Predicting eutrophication and climate change impacts on shallow lake ecology and biodiversity: disentangling the effects of temperature and nutrients

Projektinformationen

PRECISE

ID Finanzhilfevereinbarung: 255180

Abgelaufene Finanzhilfevereinbarung

Startdatum 1 Juni 2010

Enddatum 23 Dezember 2012

Finanziert unter

• Specific programme "People" implementing the Seventh Framework Programme of the European Community for research, technological development and demonstration activities (2007 to 2013)

Gesamtkosten € 205 629,00

EU-Beitrag € 205 629,00

Koordiniert durch

AARHUS UNIVERSITET
Denmark

Final Report Summary - PRECISE (Predicting eutrophication and climate change impacts on shallow lake ecology and biodiversity: disentangling the effects of temperature and nutrients)

Predicting eutrophication and climate change impact on shallow lake ecology and biodiversity: disentangling the effects of nutrients and temperature.

Shallow lakes have been profoundly affected by increases in the concentration of key nutrients for plant growth, nitrogen and phosphorus - which are the result of more intensive agriculture and increased human population. This phenomenon, termed eutrophication has impacts that are characterised by a change from clear to turbid water. This involves the loss of submerged macrophytes, which are replaced by a dense phytoplankton crop and a more species poor community at nearly all trophic levels. In combination with the effects of eutrophication fresh waters are currently threatened by climate change, both current and future. This project aimed to investigate the effects of nutrients and temperature on shallow lakes at the ecosystem level across a range of scales. The first scale used an experimental flow-through mesocosm system. This system allows the investigation of climate warming and eutrophication and the ecological structure and ecosystem function of shallow lakes. The careful design of the system allows the interactions between the two stressors to be assessed. The second phase was an intermediate temporal perspective afforded by the analysis of contemporary lake systems and the final part the long-term view was provided by palaeoecological techniques analysing remains in lake sediments.

The first section of the study used the long running climate change experiment at Aarhus University – Silkeborg (AU-S) to investigate the impact of temperature and nutrients on: a) the algal crop in different compartments of the system and b) the pathways of production. All components of the food web were sampled on two occasions to characterise seasonal variation in the food web of the mesocosms. Furthermore, in the light of previous showing that two seasonal samples might be insufficient to accurately characterise the dynamics of the different types of algal crop and their isotopic signature, the sampling programme was very significantly expanded to monthly samples across the growing season and bimonthly in the winter season. The biomass of the all the forms of primary production were measured at monthly intervals and the results demonstrate the warming does not have a clear and consistent impact on balance of benthic versus pelagic primary production. There was however, a tendency for a greater abundance of submerged plants at higher temperatures, but this abundance was less temporally stable. The second aspect of the experimental component of the project was the investigation of the isotopic composition of all components of the food web. This relied on the exhaustive monthly sampling of the isotopic signature of all components of the food-web and the nutrient sources. This change in sampling regime increased the overall focus of the project on the short-term experimental aspects of the study and increased the number of samples generated at by this phase of the project four fold. The sample were sent to UC Davis (California, USA) for analysis and the majority of samples have been analysed, however, there are some data that we are waiting to receive and therefore a full analysis not possible. The statistical analysis of the data is complicated by the non-independence of the samples, due to temporal autocorrelation. Traditional approaches such as ANOVA and multiple least squares regression are not appropriate and therefore mixed effects model have been applied to the data to determine the relative roles of nutrients and temperature in determining the variability in the isotopic composition and the shape of the food-webs. Due to the expansion of this phase of the project the full suite of results are not yet fully analysed, but a number of the objectives have been met and the potential to meet the objectives has been increased. The findings of the first section of the experimental work suggest that the variation in the relative importance of benthic and pelagic primary production increases at higher temperatures. Thus, whilst the abundance of submerged plants increases with temperature it also becomes more seasonally unstable as a result of warming with important implications for the overall biodiversity of the system. The second section of the experimental work used stable isotopic composition to investigate the role of temperature in shaping food webs. Initial results suggest that whilst the presence and abundance of aquatic plants is the most important factor affecting the food web structure, there are both direct and indirect (through the less stable macrophyte abundance) temperature effects on food web shape. Crucially it appears high resolution sampling was required to detect the between treatment differences in the experiment and thus the change in experimental design was an important step. The results derived from this objective are of direct relevance to the understanding of the process occurring in shallow lakes and the possible effects of climate change on these ecosystems.

The second part of the study had a focus on the contemporary study of lakes to investigate the impact of enrichment upon the balance of benthic and pelagic production in shallow lakes. The overall aim being to relate this change in the balance of benthic to pelagic production to biodiversity across a range trophic levels in shallow lakes. As a result of the extensive expansion of the first phase of the project, this section of the study was reduced. In place of an extensive modern survey of lakes, I analysed surface sediment remains from a 39 shallow lake sites, which had already had data on biodiversity data, for cladoceran and fish remains. Isotope analysis (13C & 15N) has been conducted on some of these samples to quantify the relative importance of benthic relative to pelagic production in each lake. Collaborations developing with colleagues in other departments in AU (in particular Geoscience and Physics) are resulting in new opportunities for analysing such low mass samples for isotopic composition. This opened up possibilities to use sedimentary remains to track long term change in shallow lake ecology using the isotopic composition of these sedimentary remains. The results show that 13C in Daphnia ephippia reflects the phytoplankton δ13C and can therefore be used, along with other elements of the food web (e.g. fish scales and invertebrate remains, to provide information on changes in food web structure over time. In particular, where fish scales are present this has allowed an estimation of change in the contribution of phytoplankton relative to benthic algae to ecosystem production. Thus, this approach provides a parsimonious means of determining the relative importance of benthic versus pelagic primary production. The results derived here increase our understanding of the effects of eutrophication on shallow lakes and how they relate to changes in biodiversity.

The final section of the study focussed on using the longer-term palaeolimnological perspective to investigate changes in biodiversity in space and over time.
Here I carried out a meta-analysis of changes in the most important aspect of biodiversity in shallow lakes, i.e. submerged macrophytes, compared with assemblage turnover of cladocerans from 20 lakes in UK and Denmark. This work demonstrated that cladoceran turnover in time is well matched to change in macrophyte assemblages, the latter being the chief control on biodiversity in shallow lakes. Furthermore, analysis of sediment cores at decadal scale resolution, for sub-fossil cladoceran, plant macrofossils analysis and 13C & 15N in Daphnia ephippia and fish scales allowed for a reconstruction of past biodiversity and aspects of ecosystem function. The novel analysis of isotopic composition of sedimentary remains allowed the balance of benthic and pelagic primary production to be inferred. This demonstrated that the shifts from benthic to pelagic primary production occurred prior to changes in the cladoceran assemblage and that changes occurred gradually rather than dramatically. The objectives of using palaeolimnological techniques to track change in biodiversity of shallow lakes over decadal to centennial scales has been achieved and resulted in a number of papers already published (Davidson et al. 2011; Davidson et al. 2013). The latter is the first published work using palaeo-remains to track centennial scale change in shallow lake diversity in a number of aspects of the food web, including primary producers and grazers. The objective of disentangling the impacts of climate change from eutrophication has proved difficult with these data, as the effects of temperature and nutrient enrichment are similar and thus establishing cause is difficult. The results of this study have increased our understanding of the effects of eutrophication on shallow lakes and how they relate to changes in biodiversity.
Davidson T.A. Bennion H., Clarke G.H. Jeppesen E., Sayer C.D. Morley D., Odgaard B.V. Rasmussen, P., Rawcliffe R., Salgado J., Simpson G.L. & Amsink S.L. (2011) The role of cladocerans in tracking long-term change in shallow lake trophic status. Hydrobiologia, 676 299-315.
Davidson T.A. Reid M. Sayer C.D. & Chilcott. (2013) Palaeolimnological records of shallow-lake biodiversity change: exploring the merits of single versus multi-proxy approaches. Journal of Palaeolimnology 49: 431–446.