of land use intensification on nutrient fluxes into a deep oligotrophic lake. Marine and Freshwater Research, 68, 1830-1844.
Schallenberg M, Schallenberg LA. 2017. A restoration and monitoring strategy for Lake Hayes. Report prepared for the Friends of Lake Hayes Society Ltd. 53 p.
Kelly D, Wood S, Waters S, Schallenberg L. 2017. Community structure and foodweb pathways in macro-algal dominated lakes, is this another stable state? Oral presentation, 5th Biennial Symposium of the International Society for River Science: Integrating Multiple Aquatic Values. Hamilton, New Zealand. 19-24 November 2017.
Kahn S. 2017. Lakes as organic matter upgraders - seasonal variation in biochemical compositions of in- and outflowing particles in pre-alpine Lake Lunz, Austria. Oral presentation, 5th Biennial Symposium of the International Society for River Science: Integrating Multiple Aquatic Values. Hamilton, New Zealand. 19-24 November 2017.
Schallenberg M, Larned S. 2017. Freshwater tipping points: What? Where? How? Why/ and When? Oral presentation, 5th Biennial Symposium of the International Society for River Science: Integrating Multiple Aquatic Values. Hamilton, New Zealand. 19-24 November 2017.
Schallenberg M. 2017. Freshwater tipping points: What? When? Where? How? Why? Keynote address presented at a workshop on "Social-Ecological Resilience of Freshwater Systems in the Anthropocene", July 8-10, Nanjing, China.
Schallenberg M, Larned S. 2017. Critical thinking about tipping points in freshwater science and management. Keynote address presented at the Environmental Defence Society conference on "Tipping Points", August 9-10, Auckland.
Verburg P, Schallenberg M, Holt K, Procter J, Bevins C, Brown L. 2017. Nutrient cycling in Lake Horowhenua and restoration by harvesting macrophytes. Oral presentation, 5th Biennial Symposium of the International Society for River Science: Integrating Multiple Aquatic Values. Hamilton, New Zealand. 19-24 November 2017.
Waters S, Kelly D, Wood S, Verburg P. 2017 Sediment geochemistry indicators of lake resilience. Oral presentation. Oral presentation, 5th Biennial Symposium of the International Society for River Science: Integrating Multiple Aquatic Values. Hamilton, New Zealand. 19-24 November 2017.
Schallenberg M, Kelly D, Verburg P, Hamilton P. 2016. Understanding how multiple stressors affect ecological resilience and integrity in New Zealand lakes. Oral Paper presented at the New Zealand Freshwater Sciences Society Annual Meeting, Invercargill, 5-8 December 2016.