Effects of global warming on shallow freshwater lakes – a long-term experimental study
Introduction
In a long-term factorial mesocosm experiment the role of interactions between simulated climate warming and eutrophication for the biological structure and ecosystem processes in shallow lake is studied.
Experimental design and set-up
Artificial shallow lake ecosystems have been established in 24 outdoor tanks, each containing 0.20m of sediment and having a water level of 1.1m (total volume: 2800 litres). Water and sediment from many lakes have been added to the tanks to assure plant establishment and a mixed benthos and plankton inoculum. A flow-through system with addition of tap water several times daily and an overflow outlet keeps the volume in the tanks constant and ensure a 3- month water residence period. A mechanically operated paddle regularly mixes the water column.
Treatments
A factorial design combining 3 temperatures with 2 nutrient levels in 4 replicates is used. Besides the natural in situ reference temperature, IPCC climate model scenario A2 and A2+50% (year 2071-2100) are employed as global warming scenarios, while high (250 µg TP l-1) and low (25 µg TP l-1) nutrient concentrations are used to induce the turbid and the clear shallow lake, respectively. Fish (three-spined-stickleback (Gasterosteus aculeatus) males) are added to the tanks in natural densities: 1 and 12 to the low and high nutrient treatment, respectively.
Heating system
Three electrically powered and computer-operated heating elements located 0.05-0.10m above the sediment warm up the water in the heated tanks to meet the required temperature difference relative to the unheated tanks. This temperature difference is adjusted monthly according to the calculations made by DMI for the global warming scenario A2 and A2+50%.
Strategy and time schedule
The experiment will focus on:
- Short-term effects of warming (2 weeks before until 8 weeks after initiating heating)
- Longer term effects of warming (= 1.5 year)
- Recovery after warming (several months after end of heating)
What is studied?
- nutrient and carbon mass balances, sediment-water exchange
- community production and respiration
- community structure in the benthic, epiphytic and pelagic habitats
- dynamics of the macrophytes
- production of selected functional groups
- phytoplankton physiology
- stable isotopes (including addition of 15N the second year)
- benthic-pelagic interactions, top-down versus bottom-up control
- Genetic analyses of Daphnia
- Daphnia-parasite interactions
Major hypotheses to be tested
We expect that higher temperature will generate:
- higher internal P-loading - lower net retention of P
- lower N loss in the turbid tanks and higher loss in the clear tanks
- higher risk of dominance by filamentous algae in summer and better survival of macrophytes during winter (if ice cover occurs naturally for prolonged periods)
- higher phytoplankton biomass and more cyanobacteria/green algae in the turbid tanks - higher risk of clear tanks to become turbid during summer
- lower top-down control on algae in turbid tanks (as higher production of phytoplankton than of grazers and less benthic facilitation of grazers are expected). Higher top-down control on phytoplankton and periphyton in low TP tanks if they stay clear (as more plants, more refuges for grazers, higher benthic facilitation and higher growth and abundance of large grazers like snails is expected).
Who?
The experiments are conducted by the National Environmental Research Institute (NERI) and the Universities of Copenhagen (KU) and Aarhus (AU) in co-operation, while the Danish Meteorological Institute (DMI) provides the modelled temperature differences downgraded to local Danish level.